Topic Last Modified: 2010-11-07

Media bypass and call admission control (CAC) work together to manage bandwidth control for call media. Media bypass facilitates media flow over well-connected links; CAC manages traffic on links with bandwidth constraints. Because Media Bypass and CAC are mutually exclusive, you must be mindful of one when planning for the other. The following combinations are supported:

• CAC and Media Bypass are both enabled. Media Bypass must be set to Use Site and Region Information. This site and region information is the same as that used for CAC.
  
  
  
  If you enable CAC, you cannot select Always Bypass, and vice-versa, because the two configurations are mutually exclusive. That is, only one of the two will apply to any given PSTN call. First, a check is made to determine if media bypass applies to the call. If it does, then CAC is not used. This makes sense, because if a call is eligible for bypass, it is by definition using a connection where CAC is not needed. If bypass cannot be applied to the call (that is, if the client’s and gateway’s bypass IDs do not match), then CAC is applied to the call.
  
  
  
  
• CAC not enabled and Media Bypass set to Always Bypass.
  
  
  
  In this configuration, both client and trunk subnets are mapped to one and only one bypass ID, which is computed by the system.
  
  
  
  
• CAC not enabled and Media Bypass set to Use Site and Region Information.
  
  
  
  Where Use Site and Region Information is enabled, bypass determination works essentially the same way, regardless of whether CAC is enabled or not. That is, for any given PSTN call, the client’s subnet is mapped to a particular site, and the bypass ID for that subnet is extracted. Similarly, the gateway’s subnet is mapped to a particular site, and the bypass ID for that subnet is extracted. Only if the two bypass IDs are identical will bypass happen for the call. If they are not identical, media bypass will not occur.
  
  
  
  Even though CAC is disabled globally, bandwidth policy needs to be defined for each site and link if you want to use site-and-region configuration to control the bypass decision. The actual value of the bandwidth constraint or its modality doesn’t matter. The ultimate goal is to have the system automatically calculate different bypass IDs to associate with different locales that are not well connected. Defining a bandwidth constraint by definition means a link is not well connected.
  
  
  
  
• CAC is enabled and media bypass is not enabled. This would apply only where all gateways and IP-PBXs are not well connected or do not meet other requirements for media bypass. For details about requirements for media bypass, see Requirements for Media Bypass.
  
  

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