Frame Relay Facts

Frame Relay Facts

Frame Relay Facts

Frame relay is a standard for packet switching WAN communications over high-quality, digital lines. Frame-relay networks:

• Provide error detection but not error recovery. It is up to end devices to request a retransmission of lost packets.
• Can provide data transfer up to 1.54 Mbps.
• Have a variable packet size (called a frame).
• Can be used as a backbone connection to LANs.
• Can be implemented over a variety of connection lines (56K, T-1, T-3).
• Operate at the Physical and Data Link layers of the OSI model.

When you sign up for Frame Relay service, you are assigned a level of service called a Committed Information Rate (CIR). The CIR is the maximum guaranteed data transmission rate you will receive on the Frame Relay network. When network traffic is low, you will likely be able to send data faster than the CIR. As network traffic increases, priority is given to data coming from customers with a higher CIR, and the effective rate may drop. In any case, you are guaranteed to have at least the amount of bandwidth specified by the CIR.

You should be familiar with the following concepts about how Frame Relay networks send data.

• Routers connect to a Frame Relay switch either directly or through a CSU/DSU.
• Frame relay networks simulate an "always on" connection with PVCs.
• Sending routers send data immediately without establishing a session.
• Frame Relay switches perform error checking but not correction.
• Corrupted packets are simply dropped without notification.
• Packets travel through the Frame Relay cloud without acknowledgments.
• Error correction is performed by sending and receiving devices.
• Frame Relay switches begin dropping packets when congestion occurs.
• Congestion is the most common cause of packet loss on a Frame Relay network.
• Packets are discarded based on information in the Discard Eligibility (DE) bit.
• Frame Relay switches set a bit in each packet to indicate that the path is experiencing congestion:
• The Backward Explicit Congestion Notification (BECN) bit is set in packets sent back to the sending device. This lets the sending device know that the path it is using to send on has experienced congestion.
• The Forward Explicit Congestion Notification (FECN) bit is set as packets are forwarded to the destination. This lets the receiving device know that the packet has experienced congestion along the path.

The action devices take in response to these messages depends on the upper-layer protocol configuration. However, a common response to a BECN message is to slow the rate of data transmissions. A common response to a FECN message might be to slow the rate of data requests.