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System components of VoIP

This is the second tip in a series excerpted from a report by Ohio State University. The report analyzes issues and challenges in the evolving VoIP technology.

This tip from Ohio State University takes a look at the major components that comprise VoIP.


The gateways are the devices that communicate between the telephone signals and the IP endpoint. The IP endpoint usually speaks H.323 for media stream and more recently Session Initiation protocol (SIP). The gateways usually perform the following six functions:

Search function

When an IP gateway is used to place a call across an IP network, it receives a called party phone number. It converts it into the IP address of the far end gateway, possibly through a table lookup in the originating gateway or in a centralized directory server.

Connection function

The originating gateway establishes a connection to the destination gateway, exchanges call setup, compatibility information and performs any option negotiation and security handshake.

Digitizing function

Analog telephone signals coming into a trunk on the gateway are digitized by the gateway into a format useful to the gateway, usually 64 KBPS PCM. This requires the gateway to interface to a variety of telephone-signaling conventions.

Demodulation functions

With some gateways the gateway trunk can accept only a voice signal or a fax signal but not both. But sophisticated gateways handle both. When the signal is a fax, it is demodulated by the DSP back into the original 2.4-14.4 KBPS digital format. This is then put into the IP packets for transmission. The demodulated information is remodulated back to the original analog fax signal by the remote gateway, for delivery to the remote fax machine.

Compression functions

When the signal is determined to be voice, it is usually compressed by a DSP from 64K PCM to a 5.3 KBPS signal, which is the G.723.1 standard.

Decompression and remodulation functions

At the same time that the gateway performs steps 1-5, it is also receiving packets. Hence this function is required.


Terminals are the LAN client endpoints that provide real time two way communications. When an endpoint is switched on, it performs a multicast discovery for a gatekeeper and registers with it. Thus the gatekeeper knows how many users are connected and where they are located. The collection of a gatekeeper and its registered endpoints is called as azone. A gatekeeper is required to perform the following functions:

Address translation

Translation of an alias address to a Transport Address using a table updated via registration messages.

Admissions control

Bandwidth management

Support for Bandwidth Request, Confirm and Reject messages, or a null function that accepts all requests for bandwidth changes.

Zone management

The Gatekeeper provides the above functions for terminals, MCUs, and gateways, which are registered in its Zone of control.

IP telephones
These are devices, which replace the existing telephones by providing enhanced services suited to VoIP. At the same time they should retain the capabilities of the original phones to keep the user comfortable.

PC software phones

This arrangement consists of a microphone connected to a PC interfaced by a card and running a software which permits voice and multimedia transfer over the Internet. Microsoft NetMeeting is an example.

View the first tip in this series: Introduction to Internet telephony

This tip is reprinted with permission from the report 'Voice over IP: Products, Services and Issues' by Vinodkrishnan Kulathumani at Ohio State University.

Authorization of LAN access, using Admissions Requests or Confirm and Reject (ARQ/ARC/ARJ) messages. Access is based on call authorization, bandwidth or some other criteria.

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