The problem may not be associated with ‘load’ so much as with ‘call attempts’. As described the problem appears to be generated by outgoing traffic on the 4 x E1. Once these 120 channels are busy the EWSD should identify the route as busy and the calll will fail. Likewise if the calls are originating from the E1’s (PABX/PBX), once all channels are busy the originating equipment will give a ‘busy-tone’ to the caller.
BUT….. if the calls are from the EWSD to the equipment on the end of the E1’s (say a PBX) there may be problems. Assume the 4E1s are connected to the PBX of a radio station that runs a competition and generates thousands of calls for a very short time period. The first call is successful and occupies a B-channel. The other 119 B channels may be free, so the EWSD tries to complete other calls ringing the competition number. There will be thousands of call attempts, maybe tens of thousands, in a vary short space of time. While there are free B-channels on the route EVERY call will generate a D-channel call set up request to the PBX, and associated D-channel messages indicating request denied because number is busy.
These set up request messages will queue in the EWSD until they are ‘serviced’ and if the processing power cannot handle it quickly, other customers in the switch will also be affected. The customers PBX may also be slowed considerably as its CPU cannot manage the avalanche of set-up requests.
I have witnessed the above scenario render a Fujitsu F640 PBX virtually unuseable (50 seconds to return dial-tone if you picked up a handset). I have no doubt other situations generating high call loads could also do similar (eg, calls to a phone-in betting shop)