Integrated Services and Routing Protocols
Over 100 years ago, people began stringing wires between houses and towns so that they could communicate by telephone. Since then, the telephone network has evolved into a global communication system which uses every communication medium we have described in this book. There is one more feature that distinguishes the telephone system from other networks. The telephone system has a large analog component. We may recall that optical fibres and digital switching devices have introduced significant amounts of digital technology into the network. Telephones, however, are still analog exchange offices. This part of the telephone network is often called the local loop or last mile.
Initially, the analog system was a logical choice because the telephone was designed to transmit a person's voice. Since then, the two fields of communications and computer science have been merging. Computers are critical to the communications systems that are used to connect them.
The data communication field is in a continual state of flux. New standards and communication network facilities are constantly being introduced and implemented. Many of the older standards are being updated to accommodate higher data rate and new technology advancements. Some of the newer technology of the past years form the backbone of today's communication systems. Among these are the Integrated Services Digital Network (ISDN), Asynchronous Transfer Mode (ATM) and Frame Relay. These are used to transport and route integrated services such as voice, data, and video information through the public phone systems (ISDN) and among numerous digital networks (ATM and Frame Relay).
ISDN uses numerous established Local Area Networks (LANs), Wide Area Networks (WANs), Metropolitan Area Networks (MANs) and other network configurations interconnecting them via common public carriers. The benefits of using an integrated network such as ISDN include line sharing, high data throughput, easy identification of data packets, and error recovery.
The establishment and termination of calls are done the same way, regardless of the configuration of the originating station. ISDN, being a digital network application, has become a popular method for accessing the Internet. Being digital, it allows for quicker connection and downloading of files from the Internet.
INTEGRATING SERVICES
Integrated Services Digital Network (ISDN) was developed by ITU-T. ISDN data services integrate voice, data and video information onto a single channel. There are a number of types of ISDN data channels used for this purpose.
Figure 8.1 shows the functionality of ISDNs basic rate. It provides three separate channels: two B channels (bearer channels) for data, voice and video information at 64 Kbps; and one D channel (delta channel) used for control signaling or for low speed packet switching transmissions at 16 Kbps such as in telemetry (remote reading of meters) or alarm systems. The three channels are time-division multiplexed onto a bit pipe providing the actual bit transmission.
The two established forms of ISDN service commonly available are: the Basic Rate Interface (BRI) specified in ITU, consisting of two B channels and one D channel (2B + D); and Primary Rate Interface (PRI) specified in ITU, consisting of 23B + D (23 B channels and one D channel).
Data in any form operating from a subscriber (customer or user) is converted into ISDN form by a terminal adapter (TA), transported onto the common carrier (telephone system) and delivered to a destination station. On the receiving side, the process is reversed, returning the data to a form usable by the receiving network customer. Because of the use of terminal adapters, the sending and receiving customers do not have to use the same type of networking or protocols at their sites. Essentially, the communication methodology is transparent to the user. What happens to the messages along the way is insignificant to the user as long as the data are successfully communicated.
ISDN SERVICES
The purpose of the ISDN is to provide fully integrated digital services to users. These services fall into three categories: bearer services, teleservices and supplementary services.Bearer services provide the means to transfer information (voice, data and video) between users without the network manipulating the content of that information. The network need not process the information and therefore need not change the content. Bearer services belong to the first three layers of the OSI model and are well defined in the ISDN standard. They can be provided using circuit switched, packet switched, frame switched, etc.
In teleservicing the network may change or process the contents of the data. These services correspond to layers 4-7 of the OSI model. Teleservices rely on the facilities of the bearer services and are designed to accommodate complex user needs without the user having to be aware of the details of the process. Teleservices include Telephony, Teletex, Telefax, Videotex and Teleconferencing. Although the ISDN defines these services by name, they have not yet become standards.
Supplementary services are those services that provide additional functionality to the bearer services and teleservices. Examples of these services are reverse charging, call waiting and message handling, all popular in today's telephone company services.
8.3 ISDN TOPOLOGY
An ISDN should work with a large variety of users and equipment, especially if it is to be integrated into an office environment. This includes both equipments designed with ISDN in mind, and current equipments whose design predates ISDN and have little in common with it. To help in the design of connection strategies, and the standardization of interfaces, ITU-T has divided the equipment into functional groups. Devices within a group provide specific capabilities. ITU-T has also defined reference points used to separate these groups-a useful aid to standardizing interfaces. Together they help categorize basic connection strategies and provide a basis on which to design more complex architectures. The following list describes the primary functional group designations.
Network Termination 1 (NTl): Non-intelligent devices concerned with the physical and electrical characteristics of the signals. They primarily perform OSI layer 1 functions such as synchronizing and timing. NT1 devices typically form the boundary between a user's site and the ISDN central office. The central office, in turn, functions much as the telephone system's central office, providing access to other sites.
Network Termination 2 (NT2): Intelligent devices capable of performing functions specified in OSI layers 2 and .3. Among this group's functions are: switching, concentration and multiplexing. A common NT2 device is a digital PBX. It can be used to connect a user's equipment together or to an NT1 to provide access to the ISDN central office.
Network Termination 12 (NT12):
NT12 is a combination of NT1 and NT2 in a single device.
Terminal Equipment 1 (TEI):
This includes ISDN devices such as an ISDN terminal, digital telephone or computer with an ISDN compatible interface. Such devices typically connect directly to a network termination device.Terminal Equipment 2 (TE2): Non-ISDN devices including printers, PCs, analog telephones or anything that has a non-ISDN interface such as RS-232 or X.21. Terminal Adapter (TA): These are devices designed to be used with TE2 equipment to convert their signals to an ISDN compatible format. The purpose is to integrate non-ISDN devices into an ISDN network.
Figure 8.2 shows typical functional groups and how they can be connected. To standardize the interfaces, ITU-T has defined reference points between the groups.
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