Service providers build Frame Relay networks using very large and very powerful switches, but devices only see the switch interface of the service provider. Customers are usually not exposed to the inner workings of the network, which may be built on very high-speed technologies, such as SONET or SDH.

From a customer’s point of view, Frame Relay is a single interface configured with one or more PVCs. Customers buy Frame Relay services from a service provider. Before considering how to pay for Frame Relay services, there are some terms and concepts to learn, as shown in the figure:

The CIR specifies the maximum average data rate that the network undertakes to deliver under normal conditions. When subscribing to a Frame Relay service, the local access rate is specified, for example 56 kb/s or T1. Typically, the provider asks the customer to specify a CIR for each DLCI.

If the customer sends information faster than the CIR on a given DLCI, the network marks some frames with a Discard Eligibility (DE) bit. The network does its best to deliver all packets; however it discards DE packets first if there is congestion.

Note: Many inexpensive Frame Relay services are based on a CIR of zero (0). A CIR of zero means that every frame is a DE frame and the network throws away any frame when it needs to. The DE bit is within the address field of the Frame Relay frame header.