Telecommunications

Network switching subsystem (NSS)

2009-03-20T23:33:00.000-07:00

Network switching subsystem (NSS) is the component of a GSM system that carries out switching functions and manages the communications between mobile phones and the Public Switched Telephone Network (PSTN). It is owned and deployed by mobile phone operators and allows mobile phones to communicate with each other and telephones in the wider telecommunications network. The architecture closely resembles a telephone exchange, but there are additional functions which are needed because the phones are not fixed in one location. Each of these functions handle different aspects of mobility management and are described in more detail below.
The Network Switching Subsystem, also referred to as the GSM core network, usually refers to the circuit-switched core network, used for traditional GSM services such as voice calls, SMS, and circuit switched data calls.
There is also an overlay architecture on the GSM core network to provide packet-switched data services and is known as the GPRS core network. This allows mobile phones to have access to services such as WAP, MMS, and Internet access.
All mobile phones manufactured today have both circuit and packet based services, so most operators have a GPRS network in addition to the standard GSM core network.

Equipment identity register (EIR)

2009-03-20T23:29:00.000-07:00

Equipment identity register (EIR)
The equipment identity register is often integrated to the HLR. The EIR keeps a list of mobile phones (identified by their IMEI) which are to be banned from the network or monitored. This is designed to allow tracking of stolen mobile phones. In theory all data about all stolen mobile phones should be distributed to all EIRs in the world through a Central EIR. It is clear, however, that there are some countries where this is not in operation. The EIR data does not have to change in real time, which means that this function can be less distributed than the function of the HLR. The EIR is a database that contains information about the identity of the mobile equipment that prevents calls from stolen, unauthorized or defective mobile stations. Some EIR also have the capability to log Handset attempts and store it in a log file.

[edit] Other support functions
Connected more or less directly to the GSM core network are many other functions.

[edit] Billing centre (BC)
The billing centre is responsible for processing the toll tickets generated by the VLRs and HLRs and generating a bill for each subscriber. It is also responsible for to generate billing data of roaming subscriber.

[edit] Short message service centre (SMSC)
The short message service centre supports the sending and reception of text messages.

[edit] Multimedia messaging service centre (MMSC)
The multimedia messaging service centre supports the sending of multimedia messages (e.g., images, audio, video and their combinations) to (or from) MMS-enabled Handsets.

[edit] Voicemail system (VMS)
The voicemail system records and stores voicemails

Visitor location register (VLR)

2009-03-20T23:28:00.000-07:00

[edit] Visitor location register (VLR)

[edit] Description
The visitor location register is a temporary database of the subscribers who have roamed into the particular area which it serves. Each base station in the network is served by exactly one VLR, hence a subscriber cannot be present in more than one VLR at a time.
The data stored in the VLR has either been received from the HLR, or collected from the MS. In practice, for performance reasons, most vendors integrate the VLR directly to the V-MSC and, where this is not done, the VLR is very tightly linked with the MSC via a proprietary interface.
Data stored include:
IMSI (the subscriber's identity number).
Authentication data.
MSISDN (the subscriber's phone number).
GSM services that the subscriber is allowed to access.
access point (GPRS) subscribed.
The HLR address of the subscriber.

[edit] Other GSM core network elements connected to the VLR
The VLR connects to the following elements:
The V-MSC to pass needed data for its procedures; e.g., authentication or call setup.
The HLR to request data for mobile phones attached to its serving area.
Other VLRs to transfer temporary data concerning the mobile when they roam into new VLR areas. For example, the temporal mobile subscriber identity (TMSI).

[edit] Procedures implemented
The primary functions of the VLR are:
To inform the HLR that a subscriber has arrived in the particular area covered by the VLR.
To track where the subscriber is within the VLR area (location area) when no call is ongoing.
To allow or disallow which services the subscriber may use.
To allocate roaming numbers during the processing of incoming calls.
To purge the subscriber record if a subscriber becomes inactive whilst in the area of a VLR. The VLR deletes the subscriber's data after a fixed time period of inactivity and informs the HLR (e.g., when the phone has been switched off and left off or when the subscriber has moved to an area with no coverage for a long time).
To delete the subscriber record when a subscriber explicitly moves to another, as instructed by the HLR.

Authentication centre (AUC)

2009-03-20T23:25:00.000-07:00

Authentication centre (AUC)

[edit] Description
The authentication centre (AUC) is a function to authenticate each SIM card that attempts to connect to the GSM core network (typically when the phone is powered on). Once the authentication is successful, the HLR is allowed to manage the SIM and services described above. An encryption key is also generated that is subsequently used to encrypt all wireless communications (voice, SMS, etc.) between the mobile phone and the GSM core network.
If the authentication fails, then no services are possible from that particular combination of SIM card and mobile phone operator attempted. There is an additional form of identification check performed on the serial number of the mobile phone described in the EIR section below, but this is not relevant to the AUC processing.
Proper implementation of security in and around the AUC is a key part of an operator's strategy to avoid SIM cloning.
The AUC does not engage directly in the authentication process, but instead generates data known as triplets for the MSC to use during the procedure. The security of the process depends upon a shared secret between the AUC and the SIM called the Ki. The Ki is securely burned into the SIM during manufacture and is also securely replicated onto the AUC. This Ki is never transmitted between the AUC and SIM, but is combined with the IMSI to produce a challenge/response for identification purposes and an encryption key called Kc for use in over the air communications.

[edit] Other GSM core network elements connected to the AUC
The AUC connects to the following elements:
the MSC which requests a new batch of triplet data for an IMSI after the previous data have been used. This ensures that same keys and challenge responses are not used twice for a particular mobile.

[edit] Procedures implemented
The AUC stores the following data for each IMSI:
the Ki
Algorithm id. (the standard algorithms are called A3 or A8, but an operator may choose a proprietary one).
When the MSC asks the AUC for a new set of triplets for a particular IMSI, the AUC first generates a random number known as RAND. This RAND is then combined with the Ki to produce two numbers as follows:
The Ki and RAND are fed into the A3 algorithm and the signed response (SRES) is calculated.
The Ki and RAND are fed into the A8 algorithm and a session key called Kc is calculated.
The numbers (RAND, SRES, Kc) form the triplet sent back to the MSC. When a particular IMSI requests access to the GSM core network, the MSC sends the RAND part of the triplet to the SIM. The SIM then feeds this number and the Ki (which is burned onto the SIM) into the A3 algorithm as appropriate and an SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet (which it should if it is a valid SIM), then the mobile is allowed to attach and proceed with GSM services.
After successful authentication, the MSC sends the encryption key Kc to the base station controller (BSC) so that all communications can be encrypted and decrypted. Of course, the mobile phone can generate the Kc itself by feeding the same RAND supplied during authentication and the Ki into the A8 algorithm.
The AUC is usually collocated with the HLR, although this is not necessary. Whilst the procedure is secure for most everyday use, it is by no means crack proof. Therefore a new set of security methods was designed for 3G phones.

Home location register (HLR)

2009-03-20T23:17:00.000-07:00

Home location register (HLR)

[edit] Description
The home location register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There can be several logical, and physical, HLRs per public land mobile network (PLMN), though one international mobile subscriber identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several physical nodes) at a time.
The HLR stores details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record.
The next important items of data associated with the SIM are the MSISDNs, which are the telephone numbers used by mobile phones to make and receive calls. The primary MSISDN is the number used for making and receiving voice calls and SMS, but it is possible for a SIM to have other secondary MSISDNs associated with it for fax and data calls. Each MSISDN is also a primary key to the HLR record. The HLR data is stored for as long as a subscriber remains with the mobile phone operator.
Examples of other data stored in the HLR against an IMSI record is:
GSM services that the subscriber has requested or been given.
GPRS settings to allow the subscriber to access packet services.
Current location of subscriber (VLR and serving GPRS support node/SGSN).
Call divert settings applicable for each associated MSISDN.
The HLR is a system which directly receives and processes MAP transactions and messages from elements in the GSM network, for example, the location update messages received as mobile phones roam around.

