The WAN link setting limits the amount of data sent per second from the RealPort driver to the Digi Ethernet device. When fabricating packets in the driver to send to the Digi unit, a running count is kept of how much data to send, along with the timing statistics, as well.
When the WAN speed threshold is reached, the data will be sent that meets the speed threshold and wait until the cps limit has been relieved.
This limits the amount of data sent to the line speed, so that flow control does not take place over the remote link.
For example, lets say you have a frame link running at 56K.
If we were to send more than 56K of data per second, the frame link might start to try to flow control, by simply dropping some of our packets.
This would cause the host operating system to re-transmit packets all the time, needlessly creating extra traffic and causing slow downs.
Even worse, in some cases communication providers will simply "drop" the remote end for awhile, because the committed rate is being violated. This could result in lost data if the RealPort socket on either end times out.
Setting the WAN speed in these situations would allows the RealPort driver to regulate the amount of data sent to ensure the speed specified is never surpassed.
Without the WAN link speed setting applied, the RealPort driver will attempt communication at LAN speeds (i.e. 10 or 100Mbps) by default. This can lead to problematic behavior including TCP re-transmits, saturation of the network link, lock up issues and/or high CPU utilization observed on the RealPort host.
Aug 23, 2018