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Reply To: ErlangB or Extended ErlangB

Donald Darden

I suppose I would be remiss if I did not make another point here.

When I refer to data communications, I am referring to a very limited concept, where data communications directly competes with or co-exists with voice communications. Principally, this would be with dialup service or the use of PBX circuits to alternate between voice and data service.

But most data communications today is over data networks, and the concept of measuring Grade of Service has a whole different meaning in that context. For example, with voice or dialup connections, a call is blocked if you do not get a dialtone. Network connections are generally always on – there is no similarity to waiting for a dialtone. If a network route is fully committed, most networks automatically provide alternate routing. If a node is currently busy, the data may be buffered for a time before being transferred. Voice communications require a simultaneous connection at both ends – computing devices can store data until a pathway can be established. If data is corrupted or lost, it can be transmitted again automatically. A whole transaction is rarely lost, as lost or corrupted packets can be retransmitted with no apparent degregation of performance.

What my last employer referred to as GOS for Data Communications was in fact, merely a determination of how many packets successfully completed a trip one way out of a total of such packets that were attempted. This may be valid, but it is not related to measuring data service in accord to ErlangB.

Actually, I have a much more precise method of determining Grade of Service for Data and Voice Over IP services, but I was unable to convence my last management to incorporate my concept in their discussions with the government, so to my knowledge their method is still seriously flawed. But right now I am out of the game, and who knows if I will ever have a chance to introduce this concept in context again.