FRTS

Frame relay traffic shaping (FRTS) limits the outgoing traffic rate and smoothes bursts for PVCs so they can transmit traffic at a nearly constant rate.

FRTS applies to the outgoing interface of a switch. You can use FRTS to remove the bottleneck created when the input rate of a device is slower than the output rate of the sending device.

As shown in Figure 43, Router B transmits packets to Router A at 128 kbps, whereas the maximum interface rate of Router A is only 64 kbps. This traffic rate disparity creates a bottleneck at the interface that connects Router A to the FR network. To avoid packet loss, you can use FRTS at the outgoing interface Serial 2/0 of Router B so the interface can transmit packets constantly at 64 kbps when no congestion is present. Even if congestion occurs in the network, Router B can still transmit packets at the rate of 32 kbps.

FRTS uses the parameters CIR ALLOW, CIR, CBS, and EBS for traffic shaping. FR PVCs can transmit packets at the rate of CIR ALLOW. In case of bursty packets, FRTS allows an FR PVC to transmit packets at a rate exceeding CIR ALLOW.

FRTS is implemented using token buckets. The meanings of the related parameters in the protocol are modified as required by the actual algorithm and principles. See Figure 44 for how a token bucket works.

In the token bucket approach, packets requiring traffic control are put into the token bucket for processing before transmission. If enough tokens are available in the token bucket for sending these packets, the packets are allowed to pass. If the number of tokens in the token bucket is not enough for sending these packets, these packets are put into the FR class queue (the FRTS queue in FRTS implementation). Once enough tokens are available in the token bucket, the packets are taken out of the FR class queue for transmission. In this way, you can control the traffic of a certain class of packets. Tokens are in the unit of bits.

The FR protocol-provisioned related parameters correspond to the FRTS parameters as follows:

• The sum of CBS and EBS equals the token bucket size.

• CIR ALLOW defines the number of tokens put into the token bucket per second.

For efficiency, the FRTS introduces the concept of dynamic Tc. Tc (Tc=size of packet/CIR ALLOW) allows for dynamic adjustment depending on the transmitted packet size. The device allocates the required tokens to the current packets waiting for transmission within the latest Tc regardless of the packet size (which is smaller than 1500 bytes).

Take sending an 800-byte packet for example. Given the CIR ALLOW of 64000 kbps, it takes Tc=6400/64000=0.1s (100ms) to put the required tokens into the token bucket. The packet is transmitted successfully after 6400 bits of tokens are put into the token bucket within 100 ms.