Admission Bandwidth Control?
In a VoIP environment the WAN circuit is generally engineered to handle X phone calls of a specific codec. For example you might plan out a circuit that supports 10 simultaneous phone calls across the WAN between sites. You select the G711 codec and plan each phone call at 82KPS per call. This would require that there be a minimum of 820KB of bandwidth available or approximately 55% of a full T span. Given that the WAN connection also supports data applications, we want to assure that Voice does not take all available bandwidth! Interestingly when people complain about the bad quality of a VoIP call, it is generally the result of exceeding a bandwidth limitation, If you engineered the circuit for 10 calls, when the 11th call is placed, not just that phone call is trashed, but all 11 phone calls are destroyed! For this reason VoIP systems in general and ShoreTel in particular have strategies for limiting the number of calls across the WAN. In ShoreTel there is a parameter entitled “Admission Control Bandwidth” located in the Sites definition in the ShorewareDirector administrative web portal. This parameter assures that a call will not be set up between this site and another site, if that phone call would exceed the bandwidth setting. This generally eliminates the 11th phone call on a circuit designed for 10 simultaneous phone calls! ShoreTel switches or media gateways, know about the bandwidth they would consume when setting up a phone call and can take action based on this ACB parameter. We need to apply solid WAN engineering practices to the circuit planning however, as the ShoreTel switches will not know if that bandwidth is actually available! So it is possible that the ABC parameter will allow a call to be setup, but bandwidth may not actually be available as other data applications might be consuming more than planned bandwidth at that point in time. For this reason, we need to prioritize voice and data with queuing strategies in our WAN routers, the subject of yet another blog!