VoIP Topology

This post is all about the functional entities that make up the VoIP solution. Topology is a pictorial representation of the logical blocks that constitute the system. Let’s take a look at a typical VoIP topology.

Typical VoIP Topology


The components can be categorized into VoIP and non-VoIP.

VoIP Components

1. Call Server : The IP phones register to the call server. Call server handles security and admission control while connecting the phones. The voice data for the call, typically carried by the transport protocol, may or may not flow through the call server.

2. VoIP Phones : Also known as Ethernet or IP phones, these phones have an IP address and support VoIP protocols such as SIP, H.323, RTP etc.

3. VoIP Protocols : VoIP protocols are of two types — signalling and transport.

The signaling protocols handle the functions derived from the telephone system architecture. Phones use the signalling protocol to register with the call server, set up, and tear down calls. Signaling protocols are also used for features such as directory services and screen displays.

VoIP Signalling Protocols

The transport protocol is used to encapsulate or carry the actual voice data, and the only protocol universally used for transport is the Real-Time Transport Protocol (RTP). The voice data packets are created with a codec and then encapsulated within RTP.

VoIP Transport Protocols

4. Codecs : Codec is a short for coder-decoder used for the purpose of converting the analog voice signal to a series of digital samples at the source and then back again at the receiver. Thus, the sending phone encodes the voice data with its codec, and the receiver decodes the voice packet with its codec.

VoIP phones always contain the codec. Codecs can also compress the voice data. While there are many different codecs, probably the most common audio codecs are from the ITU-T G series. The ITU-T H series contains the popular video codecs.

Audio Codecs

Video Codecs

5. Desktop phones and softphones : Early in the evolution of VoIP, there were attempts to get rid of the phone entirely in favor of phone applications installed on computers. However, people were used to the traditional telephone design and didn’t like the change. The application also had to compete with whatever was running on the computer at the time. Today, we have a mix of desktop VoIP phones and telephony applications, or softphones.

3CX softphone

Non-VoIP Components

The VoIP system depends on a number of services that are not VoIP specific. Many of the services, such as the Dynamic Host Configuration Protocol (DHCP), are already part of the network architecture and can be expanded to include the VoIP components. Other services include Trivial File Transfer Protocol (TFTP), Domain Name Service (DNS), and Network Time Protocol, or NTP. It is common to see these components listed in the VoIP product requirements, as it may not run without them.


About Deepak Devanand

Seeker of knowledge
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