Frame Relay Concepts

Frame Relay is a standard supported by both the American National Standards Institute (ANSI) and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) that originated from X.25. Unlike X.25, Frame Relay does not provide for error correction. The Frame Relay standard defines the encapsulation from the customer premises equipment (CPE) to the data circuit-terminating equipment (DCE) at the regional provider's switch. Once at the provider, Frame Relay stops, and something else, usually Asynchronous Transfer Mode (ATM), takes over.

Frame Relay is normally used as a way to connect to remote sites. It's cheaper than a leased line and provides more bandwidth than a Basic Rate Interface (BRI) ISDN line. DSL is currently giving it a run for its money in the ultra-value market, but Frame Relay is still the preferred choice in the mid- to large-sized corporate segment.

Frame Relay uses two types of circuits, the permanent virtual circuit (PVC) and switched virtual circuit (SVC). A PVC is a permanent connection to the service provider, like a T1. An SVC is more like an ISDN line in that it only connects when data needs to be transferred. With the adoption of ANSI standard T1.617 and ITU-T Q.933 and Q.922, the Cisco IOS supports SVCs on Frame Relay as of version 11.2, but many service providers do not offer the service. Because the majority of circuits are PVCs and because Cisco will not test on it, this chapter does not discuss SVCs in more detail.

The concept of Frame Relay revolves around a couple of different threshold levels on an individual link. The first is what you contract for.

Maximum Burst Rate

When you need a new line, you need to figure out what is the maximum amount of data that you need to transfer per second. You call a service provider and ask for a Frame Relay link of this size. This size is the maximum that your line can handle, and it is your maximum burst rate.

CIR

The next thing you need to know, which is just as important as your maximum line rate but often overlooked, is the CIR. The CIR is the guaranteed rate you receive. Now, isn't this rate the same as the maximum burst rate? No, which leads us to the next section.

Oversubscription

Service providers oversell their lines on the assumption that not everyone will use their full throughput at the same time. This process is not illegal, but many people are confused by all the acronyms and just sign the contract without asking questions. Later, they find out that the bandwidth they thought they bought really isn't a T1 equivalent at all.

A service provider adds up all the CIRs of its customers on a given wire, and as long as the total does not exceed the maximum throughput of the line in question, they're fine. Let's consider an example.

Arctic Bell has a 1.5Mbps Frame Relay circuit running into an office park where several subscribers do business. It can handle a maximum of 1.5Mbps of data at any one time. Subscriber A wants a 1.5Mbps Frame circuit with a CIR of 256Kbps, subscriber B wants a 768Kbps Frame circuit with a CIR of 384Kbps, and subscriber C wants a 384Kbps Frame circuit with a CIR of 128Kbps. Arctic Bell has total contract rates of one and three-fourth times its available bandwidth. What it has actually guaranteed each customer so far, however, is one-half of its available bandwidth.

The end result is that if each customer stays within its CIR, its packets will always be handled. Problems start occurring when customers begin to send more data than they have CIR for. If A, B, and C all start sending data at their maximum burst rates, Arctic Bell's circuit will not be able to handle the traffic. This situation results in packets getting lost.

At this time, you're probably wondering, "Why would anyone buy Frame Relay?" Not everyone is going to be maxing out its WAN link at the same time. A WAN connection bandwidth usage chart is filled with peaks and valleys, and Arctic Bell plans to fill in the valleys in subscriber A's traffic with the peaks from subscribers B and C. Because Frame Relay doesn't normally guarantee the full bandwidth, it's usually a cheaper option than a leased line.

Frame Relay also has a number of management and traffic-shaping options that aren't available on a leased line, giving you greater control over a Frame Relay circuit.

When you pay careful consideration to the CIR requirements of your business, Frame Relay is a cost-effective method of WAN access. Be on the lookout, however, for service providers who try to sell you a 0Kbps (zero) CIR. None of your packets are guaranteed to get through! By selling a 0Kbps CIR, they can oversubscribe to their heart's content and not be in breach of contract. Their only limit is how many businesses are in the area. Overall, a good rule of thumb is that your CIR should equal one-fourth of your maximum burst rate. It's always a good idea to ask about the number of packets dropped due to network congestion.