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Frame relay is a major ISDN data link protocol for end-to-end connections over the B channel. Whereas LAP-B supports packet-switched connections, frame relay is designed to support circuit-switched connections. Frame relay is an evolution of an earlier protocol, V.120, which supports multiple logical connections over the same B channel cicuit. Frame relay provides similar capabilities but removes the restriction that all the multiplexed logical connections over a B channel should be between the same two end-users.
V.120 is a popular protocol implemented by ISDN terminal adapters. It enables a TE2 to communicate with a TE1 (or another TE2 connected to a TA) over a B channel circuit.
V.120 supports asynchronous as well as synchronous transmissions. Synchronous transmission may be either as raw data or HDLC-based. Multiple logical channels between two end-users can be multiplexed onto the same B channel. Rate adaptation is provided for devices operating at less than 64 kbps. A sliding window protocol is used for flow control.
Except for the address field and the data field, the V.120 frame structure is very similar to LAP-D and HDLC. The address field contains a Logical Link Identifier (LLI) for the identification of multiple logical connections over the same B channel circuit. Certain LLI values are assigned to specific purposes. For example, 0 denotes in-channel signaling and 256 means no multiplexing.
As well as carrying user data, the data field may conatin one or two header octets. The first octet is called the terminal adaptation header and consists of a set of bit flags for controling a number of adaptation functions, including: segmentation of HDLC frames, error control, HDLC idle condition, and extension of the header with additional control information. The second octet, the control state information octet, is optional and also consists of a set of bit flags.
The V.120 procedure involves the following steps. First, a B channel circuit is established using the Q.931 call control protocol over the D channel. The resulting circuit may be used in connectionless mode by using an LLI value of 256 for the frames. For a connection-oriented mode, however, additional steps are necessary to manage multiplexed logical links. Four messages are provided for this purpose:
* Setup may be issued by either side to request a logical link to be established.
* Connect may be issued in response to a setup message in order to accept the connection.
* Release may be issued by either side to release an existing connection.
* Release Complete may be issued in response to a setup message to deciline a connection, or in response to a release message to confirm the release.
These messages may be exchanged either over the D channel (using Q.931 messages) or over the B channel (using V.120 frames with LLI set to 0). When the B channel circuit is no longer needed, it may be terminated by either side using the Q.931 call control protocol on the D channel.
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 LAPD) 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.