SIP Trunking

SIP trunks – Voice services with increased quality and reduced cost

The Integrated Services Digital Network (ISDN) is more than thirty years old and is being replaced with Public IP telephony services based on Session Initiation Protocol (SIP) lines, also known as SIP trunks.  Exponential-e enables companies with ISDN, analogue and SIP based customer telephone systems to connect to the new public IP switched telephone networks.  BT and the Alternate Network Providers, including Exponential-e, are investing heavily to update digital ISDN telephony networks.
 
Customers can use their existing telephony investments and benefit from new business-grade SIP trunking services.  This is made possible by directly connecting Private Automatic Branch Exchanges (PABXs) using gateway devices to the new next generation IP switched telephony networks.

Why change existing ISDN or analogue lines with SIP trunks?

• Economics. About 30% less in line rental costs alone.  SIP telephony trunks are delivered over data lines that are used for Internet and Wide Area Network (WAN) connections.
  By using the same infrastructure and combining voice as another application on these data lines, the existing dedicated analogue and ISDN2/30 lines are no longer required. Conventional ISDN lines are migrated to SIP trunks by porting the customer's telephone numbers to Exponential-e's next generation public switched telephone network (PSTN) voice switches.
• Capability. SIP based services offer superior features than traditional ISDN.

Enhanced features include: 

• Built-in Disaster Recovery and Data Continuity. As the telephone number no longer needs to be tied to a dedicated circuit terminating in the customer premises, the telephone number is logically programmed to ring an endpoint in a database within the next generation voice switches.
  
  Devices register with the database when they are available for service; these may be directly attached SIP PABXs, ISDN/SIP gateways with PABXs, or pure SIP endpoints.  This versatile arrangement offers a simple solution to routing calls to other destinations in the event of an unforeseen situation such as a terrorist attack, flood or other disaster.  Ifthe endpoint is not available then the next alternate endpoint registered in the database would be chosen for call termination. This capability enables your business to continue trading without interruption. Calls can be automatically routed to other destinations including other offices in different geographic locations, home phones, mobile devices or even another country if required.
• Flexibility. The current ISDN system has limited flexibility as geographic telephone numbers are tied to a physical BT or Alternate Provider telephone exchange. If you move to a different geographical area then chances are that you will lose your telephone number. 
  
  Call forwarding options exist but forwarding calls and continued rental of existing numbers and lines come at a cost. SIP trunks are truly flexible through number porting as geographic numbers are hosted in a central database and can be made to ring anywhere, on or off the network.
• Scaling. ISDN comes in circuit sizes of 30 channels per bearer with a minimum of 12 channels per bearer.   Therefore, one bearer can support 12 to 30 channels and two bearers for 30 to 60 channels.
  
  However, anything > than 30 channels but less than 41 is sub optimal.  With SIP trunks you can have as many as you need, the only limitation is the bandwidth. One SIP trunk takes up to 100 Kbps of bandwidth when in use. 10 SIP lines will use 1Mbps and so on.
  
  A quality data line with sufficient capacity is required to make and receive calls.  A major benefit is that when no calls are being made or received the bandwidth can be used for Internet or other critical business applications.

How do SIP trunks work?

As a real case scenario an organisation with an office of 100 employees has a Nortel Meridian Option 11 telephone system, two ISDN-30 circuits with 42 lines rented from BT.

The Nortel Meridian does not support SIP, but has the two ISDN 30 ports connected to the PSTN using Q931 protocol.  The  service provider is BT with a 200 number DDI range, five of which are used for departmental faxes, and eight numbers for incoming pilot groups for different departments.
The organisation has an Exponential-e business-grade Smart Internet or MPLS/VPLS WAN service delivered over a 100 Mbps bearer and would like to reduce its telephony line costs and outbound call costs. 

Exponential-e would provide the organisation with a quote to migrate the ISDN line numbers and a rate card for outbound calls.  An analysis of the telecom invoice would be provided with a recommended suitable tariff plan. Exponential-e’s Service Delivery team would manage the migration from ISDN to SIP. This is a two part process; the first being the installation of the gateway sited between the Nortel Meridian and ISDN trunks and connected to the Exponential-e Smart Internet or Corporate WAN service.  At this stage all outbound calls would traverse the IP network and would be routed over the Exponential-e PSTN voice switch. 

A PSTN migration order would be placed with the existing Voice Provider and a mutually convenient time would be agreed to cut over the customer DDI range from BT's PSTN to Exponential-e's voice switches.  Once the numbers have been migrated the ISDN circuits would be decommissioned. All calls inbound and outbound would be made transparently through Exponential-e's PSTN Voice switches and IP Data network. 

 Prerequisites for the SIP trunking service

A number of actions need to be undertaken before the SIP trunking service can be implemented:

• Assessment of DDI range to ensure it can be moved to Exponential-e. Exceptions are rare but include the DDI number range being part of a larger DDI range which may not be split. In this instance it would not be possible to port a part number range.
• Assuming that the number range can be ported, the Exponential-network is configured for Quality of Service (QoS) at the customer premises. This is software reconfiguration.  Depending on the legacy porting arrangement, this may be undertaken by Exponential-e or a third party.

SIP and VoIP done right

Previous VoIP experiences in the WAN have been poor and this has been primarily due to Service Providers using low cost broadband networks for transport delivery of voice (i.e. ADSL/XDSL).  Unfortunately, ADSL broadband networks are only best endeavour services and are not reliable enough for business quality Voice. 

In addition, most providers don’t provide QoS over their broadband services and the broadband networks suffer during peak use. With the vast majority of ISPs using voice compression to squeeze even more voice calls on their broadband lines,. the net result is poor voice quality.

However, Exponential-e takes a different approach - we use the same QoS in our WAN based solutions as most organisations do in their LANs.  Since our WAN services are Ethernet based they have very low latency. Therefore, jitter is minimal - giving exceptional quality voice.

A robust voice service

Exponential-e’s Voice services are engineered to meet business quality standards.  The availability of core Voice switches is 99.999% up time.
Single points of failure are engineered out in our designs. In addition, the voice switches are distributed over multiple geographically dispersed data centres for business continuity.

We mitigate risks in local tail access links from the exchange to the premise by adding another local tail from the customer premise to the Exponential-e network.  In the event of a local tail failure, calls would be automatically rerouted to another destination maintaining voice continuity.

Benefits

• Lower line rental cost. Typically 30% < ISDN.
• Lower cost voice calls. Typically 20% <  ISDN.
• High quality voice calls. Voice quality can be better than conventional PSTN with no compression and very low latency and minimal jitter.
• Built in disaster recovery in the event of site failure. Calls can be easily rerouted elsewhere.
• Flexible scaling. Pay only for the SIP trunks you need not the lines you have as part of a bundle.
• Number portability. Keep your number for life regardless of your office location.