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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.
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145
By contrast, the 64 kbps, 8 kHz Structured, Unrestricted service makes no assumptions about the signal’s data content and is a general purpose service. There
are also a number of other unrestricted services, offering successively higher bandwidths e.g., 384, 1536, and 1920 kbps, 8 kHz Structured, Unrestricted
services.
In addition to the above circuit-switched bearer services, there are a number of packet-switched bearer services. For example, the User Signaling service is a
packet-switched bearer service suitable for exchanging signaling information between end-users and is defined by recommendation I.451, which we will examine later in
this chapter. Another example is the Connectionless on a D Channel service which provides a datagram service on a D channel and is suitable for control-oriented or
transaction-oriented applications.
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.
11.2. Protocol Architecture
Figure 11.126 illustrates the ISDN protocol architecture in relation to the OSI reference model. Access to the ISDN is controlled by layers 1-3. The physical layer
146 Communication Networks
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defines the physical U interface for basic and primary rate access. The data link layer provides two HDLC-type protocols, one for use with B channels LAP-B and one
for use with D channels LAP-D. The network layer provides the X.25 packet level protocol for packet switching on either the B channel or the D channel and a call
control protocol for D channel signaling. Layers 4-7 are concerned with the end-to- end exchange of user data.
Figure 11.126 ISDN protocol architecture. OSI Layer
ISDN Layer Purpose
higher layers 4+ B Channel End-to-End
User Signaling Higher-level functions related to end-to-end
signaling. Network
3 Circuit-
X.25 Call
Control X.25
Circuit-switching, packet switching, and call control signaling.
Data Link 2
Switched LAP-B
LAP-D Data link control for voicedata channels and
signaling channels B and D. Physical
1 Physical
Physical interface for basic access and primary access T and S interfaces.
Below we will look at each of layers 1-3 separately.
11.2.1. The Physical Layer
The physical user interfaces for basic and primary rate access are, respectively, defined by CCITT recommendations I.430 and I.431. The S and T interfaces both
use four wires to provide full-duplex connections as two separate physical connections, one in either direction. Because of the very short distances involved
between the U interface and the S and T interfaces, this is the most effective approach to full-duplex connection.
The basic access interface at the S or T reference point consists of 2B+D channels which are multiplexed onto a 192 kbps channel using TDM. As mentioned
earlier, this carries an overhead of 48 kbps which accounts for framing and synchronization. Each frame is 48 bits long see Figure 11.127 and consists of: 16
bits from either B channel marked as B1 and B2, 4 bits from the D channel marked as D, and 12 framing and synchronization bits shown in italics. As shown
in Figure 11.127, the structure of frames sent from an NT to a TE is somewhat different from the structure of frames sent from a TE to an NT. The Framing Bit
marks the beginning of a frame. The Balancing Bit is a negative pulse and is intended to balance the DC voltage. These two bits are used for frame
synchronization. An NT echoes the most-recently received D channel bit from a TE using the Echo Bit reasons explained below. The Activation Bit is used by an NT
to activate or deactivate a TE. The Auxiliary Bit and the Auxiliary Complement Bit which is simply the logical negation of the Auxiliary Bit are used for frame
alignment. The Multiframe Bit is used for multiframing.