[edit] Other GSM core network elements connected to the HLR
The HLR connects to the following elements:
The G-MSC for handling incoming calls
The VLR for handling requests from mobile phones to attach to the network
The SMSC for handling incoming SMS
The voice mail system for delivering notifications to the mobile phone that a message is waiting
The AUC for authentication and ciphering and exchange of data (triplets)

[edit] Procedures implemented
The main function of the HLR is to manage the fact that SIMs and phones move around a lot. The following procedures are implemented to deal with this:
Manage the mobility of subscribers by means of updating their position in administrative areas called 'location areas', which are identified with a LAC. The action of a user of moving from one LA to another is followed by the HLR with a Location area update while retrieving information from BSS as base station identity code (BSIC).
Send the subscriber data to a VLR or SGSN when a subscriber first roams there.
Broker between the G-MSC or SMSC and the subscriber's current VLR in order to allow incoming calls or text messages to be delivered.
Remove subscriber data from the previous VLR when a subscriber has roamed away from it.

Mobile switching center (MSC)

2009-03-20T23:07:00.000-07:00

Mobile switching center (MSC)

Description
The mobile switching center (MSC) is the primary service delivery node for GSM, responsible for handling voice calls and SMS as well as other services (such as conference calls, FAX and circuit switched data). The MSC sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring.
In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s timeslot, known as a DS0 in America).
There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.
The gateway MSC (G-MSC) is the MSC that determines which visited MSC the subscriber who is being called is currently located. It also interfaces with the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed through a G-MSC. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufacturers design dedicated high capacity MSCs which do not have any BSSs connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.
The visited MSC (V-MSC) is the MSC where a customer is currently located. The VLR associated with this MSC will have the subscriber's data in it.
The anchor MSC is the MSC from which a handover has been initiated. The target MSC is the MSC toward which a Handover should take place. A mobile switching centre server is a part of the redesigned MSC concept starting from 3GPP Release 5.

General Architecture of the BTS.

2009-03-20T23:05:00.000-07:00

A BTS in general has the following units:
Transceiver (TRX)
Quite widely referred to as the driver receiver (DRX). Basically does transmission and reception of signals. Also does sending and reception of signals to/from higher network entities (like the base station controller in mobile telephony)
Power amplifier (PA)
Amplifies the signal from DRX for transmission through antenna; may be integrated with DRX.
Combiner
Combines feeds from several DRXs so that they could be sent out through a single antenna. Allows for a reduction in the number of antenna used.
Duplexer
For separating sending and receiving signals to/from antenna. Does sending and receiving signals through the same antenna ports (cables to antenna).
Antenna
This is also considered a part of the BTS.
Alarm extension system
Collects working status alarms of various units in the BTS and extends them to operations and maintenance (O&M) monitoring stations.
Control function
Control and manages the various units of BTS including any software. On-the-spot configurations, status changes, software upgrades, etc. are done through the control function.
Baseband receiver unit (BBxx)
Frequency hopping, signal DSP, etc..

BTS in Mobile Communication

2009-03-20T23:00:00.000-07:00

BTS in Mobile Communication
A GSM BTS network is made up of three subsystems: • The Mobile Station (MS) • The Base Station Sub-system (BSS) – comprising a BSC and several BTSs • The Network and Switching Sub-system (NSS) – comprising an MSC and associated registers
Though the term BTS can be applicable to any of the wireless communication standards, it is generally and commonly associated with mobile communication technologies like GSM and CDMA. In this regard, a BTS forms part of the base station subsystem (BSS) developments for system management. It may also have equipment for encrypting and decrypting communications, spectrum filtering tools (band pass filters) etc. antennas may also be considered as components of BTS in general sense as they facilitate the functioning of BTS. Typically a BTS will have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations). A BTS is controlled by a parent base station controller via the base station control function (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an operations and maintenance (O&M) connection to the network management system (NMS), and manages operational states of each TRX, as well as software handling and alarm collection. The basic structure and functions of the BTS remains the same regardless of the wireless technologies.

Telephone exchange

2009-03-20T22:59:00.000-07:00

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the speech information.
The term exchange can also be used to refer to an area served by a particular switch (typically known as a wire center in the US telecommunications industry). It is sometimes confused with other concepts of telephone geography, such as NPA or area code. More narrowly, in some areas it can refer to the first three digits of the local number. In the three-digit sense of the word, other obsolete Bell System terms include office code and NXX. In the United States, the word exchange can also have the legal meaning of a local access and transport area under the Modification of Final Judgment (MFJ).

Broadband In data communications

2009-03-20T22:53:00.000-07:00

Broadband in data can refer to broadband networks or broadband Internet and may have the same meaning as above, so that data transmission over a fiber optic cable would be referred to as broadband as compared to a telephone modem operating at 56,000 bits per second.
However, broadband in data communications is frequently used in a more technical sense to refer to data transmission where multiple pieces of data are sent simultaneously to increase the effective rate of transmission, regardless of data signaling rate. In network engineering this term is used for methods where two or more signals share a medium.[1]

[edit] In DSL
The various forms of digital subscriber line (DSL) services are broadband in the sense that digital information is sent over a high-bandwidth channel above the baseband voice channel on a single pair of wires.[1]

[edit] In Ethernet
A baseband transmission sends one type of signal using a medium's full bandwidth, as in 100BASE-T Ethernet. Ethernet, however, is the common interface to broadband modems such as DSL data links, and has a high data rate itself, so is sometimes referred to as broadband. Ethernet provided over cable modem is a common alternative to DSL.

[edit] In video
Broadband in analog video distribution is traditionally used to refer to systems such as cable television, where the individual channels are modulated on carriers at fixed frequencies.[2] In this context, baseband is the term's antonym, referring to a single channel of analog video, typically in composite form with an audio subcarrier.[3] The act of demodulating converts broadband video to baseband video.
However, broadband video in the context of streaming Internet video has come to mean video files that have bitrates high enough to require broadband Internet access in order to view them.
Broadband video is also sometimes used to describe IPTV Video on demand

Broadband In telecommunication

2009-03-20T22:43:00.000-07:00

Broadband In telecommunication

Broadband in telecommunications refers to a signaling method that includes or handles a relatively wide range of frequencies, which may be divided into channels or frequency bins. Broadband is always a relative term, understood according to its context. The wider the bandwidth, the greater the information-carrying capacity. In radio, for example, a very narrow-band signal will carry Morse code; a broader band will carry speech; a still broader band is required to carry music without losing the high audio frequencies required for realistic sound reproduction. A television antenna described as "normal" may be capable of receiving a certain range of channels; one described as "broadband" will receive more channels. In data communications an analogue modem will transmit a bandwidth of 56 kilobits per seconds (kbit/s) over a telephone line; over the same telephone line a bandwidth of several megabits per second can be handled by ADSL, which is described as broadband (relative to a modem over a telephone line, although much less than can be achieved over a fibre optic circuit).

