What is SIP Trunking?

White Paper On Sip Trunking – Definition And BenefitsIn direct response to frequent questioning on the topic from clients, we have prepared this ‘low tech’ white paper to help you to understand what is happening in this part of the converging worlds of IT and Telecoms. We hope that it is of interest.1. What is SIP Trunking?SIP (Session Initiation Protocol) trunking is the technology that is expected to replace ISDN ultimately as the transport mechanism for voice traffic from the PSTN (Public Switched Telephone Network) to corporate telephony systems or PABXs. It provides access to the PSTN using Voice over IP (VoIP) and there are numerous benefits to the end user from cost saving to flexibility and disaster recovery. BT’s 21st Century Network (21CN) is based around the conversion of traditional exchanges to delivering VoIP to all end users and the use of SIP will feature more and more in the years to come.2. How does it work?The service provider or carrier will port a subscriber’s number(s) into its own exchange network or hosting platform, which will be located in a secure data centre. These numbers are then re-presented across a private IP network (MPLS, VPLS) or Internet connection back to the subscriber’s premises. Quality of Service (QoS) is essential in a corporate VoIP environment and this can only be offered over leased line, MPLS or VPLS networks. Thus a leased line is provided to connect into the MPLS ‘cloud’, which in turn is linked to the carrier, who will have their links into the PSTN. In addition Virtual Route Forwarding can be set up to provide Internet access from the same leased line connection to reduce costs (See Fig 1, high level schematic).ADSL Broadband can be used in certain circumstances where user numbers are low, but it does not carry any QoS and should be used in isolation to data services. It is ideal for homeworkers and small branch offices.The service provider will usually supply both the exchange and MPLS connection as a single product, although in many cases this will be through interconnect agreements and links between different providers (as also happens with traditional telephone lines). The end user will be unaware of who is providing the various sections of their service.The SIP trunks are presented onto existing PBX equipment, either using an external media gateway or internal software if the system is quite new. The system effectively ‘sees’ the trunks as normal ISDN lines, so nothing is changed from the user perspective.3. What are the benefits?Typical uses of SIP trunks are:- Disaster recovery/Business ContinuityThe ability to re-present individual DDI numbers to any location is one of the key benefits of SIP trunking. Traditional DDI can only be routed to a single number in an emergency; in most cases, this would mean hundreds of numbers reverting to a single operator, which would be hard to manage.With SIP trunking, key DDIs can be set up to route to the user’s mobile phone or dedicated landlines in a different location; alternatively, hosted IP PBX extensions could be used as a back up and logged into from any Internet connection. The key point is that inbound numbers are not confined to a single location any more and a variety of plans can be made around this simple change.- Geographic number portabilityNumbers are also no longer confined to the local exchange. If an office relocates to what would normally be a different exchange area, it makes no difference with SIP trunks as they are just re-presented across the new network/Internet connection.- Cost ReductionThe cost of SIP trunks is approximately £5 per line per month, against £17 per line per month for ISDN 30. The leased line connection to the MPLS network must also be taken into account; however, this can be off-set if combined with VRF or an Internet gateway service, i.e. two costs are rolled into one, ISDN and Internet.There is also a potential saving of thousands when compared to traditional DR services involving the delivery of a lorry with a temporary phone system or an off site disaster recovery location, held at great cost and potentially never used.4. How will it benefit my business?There are literally dozens of scenarios where organisations can save money and improve their business continuity plans. Voice is nearly always the forgotten relation when preparing for the worst and yet is usually the operational jugular. Cut your customers’ and support staffs’ ability to talk to you and at best you can expect to flail wildly before collapsing in an undignified heap!