The Network Consultants Handbook, published by Cisco Press, helps value-added resellers (VARs) analyze and enhance their clients' basic network topology or set up a computer network from the beginning. In this excerpt from Chapter 15, author Matthew Castelli discusses best practices for managing a client's network traffic through frame relay traffic shaping.
Frame Relay Traffic Shaping
Traffic shaping supports the controlling of the traffic going out of an interface. This control
matches the flow of traffic to the speed of the remote destination (or target) interface and
ensures that the traffic conforms to policies contracted for the interface. Traffic adhering to
a particular profile can be shaped to meet downstream requirements, eliminating
bottlenecks in topologies with data-rate mismatches.
The primary reasons for using traffic shaping are to control access to available bandwidth,
to ensure that traffic conforms to the policies established for the available bandwidth, and
to regulate the flow of traffic to avoid congestion. Congestion can occur when the sent
traffic exceeds the access speed of its destination (target) interface across a VC.
Following are some examples of when to use traffic shaping:
NOTE Regarding a similar, more complicated case, a link-layer network giving indications of
congestion that has differing access rates on different attached DTE; the network might be
able to deliver more transit speed to a given DTE device at one time than another. (This
scenario warrants that the token bucket be derived, and then its rate maintained.)
Traffic shaping limits the rate of transmission of data, limiting the data transfer to one of
the following:
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The transfer rate depends on three components that constitute the token bucket: burst size,
mean rate, measurement (time) interval.
The mean rate is equal to the burst size divided by the interval, as demonstrated by the
following equation:
When traffic shaping is enabled, a maximum burst size can be sent during every time
interval. However, within the interval, the bit rate might be faster than the mean rate at any
given time.
BE size is an additional variable that applies to traffic shaping. The excess burst size
corresponds to the number of noncommitted bits—those bits outside the CIR—that are still
accepted by the Frame Relay switch but marked as DE.
The BE size allows more than the burst size to be sent during a time interval. The switch
will allow the frames that belong to the excess burst to go through, but it will mark them by
setting the DE bit. The switch configuration determines whether the frames are sent.
When BE size equals 0 (BE = 0) the interface sends no more than the burst size every
interval, realizing an average rate no higher than the mean rate. When BE size is greater than
0 (BE > 0) the interface can send as many as BC + BE bits in a burst, if the maximum amount was not sent in a previous time period. When less than the burst size is sent during an
interval, the remaining number of bits, up to the BE size, can be used to send more than the
burst size in a later interval.
Learn more about frame relay traffic shaping, as well as frame relay configuration and mesh networking, in Chapter 15 of the Network Consultants Handbook.
