Bandwidth-based simultaneous call limit

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A key factor in the deployment of a cloud PBX solution is the bandwidth available for office premises. If it is insufficient (for example, an office building has a limited bandwidth of 2 Mbps and the number of established sessions requires 2.1 Mbps), there is a severe degradation of sound quality.

An PBX environment configured in PortaSwitch can include one or more customer sites. A customer site defines a group of phone lines that are managed as a single entity and usually placed in a separate office building. Calls made within the customer site are often routed across a LAN (Local Area Network) and therefore do not require additional bandwidth allocation.

For more efficient bandwidth usage, administrators can now exclude calls that are made within the customer site from the bandwidth consumption calculation.

To do this, an administrator enables the Limit Simultaneous Calls service feature and defines a per-site bandwidth limit for a desired customer site. As a result, only external calls are scrutinized during bandwidth consumption calculation.

Setting the bandwidth limit

For example, there is an PBX environment with two customer offices (sites): in Cape Town and Johannesburg. The customer site in Johannesburg has a limited bandwidth of 1 Mbps. An administrator enables the Limit Simultaneous Calls service feature and sets the Max bandwidth value to 1 Mbps for this customer site.

  1. A user from the Johannesburg office receives an external call. The system checks if there is bandwidth available for this call. The system detects that there are 9 established external calls that consume 0.9 Mbps of bandwidth. The system allows this call because there is enough bandwidth available.
  2. Another user from the Johannesburg office makes an external call. The system checks if there is bandwidth available for this call. Now there are 10 established external calls that consume 1 Mbps of bandwidth. The system denies this call because all available bandwidth is used up. A ‘limit reached’ warning is played to the user.
  3. Users from the Johannesburg office can call each other as much as they need, since these calls do not consume bandwidth. This limitation applies to external calls only.

Playing warning prompts to end users requires additional bandwidth. Its amount depends on a number of factors like the codec chosen, the transport protocol used, silence suppression, RTCP presence, etc.

Therefore, to prevent bandwidth shortage, set aside a certain amount of bandwidth when you define the bandwidth limit. For example, to play a prompt using the G.711 codec requires approximately 85 Kbit/sec bandwidth. Thus, if you have 1 Mbps allocated for voice traffic, set the bandwidth limit to 915 Kbps (1 Mbps – 85 Kbps).

Note that the system does not exclude the following from bandwidth consumption calculations:

  • Calls within a customer site that require an RTP proxy.
  • Calls within a customer site for which call recording is enabled.
  • Calls to IVR applications.
  • Calls among customer sites.

This allows service providers to use the available bandwidth to maximum effect while deploying cloud PBX solutions for their customers.

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