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Total Throughput with IMA/ATM

The manuals say (from Cisco, by the way) that out of a T-1 the most you'll get is 70-80% of the total bandwidth (this would be typical serial interfaces - ATM could be somewhat faster, depending on a lot of variables.....).

With the IMAs, they have the advantage of discarding 0 byte data padding cells (the padding necessary for cell ordering), and so, the IMA, in theory, could actually enhance throughput, although they probably add overhead with the IMA grouping protocol.......I'm kind of curious, has anyone ever tested how much of the bandwidth you get with the IMAs in place? For instance, let's say three t-1s (ds3 at 44.7 megabits for instance)......

This sounds like a heavy techie question, but it's not. How much throughput do we get with an IMA group? All 4.5 mg?


Re: Total Throughput with IMA/ATM

The manuals would seem to be outdated, even with 2600 or 3600 routers you will get more than 70% throughput on serial lines assuming you are using the fastest switching paths through the router (CEF / fast switching). For ATM IMA interfaces, the things to consider are:

- ATM cell tax - every 53 byte ATM cell has 5 bytes of header, so from a throughput perspective, you are effectively losing 9% or so of the BW to ATM cell overhead

- IMA Control Protocol cells that are sent every 128 cells

- Cell padding

So for 4.5Mbps, you wold probably get close to 90% or so utilization. Of course you need to make sure you are using the fastest switching paths through the router as well