In SIP Trunks: A Call To the Wild, I mentioned I would get with Chris Thompson, Senior Product Manager over at ADTRAN for an explanation on how their ADTRAN 7100 IP-PBX works using Alternative Routing with service providers offering SIP trunks. It is possible to have redundant routes but this will depend upon your service provider and your IP-PBX. In most cases with most providers, you can also have a disaster route to POTS or other Telco services such as a DID number on another PBX. I asked Chris about alternative routing for SIP trunks:
It is possible to have redundant routes to the SIP trunk service provider. This can be done using DNS lookup or by having multiple SIP trunks going to multiple SIP trunk providers.
Our SIP trunking implementation is very resilient, thanks in part to the resiliency of our overall IP networking capabilities. The NetVanta 7000 series took that resiliency further by allowing for automatic rerouting of calls in setup stage and even terminating active calls if a SIP trunk were to fail.
Here are some further details on our capabilities:
1. The NetVanta 7000 series has multiple SIP server configurations for each trunk. Should one not respond, NV7000 series will try the other (primary / secondary).
2. Routing between SIP trunks during a failure case will occur if NV7000 series fail to receive an initial response to the SIP-INVITE. If it receives an alerting, pre-connect, or connect, the call will stay with that trunk and will not be re-routed by the switchboard. In the case of no response, the NV7000 series switchboard will route to another trunk.
3. The NV7000 series supports SIP keep-alives in the form of INFO or OPTION to tear down an active call if there is no response. This will keep a call from becoming stuck if the SIP server quits responding mid-call, but does not alter call routing.
4. SIP trunk routing is also resilient if FQDN's (Fully Qualified Domain Names) are used and DNS lookups fail.
5. Re-routing calls to another trunk (whether analog, digital, or SIP) is possible if conditions 1,2, or 4 are met.
Our SIP trunking implementation is very resilient, thanks in part to the resiliency of our overall IP networking capabilities. The NetVanta 7000 series took that resiliency further by allowing for automatic rerouting of calls in setup stage and even terminating active calls if a SIP trunk were to fail.
Here are some further details on our capabilities:
1. The NetVanta 7000 series has multiple SIP server configurations for each trunk. Should one not respond, NV7000 series will try the other (primary / secondary).
2. Routing between SIP trunks during a failure case will occur if NV7000 series fail to receive an initial response to the SIP-INVITE. If it receives an alerting, pre-connect, or connect, the call will stay with that trunk and will not be re-routed by the switchboard. In the case of no response, the NV7000 series switchboard will route to another trunk.
3. The NV7000 series supports SIP keep-alives in the form of INFO or OPTION to tear down an active call if there is no response. This will keep a call from becoming stuck if the SIP server quits responding mid-call, but does not alter call routing.
4. SIP trunk routing is also resilient if FQDN's (Fully Qualified Domain Names) are used and DNS lookups fail.
5. Re-routing calls to another trunk (whether analog, digital, or SIP) is possible if conditions 1,2, or 4 are met.
Now here's what's interesting. Remember what Chris said about having redundant routes--you can have multiple DNS lookup entries for the same provider and you can have multiple SIP providers. That's a pretty cool choice to have and the flexibility of using more than one provider makes sense as does using more than one SIP server location for each provider--so make sure your provider has more than one point of presence and in different regions. Consider using time of day routing to take advantage of low traffic periods. I also think that by mixing providers and additional routing choices that greater resiliency is accomplished. This is only one example and with switched wireline services, the choices are very limited. Avoid being lured to a provider that offers only redundant hardware with all the trimmings--do they have more than one presence? Diverse routing will be key especially to combat Internet growth and traffic shifts throughout the day.
I see SIP trunks carrying an advantage over "switched" calls especially in emergency and disaster situations. I recall the Baltimore harbor tunnel fire that knocked out fiber and communications, impacting just about everyone in the D.C. area--had SIP trunks been available, maybe the disaster would have been less far-reaching. Then further back there was the great Central Office fire in Chicago in the early 1980s and that was painful. It's interesting to consider and ponder still another question for customers contemplating SIP trunks is, where do their service providers have presence? On the switched network, presence is strong but you are stuck with the old way of doing business with V&H coordinates that bill on distance sensitive copper, while the SIP providers offer choices with diversity of presence. While Verizon boasts that the public switched network (PSTN) is far superior in quality and reliability to VoIP--Verizon continues to suffer from landline subscriber erosion like the other carriers and they continue to sell off portions of their copper network.
Then as a matter of strategy for mergers and acquisitions, consider how phone numbers could be more effectively absorbed into the buying organization. This is a key differentiator for providers that have the features and stickiness. Alternative routing is key to maintain diversity in the routing of voice calls to ensure business continuity. Skillfully approached and planned, M&As could become zero or near zero impact to the process and absorption of the acquisition by way of taking those calls of the acquired organization into the buying company. SIP trunking offers a new kind of presence that could be very useful. On a predatory tract--buying disconnected competitors phone numbers should be near the top of the list for some, because with SIP, it's easy.
Keep in mind that SIP trunking services are offered in many forms ranging from regional specialty to retail to services with more weight and cost that make the providers rich and yet the customers are seemingly saving money over convenience. The marketing mania that you will save 70% doesn't always mention "up to 70% savings." It depends...and you should know what that means.
Larger enterprises are going to buy SIP trunking services with enforceable SLAs while many in the SMB will opt for retail packages and some will gain enforceable SLAs while others get cheap service. It is possible to get enforceable SLAs and with hosted solutions which are at odds with CPE (Customer Provided Equipment) gear and SIP providers; the hosted guys are retaining two key silver bullets--the transfer feature without using two trunks (trunk-to-trunk transfer) and enforceable SLAs. In either case, the more dependent you are upon SIP trunks, the more diversity you'd better welcome into your configuration. Once again, alternative routing and the old ARS/LCR are must-have features.
