SIP PSTN Trunk Lines Usage

Reference Architecture

Recommendations

If possible avoid transcoding and so it is best to strip the codec list back to a single codec that is supported by all parties in this example G711alaw is used and codec lists in TPO, BT Call Server and SBC were stripped back.

Where RTP timers are used it is best to allow the carrier to fail first to avoid race conditions and get into failover loops so set any BT components to have longer RTP timeouts.

Do use session timers if supported by the carrier as it will avoid orphaned calls when there are problems in network etc.

If possible try to avoid adding IP interfaces for each service normally all SIP carriers will be in a DMZ ideally the outside of the SBC will also be in this DMZ but if not will normally be able to route to it via customer firewall or router. In some cases this was not possible and so a separate physical interface was needed for each carrier clearly this will not scale and should be avoided. The one case where this may have to be done us where carrier are delivering over the internet as opposed to a point to point circuit of MPLS. Even then would be better if customer firewall hid this.

If a customer firewall has to be used between outside of SBC and Carriers it should not NAT if possible if it has to then will need to set the SBC to expect a NAT in the path.

When a BT Call server fails calls between it and the SBC will be lost and will have to be redialed once service is recovered. The BTTrade system does not try to maintain calls in this circumstance as it would for PW so customers should be made aware that this is the expected behavior on failure.

Example Addressing

Since both carriers and the SBC were all in the DMZ no additional routing on the SBC was required however where this is not the case static routes can be added to the SBC to allow it to reach any subnet that is available on the customer network.

Session Border Controller

In thsi example BT has used an Audio Codes Virtual SBC Version 7.2. This was set to have two network interfaces an outside that faced the DMZ for the carriers and the inside that faced the BT Call Server and the BT TPO.

The SBC was initially set up set up using the Audio codes Config wizard specifying two network interfaces for use with SIP PW Carrier. This setup is not covered here but is available elsewhere in TSS help see SIP Pivate Wires Config

The PW were tested and proved to be working before any PSTN configuration was attempted.

Since the same network interfaces could be used for both PSTN and PW there was no need to create and extra SIP interfaces, in addition the same Media realm was also used for both services. In practice it may be necessary to create extra media realms if RTP port ranges have to match so this could be done if needed.