Public Switched Telephone Network

Circuit Switched Data (CSD) is the original form of data transmission developed for the time division multiple access (TDMA)-based mobile phone systems like Global System for Mobile Communications (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 to the Public Switched Telephone Network (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 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 systems.
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. 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 (PCM) signal when sent from the MSC. It is also possible to directly use the digital signal as an Integrated Services Digital 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 (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 (GPRS) provides more efficient packet-based data transmission directly from the mobile phone at speeds similar to HSCSD.
Enhanced Data Rates for GSM Evolution (EDGE) (E-GPRS) and Universal Mobile Telecommunications System (UMTS) provide improved radio interfaces with higher data rates, while still being backward compatible with the GSM core network.
CSD can be very cost effective for short data transmissions if the subscriber has a “bucket of minutes” with a low per minute cost, even regarded as free, as CSD is charged as a voice call. It is ideal for simple POP3 email checking (one does need a dial-up ISP), but Web viewing is increasingly difficult due to modern web page bandwidth requirements.
The T-Mobile system is area/city specific. It works in Denver, Colorado, Savannah, Georgia, Washington, DC, and Miami, FL and Las Vegas into SoCal roaming onto Cingular. There is no service in Sarasota, Florida or anywhere in Texas. 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 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, Optus and Telstra, however Telstra still offer CSD on Pre-Pay on their Telstra Pre-Paid Plus network, although prices are expensive.
External links
How to connect using CSD
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Mobile telephony and mobile telecommunications standards
0G (radio telephones)
MTS  MTA  MTB  MTC  IMTS  MTD  AMTS  OLT  Autoradiopuhelin
NMT  AMPS  Hicap  Mobitex  DataTAC  TACS  ETACS
GSM/3GPP family
3GPP2 family
CdmaOne (IS-95)
D-AMPS (IS-54 and IS-136)  CDPD  iDEN  PDC  PHS
2G transitional
(2.5G, 2.75G)
GSM/3GPP family
3GPP2 family
CDMA2000 1xRTT (IS-2000)
iDEN family
3G (IMT-2000)
3GPP family
3GPP2 family
CDMA2000 1xEV-DO (IS-856)
3G transitional
(3.5G, 3.9G)
3GPP family
3GPP2 family
EV-DO Rev. A  EV-DO Rev. B
Mobile WiMAX (IEEE 802.16e-2005)  Flash-OFDM  IEEE 802.20
4G (IMT-Advanced)
3GPP family
LTE Advanced
WiMAX family
IEEE 802.16m
Related articles
History  Cellular network theory  List of standards  Comparison of standards  Spectral efficiency comparison table  Cellular frequencies  GSM frequency bands  UMTS frequency bands  Mobile broadband
Categories: Global System for Mobile communicationsHidden categories: Articles that may contain original research from September 2007 | All articles that may contain original research