ntation Considerations 137 some of the other SIP trunk redundancy mechanisms in preparation for a future state where your network might no longer have TDM connectivity. The operation of this mechanism is shown in Figure 7-10. Primary Path to VolP (SIP) Dial-peer, Preference 1 Max-conn 300 POTS Dial-peer, Preference 2 Secondary Path to PSTN on Failover or Congestion Figure 7-10 SIP Trunk to PSTN Failover Configure call routing to use the SIP trunk as the primary method of access (using a higher preference dial-peer) and the TDM gateway as the secondary path (using a lower- preference dial-peer). You can use the same physical Cisco platform for both functions SO that adding a SIP trunk to your PSTN gateway does not mean adding equipment to the network. SIP Trunk Capacity Engineering Part of the scalability assessment for your network is to determine how many concurrent sessions should be supported on the SIP trunk service offering that you get from a serv- ice provider. If you have current PSTN traffic statistics on your TDM gateways, this assessment is somewhat easier as the ratios of phones to trunks do not change with SIP trunking. But many enterprise networks do not have detailed current statistics of these call patterns. SIP trunk session sizing is also affected if you choose a centralized model, as opposed to the distributed model of traditional TDM trunking where there is often oversubscription at each site. This oversubscription can be consolidated with a centralized SIP trunk facili- ty, but you still have to engineer with some level of bursting of call traffic for unusual situations. As a ballpark assessment, you can use the same method of estimating trunk (which is equivalent to a SIP trunk session) capacity as you used in the traditional voice traffic engineering exercises. An average enterprise business can use a 5:1 trunking ratio, mean- ing for every five phones, provision one trunk (SIP session). Enterprises that are primarily 138 SIP Trunking internally focused (for example research facilities or e