Converting voice to digital packets

VoIP sends digitized voice across computer networks.

When converting an analog signal (be it speech or another noise), you need to consider two important factors: sampling and quantization. Together, they determine the quality of the digitized sound.

Real Time Protocol

he protocol used to send real-time streams of data across a network is simply called the Real Time Protocol (RTP for short). RTP has been originally defined by IETF in RFC1889 and the up-to-date definition is in RFC3550.

When transmitting the streams of data, the protocol needs to handle the following conditions in the network:

Session Description Protocol

The Session Description Protocol was first published in 1998 in RFC2327, one year before SIP itself. The protocol was updated in 2006 with RFC4566. Like SIP, SDP is also a product of the MMUSIC working group. Aside from SIP, SDP was also used in Mbone.

SIP Call Flow

Registration

Before we describe the flow of a typical SIP call, let's have a look at how SIP user agents register with a SIP registrar. The example below shows a situation where an SIP softphone (namely, the Ekiga client) registers with an Asterisk PBX. The Asterisk's IP address is 10.10.1.99, while the client is at 10.10.1.13 and wants to register the telephone number 13.

In order to register, the SIP telephone needs the send the REGISTER request:

SIP Messages

SIP is a text-based protocol. The formatting of SIP messages is based on the syntax of HTTP version 1.1. There are two types of messages: requests and responses.

SIP a Voip Protocol

The Session Initiation Protocol (SIP for short) is a Voice over IP protocol designed by theInternet Engineering Task Force. SIP was created by the MMUSIC group of the IETF (MMUSIC stands for Multi-party Multimedia Session Control). Formally, the protocol is intended for creating, modifying and terminating sessions with one or more participants. The sessions are mainly VoIP telephone calls or conferences.

Voice Over Internet Protocol

Voice over Internet Protocol (VoIP), is a technology that allows you to make voice calls using a broadband Internet connection instead of a regular (or analog) phone line. Some VoIP services may only allow you to call other people using the same service, but others may allow you to call anyone who has a telephone number - including local, long distance, mobile, and international numbers. Also, while some VoIP services only work over your computer or a special VoIP phone, other services allow you to use a traditional phone connected to a VoIP adapter.