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What is the relation between Burst Size & Average rate on interfaces when using Traffic Shaping ?
Traffic shaping prevents the bit rate of the packets exiting an interface from exceeding a configured shaping rate. To do so, the shaper monitors the bit rate at which data is being sent. If the configured rate is exceeded, the shaper delays packets, holding the packets in a shaping queue. The shaper then releases packets from the queue such that, over time, the overall bit rate does not exceed the shaping rate.
Routers can send bits out an interface only at the physical clock rate. To average sending at a lower rate, the router has to alternate between sending packets and being silent. For instance, to average sending at a packet rate of half the physical link speed, the router should send packets half of the time, and not send packets the other half of the time. Over time, it looks like a staccato series of sending and silence.
A shaper sets a static time interval, called Tc. Then, it calculates the number of bits that can be sent in the Tc interval such that, over time, the number of bits/second sent matches the shaping rate.
The number of bits that can be sent in each Tc is called the committed burst (Bc), an 8000-bit Bc can be sent in every 125-ms Tc to achieve a 64-kbps average rate. In other words, with a Tc of 125 ms, there will be eight Tc intervals per second. If Bc bits (8000) are sent each Tc, then eight sets of 8000 bits will be sent each second, resulting in a rate of 64,000 bps.
Because the bits must be encoded on the link at the clock rate, the 8000 bits in each interval require only 62.5 ms (8000/128,000) to exit the interface onto the link. The interface sends at the line rate (access rate) for 62.5 ms, and then waits for 62.5 ms, while packets sit in the shaping queue.
So by definition: Bc Committed burst size, measured in bits. This is the amount of traffic that can be sent during the Tc interval. Typically defined in the traffic contract.
Relation between Average Rate & Bc is like the following
Average Rate on Link = N x Bc
Where N = 1/Tc (where Tc measured in seconds)
Typical Value of N 1/0.125sec = 8
So, Average Rate = Bc/Tc