Frame has class of service defined so that the throughput of any given DLCI can be controlled. There are two reasons for this. Firstly a customer pays for a given amount of bandwidth, so why should they be given more?, and secondly, that any customer using a large amount of bandwidth (more than they should be) is depriving another customer of it. This occurs especially where a network is over-subscribed.
At a lower level again, classes of service are required where there are more than 1 PVC's coming into a customer port. It is important to ensure that one PVC does not consume all the available bandwidth of the physical pipe and deprive others the use of it. There is a general rule when establishing a Frame Relay network for customers that I use:
The CIR's of the PVC's should be equal to or less than the port speed.
Committed Information Rate (CIR) is rate the rate which a telco commits to having available for transmission at all times for a PVC. The CIR is also generally referred to as throughput. Many large corporate customers require a Frame Relay reporting tool to show that they are infact getting their "value for money."
Committed Burst Size (Bc) is the maximum number of bits of data that a network will commit to transmitting at the CIR for a given interval.
Excess Burst Size (Be) is the number of bits of data above the CIR, in excess of Bc, that will occur over the measurement interval.
(Tc) is the committed rate measurement interval
The formula is as follows: CIR = Tc x Bc
The measurement can be for a certain period or on a sliding window basis.
It is important to remember that data is bursty in nature and will not usually be transmitted constantly for long periods.
For a more graphical explanation, click the graphic on the right.
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