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Justin Brawo

JustinBravo|United States

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What is the Basic Concept of Direct Inward Dialing?

Jun 06, 2018

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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 precept 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 a 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 dialed, 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 Direct Inward Dialing service was a lot less as compared to individual telephone lines. For the service provider, the same infrastructure could be used for multiple telephone numbers.

DID number is used as a way, so you can reuse the number which is limited to physical phone lines and can handle calls of other published numbers. The telecommunication companies used DID for their business needs and identify the numbers through this you can connect the business to PBX. Analog DID uses a different type of lines than the regular lines.

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 digitized. 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 Direct Inward Dialing 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.