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10 GSM Networks: Protocols, Terminology, and Implementation

multiplexing technique, such a 2-Mbps PCM link can, among others, be parti-tioned into 32 independent channels, each capable of carrying 64 kilobits per second (Kbps).

Another aspect of the change that the digital technology has enabled reveals its advantage only after a second look. Almost all signaling standards, like Signaling System Number 7 (SS7) and Link Access Protocol for the D-channel (LAPD) separate the traffic channel from the signaling or control channel. This is referred to as outband signaling, in contrast to inband signaling. In the case of inband signaling, all the control information is carried within the traffic channel. Although outband signaling reąuires the reservation of a traffic channel, it makes a morę efficient use of resources overall. The reason for that lies in the reduced occupation time of the traffic channel, which is not needed during cali setup. Both cali setup and cali release can be carried out for many connections via one control channel, sińce signaling data use the resources morę economically. One 64-Kbps time slot out of a 2-Mbps PCM link typically is used for signaling data; a cali setup consumes about 1 to 2 Kbps.

1.4.3 What is Signaling Used For?

The main task of signaling is still to set up and to elear a connection between end users or machines. Today, constantly new applications are added. Among them are automated database accesses, in which telecommunications systems cali each other and which are fairly transparent to a caller, or the wide area of supplementary services, of which only cali forwarding is mentioned here as an example. The glossary provides a list of all GSM supplementary seryices.

1.5 Representation of Messages

When working with protocol test equipment and in practical work, message names usually are abbreviated. Most GSM and ITU Telecommunication Standardization Sector (ITU-T) Recommendations list the well-defined abbreviations and acronyms, which this book also uses to a large extent. The complete message names and explanations can be found in the respective chapters.

Since a picture often expresses morę than a thousand words, this book contains a large number of figures and protocol listings. The various messages illustrated in the figures show parameters, which are formatted per interface and are presented as shown in Figures 1.4(a) through 1.4(e).