Instrumentation and Telecom Engineering

Advanced Voice Over IP

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Advanced Voice Over IP Course

Introduction:

For years, many different data networking protocols have existed, but now, data communications have firmly found its home in the form of IP, the Internet Protocol. On the other hand, even since telecommunications began, voice communications have rigidly stuck to a classic circuit-switched networking environment.

Course Objectives:

This course looks at the subject of voice over IP, covering various methods of its implementation and looking at many of the relevant standards. We also cover many of the issues that need to be considered when deploying voice over IP and discuss how these may be dealt with. In order to present the information in such a way that it is best understood, we make significant use of live demonstration equipment to provide a real-life and tangible perspective to the course. This course covers live demos of IP phones, IP PBX systems, gateways, voice compression systems and covers both the H.323 and SIP protocols. We also make use of protocol analysis equipment to show what is actually happening in the IP network.

Who Should Attend?

This course is aimed at the voice and/or data professionals who wish to extend their knowledge and experience into the area of voice over IP. Detailed knowledge of neither voice nor data (IP) is required for a student to derive benefit from the course. A general understanding of voice and/or data (IP) will however be of help.

Course Outlines:

Introduction to voice over IP (VoIP)

This section gives a basic introduction to voice over IP, looking at some fundamental applications and business reasons as to why one might consider deploying it.

Why VoIP? A view from a business perspective as to why voice over IP may be an appropriate technology to deploy in many voice-related applications
A look at voice over IP as deployed across the Internet or across a private IP network.
An introduction to some of the standards for voice over IP:

H.323 and related protocols
Session Initiation Protocol (SIP)
Media Gateway Control Protocol (MGCP) and Megaco / H.248

An introduction to some of the devices that may be used to implement a voice over IP network

IP phones

Power to the IP phone - powered Ethernet

Gateways
Call control systems (e.g. the Gatekeeper)
IP enabled voice switches (PBX and public network switches)

An introduction to some of the challenges of deploying voice over IP

Overview of the voice technologies relevant to the voice over IP

This section gives a simple overview of the voice-related technologies that we see as relevant to the voice over IP. It is not our intention to provide an in-depth voice technology course coverage as this would take too long. Having said that, the course tutor is very experienced in voice technology training and, if appropriate, can spend extra time with students if required to help them understand the important aspects (This typically would be done during a break so as not to impact others on the course).

Interface types:

Foreign Exchange Station (FXS) and Foreign Exchange Office (FXO) interfaces
Ear & Mouth - E&M 2 wire and 4 wire
2.048Mbit/s digital interface - G.732/G.704/G.703
Channel Associated Signalling ( CAS).
Common Channel Signalling (CCS) with a summary of some of the key types including Q.931, QSIG, DPNSS, SS7

Voice Coding:

G.711 Pulse Code Modulation (PCM) - 64kbit/s
Compressed voice:

G.728 Low-Delay Code Excited Linear Prediction (CELP) - LD-CELP
G.729 Conjugate Structured Algebraic CELP (CS-ACELP) - Also G.729a and b
G.723.1 MP-MLQ and ACELP

The use of Mean Opinion Score (MOS) to quantify the quality of voice communications

Overview of the IP technologies relevant to the voice over IP

This section gives a simple overview of the IP related protocols that are relevant to the voice over IP. It is not our intention to provide an in-depth IP course coverage as this would take too long. Having said that, the course tutor is very experienced in IP technology training and, if appropriate, can spend extra time with students if required to help them understand the important aspects (This typically would be done during a break so as not to impact others on the course).

The IP protocol and the key parts of the IP header relevant to the voice over IP. We discuss this in relation to IP version 4 and IP version 6
Overview of IP addressing and network and subnet addressing
Other key protocols:

Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Real-Time Protocol (RTP) and Real-Time Control Protocol (RTCP)

The H.323 framework for voice over IP

H.323 is the oldest and most commonly used protocol suite used to deliver voice over IP. The reasons for this are manifold including the fact that the protocol specifications have been in place for a long time and others such as Microsoft's inclusion of H.323 in its Netmeeting software, an application residing on virtually every desktop computer in the world. The original version (V1) of H.323 released in 1996 was referred to as "Visual telephone systems and equipment for local area networks which provide a non-guaranteed quality of service". Its scope goes far beyond just voice over IP and in recognition of its expanded scope to cover not just LANs, it was renamed in version 2 as "Packet-based multimedia communications systems". Since then, it has evolved further, adding various new features and improvements plus correcting issues that were present in its earlier versions.

Overview of H.323
H.323 architecture:

The H.323 Terminal
The H.323 Gateway
The H.323 Gatekeeper
The H.323 Multipoint Control Unit (MCU)

H.225.0 - This deals with a number of aspects of establishing communication across the data network including Registration, Admissions & Status (RAS) operations, along with Q.931 signaling messages for call setup, call control, and communications between terminals, gateways, gatekeepers, and MCUs.
H.245 - This specifies the syntax and semantics of terminal information messages as well as procedures to use them for in-band negotiation at the start of or during communication.
H.323 examples

Session Initiation Protocol - SIP for voice over IP

The Session Initiation Protocol (SIP) is a signaling protocol used for establishing sessions in an IP network. A session could be a simple two-way telephone call or it could be a collaborative multi-media conference session. It is basically a very simple protocol having been developed purely as a mechanism to establish sessions. It does not know about the details of a session, it just initiates, terminates, and modifies sessions. In order to handle other aspects of end-to-end communications, SIP was designed to reuse many existing protocols and protocol design concepts.

SIP architecture:

SIP user agents (user devices) - phones, workstations, PDAs etc.
SIP servers - Proxy server, redirect server, registrar server
SIP location servers
SIP gateways

SIP functions:

Address resolution
Session setup
Media negotiation using the Session Description Protocol (SDP)

SIP examples - We shall look at a number of examples of SIP's use including basic call setup, call forward plus others.

MGCP and Megaco / H.248 for voice over IP

Media Gateway Control Protocol (MGCP) and Megaco / H.248 are control protocols designed to be used between a Media Gateway Controller or call agent and a media gateway. Megaco / H.248 is newer than MGCP and is seen by many as MGCP's replacement. That said, many products already employ MGCP and therefore, for some time, both protocols will be used.

Media Gateway Controllers
Media Gateways
Endpoints
MGCP & Megaco / H.248 message examples