1.1.1. GSM CSD

The usually referred GSM CSD bearer service is the most widely used data service providing a non-transparent data rate of 9.6 kbit/s. It provides error correction and flow control, which is what the "non-transparent" means in this context. Due to the special technologies employed for error protection and correction (FEC, ARQ, interleaving) on the air interface (physical layer) the measurable two way round trip time (RTT) is typically around 1 second for a GSM network operating in the 900 MHz frequency band and 500 ms for a GSM network operating in the 1800 MHz frequency band. This RTT can in general be measured when a mobile host (laptop connected to a GSM mobile phone via the infrared port or a serial cable) is connected to a fixed host in the Internet via the GSM non-transparent bearer service, by pinging the fixed host. The reason for the lower RTT in the 1800 MHz GSM network is the smaller interleaving depth, which becomes possible due to the usually half sized cell sizes compared to the 900 MHz cells. This 9.6 kbit/s service is what is most often used to connect laptop computers, PDA and WAP phones to the services they use.

Commonly, GSM network operators support the non-transparent CSD bearer service through a modem interworking function. This means that a mobile station (mobile phone) initiates a data call and the network routes the call to the modem interworking function, which is located at the Mobile Switching Centre (MSC) of the GSM network, which then dials the number supplied by the mobile station. This is different from voice calls, where the GSM network routes the call itself, often to another mobile station on the same network. The GSM network doesn't route data calls; it dials the requested number on behalf of the mobile station and leaves the routing to the external wireline telephone network. The main reason for this is that the GSM network has information about what the user wants to do with the data call. Maybe the user is contacting his Internet Service Provider (ISP) to send email or maybe he is dialling his corporate intranet to set up a virtual private network (VPN) connection to retrieve confidential customer information from a company database. The GSM network also does not know what speeds and compression are supported by the connection it dials. It has to take a "lowest common denominator" approach, just like a wireline modem does when it dials an ISP. Like a wireline modem, it has to conduct a complex conversation modem-to-modem, to configure speeds and compression, every time it dials. This modem-to-modem conversation accounts for most of the delay when a call from a laptop or WAP phone is established.

In addition to modem-to-modem calls most of the GSM networks today can also use ISDN to set-up calls from the MSC interworking function. In this case, there is no need for the long CSD call set-up. The selection whether an ISDN or a modem-to-modem call is established is done by adding a special AT command set in the dial-up networking settings. The details about the AT command settings for the mobile station in use calls, can be found on the web pages of the mobile phone vendors.

Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann