Satellite Internet Forum

[eccc]

Please, what is the committed information rate (CIR) and how does this apply to a shared satellite Internet platform?

How can one use this figure to calculate the number of computers that a particular bandwidth tier can support.

For instance, 1024/128 shared platform with 8:1 contention. what is the CIR, and how many systems can hook up?

thank you in advance for the answers!

[ninosat]

The committed information rate (CIR) is the rate at which the network supports data transfer under normal operations. Its name is descriptive: you have a contract with your carrier, who has committed to providing a given throughput, here called the committed information rate. The CIR is measured in bits per second. You configure this value that the carrier provides per virtual circuit.

When configuring the CIR, consider the following:

· CIR of 0

You can contract with a carrier for a CIR of 0, which yields best-effort service at low cost. The carrier transmits data, but does not commit to providing a specified throughput. To configure a CIR of 0, set both the throughput (which is the CIR) and the committed burst (Bc) to 0, and set the excess burst (Be) to a value greater than 0.

· Maximum CIR

The maximum CIR should not be greater than the speed of the access line on the slower end of a virtual circuit. If you configure CIRs for these virtual circuits at the central site, you can use CIR enforcement (described in the next topic) to prevent the big pipe from sending traffic that exceeds the PVC CIRs.

· CIR enforcement

CIR enforcement means restricting the speed of outbound traffic to a rate no faster than the CIR. It is the major component of traffic shaping. You can configure CIR enforcement to operate over Synchronous, High-Speed Serial Interface (HSSI), T1, E1, and Integrated Services Digital Network (ISDN) lines, for frame relay backup, demand, bandwidth-on-demand, and leased lines at the virtual circuit level. CIR enforcement operates on whole frames only. It controls congestion either by bringing down the virtual circuit, or by throttling the traffic.

Committed burst rate and excess burst rate
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The committed burst size (Bc) defines the number of bits that the router can transmit over a specified time interval (Tc) when congestion is occurring. The excess burst size (Be) defines the number of extra bits that the router attempts to send over the Tc when there is no congestion. Both the Bc and the Be are values that you configure.

The sum of the Bc and the Be is the maximum amount of traffic that can travel across the network per Tc when there is no congestion. If you set the Be to a value greater than zero, the router can send traffic exceeding the CIR. To enforce the CIR, that is, to limit traffic that the router can send to the amount of the CIR, set the Be to 0.

If you enable congestion control and set the congestion method to throttle, the virtual circuit sends only Bc bits of data over the time interval Tc when congestion occurs, even if you have configured the Be to a value greater than 0. It queues the excess data until congestion abates. If you set the congestion method to throttle-then-shutdown, the virtual circuit first queues traffic when congestion occurs, and then terminates the virtual circuit if throttling does not alleviate congestion.