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SIP Trunking Drilldown: Bandwidth Considerations

Communications Server 2007 R2

Topic Last Modified: 2009-07-14

SIP trunking services are typically priced according to the maximum number of simultaneous calls. Bandwidth availability needs to be taken into account so that you can take full advantage of the peak capacity that you have paid for. The bandwidth needs are determined as follows:

  • SIP Trunk Peak Bandwidth = Max Simultaneous Calls x 80kbps

The 80kbps value reflects 64kbps for the G711 codec plus 16kbps of packet overhead. In reality, the bandwidth will be somewhat lower due to silence suppression. That means the Mediation Server and the service provider stops sending real-time transport protocol (RTP) media packets when the respective participant is not talking. For audio sent by the Mediation Server, silence suppression will reduce the bandwidth by roughly 35%. The challenge is that bandwidth planning is usually symmetric, and each service provider implements different degrees of silence suppression (or perhaps no silence suppression at all). Therefore, it is best to plan for the full 80kbps bandwidth unless you have further information regarding the silence suppression characteristics of your particular service provider.

In the private connection topology, it is easy to plan your bandwidth because it is a dedicated link. Simply compute your SIP trunk peak bandwidth, and then obtain at least that much bandwidth in your dedicated connection.

In the virtual private network (VPN) connection topology, how you plan your bandwidth depends on the network that the VPN is running over. If the VPN is running over a controlled link between your company and the service provider (for example, a corporate MPLS link), compute your SIP trunk peak bandwidth and set aside that much bandwidth on the link for the VPN connection. If the VPN is running over an uncontrolled link (for example, a public internet connection), compute your SIP trunk peak bandwidth and reserve that much bandwidth on your link for the VPN connection. This prevents a saturated link from causing bandwidth issues for your SIP trunk. However, this does not guarantee that your SIP trunk traffic will not be affected by congestion on the Internet. Therefore, running a VPN over an Internet connection is not recommended for deployments that require a high service level agreement (SLA).

From a bandwidth standpoint, the public connection topology is handled similarly to running a VPN connection over an Internet connection. Compute your SIP trunk peak bandwidth and reserve that much bandwidth on your public connection link. This prevents a saturated link from causing bandwidth issues for your SIP trunk. However, this does not guarantee that your SIP trunk traffic will not be affected by congestion on the Internet. Again, running a VPN over an Internet connection is not recommended for deployments requiring a high SLA.

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