Understanding Concept Of DID (Direct Inward Dialing)

Most modern enterprises opt in for DID (Direct Inward Dialing) option as a service from their telephony service provider. This is not a new concept; it has been around for a long time. While the supporting technology delivering this service has evolved, the basic concept remains the same.

The basic percept of telephony is dedicated resource. What this means is that every customer number would be assigned a dedicated resource from end to ends, so even subscriber line had an associated cable. Now this was fine for retail customers who have single line. For enterprise customers with many employees, individual subscriber lines were quite expensive.

With DID, this problem was resolved. This allowed the enterprises to install a PBX that could support this service and lease a single trunk from the CO to that PBX. The CO switch allocated a block of numbers on the trunk. Whenever one of these numbers were dialled, CO would send it over the trunk and PBX will forward the call to the right extension.

This was a great solution for the company as well as the service provider. For the enterprise, this meant fewer operators to handle the calls; besides the cost of DID service was lot less as compared to individual telephone lines. For the service provider, same infrastructure could be used for multiple telephone numbers.

When telephony and communication networks evolved into ISDN networks, DID circuits moved from analog trunks to ISDN based solution. Typically, these numbers would be supported over an ISDN trunk. This trunk would connect the PBX to the CO switch just like the analog trunks, but it was digitised. This solution is quite common even today.

The last point here is the importance of this concept in VoIP networks too. Not many people would believe DID would be relevant in VoIP scenario. But it can be used to allocate number block to the communication gateway. When new VoIP phones are connected and configured, the gateway assigns the numbers to respective handsets.