The ANT Telecom Blog

ISDN v SIP

Written by ANT Telecom | 26 Sep 2014

The use of Integrated Digital Networks (ISDN), which provides the infrastructure for the simultaneous transfer of voice, video and other data, has been the norm since the late '80's. However, it is fast becoming outdated and is starting to be replaced by Session Initiation Protocol (SIP) trunking.

So what are the benefits of SIP v ISDN and is it time for you to start thinking about making the switch?

Benefits of SIP

The main benefit to SIP is the increase in cost efficiencies it can offer in comparison to ISDN. SIP trunks are far cheaper to install and the on-going rental cost of a SIP trunk is considerably less on a per channel basis than ISDN. There isn't a huge difference in call costs but on the whole a saving should be realised.

SIP trunks can be setup far quicker than ISDN too. A SIP trunk can be set up by a carrier within 48 hours whereas an ISDN circuit would take approximately 30 days to install, however, this is on the proviso that a data circuit for delivering the SIP trunk is already in place.

Redundancy or resiliency is also another consideration. Duplicating ISDN circuits is fairly costly and doesn't always overcome the issue as a fault at the exchange can easily effect both circuits. When a circuit is affected, routing calls to the backup circuit or an alternative number can be quite timely.

This is far simpler with SIP as the calls can be simply pointed to another data circuit / IP address or telephone and is more cost effective as a temporary measure and provides more control.

On nearly all occasions you can also keep your existing number range should you decide to switch to SIP. In fact, even if you decide to up sticks and move hundreds of miles to a new location you can keep your existing numbers. What's more, you can choose any number range regardless of geographic location of your office. The only limitation is whether the numbers are available with the exchange or carrier.

Additionally, we often find that organisations that set up SIP as the primary infrastructure see a larger return on investment (ROI), both in the short and long-term.

Future proofing

SIP Trunks are just another element of the convergence story. First there was VoIP and IP trunks, then IP PBXs and converged networks and now continuing that trend there are SIP trunks. The driver is the same, why have separate voice and data networks when you don't need to? Not only do you pay for 2 services, it takes you twice as long to manage them as the theory goes.

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It is clear that SIP can provide a ROI and a future plan for organisations but it isn't always that simple. So despite the benefits, it is always worth taking a minute before you go ahead and implement SIP trunks.

ISDN has been in use for many years and it is very reliable. SIP Trunks are too though, and require a connection to the Internet to deliver the voice calls. Any circuit with the necessary bandwidth will work, however some are more reliable for delivering voice traffic than others. For example ADSL is a contended circuit whereby the carriers are unable to provide any quality of service or prioritise voice over data traffic. This means that when internet use is high, it is likely to cause slower connection speeds that affect the voice packets so that they can't be delivered in a timely fashion. When this occurs users may experience different problems from crackling background noises to the odd word sounding faint, to complete distortion and in some cases the line disconnecting. This may not be a problem for some businesses whereas it would be for others. The only way to really get an indication of your current ADSL's ability and to find out what issues you may face, is to experiment with routing some calls via the SIP trunks and other calls via ISDN at different times of the day and days of the week.

Alternatively, uncontended circuits such as MIA or EFM can guarantee bandwidth and as a result are more reliable for delivering SIP calls. However, these circuits are more costly than ADSL and may make swapping from ISDN to SIP less attractive from a cost saving perspective.

Reliability vs modern technology

ISDN remains the more reliable and traditional of the two, however you may find SIP could be the only choice, as ISDN becomes ever more out-dated. The benefits of SIP are significant, and for many organisations, it will undeniably be the right direction to take. However, each organisation is different and you should give careful consideration when choosing which infrastructure to embrace. And if in any doubt, seek advice from a trusted communications partner.

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