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Integrated Services Digital Network
Current trends in telecommunication are toward integration of voice and data services. So far these services have been available separately, requiring separate subscription, communication links, and equipment. It has long been acknowledged that the integration of these services will result in significant flexibility and cost benefits to both service users and service providers. The Integrated Service Digital Network (ISDN) is a major attempt to realize these objectives.
The three most important ingredients of ISDN have already been discussed in earlier chapters: circuit switching, packet switching, and common channel signaling (SS7). This chapter looks at the rest of the ISDN technology. We will start with some basic ISDN concepts, including its channels, reference points, functional groupings, and services. We will then describe the ISDN protocol architecture in relation to the OSI model, and discuss various ISDN standards. Finally, we will examine the potential future of ISDN within the context of global communication networks.
ISDN provides a fully integrated digital network for voice and data communication. It supports both circuit and packet switching.
Each ISDN switch consists of an exchange termination part, which performs the necessary circuit switching functions, and a packet handler, which performs the necessary packet switching functions. The packet handlers implement X.25 and are connected to a public packet switched network via X.75. The exchange terminations are interconnected via tandem exchanges. STPs and SCPs provide network intelligence, and were described in the previous chapter. Subscriber access is provided via a network termination and/or terminal adapter (NT/TA). This provides the connectivity for a variety of user devices, including ISDN phones, Plain Old Telephone Sets (POTS), LANs, PBXs, and X.25 terminals.
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 computergenerated 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 endpoints 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)
* 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.
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 equivalent to those denoted by one or more OSI layers. The interfaces between the functional groupings are called reference points.
The U (User) interface is a 2-wire physical interface to the network.
The Network Termination 1 (NT1) functional grouping provides OSI layer 1 capabilities and deals with signal transmission and physical connectors for interfacing Customer Premises Equipment (CPE) to ISDN. The NT1 transforms the U interface into a 4-wire subscriber S/T interface which supports 2B+D channels (in case of basic access) or T interface which supports 23B+D or 30B+D (in case of primary access). NT1 multiplexes these channels using TDM into a continuous bit stream for transmission over the U interface. NT1 also supports up to eight CPEs connected in a multidrop line arrangement to basic access. The NT1 device may be owned and operated by the service provider, baring the customer from direct access to the U interface, or it may be a CPE.
The Network Termination 2 (NT2) functional grouping provides additional OSI layer 2 and 3 capabilities on top of NT1. NT2 is a CPE which transforms the T (Terminal) interface into an S (System) interface. The S interface supports 2B+D channels. NT2 may perform switching and concentration functions. A typical NT2 device would be a digital PBX, serving a set of digital phones, or a LAN, serving a set of personal computers.
Two types of terminal equipment may be used for ISDN access.
Terminal Equipment 1 (TE1) denotes ISDN terminals which use a 4-wire physical link to the S or S/T interface. TE1 devices conform to ISDN standards and protocols and are especially designed for use with ISDN. A digital ISDN telephone and a PC with an ISDN card are examples.
Terminal Equipment 2 (TE2) denotes non-ISDN terminal equipment. Ordinary terminals and personal computers are examples. These devices can be connected to ISDN at the R (Rate) reference point. RS-232 and V.21 are examples of the type of standards that may be employed for the R reference point. The mapping between the R interface and the S or S/T interface is performed by a Terminal Adapter (TA), which performs the necessary protocol conversions and data rate adaptations between the two interfaces.
It is worth pointing out that although NT1, NT2, and TAs may be offered as separate devices, in practice this is not always the case. For example, some CPE manufacturers produce TAs that have NT1 and NT2 capabilities, as well as additional interfaces for other devices (e.g., analog telephones).
ISDN provides three types of services:
* Bearer services
* Supplementary services
Tele and supplementary services represent the type of features and functions which are visible to end-users, while bearer services represent the parts of the network which remain hidden from end-users.
Bearer services facilitate the real-time communication of digital information between end-users. These services mainly relate to network functions and account for OSI layers 1-3. An example of a bearer services is the 64 kbps, 8 kHz structured, speech service. This service uses a data rate of 64 kbps together with 8 kHz timing information (which structures the data into octet intervals) for transmitting a Pulse Code Modulated (PCM) speech signal. The fact that the signal represents speech is known to the network, allowing it to employ transformations which may not preserve bit integrity but will result in good quality audio reproduction.
Teleservices provide a set of higher-level functions on top of bearer services. These services account for OSI layers 4-7. Examples of teleservices are:
* Telephony services which provide speech communication over a B channel with control signaling over the D channel.
* Facsimile services which facilitate the communication of bitmap images over a B channel with control signaling over the D channel.
* Teletex services which facilitate the interchange and communication of textual as well as formatted documents over a B channel with control signaling over the D channel.
Supplementary services enhance bearer and teleservices in an independent fashion. Examples of supplementary services are:
* The Centrex service emulates a private network and provides specialized features to a set of subscribers.
* The Call Transfer service allows a user to transfer an active call to a third-party.
* The Call Waiting service allows a user already engaged in a call to be informed of another incoming call.
* The Calling Line ID service provides the calling party's address information to the called party.
Although these services all appear geared toward circuit-switched telephone calls, they are equally applicable to packet-switched data calls.