When a company provisions a leased line from a service provider, that link provides a fixed, dedicated amount of bandwidth. For example, a company might implement a dedicated 256 Kbps link between two locations. Things work a little bit differently in the world of Frame Relay. Instead of providing fixed bandwidth over a dedicated circuit, bandwidth is allocated using what is referred to as a Committed Information Rate (CIR).
A CIR is an average transmission capacity agreed upon for a virtual circuit that connects one company location to another. For example, a company may provision a circuit with a CIR of 64Kbps. What this means is that the company is basically guaranteed that they will always have at least 64 Kbps of bandwidth available to them. However, because the physical circuits of the Frame Relay network are shared amongst many subscribers, there will often be excess bandwidth available at any point in time. This allows a customer’s traffic to actually “burst” to higher speeds, as available network bandwidth permits. The speed to which data can burst is referred to as the Committed Burst Information Rate (CBIR). For example, a virtual circuit with a CIR of 64 Kbps may have a CBIR of 128 Kbps.
The fact that companies can potentially use more bandwidth than they have paid for makes Frame Relay an attractive option, but it obviously presents an issue. What would stop one company’s traffic from monopolizing available bandwidth? This issue is dealt with by designating certain frames “discard eligible” (DE). For example, let’s say that a company has paid for a CIR of 64Kbps. When sending data, all frames that are within this 64 K bandwidth allotment are sent out normally. However, once the CIR is met, any additional frames that need to be sent have a special bit (known as the DE bit) in their frame header set to 1. The DE bit marks the frame as eligible to be discarded, should the network be congested. This is illustrated in the figure below.
Another benefit of Frame Relay is that CIRs need not necessarily be the same throughout a customer’s network. For example, a company may have two branch office locations connected to their head office, as illustrated in the figure below. In this example, the PVC between the Montreal branch office and the head office has a CIR of 64Kbps, while the PVC connecting the head office to the much smaller Utah office has a CIR of only 16Kbps. CIR values are usually made available in multiples as small as 4 or 8 Kbps. In fact, it is also possible to provision Frame Relay services with a CIR of 0 – while this connection would be valid; all frames sent over the PVC with this CIR would be marked discard eligible. In reality, many Frame Relay networks have enough excess capacity that a CIR of 0 is not necessarily a bad choice for connecting to small offices, or as an interim solution while a more reasonable CIR is being determined.