CDMA Technology

2009-03-20T22:40:00.000-07:00

CDMA Technology

Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), which use CDMA as an underlying channel access method.
One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a bandwidth of frequencies. This concept is called multiplexing. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.
An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

TDMA Technology

2009-03-20T22:37:00.001-07:00

TDMA Technology

Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using his own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, and combat-net radio systems. For usage of Dynamic TDMA packet mode communication, see below.

TDMA frame structure showing a data stream divided into frames and those frames divided into time slots.
TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.

FDMA Technology

2009-03-20T22:31:00.000-07:00

FDMA Technology

Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, allowing them to utilize the allocated radio spectrum without interfering with each other. Multiple Access systems coordinate access between multiple users. The users may also share access via different methods such TDMA, CDMA, or SDMA. These protocols are utilized differently, at different levels of the theoretical OSI model.

FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
FDMA is not vulnerable to timing problems as TDMA. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.
Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA.

Control System

2009-03-20T22:28:00.000-07:00

Control System

A control system is a device or set of devices to manage, command, direct or regulate the behavior of other devices or systems.
There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls. There is also fuzzy logic, which attempts to combine some of the design simplicity of logic with the utility of linear control. Some devices or systems are inherently not controllable.
The term "control system" may be applied to the essentially manual controls that allow an operator to, for example, close and open a hydraulic press, where the logic requires that it cannot be moved unless safety guards are in place.
An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. For example various electric and pneumatic transducers may fold and glue a cardboard box, fill it with product and then seal it in an automatic packaging machine.
In the case of linear feedback systems, a control loop, including sensors, control algorithms and actuators, is arranged in such a fashion as to try to regulate a variable at a setpoint or reference value. An example of this may increase the fuel supply to a furnace when a measured temperature drops. PID controllers are common and effective in cases such as this. Control systems that include some sensing of the results they are trying to achieve are making use of feedback and so can, to some extent, adapt to varying circumstances. Open-loop control systems do not directly make use of feedback, but run only in pre-arranged ways.

RF Engineering

2009-03-20T22:22:00.000-07:00

RF Engineering, also known as Radio Frequency Engineering is a field, in particular within electronics, that deals with devices which are design to operate in the Radio Frequency spectrum. These devices operate within the range of about 3 Hz up to 300 GHz.
RF Engineering is incorporated into almost everything that transmits or receives a radio wave which includes, but not limited to, Mobile Phones, Radios, WiFi and walkie talkies.
RF Engineering is a specialized field, to properly produce quality results an in-depth knowledge of Mathematics, Physics and general electronics theory is required. Even with this, the initial design of an RF Circuit usually bears very little resemblance to the final physical circuit produced as alteration to the design is often required to achieve the required results.

Teletraffic Engineering

2009-03-20T22:16:00.000-07:00

Introduction
Traffic engineering uses statistical techniques such as queuing theory to predict and engineer the behaviour of telecommunications networks such as telephone networks or the Internet.
The field was created by the work of A. K. Erlang in whose honour the unit of telecommunications traffic intensity, the Erlang, is named. The derived unit of traffic volume also incorporates his name. His Erlang distributions are still in common use in telephone traffic engineering.
The crucial observation in traffic engineering is that in large systems the law of large numbers can be used to make the aggregate properties of a system over a long period of time much more predictable than the behaviour of individual parts of the system.
The queueing theory originally developed for circuit-switched networks is applicable to packet-switched networks.
The most notable difference between these sub-fields is that packet-switched data traffic is self-similar. This is a consequence of the calls being between computers, and not people.

Teletraffic Engineering

Teletraffic engineering is the application of traffic engineering theory to telecommunications. Teletraffic engineers use their basic knowledge of statistics including Queueing theory, the nature of traffic, their practical models, their measurements and simulations to make predictions and to plan telecommunication networks at minimum total cost. These tools and basic knowledge help provide reliable service at lower cost. Because the approach is so different to different networks, the networks are handled separately here: the PSTN, broadband networks, mobile networks, and networks where the possibility of traffic being heavy is more frequent than anticipated

Air Traffic Control

2009-03-20T22:14:00.001-07:00

Air Traffic Control
Air traffic control radar at London Heathrow Airport
Air Traffic Control Systems
Air Traffic Control (ATC) Radars
Secondary Surveillance Radar (SSR) (Airport Surveillance Radar)
Ground Control Approach (GCA) Radars
Precision Approach Radar (PAR) Systems
Distance Measuring Equipment (DME)
Radio Beacons
Radar Altimeter (RA) Systems
Terrain-Following Radar (TFR) Systems

Radar Antenna System

2009-03-20T22:07:00.000-07:00

Antenna design
Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.
Early systems tended to use omni-directional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other shows a minimum.
One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional

Radar System

2009-03-20T21:54:00.000-07:00

Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for radio detection and ranging.[1][2][3] The term has since entered the English language as a standard word, radar, losing the capitalization. Radar was originally called RDF (Radio Direction Finder) in the United Kingdom.
A radar system has a transmitter that emits either microwaves or radio waves that are reflected by the target and detected by a receiver, typically in the same location as the transmitter. Although the signal returned is usually very weak, the signal can be amplified. This enables radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar is used in many contexts, including meteorological detection of precipitation, measuring ocean surface waves, air traffic control, police detection of speeding traffic, and by the military.

Optical Fiber Communication

2009-03-20T04:19:00.000-07:00

An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.
Light is kept in the core of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those which can only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 550 meters (600 yards).
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections

Wireless Local Loop

2009-03-19T09:46:00.000-07:00

Definition and Overview
DefinitionSometimes called radio in the loop (RITL) or fixed-radio access (FRA), WLL is a system that connects subscribers to the public switched telephone network (PSTN) using radio signals as a substitute for copper for all or part of the connection between the subscriber and the switch. This includes cordless access systems, proprietary fixed radio access, and fixed cellular systems.
OverviewIndustry analysts predict that the global WLL market will reach millions of subscribers by the year 2000. Much of this growth will occur in emerging economies where half the world's population lacks plain old telephone service (POTS). Developing nations like China, India, Brazil, Russia, and Indonesia look to WLL technology as an efficient way to deploy POTS for millions of subscribers—without the expense of burying tons of copper wire.
In developed economies, WLL will help unlock competition in the local loop, enabling new operators to bypass existing wireline networks to deliver POTS and data access. So the question isn't will the local loop go wireless, but when and where. This tutorial discusses the basics of WLL and examines the markets and future for this technology.

Cordless Systems

2009-03-19T09:39:00.000-07:00

Cordless Systems
nResidential – a single base station can provide in-house voice and data support
nOffice
nA single base station can support a small office
nMultiple base stations in a cellular configuration can support a larger office
nTelepoint – a base station set up in a public place, such as an airport .

nModest range of handset from base station, so low-power designs are used
nInexpensive handset and base station, dictating simple technical approaches
nFrequency flexibility is limited, so the system needs to be able to seek a low-interference channel whenever used

Satellite Communications

2009-03-19T09:34:00.000-07:00

Satellite ,is in effect,a microwave relay station.It is used to link two or more ground–base microwave transmitter/receiver,known as earth stations
Some satellites are natural, like the Moon, which is the natural satellite of the earth. Other satellites are made by scientists and technologists to go around the earth and do certain jobs

A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits.
For fixed (point-to-point) services, communications satellites provide a microwave radio relay technology complementary to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio broadcasting, for which application of other technologies, such as cable, is impractical or impossible.

What is ATM ?

2009-03-19T09:30:00.000-07:00

ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.
Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).
ATM also stands for automated teller machine, a machine that bank customers use to make transactions without a human teller.

Short for Asynchronous Transfer Mode, a network technology based on transferring data in cells or packets of a fixed size. The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line.

