GSM Global System for Mobile Communications

This chapter discusses the Global System for Mobile communications (GSM). The overall system consists of Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and Operation Support System (OSS). The BSS consists of Base Transceiver Stations (BTSs) and the Base Station Controllers (BSCs). The NSS consists of Mobile-services Switching Center (MSC), and several databases (e.g., HLR, VLR, EIR, and AUC). The OSS is responsible for organization and maintenance. We then discuss the air interface. Multiple access uses a combination of FDMA (with optional frequency hopping) and TDMA; FDD is used for duplexing. The time axis is divided into timeslots, which are grouped into frames, multiframes, and superframes. A combination of frequency band and timeslot index is a physical channel. GSM uses GMSK modulation; equalization is helped by a training sequence (midamble) in each timeslot. Coding employs unequal error protection, based on convolutional and/or block codes codes for bits created by the speech coder (regular pulse excited – long term prediction RPE-LTE). Interleaving over multiple slots is used. We next discuss the mapping of logical channels (types of data) to physical channels (time/frequency resources). Besides the (payload) traffic channels (TCHs), logical channels include various control channels. Next, we discuss time synchronization, including timing advance, and frequency synchronization. Data transmission in GSM includes circuit-switched transmission for low data rates, e.g., in Short Message Service (SMS) and packet-switched (e.g., General Packet Radio Service GPRS, see appendix). We then describe establishing connections and performing handovers. Various identity numbers, including MS ISDN, IMSI, MSRN, and IMEI are defined. The role of the SIM (Subscriber Identity Module) is analyzed. A discussion of various services enabled in GSM, and aspects of billing, conclude the chapter.