One of the most frequent questions asked by clients is if they should move from traditional ISDN to SIP based telephone trunks. But what are the advantages and limitations of each approach?
ISDN
Advantages:
Reliable – ISDN circuits are a well proven technology, they are reliable and secure.
Direct Dial-in (DDI) – DDI’s allow an almost unlimited volume of numbers to be carried across an ISDN30 circuit.
Limitations:
Inflexible – The minimum number of channels is typically 8 and although suppliers allow customers to increase the number of channels, usually they will not decrease the number for a period of 12 months. A single ISDN30 also has a maximum of 30 channels.
Geographic limitations – ISDN’s are connected to a local exchange and are geographically inflexible. You can only route calls in and out using telephone numbers in that area, it’s not possible, for example, to route a Glasgow number into a ISDN30 in Birmingham.
Resilience – If there is a fault on an ISDN circuit the network provider will divert all of the numbers on an ISDN (including perhaps hundreds of DDI numbers) to a single disaster recovery location.
Expensive – ISDN services are an old technology and therefore relatively expensive to manage.
SIP Trunking
Advantages:
Geographically Independent – SIP trunks are not limited to a particular Exchange – A Glasgow number can be delivered to a SIP circuit in Birmingham!
Flexible Routing – As the routing is made in the cloud, calls can be routed via a number of different circuits which allows for resilient routing.
Resilient Numbering – DDI numbers in a disaster recovery scenario can route to separate locations. i.e. the whole circuit does not have to be diverted to a single number as with ISDN services.
Cost – In many cases, reduced call rates.
Limitations:
New Technology – SIP is not as widely deployed as ISDN.
Suppliers – There are relatively few players in the market and so choice is more limited.
Implementation – There are frequent implementation issues with new installations.
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