How It Works
Frame relay technology is a packet-switching service that is similar in operation to, and considered the replacement for, the older X.25 packet-switching technology—but it provides higher performance and has a greater efficiency because it is a more streamlined protocol. For example, while X.25 includes error-correction functions, frame relay leaves error correction up to the station endpoints in order to speed up WAN communications. When errors do occur, frame relay drops the offending frame and retransmits the data. Frame relay also does not support the hop-by-hop flow control functions that X.25 supports, which further streamlines frame relay operation.
As in other packet-switching networks, frame relay operates by breaking network data into “packets” and tagging each packet with a destination address. Each packet is then relayed through the switching nodes that make up the packet-switching network. The packets are reassembled into the correct order at their destination. Frame relay is a protocol-independent service that uses special frame relay devices for encapsulating network data into variable-length packets called frames using the data-link layer protocol called High-level Data Link Control (HDLC). Frame relay links can have unpredictable latency for forming connections because frame relay networks have no prioritization scheme.
Instead of relaying each packet individually through the frame relay network, frame relay uses virtual circuits that act as temporary paths through the network. These virtual circuits can be either switched virtual circuits (SVCs) that are set up and torn down on a call-by-call basis, or permanent virtual circuits (PVCs) that are established in advance. PVCs are preferred because they provide a more reliable grade of service for the customer. PVCs provide dedicated point-to-point connections between local and remote customer premises through a frame relay cloud. By establishing multiple PVCs, you can run multiple logical WAN links over a single physical frame relay connection. PVCs are managed using the Local Management Interface (LMI) protocol, which provides features for verifying link integrity and managing the status of PVCs.
PVCs function in a way similar to private leased lines and provide the customer with a level of service that is agreed upon, called the Committed Information Rate (CIR). The CIR, a negotiated level of service you purchase from the carrier, specifies your maximum transmission speed over the frame relay network. Speeds typically range from 56 Kbps to T3 speeds and higher, depending on your requirements. CIR acts as a kind of bandwidth throttling mechanism that facilitates the use of shared frame relay circuits by different users. Some service providers allow temporary bursts of traffic to exceed the CIR, but any traffic above the Committed Information Burst Rate (CIBR) is dropped and requires retransmission.
Frame relay runs over T1 and fractional T1 carriers with transmission speeds ranging from 56 Kbps to 1.544 Mbps or higher. Since frame relay is independent of network protocols such as TCP/IP and IPX/SPX-Compatible Protocol, it has more flexibility than X.25.
Frame relay implementations usually follow one of two networking topologies:
To connect a network to a telco Frame Relay Bearer Service (FRBS), use a special bridge, router, or CSU/DSU (Channel Service Unit/Data Service Unit) device called a frame relay access device (FRAD). The FRAD connects your customer premises to an Edge Switch (ES) on your provider’s frame relay cloud (the collection of all frame relay circuits belonging to your provider). See the illustration for an example.
Frame relay technology is more popular in North America than slower packet-switching technologies such as X.25, while in Europe, X.25 has traditionally been a more popular solution. Frame relay services were first offered in 1992 by AT&T, Sprint, and other carriers, which have installed frame relay points of presence (POPs) for connections to the central office (CO) of local telcos in major metropolitan locations around the United States.
An important consideration in frame relay communication is a set of extensions to the frame relay encapsulating protocols called the LMI, developed by Cisco, DEC, and others. Frame relay routers from Cisco use a “Cisco LMI” while many other vendors use the “ANSI LMI,” which can create incompatibilities.
Bursts of traffic above the CIR are typically short (less than two seconds in duration) and are generally possible only during off-peak utilization times. When access to the service provider’s frame relay network is heavy, your maximum bandwidth will be your CIR.
Graphic F-20. A frame relay WAN link.
Some possible strategies for troubleshooting frame relay links in different kinds of situations appear in the following table.
Troubleshooting Frame Relay Links
|Problem ||Suggestions |
Frame relay link is down (connections fail)
Check cabling and connections, make sure you are using a data terminal equipment (DTE) cable, try connecting the cable to a different port, or try a different cable. Make sure you are using IETF encapsulating if mixing frame relay devices from different vendors.
Cannot ping remote router
Check the status of PVC; contact carrier if this is down. Check the router’s access list, disable access list, and retry. Make sure you are using IETF encapsulating if mixing frame relay devices from different vendors. Check the configuration of the frame relay address map.
Cannot ping device on remote network
Try pinging local router’s frame relay address; check that a default gateway is specified. Check for split horizon conditions in a hub-and-spoke frame relay implementation.