What is VOIP?

2009-03-19T09:20:00.000-07:00

What is VoIP?

VoIP (voice over IP) is an IP telephony term for a set of facilities used to manage the delivery of voice information over the Internet.VoIP involves sending voice information in digital form in discrete packets rather than by using the traditional circuit-committed protocols of the public switched telephone network (PSTN). A major advantage of VoIP and Internet telephony is that it avoids the tolls charged by ordinary telephone service

Introduction VOIP is an acronym for Voice Over Internet Protocol, or in more common terms phone service over the Internet. If you have a reasonable quality Internet connection you can get phone service delivered through your Internet connection instead of from your local phone company. Some people use VOIP in addition to their traditional phone service, since VOIP service providers usually offer lower rates than traditional phone companies, but sometimes doesn't offer 911 service, phone directory listings, 411 service, or other common phone services. While many VoIP providers offer these services, consistent industry-wide means of offering these are still developing.

ISDN Technology

2009-03-19T09:14:00.000-07:00

Integrated Services Digital Network (ISDN)

Definition: ISDN is a network technology that supports digital transfer of simultaneous voice and data traffic. Similar to DSL in this respect, an ISDN Internet service works over ordinary telephone lines. ISDN Internet service generally supports data rates of 128 Kbps.
ISDN emerged as an alternative to traditional dialup networking during the 1990s. The relatively high cost of ISDN service, though, limited its popularity with residential customers at the outset. More recently, the much higher network speeds supported by newer broadband technologies like DSL have drawn many consumers away from ISDN service.
ISDN technology today has limited applications as a networking solution. Some customers who live in rural areas of the U.S. subscribe to ISDN Internet as an alternative to satellite Internet. ISDN phone service also remains fairly common in some European countries.
Also Known As: Integrated Services Digital Network

Metropolitan Area Network (MAN) Network

2009-03-19T06:28:00.000-07:00

Metropolitan Area Network (MAN)

A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. MANs can also depend on communications channels of moderate-to-high data rates. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations. MANs might also be owned and operated as public utilities. They will often provide means for internetworking of local networks. Metropolitan area networks can span up to 50km, devices used are modem and wire/cable

A Metropolitan Area Network (MAN) is a large computer network that spans a metropolitan area or campus. Its geographic scope falls between a WAN and LAN. MANs provide Internet connectivity for LANs in a metropolitan region, and connect them to wider area networks like the Internet.

WAN Networks

2009-03-19T06:20:00.000-07:00

What is a WAN?

A wide area network (WAN) is a geographically dispersed telecommunications network. The term distinguishes a broader telecommunication structure from a local area network (LAN). A wide area network may be privately owned or rented, but the term usually connotes the inclusion of public (shared user) networks. An intermediate form of network in terms of geography is a metropolitan area network (MAN.

WAN (Wide Area Network)
Covers a long distance [more than 1 kilometer ( > 1 km)]
Usually spans several locations (even world wide)
Usually has a larger number of users (100's or even 1000's)
Implemented as a client-server model

LAN Network

2009-03-19T06:15:00.000-07:00

Definition: A local area network (LAN) supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. A LAN is useful for sharing resources like files, printers, games or other applications. A LAN in turn often connects to other LANs, and to the Internet or other WAN.
Most local area networks are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs. Wireless LAN and other more advanced LAN hardware options also exist.

Definition: local area network (LAN): A computer network that covers a relatively small area. Most LANs cover a single building or group of buildings. A system of LANs can be connected over any distance through telephone lines and radio waves, creating a wide-area network.

2009-03-19T06:10:00.000-07:00

NETWORK: Two or more computers which are linked together and which share resources like data, programs and peripherals (like printers) are networked.

Types of Networks
LAN (Local Area Network)
Covers a short distance (usually less than 1 kilometer [<> 1 km)]
Usually spans several locations (even world wide)
Usually has a larger number of users (100's or even 1000's)
Implemented as a client-server model Peer to Peer Networks
All computers on the network have the potential to share resources that they have control over
NOT a client-server model (in the strict sense of that concept)
usually confined to a small area (an office or a lab)
Usually has a small number of users (around 2 to 25 users)
considered to be less secure than a LAN or a WAN
see above for more details NOTE: you may have hybrids (combinations) of LANs, WANs and Peer-to-Peer networks
Intranets vs The Internet
The Internet is a collection thousands of Wide-Area and Local-Area networks with servers (http, ftp, chat, ...) on them. The Internet uses TCP/IP as the primary networking protocol to communicate with all these machines.
Intranets are local networks that do not have access to the outside world (they are self-contained). They are like the Internet but they are private.

DSL service of Internet

2009-03-19T05:49:00.000-07:00

DSL (Digital Subscriber Line ):-

•DSL is a broadband data communications technology that uses uses existing copper local loop telephone connections.
•The term “DSL” has relevance that also extends across both ISDN and “T Carrier” service.
•In the majority of new installations, and for purposes here, DSL is a digital data service that uses frequencies above the voice band to provide fast (up to 1.5 Mbs) network connections

DSL Las Vegas NV services is a method for moving data over regular phone lines. A dsl internet service provider offers a DSL circuit that is much faster than a regular phone connection, and the wires coming into the subscriber?s premises are the same (copper) wires used for regular phone service. A DSL circuit must be configured to connect two specific locations, similar to a leased line. A commonly discussed configuration of Las Vegas dsl allows downloads at speeds of up to 1.544 megabits (not megabytes) per second, and uploads at speeds of 128 kilobits per second. This arrangement is called ADSL: ?Asymmetric? Digital Subscriber Line. Another common configuration is symmetrical: 384 Kilobits per second in both directions. In theory ADSL allows download speeds of up to 9 megabits per second and upload speeds of up to 640 kilobits per second. DSL connections are now a popular alternative to Leased Lines and ISDN, being faster than ISDN and less costly than traditional Leased Lines. An advantage of using Las Vegas dsl connections over traditional cable lines is that DSL is more secure. Las Vegas DSL service is not a shared network service like cable modems, which means that one end-user can not see what's on a neighbor's computer using DSL. The dsl internet service is a private dedicated connection from the end-user to the network.

Mobiles Theory

2009-03-18T04:08:00.000-07:00

A mobile phone (also known as a handphone,[1] wireless phone, cell phone, cellular phone, cellular telephone or cell telephone) is a long-range, electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites. In addition to the standard voice function of a mobile phone, telephone, current mobile phones may support many additional services, and accessories, such as SMS for text messaging, email, packet switching for access to the Internet, gaming, Bluetooth, infrared, camera with video recorder and MMS for sending and receiving photos and video, MP3 player, radio and GPS. Most current mobile phones connect to a cellular network of base stations (cell sites), which is in turn interconnected to the public switched telephone network (PSTN) (the exception is satellite phones).
A mobile phone proper typically has a telephone keypad, more advanced devices have a separate key for each letter. Some mobile phones have a touchscreen.
Contents[hide]
1 History
2 Handsets
2.1 Features
2.2 Applications
2.3 Media
2.4 Power supply
2.5 SIM card
3 Terminology
3.1 Related non-mobile-phone systems
4 Privacy
5 See also
6 References
7 Further reading
8 External links
//

Types of Laptops

2009-03-18T03:58:00.000-07:00

Laptops

Laptops have evolved from heavy, single-purpose models which ran basic software and functions. You are now able to buy a laptop which is more suited to your personal computing needs, which means there are a variety of laptop types on the market.The standard power laptop provides you with a fast and efficient computing experience. For more graphical and dynamic features, you may want to consider the gaming laptops. If you're after a super light, portable machine go for a lightweight laptop which won't hold you down. Alternatively, you could think about using one of newer laptop types around called a dual-core laptop, which provides faster CPU and better performance than ever.

