Circuit Switched Data
Circuit Switched Data is the original form of data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

 transmission developed for the time division multiple access
Time division multiple access
Time division multiple access 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 its own time slot. This...

 (TDMA)-based mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 systems like Global System for Mobile Communications
Global System for Mobile Communications
GSM , is a standard set developed by the European Telecommunications Standards Institute to describe technologies for second generation digital cellular networks...

 (GSM). CSD uses a single radio time slot to deliver 9.6 kbit/s data transmission to the GSM Network and Switching Subsystem where it could be connected through the equivalent of a normal modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

 to the Public Switched Telephone Network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

 (PSTN) allowing direct calls to any dial-up service.

Prior to CSD, data transmission over mobile phone systems was done by using a modem, either built into the phone or attached to it. Such systems were limited by the quality of the audio signal to 2.4 kbit/s or less. With the introduction of digital transmission in TDMA-based systems like GSM, CSD provided almost direct access to the underlying digital signal, allowing for higher speeds. At the same time, the speech oriented audio compression
Audio compression
Audio compression may refer to:*Audio compression , a type of lossy compression in which the amount of data in a recorded waveform is reduced for transmission with some loss of quality, used in CD and MP3 encoding, Internet radio, and the like...

 used in GSM actually meant that data rates using a traditional modem connected to the phone would have been even lower than with older analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...


A CSD call functions in a very similar way to a normal voice call in a GSM network. A single dedicated radio time slot is allocated between the phone and the base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

. A dedicated "sub-time slot" (16 kbit/s) is allocated from the base station to the transcoder, and finally another time slot (64 kbit/s) is allocated from the transcoder to the Mobile Switching Centre (MSC).

At the MSC, it is possible to use a modem to convert to an "analog" signal, though this will typically actually be encoded as a digital pulse-code modulation
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...

 (PCM) signal when sent from the MSC. It is also possible to directly use the digital signal as an Integrated Services Digital Network
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

 (ISDN) data signal and feed it into the equivalent of a remote access server.

GSM data transmission has advanced since the introduction of CSD:
  • High-Speed Circuit-Switched Data
    High-Speed Circuit-Switched Data
    High-speed circuit-switched data , is an enhancement to Circuit Switched Data , the original data transmission mechanism of the GSM mobile phone system, four times faster than GSM, with data rates up to 38.4 kbit/s....

     (HSCSD) is a system based on CSD but designed to provide higher data rates by means of more efficient channel coding and/or multiple (up to 4) time slots.
  • General Packet Radio Service
    General Packet Radio Service
    General packet radio service is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications . GPRS was originally standardized by European Telecommunications Standards Institute in response to the earlier CDPD and i-mode...

     (GPRS) provides more efficient packet-based data transmission directly from the mobile phone at speeds similar to HSCSD.
  • Enhanced Data Rates for GSM Evolution
    Enhanced Data Rates for GSM Evolution
    Enhanced Data rates for GSM Evolution is a digital mobile phone technology that allows improved data transmission rates as a backward-compatible extension of GSM...

     (EDGE) (E-GPRS) and Universal Mobile Telecommunications System
    Universal Mobile Telecommunications System
    Universal Mobile Telecommunications System is a third generation mobile cellular technology for networks based on the GSM standard. Developed by the 3GPP , UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for...

     (UMTS) provide improved radio interfaces with higher data rates, while still being backward compatible with the GSM core network.


The T-Mobile system is area/city specific. It works in Denver, Colorado
Denver, Colorado
The City and County of Denver is the capital and the most populous city of the U.S. state of Colorado. Denver is a consolidated city-county, located in the South Platte River Valley on the western edge of the High Plains just east of the Front Range of the Rocky Mountains...

, Savannah, Georgia
Savannah, Georgia
Savannah is the largest city and the county seat of Chatham County, in the U.S. state of Georgia. Established in 1733, the city of Savannah was the colonial capital of the Province of Georgia and later the first state capital of Georgia. Today Savannah is an industrial center and an important...

, Washington, DC, and Miami, FL and Las Vegas
Las Vegas, Nevada
Las Vegas is the most populous city in the U.S. state of Nevada and is also the county seat of Clark County, Nevada. Las Vegas is an internationally renowned major resort city for gambling, shopping, and fine dining. The city bills itself as The Entertainment Capital of the World, and is famous...

 into SoCal roaming onto Cingular. There is no service in Sarasota, Florida
Sarasota, Florida
Sarasota is a city located in Sarasota County on the southwestern coast of the U.S. state of Florida. It is south of the Tampa Bay Area and north of Fort Myers...

 or anywhere in Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

. Fax services required a second number for an additional $10/mo.

The AT&T Mobility GSM EDGE network supports CSD in legacy Cingular and AT&T Wireless
AT&T Wireless
AT&T Wireless Services, Inc., founded in 1987 as McCaw Cellular Communications, Inc., and now legally known as New Cingular Wireless Services, Inc., formerly part of AT&T Corp., is a wireless telephone carrier in the United States, based in Redmond, Washington, and later traded on the New York...

 footprint areas. It is not generally available but can be provisioned for selected customers with specific needs requiring the service.

Currently in Australia, CSD is only available on Mobile Plans with Vodafone
Vodafone Group Plc is a global telecommunications company headquartered in London, United Kingdom. It is the world's largest mobile telecommunications company measured by revenues and the world's second-largest measured by subscribers , with around 341 million proportionate subscribers as of...

, Optus
SingTel Optus Pty Limited is the second largest telecommunications company in Australia, and is a wholly owned subsidiary of Singapore Telecommunications...

 and Telstra
Telstra Corporation Limited is an Australian telecommunications and media company, building and operating telecommunications networks and marketing voice, mobile, internet access and pay television products and services....

, however Telstra is in the process of shutting down their CSD network as of the 4th May 2010.

In the UK CSD is available with Vodafone
Vodafone Group Plc is a global telecommunications company headquartered in London, United Kingdom. It is the world's largest mobile telecommunications company measured by revenues and the world's second-largest measured by subscribers , with around 341 million proportionate subscribers as of...

, but requires activation by Vodafone customer services.
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