Frame Relay

Frame relay is an evolution of V.120 and provides similar capabilities to X.25, but in a considerably streamlined form, thus facilitating higher data rates. Simplifications employed by frame relay include: use of two protocol layers (as opposed to three in X.25), end-to-end (as opposed to node-to-node) error control and flow control, and use of a separate logical connection for control signaling. The main argument in support of frame relay is the fact that ISDN utilizes highly reliable transmission media (e.g., optical fiber) which do not justify the high overheads of X.25. Like X.25 and V.120, frame relay supports the multiplexing of multiple logical connection over the same B or H channel. Unlike V.120, it does not requires these connections to be between the same two end-users.

As with ISDN, frame relay uses I.430 or I.431 for its physical layer. The data link layer is sublayered. The core functions of Q.922 (an enhanced version of LAP- D) are used for end-to-end link control. These are essentially the same as LAP-D, except that they also include congestion control functions. At the UNI, the whole of Q.922 (including error control and flow control functions) is used for the reliable transfer of Q.931 messages.

End-user access to the network is always via a frame handler. The frame handler may be an integrated component of the local exchange or a completely separate node in the network. A B or H channel end-user connection to a frame handler is established by exchanging Q.931 messages over the D channel. Multiple logical frame relay connections may then be established over this connection.

Frame relay uses a frame structure very similar to that of V.120, except that there is no control field. Consequently, there are no control frames, which means that functions such as inband signaling, flow control, and error control are not supported. Error handling is limited to checking the FCS of a frame and discarding it if it is incorrect.

Given that the role of frames in a frame relay network is similar to packets in a pcaket-switched network, some means of congestion control is needed to ensure

adequate performance. Because a frame relay network has no means of flow control, it cannot assume full responsibility for congestion control. Instead, this is jointly handled by the user and the network. Two forms of signaling are provided to facilitate this: explicit signaling and implicit signaling.

Explicit signaling is based on two bits in the frame address field. These bits may be set by the frame handlers in the network in order to communicate congestion information to the user. The Backward Explicit Congestion Notification (BECN) bit serves as a warning to the user that frames transmitted by the user in the opposite direction to this frame may experience congestion situations. The user can respond to this by simply reducing the frame transmission rate. The Forward Explicit Congestion Notification (FECN) bit warns the user of congestion in the same direction as the frame itself. To respond to this, the user can submit a request (via Q.922) to its peer user to reduce its frame transmission rate.

Implicit signaling involves the user detecting that the network has discarded a frame due to congestion. In response, the user can reduce its transmission rate. An additional bit in the address field of a frame, called Discard Eligibility (DE), can be set by the user to indicate to the network that when discarding frames due to congestion, preference should be given to this frame.

11.4.

Internetworking

For economic and practical reasons, the introduction of ISDN involves a rather long transitory phase, during which time ISDN and earlier networks have to coexist. For this to work, some means of internetworking is needed. For examples, subscribers who are served by ISDN may need to contact subscribers who are served by PSTN. Such internetworking involves a number of considerations, including the following:

• Mapping between the numbering plans of the two networks.

• Mapping between the services offered by the two networks.

• Mapping between the control signals employed by the two network.

• The physical interfacing of the two networks.

An additional set of reference points have been defined by CCITT to address the above. These reference points define interfaces between ISDN and other networks:

• The K reference point is used for interfacing ISDN to an existing, non-ISDN voice or data network, where the internetworking mappings are performed by ISDN.

• The L reference point is used for interfacing ISDN to an existing, non-ISDN voice or data network, where the internetworking mappings are performed by the non-ISDN network.

• The M reference point is used for interfacing ISDN to a specialized network (e.g., MHS), where the internetworking mappings are performed by the latter.

• The N reference point is used for interfacing two ISDN networks, where both networks may need to do internetworking mappings to provide a common set of services.

11.5.

ISDN Standards

Figure 11.134 provides a high-level view of CCITT ISDN recommendations. Each of the series consists of many parts. The I.100 series provides a general introduction to ISDN, its principles, objectives, and envisaged evolution path. The I.200 series defines the ISDN end-user services and a set of attributes for each service. The I.300 series defines the ISDN network and its functions and protocols in relation to ISDN services. The I.400 series defines the physical and logical interfaces between ISDN and its users. The I.500 series deals with the issues of internetworking between ISDN networks and between ISDN and non-ISDN networks (e.g., PSTN). The I.600 series deals with the issues of maintenance of subscriber access and installation.

Figure 11.134 High-level view of CCITT ISDN recommendations.

Part / Series Description

I: I.100 Series General description of ISDN concepts II: I.200 Series Service capabilities

III: I.300 Series Overall network aspects and functions IV: I.400 Series User-network interfaces

V: I.500 Series Internetwork interfaces VI: I.600 Series Maintenance principles

Figure 11.135 summarizes the major ISDN-related standards for four areas of basic rate, primary rate, SS7, and packet-switched network.

Figure 11.135 Major ISDN-related standards.

Layer Standard Description

1 I.430 2B+D channels (D=16 kbps)

Basic Rate 2 I.441 LAP-D (with multiple TEs) 3 I.451 User-network interface

1 I.431 23B+D or 30B+D channels (D=64 kbps)

Primary Rate 2 I.441 LAP-D (point-to-point) 3 I.451 User-network interface

1-3 Q.701-Q.708 Message Transfer Part (MTP)

Signaling System 7 4 Q.711-Q.714 Signaling Connection Control Part (SCCP) 4+ Q.761-Q.766 ISDN User Part (IUP)

4+ Q.791-Q.795 Operation, Admin and Maintenance (OA&M) 1 X.21 Physical interface

Packet Network 2 LAP-B Link access 3 X.25, X.75 Packet interface

11.6.

Further Reading

Black (1989) and Freeman (1989) both contain introductory chapters on ISDN. Griffiths (1990), Helgert (1991), Stallings (1992), and Kessler (1993) provid detailed discussions of ISDN architecture, services, and protocols. Dickson and LLoyd (1992) discuss many of ISDN protocols within the context of the OSI model. Deniz (1993) describes the use of ISDN for interconnecting LANs. Lai (1989) provides a useful introduction to frame relay. Deatiled descriptions of frame relay applications and protocols can be found in Chen and Rege (1989), Smith, P. (1993), and Black (1994).