Multimedia Laptops
If you are looking for a laptop to perform multimedia tasks like record TV, play movies, organize and play music, etc… then you may also want to check for the proper connectivity for the devices you will be using with it. For example, if you want to plug your laptop into your 52" high definition TV, make sure you have an HDMI port on the laptop, or a composite video out.
If you want to record live TV, make sure the laptop has a video in ports so you can connect the TV signal. Multimedia machines do not always offer these features on lower priced models so make sure to check. Multimedia laptops also need to have a decent amount of memory in them; I would recommend no less than 1GB. Multimedia computers usually have a DVD burner and a good high definition widescreen display as well for watching movies and videos.

wimax technologies

2009-03-17T10:11:00.000-07:00

WiMAX is a wireless digital communications system, also known as IEEE 802.16, that is intended for wireless "metropolitan area networks". WiMAX can provide broadband wireless access (BWA) up to 30 miles (50 km) for fixed stations, and 3 - 10 miles (5 - 15 km) for mobile stations. In contrast, the WiFi/802.11 wireless local area network standard is limited in most cases to only 100 - 300 feet (30 - 100m). With WiMAX, WiFi-like data rates are easily supported, but the issue of interference is lessened. WiMAX operates on both licensed and non-licensed frequencies, providing a regulated environment and viable economic model for wireless carriers.WiMAX can be used for wireless networking in much the same way as the more common WiFi protocol. WiMAX is a second-generation protocol that allows for more efficient bandwidth use, interference avoidance, and is intended to allow higher data rates over longer distances. The IEEE 802.16 standard defines the technical features of the communications protocol. The WiMAX Forum offers a means of testing manufacturer's equipment for compatibility, as well as an industry group dedicated to fostering the development and commercialization of the technology.WiMax.com provides a focal point for consumers, service providers, manufacturers, analysts, and researchers who are interested in WiMAX technology, services, and products. Soon, WiMAX will be a very well recognized term to describe wireless Internet access throughout the world.

Bluetooth Technology

2009-03-17T06:23:00.000-07:00

Definition: Bluetooth is a specification for the use of low-power radio communications to wirelessly link phones, computers and other network devices over short distances. The name Bluetooth is borrowed from Harald Bluetooth, a king in Denmark more than 1,000 years ago.
Bluetooth technology was designed primarily to support simple wireless networking of personal consumer devices and peripherals, including cell phones, PDAs, and wireless headsets. Wireless signals transmitted with Bluetooth cover short distances, typically up to 30 feet (10 meters).

Why is the technology called Bluetooth
The heart of the Bluetooth brand identity is the name, which refers to the Danish king Harald "Bluetooth" Blaatand who unified Denmark and Norway. In the beginning of the Bluetooth wireless technology era, Bluetooth was aimed at unifying the telecom and computing industries. Bluetooth can be used to wirelessly synchronize and transfer data among devices. Bluetooth can be thought of as a cable replacement technology. Typical uses include automatically synchronizing contact and calendar information among desktop, notebook and palmtop computers without connecting cables. Bluetooth can also be used to access a network or the Internet with a notebook computer by connecting wirelessly to a cellular phone.
Types of Bluetooth Devices
Bluetooth Dongle
Bluetooth Dongle : Installing a Bluetooth dongle is easy; simply insert the CD that came with it, follow the on screen prompts and then plug the dongle into a free USB port. If you had a Bluetooth compatible laptop you could just plug the dongle into an internet enabled personal computer and check your e-mail, download Windows updates, or transfer files. On the same lines you could also synchronize your PDA with your personal computer and download the latest appointments, e-mails or send text messages

EDGE services

2009-03-17T06:07:00.000-07:00

Enhanced Data Rates for GSM Evolution
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"EDGE" redirects here. For other uses, see Edge.
Enhanced Data rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC) is a backward-compatible digital mobile phone technology that allows improved data transmission rates, as an extension on top of standard GSM. EDGE is considered a 3G radio technology and is part of ITU's 3G definition,[1]. EDGE was deployed on GSM networks beginning in 2003— initially by Cingular (now AT&T) in the United States.[2]
EDGE is standardized by 3GPP as part of the GSM family, and it is an upgrade that provides more than three-fold increase in both the capacity and performance of GSM/GPRS networks. It does this by introducing sophisticated methods of coding and transmitting data, delivering higher bit-rates per radio channel.
EDGE can be used for any packet switched application, such as an Internet connection. EDGE-delivered data services create a broadband internet-like experience for the mobile phone user. High-speed data applications such as video services and other multimedia benefit from EGPRS' increased data capacity.
Evolved EDGE continues in Release 7 of the 3GPP standard providing reduced latency and more than doubled performance e.g. to complement High-Speed Packet Access (HSPA). Peak bit-rates of up to 1Mbit/s and typical bit-rates of 400kbit/s can be expected.
Contents[hide]
1 Technology
1.1 Transmission techniques
1.2 EGPRS modulation and coding scheme (MCS)
1.3 Classification
1.4 EDGE Evolution
2 Networks
2.1 Americas
2.2 Europe and CIS
2.3 Africa
2.4 Middle East
2.5 Asia
2.6 Asia and Far East
2.7 Australasia
3 See also
4 References
5 External links
//

[edit] Technology
EDGE/EGPRS is implemented as a bolt-on enhancement for 2G and 2.5G GSM and GPRS networks, making it easier for existing GSM carriers to upgrade to it. EDGE/EGPRS is a superset to GPRS and can function on any network with GPRS deployed on it, provided the carrier implements the necessary upgrade.
EDGE requires no hardware or software changes to be made in GSM core networks. EDGE compatible transceiver units must be installed and the base station subsystem needs to be upgraded to support EDGE. If the operator already has this in place, which is often the case today, the network can be upgraded to EDGE by activating an optional software feature. Today EDGE is supported by all major chip vendors for both GSM and WCDMA/HSPA.

[edit] Transmission techniques
In addition to Gaussian minimum-shift keying (GMSK), EDGE uses higher-order PSK/8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.
EDGE can carry data speeds up to 236.8 kbit/s (with end-to-end latency of less than 150 ms) for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots) in packet mode. This means it can handle four times as much traffic as standard GPRS. EDGE meets the International Telecommunications Union's requirement for a 3G network, and has been accepted by the ITU as part of the IMT-2000 family of 3G standards. It also enhances the circuit data mode called HSCSD, increasing the data rate of this service.

[edit] EGPRS modulation and coding scheme (MCS)
EDGE is four times as efficient as GPRS. GPRS use four coding schemes (CS-1 to 4) while EDGE uses nine Modulation and Coding Schemes (MCS-1 to 9).
Coding and modulation scheme (MCS)
Speed (kbit/s/slot)
Modulation
MCS-1
8.80
GMSK
MCS-2
11.2
GMSK
MCS-3
14.8
GMSK
MCS-4
17.6
GMSK
MCS-5
22.4
8-PSK
MCS-6
29.6
8-PSK
MCS-7
44.8
8-PSK
MCS-8
54.4
8-PSK
MCS-9
59.2
8-PSK

[edit] Classification
EDGE is part of ITU's 3G definition and is considered a 3G radio technology.[1]

[edit] EDGE Evolution
EDGE Evolution improves on EDGE in a number of ways. Latencies are reduced by lowering the Transmission Time Interval by half (from 20 ms to 10 ms). Bit rates are increased up to 1 MBit/s peak speed and latencies down to 800 ms using dual carriers, higher symbol rate and higher-order modulation (32QAM and 16QAM instead of 8-PSK), and turbo codes to improve error correction. And finally signal quality is improved using dual antennas improving average bit-rates and spectrum efficiency. EDGE Evolution can be gradually introduced as software upgrades, taking advantage of the installed base. With EDGE Evolution, end-users will be able to experience mobile internet connections corresponding to a 500 kbit/s ADSL service.

