Further Reading Telephone Networks

www.pragsoft.com Chapter 11: Integrated Services Digital Network 141 Figure 11.123 ISDN network overview. Packet Handler Exchange Termination ISDN Switch Packet Handler Exchange Termination ISDN Switch Public Packet Switched Network Tandem Exchange SCP STP To other ISDN exchanges To other ISDN exchanges ISDN Phone POTS PBX NTTA LAN X.25 Access U X.75 X.75

11.1.1. ISDN Channels

Subscriber access to ISDN is via digital channels, of which there are three types: • B channels are used for carrying user data digitized voice or computer- generated data at 64 kbps. This data rate is more than necessary in many situations e.g., compressed digitized voice can be transmitted using less bandwidth. Consequently, a B channel is sometimes subdivided into smaller subchannel. Whether there is a subdivision or not, the network treats the whole thing as one channel. All subchannels therefore are between the same two end- points and follow the same route. • D channels are primarily used for common channel signaling purposes. They are typically associated with B channels and carry the control signals for B channel calls. D channels are also used for packet-switched data communication. A D channel may operate at 16 or 64 kbps. • H channels are used in a high-speed trunk capacity. They are suitable for applications that require higher than 64 kbps data rates. Multi-media applications e.g., audio, video, and graphics multiplexed over the same channel are examples. H channels are divided into three categories depending on their speed: • H0 operates at 384 kbps = 6 B channels 142 Communication Networks Copyright © 2005 PragSoft • H11 operates at 1536 kbps = 23 B channels • H12 operates at 1920 kbps = 30 B channels Only D channels can be used for carrying signaling information. B and H channels can only be used for carrying user data. In practice, channels are offered to users in a packaged form. Two such packages have been defined: basic access and primary access. The Basic Rate Access BRA package also called 2B+D is primarily intended for residential subscribers and consists of the following: • Two B channels • One 16 kbps D channel • Overhead of 48 kbps for framing, synchronization, etc. This produces a total bit rate of 192 kbps. The channels may be used for a variety of purposes. For example, the two B channels can be used for two independent voice services, or one of them can be used for voice and the other for a data service such as fax, teletex, or remote LAN access. Modest data communication requirements e.g., remote banking transactions may be met by the D channel alone. Other permitted combinations for basic access are: B+D or just D. The Primary Rate Access PRA package is aimed at business users with greater bandwidth requirements. Primary access comes in two configurations: • At a bit rate of 1.544 mbps North America and Japan and consisting of: • 23 B channels • One 64 kbps D channel • Overhead of 8 kbps • At a bit rate of 2.048 mbps Europe and consisting of: • 30 B channels • One 64 kbps D channel • Overhead of 64 kbps As with the basic access, lower configurations are also possible, depending on requirements. Primary access can also support H channels.

11.1.2. Functional Groupings and Reference Points

User access to ISDN is provided at a number of different levels of abstraction. These levels are defined by functional groupings, which encompass functions