ISDN Channels Basic Concepts

www.pragsoft.com Chapter 11: Integrated Services Digital Network 143 equivalent to those denoted by one or more OSI layers. The interfaces between the functional groupings are called reference points see Figure 11.132. Figure 11.124 ISDN functional groupings and reference points. TA NT1 NT2 ISDN Network NT1 TA U T S ST R R U TE1 TE2 TE1 TE2 Functional groupings Reference points Primary access Basic access 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 ST 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 ST 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 144 Communication Networks Copyright © 2005 PragSoft R interface and the S or ST 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. Figure 11.125 illustrates an example of such a device: an ISDN point-of-sale terminal adapter designed as a retail environment CPE which provides a variety of interfaces. Figure 11.125 ISDN point-of-sale terminal adapter. ISDN POS-TA U interface RS232 ST interface POS Terminal Analog interface ISDN Network

11.1.3. ISDN Services

ISDN provides three types of services: • Bearer services • Teleservices • 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.