[edit] Networks
The Global mobile Suppliers Association (GSA) states that,[3] as of January 2009, there were 413 GSM/EDGE networks in 177 countries, from a total of 441 mobile network operator commitments in 184 countries.
The following companies have EDGE networks in operation:

Cellular Service Information by Carrier

2009-03-17T06:03:00.000-07:00

General GSM Information
What is GSM?
What is a SIM card?
Is this like a calling card?
I currently have a GSM Phone. Will it work overseas?
What is a locked cell phone?
How is the cellular coverage in other countries?
What are PIN and PUK numbers?
How do I install a SIM card?
What is Bluetooth™?
Cellular Abroad Questions
Is it better to buy or rent the phones from you?
How can I be sure the technology won't become obsolete in the future?
How many minutes come with the SIM card?
If the SIM card costs $80, do I get $80 worth of talk time?
What are the calling rates and purchase and rental prices?
How do I calculate my rental period?
I live in the LA area. Can I pickup a rental or make a purchase at your offices?
After my trip, could I use the phone I buy from Cellular Abroad at home?
GSM Phone Usage
Will my phone come with a charger?
What is my new cell phone number?
Can somebody else use my SIM card?
Can I retain my US cellular phone number?
Will I have voicemail?
What about email and internet (SMS, WiFi, GPRS)?
Can I use multiple SIM cards with my phone?
If I run out of call credit, do I buy another SIM card?
How do I add more talk time to my SIM card?
Will my country-specific SIM card work outside the country of origin?
How do I roam with my country-specific SIM card?
How do I know how much call credit I have remaining?
What's a "Service Life"?
Troubleshooting
I am currently overseas and I charged the phone and inserted the SIM card, but I still can't make any calls. What's wrong?
What if my cell phone or SIM card doesn't work?

