Telephony concepts

What is CENTREX?

CENTRAL Office Telephone Exchange (CENTREX) is a business phone service that provides switching functionality at a telephone company’s Central Office rather than at the client site. The communications infrastructure and required software are typically owned and managed by Telephone Company, which then sells the service to its customer. The Telephone Company manages the service from it’s own site to the client site either through individual copper pair lines or through multiplexed lines that use fiber optics or copper link. To deploy Exchange Unified messaging in a traditional CENTREX environment, you must have a VOIP Gateway.

IP CENTREX: It is an updated version of CENTREX which uses IP/PBX based telephony system. The client site is connected through a data network instead of telephone wires. IP CENTREX provides VOIP and other IP based services besides its connectivity to PSTN. A VOIP gateway may not be required to deploy Exchange Unified Messaging.

What is PBX?

PBX sometimes known as telephone switch or telephony switching device is a device that connects office telephones to the public telephone network. It routes incoming calls to the appropriate extension. PBX users share a certain number of external lines for making calls external to PBX. For PBX to be integrated with Exchange unified messaging you need a VIOP gateway. For IP/PBX which supports SIP over TCP and T.38 for FAX you don’t need VOIP Gateway.

Types of PBX

Analog PBX: It sends voice and signaling information such as touchtone of dialed phone number, as analog signals, technically known as Dual Tone Multi-Frequency (DTMF).

Digital PBX: It encodes analog signals to digital signal using audio codec G.711. after it is encoded, the digitized voice signal is sent on a channel by using circuit switching. It can also support analog Trunks.

Hybrid PBX: It can provide both digital and IP/PBX capabilities. Depending on VOIP protocol compatibility of Hybrid PBX with Exchange unified messaging, you may or may not require VOIP Gateway.

IP PBX: It carries voice over data network by using packet switching technology that allows a single network channel to handle multiple connections. The IP phone contains NIC which is connected to the data network. The phone converts the voice signal to the digital signal before being transmitted to the data network. These digital signals are transmitted using Real-time Transport Protocol (RTP) so that the delivery order and timing of data packets are maintained.

PBX concepts

Direct Inward Dialing (DID): In DID, a unique phone number is assigned to a user within the Organization, so the users can be directly called from the external number without the need to transfer the call. Exchange unified messaging requires DID number for configuring Auto Attendant.

Hunt Group: It is a set of extensions that are defined as a group, that have identical endpoint such as sales or support so that callers receive busy signal less often.

Pilot Number: It is a method by which PBX identifies the Hunt Group. When used with Exchange unified messaging, PBX uses pilot number to target a diverted ring no answer or busy call to Exchange unified messaging so that voice messages can be taken and diverted to the intended user’s mailbox.

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