8.4 Frame Relay Traffic Shaping
8.4.2 Traffic shaping and flow terminology
The traffic shaping over Frame Relay feature applies to Frame Relay PVCs and SVCs.

You should be familiar with some terminology related to Frame Relay traffic flow. Common Frame Relay terms related to traffic flow follow:

  • Local access rate - The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network, regardless of other settings.
  • CIR - The rate, in bits per second, that the Frame Relay switch agrees to transfer data. The rate is usually averaged over a period of time, referred to as the committed rate measurement interval (Tc). In general, the duration of Tc is proportional to the "burstiness" of the traffic.
  • Oversubscription - Oversubscription is when the sum of the CIRs on all the VCs coming in to a device exceeds the access line speed. Oversubscription can occur when the access line can support the sum of CIRs purchased, but not of the CIRs plus the bursting capacities of the VCs. If oversubscription occurs, packets are dropped.
  • Committed Burst (Bc) - The maximum number of bits that the switch agrees to transfer during any committed rate measurement interval (Tc). The higher the Bc-to-CIR ratio, the longer the switch can handle a sustained burst. For example, if the Tc is 2 seconds and the CIR is 32 Kbps, the Bc is 64 Kbps. The Tc calculation is Tc = Bc/CIR.

Note: Tc is not a recurrent time interval. It is used strictly to measure inbound data, during which time it acts like a sliding window. Inbound data triggers the Tc interval.

  • Excess Burst (Be) - The maximum number of uncommitted bits that the Frame Relay switch attempts to transfer beyond the CIR. Be is dependent on the service offerings available from your vendor, but it is typically limited to the port speed of the local access loop.
  • FECN - When a Frame Relay switch recognizes congestion in the network, it sends a FECN packet to the destination device, indicating that congestion has occurred, as shown in Figure .
  • BECN - When a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router, instructing the router to reduce the rate at which it is sending packets, as shown in Figure . With Cisco IOS Release 11.2 or later, Cisco routers can respond to BECN notifications. This topic is discussed later in this chapter.
  • DE indicator - When the router detects network congestion, it will mark the packet "Discard Eligible." The DE bit is set on the oversubscribed traffic-that is, the traffic that was received after the CIR was met. These packets are normally delivered, but in periods of congestion, the Frame Relay switch will drop packets with the DE bit set first.

Note: These are generic Frame Relay terms. They may be the same or slightly different from the terms your Frame Relay service provider uses.

The CIR, by itself, does not provide much flexibility when dealing with varying traffic rates. In practice, the Frame Relay switch measures traffic over a time interval that is specific to each logical connection, as you can see in Figure .

Bc and Be are amounts of data that a Frame Relay network agrees to transfer over a time interval, T. Be is the maximum amount in excess of the Bc that the network attempts to transfer under normal conditions. However, the traffic beyond the Bc will be marked DE.

Notice in the Figure that the actual frame transfer rate parallels the access rate; when a frame is being transmitted on a channel, that channel is dedicated to that transmission. The horizontal lines represent no frames being transmitted. In general, the sum of CIRs for all logical connections must be less than or equal to the physical line rate.