Information Technology Reference
In-Depth Information
protocol layers. Greater performance and efficiency is therefore possible without sacrificing data
integrity. Frame Relay is designed with this approach in mind. It includes a cyclic redundancy check
(CRC) algorithm for detecting corrupted bits (so that the data can be discarded), but it does not include
any protocol mechanisms for correcting bad data (for example, by retransmitting it at this level of
protocol).
Another difference between Frame Relay and X.25 is the absence of explicit, per-virtual-circuit flow
control in Frame Relay. Now that many upper-layer protocols are effectively executing their own flow
control algorithms, the need for this functionality at the link layer has diminished. Frame Relay,
therefore, does not include explicit flow control procedures that duplicate those in higher layers. Instead,
very simple congestion notification mechanisms are provided to allow a network to inform a user device
that the network resources are close to a congested state. This notification can alert higher-layer
protocols that flow control may be needed.
Current Frame Relay standards address permanent virtual circuits (PVCs) that are administratively
configured and managed in a Frame Relay network. Another type, switched virtual circuits (SVCs), has
also been proposed. The Integrated Services Digital Network (ISDN) signaling protocol is proposed as
the means by which DTE and DCE can communicate to establish, terminate, and manage SVCs
dynamically.
LMI Extensions
In addition to the basic Frame Relay protocol functions for transferring data, the consortium Frame
Relay specification includes LMI extensions that make supporting large, complex internetworks easier.
Some LMI extensions are referred to as “common” and are expected to be implemented by everyone who
adopts the specification. Other LMI functions are referred to as “optional.” A summary of the LMI
extensions follows:
•
Virtual circuit status messages (common)
—Provide communication and synchronization between
the network and the user device, periodically reporting the existence of new PVCs and the deletion
of already existing PVCs, and generally provide information about PVC integrity. Virtual circuit
status messages prevent the sending of data into black holes—that is, over PVCs that no longer exist.
•
Multicasting (optional)
—Allows a sender to transmit a single frame but have it delivered by the
network to multiple recipients. Thus, multicasting supports the efficient conveyance of routing
protocol messages and address resolution procedures that typically must be sent to many
destinations simultaneously.
•
Global addressing (optional)
—Gives connection identifiers global rather than local significance,
allowing them to be used to identify a specific interface to the Frame Relay network. Global
addressing makes the Frame Relay network resemble a local-area network (LAN) in terms of
addressing; Address Resolution Protocols, therefore, perform over Frame Relay exactly as they do
over a LAN.
•
Simple flow control (optional)
—Provides for an XON/XOFF flow control mechanism that applies
to the entire Frame Relay interface. It is intended for devices whose higher layers cannot use the
congestion notification bits and that need some level of flow control.
Frame Format
The Frame Relay frame is shown in Figure 18-1. The Flags fields delimit the beginning and end of the
frame. Following the leading Flags field are 2 bytes of address information. Ten bits of these 2 bytes
make up the actual circuit ID (called the DLCI, for data link connection identifier).
