compatible to allow was completed in 1990, but work continues on further phases roaming. incorporating new features and services. In 1987, the majority of NMT900 was developed as a necessity as capacity became ex- operators participating in GSM signed a Memorandum of Under- hausted on the NMT450 networks, and has been deployed since standing (MoU) committing them to make GSM a reality by install- 1987 as an overlay network in several countries, and in Switzerland ing networks and opening commercial service by 1991. Since that as their main network. time further operators have signed the MoU, bringing the total to date to 25. The GSM technical standard makes full use of currently available 47.4.4 C450 levels of technology, incorporating features such as low bit rate speech, convolutional channel coding with bit interleaving and C450 (also known as Netz-C) was developed by Siemens during the frequency hopping. The standard is intended to endure for many early 1980's under the direction of the (West) German PTT, Deut- years to come. sche Bundespost. Commercial service opened in 1985 following a trial period. C450 has a channel spacing of 20kHz, in common with other mobile services in Germany at 450MHz and speech modulation is Detailed description of GSM analogue FM with a frequency deviation of 4.0kHz. Signalling for call control is transmitted at 5.28kbit/s by direct FSK. Error protec- 47.5.1 GSM architecture tion of the signalling is by bit interleaving with a BCH block code backed by an acknowledgement protocol. The basic architecture of GSM is not dissimilar to other cellular In addition, C450 uses continuous signalling between base station radio systems and comprises base transceiver stations (BTS), Base and mobile during a call, achieved by time compressing the speech Station Controllers (BSC), Mobile Switching Centres (MSC), a in bursts of 12.5ms, each burst being compressed into 11.4ms. This variety of registers and a network management system, as shown in process opens up slots of 1.1ms duration every 12.5m