Circuit Switched Data
Circuit Switched Data, often referred to as CSD, is the original form of data transmission developed for the TDMA-based mobile phone systems like GSM. CSD uses a single radio time slot to deliver 9.6 Kbits/sec data transmission to the GSM Network and Switching Subsystem where it could be connected through the equivalent of a normal modem to the PSTN allowing direct calls to any dial up service. As of 2006, some GSM-based service providers, Cingular et al, still provide CSD by request.
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 Kbits/sec 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 analogue 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 Kbits/sec) is allocated from the base station to the transcoder, and finally another time slot (64 Kbits/sec) 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 PCM signal when sent from the MSC. It is also possible to directly use the digital signal as an ISDN data signal and feed it into the equivalent of an remote access server.
GSM data transmission has advanced since the introduction of CSD:
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.
GPRS provides more efficient packet-based data transmission directly from the mobile phone at speeds similar to HSCSD.
Finally EDGE (E-GPRS) and UMTS provide improved radio interfaces with higher data rates, while still being backward compatible with the GSM core network.
List of telephony terms:
3gpp - a-law - abbreviated dialing - adsl - ani - answering machine - apn - automatic ring back - b-channel - baud - bell 202 modem - bit rate - bonding - bri - busy signal - cable modem - call-progress tones - call accounting - call capture - call forwarding - call originator - call park - call pick-up - call transfer - call waiting - call waiting deluxe - called party - caller id - caller id spoofing - calling party - carrier wave - cbr - ccitt - cdma - cdma2000 - cellular repeater - celp - channel - clec - clock rate - codec - collect call - conference - conference call - crc - csd - d-channel - data compression - device driver - dial-up - dial - dial tone - direct-inward-dialing - direct distance dialing - distinctive ring - dnis - dsl - dsp - dtmf - dtr - duplex - echo cancellation - edge - extension - fax - fcc - fdma - fidonet - follow-me - g.711 - g.723.1 - g.723 - g.726 - g.lite - gprs - gps - gsm - h.323 - harmonic - headphones - hscsd - hspda - iad - idsn - internet call waiting - isp - ivr - jack plug - local loop - long distance - microcontroller - mobile phone - modem - modulation - mu-law - music-on-hold - night service - off-hook - on-hook - pabx - pager - payphone - pbx - pcm - pots - prank call - precise tone plan - pstn - pulse dialling - push to talk - ring modulation - ring tone - ringback - ringing signal - rj11 - roaming - serial communications - serial port - signal noise - sim - simplex - sit - sms - softmodem - switchboard operator - tapi - tdma - telecommunications - telemarketing - telephone - telephone call - telephone card - telephone company - telephone exchange - telephone line - telephone number - telephone numbering plan - telephone operator - telephone switchboard - telephony - tts - twisted pair - umts - v.32 - v.32bis - v.34 - v.42bis - vbr - vertical service code - voicemail - voip - vox - wap - wav - wi-fi - wimax - wire
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