3Com MSR 50 Series Configuration Manual page 1712

1712 C 91: F
HAPTER RAME
R Q S C
ELAY O ONFIGURATION
For example, to send an 800-byte packet, the Router needs a 6400-bit token,
where 6400 = 800 × 8. Given the CIR of 64000 bps, the time needed to add the
required tokens into the token bucket is 6400/64000 = 100 ms, that is, the Tc for
the packet is 100 ms. The packet hence will be transmitted after 6400 bits of
tokens are added into the token bucket. In some special cases, for example, if CIR
is 8000 bps, the Tc for the packet should be 6400/8000 = 800 ms > 100 ms.
However, as Tc is defined to be in the range of 10 ms to 100 ms, the Tc for the
packet will take the lower threshold value, that is, 100 ms, rather than 800 ms.
Likewise, if CIR is 1024000 bps, the Tc for the packet should be 6400/1024000 =
6.25 ms < 10 ms, but the actual Tc will take the upper threshold value, that is, 10
ms.
As mentioned above, the size of token bucket equals CBS plus EBS and the tokens
required for sending a packet are distributed at a time on the device. To ensure
there are always adequate tokens in the token bucket for sending packets of any
sizes, especial large packets like 1500-byte packets that require 1500 × 8 = 12
Kbits of tokens, the CBS on the routers must not smaller than 15K and
recommends the users to set CBS to the same size of CIR.
As provisioned in the standard protocol, given Tc = 20 ms and CIR = 64000 bps,
only 1280 bits (0. 02 × 64000 bits) of tokens can be put into the token bucket
within each Tc. Therefore, to send an 800-byte packet, the device needs to add
tokens for five times in order to send the packet. Compared to the
implementation of the standard protocol, the device in our implementation can
add all the tokens required for sending the same packet at a time, hence
significantly improving the efficiency.
When the network congestion occurs, if the Frame Relay switch device has been
configured with the function of congestion management, it will notify the device
of network congestion. Upon receiving the notification, the device will eventually
slow down the transmit rate to the CIR so as to ease the congestion, when users
can transmit data at the rate of CIR. After this, if no notifications of network
congestion are received within a certain period of time, the device will eventually
raise the transmit rate from the CIR back to the CIR ALLOW.
Figure 490 Fundamentals of Frame Relay traffic shaping
CIR ALLOW+PIR 128 Kbps CIR ALLOW+PIR 128 Kbps
CIR ALLOW 64Kbps CIR ALLOW 64Kbps
CIR 32Kbps CIR 32Kbps
As shown in the above figure, the parameters of FRTS are respectively set to be:
CIR ALLOW= 64 kbps, CIR = 32 kbps, CBS = 64000 bits, and EBS = 64000 bits.
The token bucket is 128000 bits, the speed of token addition is 64 kbps before 4s,
and packets are sent at 64 kbps on the PVC. At the 4s, the device receives the
0s 0s 1s 1s 2s 2s 3s 3s
TIME TIME
4s 4s 5s 5s 6s 6s