There are several things that can cause this. If the link is full duplex (communication in both directions at link speed is possible - usually possible with 100mb ethernet) and you're using the LanHealthMeters, then you can see volumes up to double the expected speed (200%). Also, the numbers for utilization variables are based on the ifSpeed of the port. If you look at the IfEntry (interface) table, you can see what this value is actually set to. With PVCs, it may be listing the link speed and not just the bandwith available to this connection. We look at the in and out bytes sent on each interface and divide them by the speed of the port and the poll interval. For frame relay, it is actually possible for utilization to exceed the CIR rate for a frame relay port and for you to get utilizations up to 200% or so. If you are getting numbers much larger, then it may be something else so let us know. A detailed description follows:
Frame Relay is optimized for use over higher-speed (such as T1) and very low error-rate data circuits. For example, to reduce the processing load on, and latency through, frame relay networks, the switches do not perform any error correction (other than discarding corrupted frames) nor flow control (other than setting the Forward Explicit Congestion Notification and Backward Explicit Congestion Notification bits in the frame header). If the user equipment does not react fast enough (or at all) to the flow control, then the network discards frames when it gets congested. Whether due to corrupted frames or a congested network, discarded frames must be retransmitted by the user-equipment (which is typically a router).
Port speeds (the speed of the physical data circuit access line from the user to the service provider's frame relay switch and the speed of the switch port) are typically between DS-0 and T1, though there are usually only two choices:
- 56,000 or 64,000 bits/s (whichever is offered)
- T1 (1.544 Mbits/s)
As was stated above, the network does not use conventional window-based flow control (which only lets end-stations transmit a predetermined number of messages before they are acknowledged). Instead, carriers commit to being able to carry a prespecified data rate (the committed information rate) for each PVC. The CIR (which will usually be up to one-half of, but could be equal to, the speed of the access line) is specified as a bit rate averaged over a 1-second period. Carriers charge more for higher CIRs.
Users can always send data at up to the speed of the access line, and if the carrier's network has extra capacity, then the network will carry these "burst" data (that is, data coming in, in excess of the CIR). Otherwise, the network can discard the burst data. Some carriers offer PVCs with zero CIR (at a cost savings to the user). Users find that these services drop less than 0.01% of traffic and so are often a cost-effective choice.