Cellular Service Information by Carrier
Information on Sprint International Service Information on Nextel International Service Information on T-Mobile International Service Information on Verizon International Service Information on Cingular International Service Information on AT&T International Service
General GSM Information
What is GSM?GSM (Global System for Mobile Communications) is the world's predominant mobile phone standard, first adopted in Europe and then quickly spreading throughout Asia, Africa and the Pacific Rim (Australia, N. Zealand, etc.). GSM enables the same cell phone to work in London, Johannesburg, Beijing, Sydney and elsewhere.
The United States however, did not standardize GSM as a part of it's infrastructure, instead leaving the development of cellular service to competing wire carriers which created whatever standard they wished including CDMA and TDMA. Only recently has the US aggressively embraced the GSM standard, which makes the Cellular Abroad service that much better.
Carriers such as T-Mobile, Cingular and AT&T are now providing GSM service in the majority of the markets they serve. However, the GSM standard that we use in the US and Canada is not always compatible overseas. This explains why many US travelers have difficulty getting wireless coverage overseas without paying $1-$7/minute and why you should consult with the experts here at Cellular Abroad.
What is a SIM card?The SIM (Subscriber Identity Module) card also known as a "smart chip" is basically the "brain" of the phone, providing cellular service as well as many other functions including call log, voice mail and SMS and GPRS (internet connection standards). SIM cards are the size of a credit card but they have a pop-out chip about the size of a postage stamp which fits into all modern GSM phones. The smart chip is embedded with circuitry and when inserted into a GSM mobile phone provides wireless phone service on a particular GSM network. SIM cards are usually available prepaid or on a contractual basis, however Cellular Abroad only offers the prepaid version which is the most popular because of the user's complete control over how much they spend. There are no hidden fees or surprise charges. To see what a SIM card looks like, click here for our visual guide.
Is this like a calling card?Not really. You cannot receive a call with a calling card. Calling cards are a bit risky and often provide poor customer service. With the increasing popularity of mobile phones, it is not uncommon to find broken pay phones that go unrepaired by companies that own them.
I currently have a GSM Phone. Will it work overseas?It is possible, though unlikely, that your current US GSM cell phone will work overseas. Most GSM phones sold in the US are only compatible with the nation-wide operating frequency of 1900 MHz (or the emerging 850MHz band). Overseas services use 900 and/or 1800 MHz frequency standards, which is why we carry only (unlocked) tri-band phones. Most carriers offer global tri-band phones, however they provide them "locked" which is a distinct disadvantage for the consumer. All GSM cell phones sold through Cellular Abroad are unlocked and ready to use on practically all GSM networks and with all SIM cards. At Cellular Abroad we believe in freedom of choice for our customers in order to provide the best possible price on airtime and network service. Most other plans force you to use one system with no control over airtime pricing nor the lowest rates available.
What is a locked cell phone?A locked cell phone is one that has been altered in order to use only one particular cellular network. This is often referred to as a service provider lock or "SP lock" for short. All GSM network operators lock the GSM cell phones they sell or offer with their cellular services. This is the hidden cost of the otherwise "free" or heavily-discounted phone you might receive directly from a network operator by signing a service contract. The cost of providing you with a "free" or low cost phone is subsidized over time through more expensive phone rates when you make calls domestically or roaming overseas. In the end, you end up paying much more for your "free" phone and service than you would if you purchased a package through Cellular Abroad and had the rock-bottom prices available everywhere. An unlocked phone is therefore one where this software restriction has been removed.
How is the cellular coverage in other countries?Generally speaking, cellular coverage overseas is excellent-much better than average coverage in the US and Canada. Cellular usage is more common overseas than it is in the US and the GSM standard is a higher quality than many of the technologies available domestically. One point to make, however, is that buildings in Europe are sometimes many hundreds of years old. Thus they have much thicker walls than relatively modern US architecture, so if you are indoors and away from a window, you may have difficulty getting reception.
What are PIN and PUK numbers?Some prepaid SIM cards have a security mechanism in place that requires the subscriber to enter in a PIN (personal identification number) code every time the GSM cell phone is turned on. When a PIN code has been assigned it is important not to forget this code. If you enter the PIN code incorrectly into the phone 3 times in a row then your GSM cell phone will display a Blocked message on its LCD. When this occurs you will then be required to unblock your phone using the PUK code. If you enter the PUK phone incorrectly 10 times in a row you will block your SIM card indefinitely. All phones have a different set of procedures to unblock and you will need to consult your GSM cell phone manual when this occurs. We strongly suggest that if you have the PIN-check feature enabled on your SIM card, that you turn it off. Please consult your phone manual's index under the "security" or "lock" section if unsure how to disable PIN-check.
How do I install a SIM card?SIM cards typically are credit-card sized cards with a smaller SIM chip almost completely floating within the card and attached on one edge of the chip by some perforated plastic. The chip is approximately the size of a standard postage stamp folded in half. You can refer to this installation animation to get an idea of how easy it is to install a SIM chip into your GSM phone.
What is Bluetooth™?Bluetooth is a wireless technology which provides a way to connect and exchange information between devices like personal digital assistants (PDAs), mobile phones, laptops, PCs, printers and digital cameras via a secure, low-cost, globally-available short range radio frequency. Bluetooth™ cell phones also allow you to use popular wireless hands-free earpieces and wireless speaker car kits.
Cellular Abroad Questions [back to top]
Is it better to buy or rent the phones from you?There is no simple answer for this. If your rental period approaches 2 months, you would be better off buying the phone. Also, if you travel often, it makes sense not having to keep paying for shipping. Renting a phone is a good way to try out our phone and see if you like it. If you decide to keep the rental, you pay no rental fee. You simply forfeit the deposit on the phone. On the other hand, purchasing a phone ensures you will be acquiring an unused handset.
How can I be sure the technology won't become obsolete in the future?This certainly won't occur in the near future. In fact, this is a relatively new technology in the US that many of the cellular service providers are quickly embracing. Sure, there will always be newer, smaller and "better" cell phones introduced in the future, but the underlying technology powering the service will remain the same. Consider classic cars as an analogy; while they may be less modern, they still get you around because even the oldest vehicles use gasoline for power.
How many minutes come with the SIM card?The SIM cards usually come with call credits measured in monetary units such as euros rather than increments of time. This is because rate charges for calls vary depending on time of day, destination of the calls and other discounts or charges the carrier may apply. Also, with most GSM carriers providing free incoming calls to the user, the number of minutes on the SIM card could be substantial without the need to add more call credits.
If the SIM card costs $80, do I get $80 worth of talk time?When you purchase the prepaid SIM card, you are buying the brain that inserts into your GSM cell phone and makes it work. There is always some credit included with new SIM cards, but the amounts vary from country to country. Additional airtime is purchased locally and usually available at or near face value. For example, a 25 Euro voucher will credit your SIM card 25 Euros. The remaining cost for the SIM card is a set-up fee which pays for service on this particular pre-paid account.
What are the calling rates and rental and purchase prices?Rates vary from country to country and whether you are utilizing a country-specific or international SIM card. Please check the exact rates by visiting our Prices and Rates page and selecting the name(s) of your destination country(ies). Results will display per-minute calling rates, as well as handset and SIM card rental and purchase prices.Ê Remember, with the exception of just a few countries, incoming calls (received within-country) are FREE - regardless of origin.
How do I calculate my rental period?It is important to remember that your phone rental is measured from the date of your stated departure until the date it is sent back (not when it arrives) or returned in person. We always ship FedEx Express 2-Day (unless faster service is required) and do our best to have the package arrive 2 business days before departure. We use FedEx tracking information to document the period of time for shipped phones. The number of days the rental is out between these two ship times minus 4 business days will be rounded up or down to the closest weekly rental period.
I live in the LA area. Can I pickup a rental or make a purchase at your offices?Cellular Abroad operates as an Internet business, not a retail location. Walk-in sales or will-call service is possible, but a $10 service fee will be charged for this service. If you're planning on stopping in, please call ahead for directions and availability of the item you seek. It's also extremely helpful to us—if you've already made your purchase decision prior to arriving—to supply your name, billing address and other pertinent information. This will help us to expedite your purchase. Our office hours are 8 AM to 6 PM Pacific Time. During our winter low-season months, we close at 5 PM.
After my trip, could I use the phone I buy from Cellular Abroad at home?The answer to this question depends on what network you currently use and if you're planning on replacing an old phone or looking to establish a new phone plan. All of our package phones will work only on GSM networks. (The phones cannot work on Sprint, Verizon or Nextel networks.) A lot of our customers find a prepaid card from T-Mobile is a great solution for infrequent phone users or as a starter or emergency phone for children. Here in the US, our tri-band phones work on the 1900 MHz GSM band, which is the predominant GSM band in use in the U.S. (At this time, T-Mobile operates only on this frequency.) While GSM technology is the world standard for mobile phone communications, you may find that your local GSM provider has poor coverage in your area. We at Cellular Abroad cannot determine this information for you. Your best bet is to ask people you know who use a local GSM carrier what what their experience has been. Another possibililty is that part of your local provider's network may be setup on the 850 MHz band. (Cingular uses the 850 MHz band in some areas.) If this is the case in your area and you want to buy a phone for use in the US as well as abroad, you may want to invest in a quad-band phone. You will also want to check directly with the desired GSM carrier to find out their rate plans. Ask a service rep about the phone model you intend on purchasing and what's involved in integrating that phone with their service. If you are told that the phone is not supported, this will mean either that they do not have service reps trained on that model phone or that you will need a quad-band phone to give you coverage in areas serviced by the 850 MHz network.
GSM Phone Usage [back to top]
Will my phone come with a charger?Yes! All of our cell phones (for rent or for sale) come with a variable-voltage travel charger plus wall-plug adapters for overseas. Our rental phones are shipped pre-charged.
What is my new cell phone number?Your new cell phone number is a local number for the country pertaining to your SIM card. For example, if you have an Italian SIM card, your number will be a local Italian cell phone number.
Can somebody else use my SIM card?Anyone can use your card. If you lose your phone with the SIM card inside, you can usually call customer service to deactivate the card so that nobody can use the remaining credit, though they won't be able to issue you a new card. You can also safely lend your phone and SIM card to friends without worrying about incurring any out-of-pocket costs. If they need more talk time, they can simply add some by purchasing a recharge card and using it with your SIM card.
Can I retain my US cellular phone number?Unfortunately you cannot. We recommend that you leave your overseas cellular number on your domestic voice mail. This way, whoever needs to reach you while you are overseas can still do so. You might be able to forward calls, providing someone can program forwarding for you back in the States. The problem with this is that all wrong numbers, telemarketers and everyone else you don't want to hear from when you're abroad will be able to reach you and that will end up being a nuisance.
Will I have voicemail?Most GSM phones have voicemail capacity, but it depends on what services are available through the local provider. In almost all cases voicemail is included with your service.
What about email and internet (SMS, WiFi, GPRS)?These options depend on the local service provider and the hardware (phone handset) being used. Almost all GSM providers support SMS (Short Message Service or "text messaging"). And in countries where incoming calls are free, incoming SMS messages tend to be free. A general rule of thumb is that SMS rates are usually about half the cost of an outgoing per-minute rate, whether local or international.
GPRS-enabled phones can access the internet when used with the SIM card from a carrier that offers that service. Rates and configuration are obtained through the carrier and can vary widely. Some GPRS phones can be used as a modem when connected to a laptop computer. Again, rates and services offered vary and are determined by the carrier whose SIM is being used in the phone at the time.
For full connectivity, a laptop computer with a WiFi access card will work in some destinations. Wireless access is becoming more prevalent in all countries, though accessing it will vary depending on the terms of the service provider. A great book on the subject is "Global Mobile - Connecting without Walls, Wires or Borders" by Fred Johnson.
For simple email Cellular Abroad offers the Email Abroad. It's unique little Composer isn't much larger than a phone and stores outgoing email that you compose when you want to record memories of sightseeing from a sidewalk café, for example. To send outgoing emails and download incoming emails, simply dial a local access number from any phone, hold the device to the phone and the messages are sent. It's cheap, light and has a calendar and other organizational features.
Additionally, internet cafes and terminals are not hard to find almost everywhere. We've seen coin operated terminals in beachside convenience stores and an internet café in the Andes Mountains of Colombia. These are great for logging on to the internet to check emails and catch up on the news or to research the next destination. Rates are often only a few dollars an hour.
Can I use multiple SIM cards with my phone?Since we offer only unlocked phones, you can swap SIM cards as often as you like. However if you are traveling to multiple countries, you might consider that each SIM card has its own unique phone number. In the case of traveling to many countries during one trip, an International SIM card is the optimal choice for many travelers as you keep the same phone number regardless of your position on the globe. Note that you will be roaming when outside the country of origin for the card.
If I run out of call credit, do I buy another SIM card?NO! Remember, the SIM card is the "brains" of your phone and is rechargeable. Once you have a SIM card all you need to do is add credit for more talk time. Make sure that you do not wait beyond the service life of your SIM card to add more call credit.
How do I add more "talk time" to my SIM card?In prepaid phone lingo, adding more talk time is known as "recharging" or "topping-up" a SIM card. Recharging a SIM card is as easy as entering a code number into the handset. Recharge voucher cards with recharge codes on them can be purchased at many convenience stores, magazine kiosks and cell phone stores. They are also often found at post offices, train stations and airports. Please note that you can only purchase recharge cards within the country of origin. (If you have a country-specific SIM for Germany, for instance, you can only purchase recharge cards for that SIM card within Germany.)
Will my country-specific SIM card work outside the country of origin?When you use a country-specific SIM card outside of its home country, this is known as "roaming". While this is often possible to do, there are some problems that make this undesirable for longer side-trips. First of all, the rates are much more expensive for making local and international calls and you don't get free incoming calls. Also, if you run out of credit, you rarely can purchase recharge vouchers outside of the country of origin.
How do I roam with my country-specific SIM card?First of all, Cellular Abroad does not support roaming out-of-country with country-specific SIM cards. (There are too many possibilities and variables for us to do this adequately.) That said, here are some rules-of-thumb for you to use if you plan to roam:
Call customer service for your SIM card carrier to request or confirm that your SIM has roaming capabilities enabled. Roaming with some SIM cards may NOT be possible at all.
Make sure to ask customer service if there are any variations in dialing to or from your number when roaming. (Perhaps the use of a different country code or dialing prefix, etc.)
Ask customer service to provide dialing rates for making and receiving calls when roaming for all scenarios of interest. (Customer service may not be available from the U.S.)
Make sure you have a good guesstimate of how much time you'll use the phone when roaming.
Buy and apply adequate call credits (via recharge vouchers) for use when roaming. You may want to purchase a "backup" recharge voucher to use in case your SIM card runs out of call credits when you are roaming.
How do I know how much call credit I have remaining?You can always check your remaining call credit. The instructions vary from country to country and are always provided with the SIM cards you purchase through us. They can also be found at the bottom of the Customer Service page. Typically you either call a special number or send an SMS message and the carrier answers back with your remaining credit.
What's a "Service Life" ?Prepaid calling plans provide users with the easiest commitment-free way to obtain a cellular phone number. For obvious reasons, service providers only have a limited quantity of phone numbers to distribute and it only makes sense to diable an account which has remained dormant for a reasonable amount of time. Most carriers refer to a service life as the time from activation (or recharge) of the prepaid account to a pre-designated cut-off time whereby there has been no additional recharge credit applied to the prepaid account. (Account activation usually takes place at the time of the first outgoing call.) Different carriers take different actions at the end of the service life. Most choose to decommission the SIM card, effectively disassociating the phone number from the SIM. Afterwards, the customer may be able to reactivate the account but will need to obtain a new number. Many services simply force the customer to obtain a new SIM card if the service life has lapsed. As long as regular recharge credits are being applied to the prepaid account (within the service life period), you should be able to keep the account active and retain the phone number associated with the account.
Troubleshooting [back to top]
I am currently overseas and I charged the phone and inserted the SIM card, but I still can't make any calls. What's wrong?Generally this problem can be rectified by changing the band the phone is currently operating on. For example, if you are in Europe and the band of your GSM cell phone is set to the 1900 frequency, you will need to reset it to the 900/1800 frequency for it to work properly. Usually you'll know you're using the incorrect frequency if you do not a get a signal on the LCD display of your cell phone. However this is not always the case as it could also mean you are simply not in a good coverage area or indoors. Many European buildings have thicker walls than in the US, so your phone may not work if you are not near a window. If you are roaming and your carrier has different roaming agreements with other carriers, you may be able to improve your coverage by manually switching GSM networks. For details on how to do this, consult your manual.
What if my cell phone or SIM card doesn't work?It is rare that one of our cell phones wouldn't work and extremely rare for a SIM card not to work. We personally test all of our phones prior to sending them out to ensure quality service and no defects. You can double-check this after you receive your cell phone and a SIM card for your overseas trip. Simply charge the phone and then see if it powers on. While you can't use the foreign SIM card here in the U.S., if your phone powers on (after you have charged it), it should work fine. Less than 1% of all GSM cellular phones are defective and less than 1 in 10,000 SIM cards are defective. If for some reason a phone or SIM card purchased from Cellular Abroad proves defective, it will be replaced ASAP. If you encounter problems while on the road, you will first want to determine if the problem is a network problem or if it is hardware-related. If you can use someone else's GSM phone and reproduce the problem, then you should contact customer service for the local carrier to see if there is a problem with the network. If the problem seems to be with the handset, please contact Cellular Abroad by phone at 00-1-310-862-7100 or email us at http://www.cellularabroad.com/xcart/help.php?section=contactus&mode=update. Our office hours are daily from 8 AM to 5 PM Pacific Time. We will be glad to return the call to save you money while troubleshooting your issue.

