GSM

GSM Overview

History:

In 1982, the Conference Europenne des Postes et Telecommunications (CEPT) established the Groupe Special Mobile (GSM) committee. The GSM committee was established to oversee the development of a European cellular phone standard based on a digital system.

Previous cellular phone standards were analog, incompatible from country to country, and were expensive mainly due to low sales volumes. In 1987, a narrowband Time Division Multiple Access system was adopted. Unlike the IS-54B/IS-136 standards that have included compatibility with the older analog standard, GSM is completely digital and does not have the burden of supporting an older analog standard.

After ten years of development, the cellular phone system based on the GSM (now standing for Global System for Mobile communications) was put into service in 1992. At the end of 1994, there are more than 4.8 million subscribers and seventy countries either operate or are planning to implement GSM systems.

Physical Channels:

The spectrum allocated for GSM is 890-915 MHz for the mobile to base station (uplink) and 935-960 MHz for the base station to mobile transmission (downlink). The uplink and downlink frequencies are divided into 124 channels of 200 kHz each. Uplink and downlink channels are paired with 45 MHz of separation. Information is transmitted at 270.83 kbits/sec on each channel.

To increase the GSM system capacity, there is an option for Extended GSM. The frequency range is increased by 10 MHz (50 channels) for the uplink and downlink.

Each channel is time-sliced into frames. A frame has a duration of 4.615 msec. Each frame is further divided into eight slots, and the slot is assigned to a particular user for voice or data communications. In this way, the user gets 1/8 of the time to transmit or receive information. Configuring the communication protocol in this manner is know as Time Division Multiple Access (TDMA).

Information is transmitted in the slot with a burst. Figure 1 shows the format of a Normal burst.

 Tail    ENCRYPTED DATA      Equalizer     ENCRYPTED DATA      Tail    Guard   
 Bits         BITS           Training           BITS           Bits    Period  
                             Sequence                                          

  3            58               26               58             3       8.25

Figure 1: Normal burst format

Logical Channel Types:

To facilitate signaling and traffic, an elaborate set of logical channels has been developed. The logical channels can be broken into two major categories, traffic and control.

Traffic channels are used for voice and data services at the full rate of 22.8 kbits/sec or at the half rate of 11.4 kbits/sec. The traffic channels have been designed to handle future reduced bit rate vocoders when they become available. A system utilizing half rate vocoders could support twice the capacity.

Control channels carry signaling or synchronization information. There are four categories of control channels: the broadcast control channel (BCCH), the common control channel (CCCH), the stand-alone dedicated control channel (SDCCH) and the associated control channel (ACCH).

The BCCH is used for downlink communications for mobile frequency correction, frame synchronization and base station identification, and finally, general information about control channel configuration.

The CCCH is used for paging the mobile station when a call is originated on the network side to the mobile station. Additionally, the mobile station can request a traffic channel or a dedicated control channel for call initiation.

SDCCH channels are used for setting up the services that are required by a user. The mobile station communicates the services required using the SDCCH message. The base station determines availability of services and allocates and assigns an available traffic channel.

There are two basic types of ACCH channels, the fast associated control channel (FACCH) and the slow associated control channel (SACCH). FACCH are used to transmit urgent information such as hand-off commands. The FACCH commands are accomplished by stealing slots from a traffic channel. SACCH commands are built into the TDMA frame structure and are used to convey mobile received power levels and the signal strength of adjacent cells.

Convolutional encoding is used to protect data from transmission errors in the control channel messages in the same way traffic data is protected.