EIR clarification

It seems that our attempt to clarify the frame relay Excess Information Rate may have raised more questions than it answered, so we'll attempt to clear things up a bit.

In the November 10 newsletter, we discussed the genesis of EIR. In particular, we discussed B(e), which is the number of excess bits above B(c) that can "fly standby" if there's available bandwidth. However, any frames that contain bits that spill into B(e) will be marked as lower priority with the discard eligible bit. And if B(e) is divided by T(c), the timeframe over which the committed information rate (CIR) is measured, you end up with the EIR. Thus, you are guaranteed that within timeframe T(c) you may send B(c) bits, and, if bandwidth is available, then you may also within that timeframe send additional B(e) bits. However, any bits in excess of B(c) plus B(e) will be discarded, and delivery will not be attempted.

We blew it when we said that "most of the services that we're aware of set the EIR to be equal to the port speed." We should have said that "...most of the services that we're aware of set the EIR so that the EIR plus the CIR equals the port speed." Thus, for all bits transmitted, the network will either guarantee transmission (in the CIR) or attempt transmission (in the EIR).

So, in subsequent discussions, we continued this alternate interpretation of EIR. For instance, we mentioned on November 15 that "...you could be connected to your service provider with a port speed of 1.536M bit/sec (T-1), with a CIR for a given permanent virtual circuit of 256K bit/sec and an EIR for that PVC of 512K bit/sec. In this situation, traffic up to 256K bit/sec is assured the committed bandwidth, and traffic above 256K but less than 512K bit/sec would be transmitted on a discard-eligible basis. But transmission would not even be attempted for the traffic in a burst that exceeded 512K bit/sec."

We should have specified the EIR as 256K bit/sec - the increment above 256K bit/sec, rather than the full 512K bit/sec. Thus, the above would correctly read: "...you could be connected to your service provider with a port speed of 1.536Mbit/sec (T-1), with CIR for a given PVC of 256K bit/sec and an EIR for that PVC of 256K bit/sec. In this situation, traffic up to 256K bit/sec is assured the committed bandwidth, and traffic above 256K but less than 512K bit/sec would be transmitted on a discard eligible basis. But transmission would not even be attempted for the traffic in a burst that exceeded 512K bit/sec."

The bottom line is that nothing really changes in terms of the possible uses of the EIR as discussed on November 15 and beyond. However, the arithmetic was off due to our alternative usage of the (admittedly undefined) EIR terminology.

Our thanks to Vincent Surillo for bringing this to our attention in the frame relay newsletter discussion area at Webtorials.Com. To discuss this and other WAN issues with Steven Taylor, Joanie Wexler and your fellow frame relay newsletter subscribers, please visit the Webtorials.Com Public Forum at www.webtorials.com/forum.htm.

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