Types of Telephones at the NIH

2009-03-16T01:45:00.000-07:00

CIT-Telecommunications supports ISDN and analog telephone services at the NIH. These services are used for voice, data, and video applications:

Analog Lines - This type of service may be used for modems, facsimile machines, TTY equipment, teleconferencing telephones, single line telephone sets (i.e. courtesy phones). For analog equipment offered at the NIH, see our Information Sheet (Adobe 78kb)

ISDN lines - The Integrated Services Digital Network service is used for voice and data applications which provide high bandwidth. This application is used primarily with an ISDN phone set. These lines are used with video applications as well for video conferencing capabilities. For ISDN equipment offered at NIH, see our Information Sheet (Adobe 78kb)

Also see:

How to Request ISDN/Analog Services

For additional information, please contact the NIH Help Desk:

Phone:	301-496-4357 (301-496-HELP) 866-319-4357
TDD/TTY:	1-800-438-8832

Web:	ITHelpDesk.nih.gov

Back-up Red Phones.

2009-03-16T01:38:00.000-07:00

NIH Emergency Back-up System (“214” Numbers and Red Phones)

The Red Emergency Backup Telephones or “Red Phones” are to be used only in the event the NIH telephone system is not working. These telephones are area code 301, beginning with a 214 exchange and are found throughout the NIH community. Contact your Administrative Officer should you require one in your area.

View a more detailed list of Back-up Emergency Numbers (PDF, 31kb)

Dialing Instructions for Red Telephones with a 214-exchange

To dial from a red phone to an in-house phone
Dial 9, wait for second dial tone, dial the 10 digit number
To dial from a red phone to a red phone
Dial the 10-digit number
To dial from an in-house phone to a red phone
Dial 9 + (area code) + 7 digit number
To dial from a red phone to a local outside line
Dial 9, wait for second dial tone , dial (area code) + number
To dial from a red phone to a long distance outside line
Dial 9, wait for second dial tone , dial 1 + (area code) + number

When using the red phone for NIH pagers dial 301-214-1817 and follow the voice activated instructions.
All 214 exchange numbers are in area code 301.
To reach the NIH operator from a red phone, dial “0”.

For additional information, please contact the NIH Help Desk:

Phone:	301-496-4357 (301-496-HELP) 866-319-4357
TDD/TTY:	1-800-438-8832

Web:	ITHelpDesk.nih.gov