Nokia D211 Developer's Manual
Also see for D211: User guideUser guide
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Contents
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10/1132.2.2 Further ReadingFor more information on WECA and on the Wi-Fi standard, check the Wi-Fiô web site athttp://www.wi-fi.com/. The web site also contains a list of Wi-Fi certified products.2.3 GSMGPRS and HSCSD operate in GSM networks at the 900 MHz and 1800 MHz bands. Themaximum transmit power is 2 W for GSM 900 and 1 W for GSM 1800.The data services in the first commercial GSM systems in 1991-1992 were based on circuitswitched technology. The connection over the air interface used one time slot per useroffering a maximum user data rate of 9.6 kbit/s. Since then, mainly due to the increasedInternet usage and ever increasing amount of information in the Internet, there has been aconstantly increasing user demand for higher data rates. The fixed telephone modems haveresponded to these requirements with increased data rates ñ modem data rates have in factincreased over time from 1.2 kbit/s (V.23) to current 56 kbit/s (V.90). In the light of this, it hasbeen very clear that, in order to remain competitive and to meet the customer requirements,the GSM system has to offer higher bit rates. Therefore, the work on GSM High SpeedCircuit Switched Data (HSCSD) was started in 1994.HSCSD is an enhancement of the original circuit switched data service to enable higher userdata rates. The increased data rates are achieved by using multiple time slots per a userconnection. In theory, one to eight time slots can be combined dynamically giving maximumspeed of 76.8kbit/s (9.6 kbit/s * 8) for one user. It should be noted that GSM circuit switcheddata shares the capacity with voice services, i.e. a data user using one time slot decreasesthe maximum amount possible for voice calls in that particular cell by one. Furthermore, if thedata user uses several time slots for the connection, the maximum amount of voice calls isdecreased by the amount of the used time slots.In 1997, as a parallel activity to HSCSD, the GSM user data rate per single time slot wasincreased from 9.6 kbit/s to 14.4 kbit/s by applying different error coding mechanisms in thestandard. The motivation for this activity was: 1) to better meet the data rates in the fixedtelephone domain (that developed as multiples of 14.4 kbit/s) and 2) to further increase themaximum offered bit rates. The GSM 14.4 kbit/s data service gives a theoretical maximumuser data rate of 115.2 kbit/s with eight time slots.Fax is also a part of GSM data services. Fax uses the above mentioned data channels.Because the transmission over GSM is digital, there is no need for a modem in a mobilestation. Instead, during the communication over PSTN, a modem located at the GSM corenetwork is used.For HSCSD, the corresponding modem rates at the fixed telephone network side are forexample V.90 and V.110. For example, with a V.90 modem the maximum bit rate of 43.2kbit/s can be achieved, which matches with a GSM HSCSD mobile using 3 time slots.An important feature in HSCSD is that it supports asymmetric connections: data rates (theamount of time slots) can be different in uplink (to the network) and in downlink (to themobile). This is important for example in Internet browsing, where the nature of the traffic isasymmetric (more data comes to the terminal than leaves the terminal). In HSCSD, it ispossible to reserve more time slots for downlink traffic than for uplink. Also streaming ispossible over HSCSD since it offers guaranteed bit rate service. |
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