Technical[ edit ] CSD uses a single radio time slot to deliver 9. 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. 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 single dedicated radio time slot is allocated between the phone and the base station. 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.
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A list of related standards in the public domain is accessible via the ETSI standards search. Although originally designed for operation in the MHz band, it was soon adapted for MHz. Over the years, the versatility of GSM has resulted in the specifications being adapted to many more frequency bands to meet niche markets. At the time of the original system design, this rate compared favourably to those available over fixed connections. However, with the passage of time, fixed connection data rates increased dramatically.
In the course of the next few years, the General Packet Radio Service GPRS was developed to allow aggregation of several carriers for higher speed, packet-switched applications such as always-on internet access. The first commercial GPRS offerings were introduced in the early s.
Meanwhile, investigations had been continuing with a view to increasing the intrinsic bit rate of the GSM technology via novel modulation techniques. Since the four channels are tied up in a circuit-switched connection, the network operator was likely to charge an HSCSD call at a considerably higher rate than a simple, single channel call, and so the service was never enormously popular.
GPRS is the essential enabler for always-on data connection for applications such as "web browsing" and "Push-to-Talk over Cellular". Most operators do not offer such high rates, because obviously if a slot is being used for a GPRS bearer, it is not available for other traffic. Also, not all mobiles are able to aggregate all combinations of slots. This is important, because it allows EDGE to be introduced piecemeal into existing GSM networks without disrupting the frequency reuse plan of the existing deployment.
In fact, to cater for the potentially increased sensitivity to noise in marginal coverage areas, EDGE uses a combination of 8PSK and GMSK, to give a balanced improvement in bit rate under virtually all radio conditions. The four coding schemes of GPRS are increased to nine in EDGE, and new segmentation techniques can radically improve throughput by permitting the coding scheme to be changed on the fly in case of retransmission of a segment in rapidly changing radio conditions.
In addition, the packet window size increased to compared with the 64 for GPRS, resulting in more robust transmission and reception. TR It remains, however, a very useful reference document.
2nd Generation (GERAN)
Circuit Switched Data