We have a point-to-point OC-3 that we brought up using 2x 7204VXR routers. The interfaces are ATM (which I have little experience with). The link is running perfectly clean, no errors, drops etc and we're seeing 12ms average round trip when pinging between the Headquarter and the DR site. The problem is when people try to copy file across that link they're only getting 22.8Mbps throughput at best. Is there any configuration in ATM that would prevent the whole pipe to be utilized? One router is providing clocking and I specified 155000. When I do a show int ATM 1/0 it also show bandwidth 155000.
Chances are you're seeing a limitation in the Segmentation-and-Reassembly (SAR) process in your routers.
SAR is the process that chops the frames into cells at the transmit end, then re-assembles them at the receive end.
Back in the LANE/ATM-at-the-Core days, about the best SAR we ever measured (through a single system) in the Lab was ~50Mbps (using Fore Systems, Cisco, Bay Networks/Nortel, Xylan (now Alcatel) and IBM).
SAR is a fairly intensive task. To fill the OC3, you may want to add another layer of router(s) such that you have a layer that handles the several Ethernet segments-to-ATM and they feed the OC3-connected router(s) native ATM. That system aggregate the streams to fill the pipe.
This way you distribute the SAR function and relieve the processing load on the OC3 gateway system(s).
To test the system, check out a free application called Qcheck. It runs on a PC endpoint (one at each end) and operates from RAM (to rule out disk and other I/O systems). Qcheck will give you pretty good stats for throughput, latency, and packet drop for TCP and UDP traffic streams.
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