126
Figure 45 FRTS implementation
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.
How FRTS works
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
for how a token bucket
works.
Figure 46 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.
Token bucket
Classification
Packets to be sent
through this interface
Packets sent
Tokens are put
into the bucket at
the set rate
FRTS queue
Tokens
