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Direct inward dialing (DID), also called direct dial-in (DDI) in Europe and Oceania, is a feature offered by telephone companies for use with their customers' private branch exchange (PBX) systems.
The term DID originated in conventional telephony, where it allowed an individual extension on a private branch exchange to be reached directly via its own standard outside number. In modern usage, voice-over-IP providers commonly refer to individual numbers provided to their subscribers as DIDs.
In DID service the telephone company provides one or more trunk lines to the customer for connection to the customer's PBX and allocates a range of telephone numbers to this line (or group of lines) and forwards all calls to such numbers via the trunk. As calls are presented to the PBX, the dialed destination number (DNIS) is transmitted, usually partially (e.g., last four digits), so that the PBX can route the call directly to the desired telephone extension within the organization without the need for an operator or attendant. The service allows direct inward call routing to each extension while maintaining only a limited number of subscriber lines to satisfy the average concurrent usage of the customer.
Traditionally, DID circuits were analog. These types of DID trunks had to be powered by the customer premises equipment. The central office equipment detects the power state of the line and disables service if the circuit is not powered up. This is the reverse arrangement from standard plain old telephone service (POTS) lines which are powered by the central office. Nowadays, it is far more common to deliver DIDs on a PRI circuit.
DID service is usually combined with direct outward dialing (DOD) allowing PBX extensions direct outbound calling capability with identification of their DID number.
This system is also used by fax servers. A telephone line is terminated in a computer running fax server software and fax modem cards. A set of digits of the assigned phone numbers are used to identify the recipient of the fax. This allows many recipients to have an individual fax number, even though there is only one fax machine available.
Some voice over IP vendors have used one central, remotely located fax server as a means of offering Internet fax service to their clients. In theory, standards such as T.38 should have allowed VoIP subscribers to keep their existing fax equipment working locally; in practice, T.38 at the subscriber's site offers no benefit if the upstream provider is least-cost routing to gateways that don't support T.38 and cannot reliably send or receive fax/modem traffic. A fax server at a central location, connected directly to PSTN T-carrier primary rate interface lines and using direct inward dial to identify the intended addressee can convert a fax to an Internet document (such as .tif or .pdf) for web or e-mail delivery. The fax traffic never passes through the VoIP infrastructure as a dial-up modem call and therefore arrives reliably even if T.38 is not properly supported at some points in the network.
Direct inward dial has similar relevance for Voice over Internet Protocol (VoIP) communications. To allow PSTN users to directly reach users with VoIP phones, DID numbers are assigned to a communications gateway. The gateway connects the public switched telephone network (PSTN) to the VoIP network, routing and translating calls between the two networks for the VoIP user.
Calls originating on the PSTN will ring directly to the VoIP subscriber who holds the corresponding assigned DID number.
DID is not required for outbound calls, although commonly one of the subscriber's DID numbers will be sent as caller ID on outgoing calls if available.
In countries with multiple competing local providers, DID numbers and services can be purchased in bulk from a Competitive Local Exchange Carrier (CLEC).
For voice over IP resellers, some specialised CLECs (for local numbers) or interexchange carriers (for toll-free numbers) will deliver blocks of direct inward dial calls already converted to Session Initiation Protocol or common VoIP formats. The individual VoIP provider need only obtain an inventory of local or freephone numbers from VoIP-aware carriers in various regions, import them in bulk to a VoIP PBX (such as Asterisk) and issue them individually to end users. International DID numbers can be purchased in bulk from international providers.
UK Geographic DID numbers can be obtained for free, terminated over SIP and also generate a revenue for inbound traffic. The number is not free for the caller; sometimes low-premium (+44 845) numbers are used. A few US DIDs are available without monthly charges from vendors like Ipkall, but at the expense of the caller paying for a call to some expensive, rural location.
The majority of vendors charge a nominal amount per number per month (as little as $1/month in small quantities) and then bill per-minute or per number of channels which can be simultaneously in use. For the caller, these numbers can be assigned to locations which are a local call.
The corresponding service to DID for outgoing calls from a PBX to the central office exchange is called direct outward dialing (DOD) or Direct Dial Central Office (DDCO). This service is often combined with DID service and allows direct dialing of global telephone numbers by every extension covered by the service without the assistance of an operator. The caller line identification (CLI) or caller-id of extensions for outgoing calls is often set to the extension DID number, but may be the organization's central switch board number.