ISDN services guarantee the Quality, Security and Reliability for calls that are essential in business-to-business communications. There are a number of reasons why ISDN has been the connection of choice for the last 20 years for voice, data and video services from both commercial and technical points of view.
Any technology which claims to be an actual ISDN replacement service would have to be absolutely compelling both commercially and technically.
Commercially, ISDN is delivered as a managed service from the Network Operator (e.g. BT). It follows a straightforward, modular cost structure where each channel costs the same monthly amount. The infrastructure cost to the Operator is not proportional to the rental charged, which is fixed per channel per month.
This is the first point at which the commercial SIP model struggles if the connectivity is via a leased line between the client and the SIP provider. ISDN rental is cheaper in the 2 to 30 channel price band, which removes a compelling commercial argument for changing to SIP.
If the leased line is an Internet connection, or any part of the SIP service is delivered over the public Internet, then there is no Quality of Service (QoS). So we have a product which is more expensive than ISDN, and does not meet the existing Quality, Security and Reliability standards which ISDN clients have not only enjoyed for the last 20 years, but come to expect from a business-grade telephone service.
For smaller ISDN installations where the delivery of SIP is attempted over a standard ADSL Internet connection, there is again no Quality of Service and the service is often “wires only”. Reliance on the Internet for real-time voice transmissions can result in a noticeable and unpredictable degradation in the Quality, Security and Reliability of the voice service compared with ISDN. For many ISDN users their experience of SIP was poor quality, call cut-offs, one way speech, and no way to fix the problems.
Whilst SIP has it’s place in the market, it will remain primarily a LAN or private WAN technology whilst the external access network problem prevails. A SIP service should only be considered as an ISDN replacement if it is fully managed by the supplier, on a management platform which can provide trend reports and utilisation data, and an end-to-end Quality of Service guarantee just like the ISDN service it replaces.
Until a Service Provider introduces a managed SIP service with an ISDN gateway which guarantees end-to-end Quality of Service for voice, ISDN certainly has a bright and sustainable future.