Layer-2 QoS Solutions 41 Frame Relay QoS Roughly speaking, Frame Relay QoS can be considered a subset of ATM QoS. Frame Relay is also connection-oriented and virtual circuit based. Each Frame Relay VC has a Committed Information Rate (CIR). At the ingress of the VC, policing will be performed and nonconforming frames will be marked with a high Discard Eligibility (DE), which is very similar to ATM Cell Loss Priority. Frame Relay also has a congestion notification mechanism using the Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) bits in the frame header. Such explicit congestion notifications are supposed to notify the traffic senders to slow down when there is congestion in the network. However, because such notifications are not hooked up with the higher-layer protocol, for example, the TCP/IP stack, they are not really useful [Huston]. Frame Relay as a service rarely has its own network. Most of the time, Frame Relay is just a service interface to the users, and is carried over an ATM network. Therefore, Frame Relay does not really have its own independent QoS solution. Ethernet QoS