IT-3303-3 Broadband Networks

Punjab University College of Information Technology,University of the Punjab.

VoIP and H.323

Lecture - Objectives

• VoIP• Advantages of VoIP• How to Use VoIP?• VoIP Protocols• Background to H.323• Components of H.323• H.323 Protocol Overview• H.323 Call establishment

VoIP

• VOIP is an acronym for Voice over Internet Protocol, or in more common terms phone service over the Internet. If you have a reasonable quality Internet connection you can get phone service delivered through your Internet connection instead of from your local phone company.

• VoIP, is a method for taking analog audio signals, and turning them into digital data that can be transmitted over the Internet.

Advantages of VoIP

• There are 3 main causes for the evolution of the Voice over IP market: – Low cost phone calls – Merging of data/voice infrastructures

How to Use VOIP? • There are three different ways of VoIP service in common use

today: • ATA - The simplest and most common way is through the use

of a device called an ATA (analog telephone adaptor). The ATA allows you to connect a standard phone to your computer or your Internet connection for use with VoIP. The ATA is an analog-to-digital converter. It takes the analog signal from your traditional phone and converts it into digital data for transmission over the Internet.

• IP Phones - These specialized phones look just like normal phones with a handset, cradle and buttons. But instead of having the standard RJ-11 phone connectors, IP phones have an RJ-45 Ethernet connector. IP phones connect directly to your router and have all the hardware and software necessary right onboard to handle the IP call.

How to Use VOIP?

• Computer-to-computer - This is certainly the easiest way to use VoIP. You don't even have to pay for long-distance calls. There are several companies offering free or very low-cost software that you can use for this type of VoIP. All you need is the software, a microphone, speakers, a sound card and an Internet connection, preferably a fast one like you would get through a cable or DSL modem. Except for your normal monthly ISP fee, there is usually no charge for computer-to-computer calls, no matter the distance.

• A softphone is client software that loads the VoIP service onto your desktop or laptop.

VoIP Protocols

• There are several protocols currently used for VoIP. These protocols define ways in which devices connect to each other and to the network using VoIP. They also include specifications for audio codecs.

• The most widely used protocol is H.323, a standard created by the International Telecommunication Union (ITU). H.323 is a complex protocol that was originally designed for video conferencing.

VoIP Protocols

• An alternative to H.323 is Session Initiation Protocol (SIP). SIP is a much more streamlined protocol, developed specifically for VoIP applications. Smaller and more efficient than H.323, SIP takes advantage of existing protocols to handle certain parts of the process.

• Media Gateway Control Protocol (MGCP) is a third commonly used VoIP protocol that focuses on endpoint control. MGCP is geared toward features like call waiting.

H.323

• The first successful set of protocols for VoIP was developed by the International telecommunications Union (ITU). This set is known as H.323 and has the title, “Packet-based Multimedia Communications Systems.”

• H.323 is a comprehensive and very complex protocol that was originally designed for video conferencing.

• It provides specifications for real-time, interactive videoconferencing, data sharing and audio applications such as VoIP.

H.323

• Actually a suite of protocols, H.323 incorporates many individual protocols that have been developed for specific applications.

• H.323 is a large collection of protocols and specifications. That's what allows it to be used for so many applications.

H.323 Network Architecture

H.323 Terminal

H.323 MCU

H.323 Gatekeeper

H.323 Terminal

H.323 Gateway

ISDNPSTN

Packet Network

H.323 Components

H.323 Components

• The H.323 standard specifies its network architecture. This architecture consists of four components:– Terminals – Gateways – Gatekeepers– Multipoint controller units (MCUs)

Terminals

• H.323 terminals are primarily IP telephones and PCs, not traditional telephones. An H.323 terminal must have:– A network interface– Audio codecs– H.323 software

Terminals

• H.323 terminals must support audio (G.711 mandatory, G.723.1 and G.729 are recommended for networks of low bandwidth). Video and data support is optional; H.261 is mandatory when video is supported. H.245 and H.225 are required for control functions, and the RTP is required for sequencing media packets.

H.323 Gateways

• Gateways provide a means for interoperability among different telecommunications systems. Gateways provide many functions, including:

• H.323 gateway provide translation services between the H.323 network and another type of network, such as an Integrated Services Digital Network (ISDN) or the Public Switched Telephone Network (PSTN).

• One side of the gateway supports H.323 signaling and terminates packet media according to the requirements of H.323.

H.323 Gateways

• The other side of the gateway interfaces to a circuit-switched network and supports the transmission characteristics and signaling protocols of the circuit-switched network.

• On the H.323 side, the gateway has the characteristics of an H.323 terminal. On the circuit-switched side , it has characteristics of a node of circuit-switched network.

Gateway

Gatekeepers

• A gatekeeper is an optional entity within an H.323 network, however, if they present, they controls a number of H.323 terminals, gateways, and multipoint controllers (MCs). The gatekeeper provide the following services:

• Address Translation: call originating within an H.323 network may use an alias to address the destination terminal. So the gatekeeper must be able to translate an H.323 alias addresses into a transport address. – H.323 alias = email-ID (e.g. schlatter@switch.ch), or

e164Number (e.g. 004112681549)

Gatekeepers

• Endpoints do register with their H.323 aliases1 and call signaling IP address

• A GK translates H.323 aliases into call signaling IP addresses (especially useful for endpoints with dynamic IP addresses)

• This function is particularly important in scenarios where a phone on the circuit-switched network is attempting to call a PC on an IP network. (an E.164 number like 212-555-2121 will be translated into an IP network address like 180.23.12.78)

• Admission Control: the gatekeeper can control the admission of the endpoints into the H.323 network. It uses RAS messages, admission request (ARQ), admission confirm (ACF), and admission reject (ARJ) to achieve this. Admission control can be a null function that admit all endpoints to the H.323 network.

• Bandwidth Control and Management: the gatekeeper provides support for bandwidth control by using the RAS messages, bandwidth request (BRQ), confirm (BCF), and reject (BRJ). For instance, if a network manager has specified a threshold for the number of simultaneous connections on the H.323 network, the gatekeeper can refuse to make any more connections once the threshold is reached.

Gatekeepers

• Zone Management: the set of terminals, gateways and MCs controlled by a single gatekeeper is known as a zone. Within its zone, a gatekeeper must provide required functions (for example: address translation, admission control, bandwidth control) to all endpoints that have registered with it.

• Call Authorization: the gatekeeper can decide to authorize or reject a given call; the provider of the H.323 service specifies the reasons for authorization and rejection. Reasons may include the time of the day, type of service subscription, desire to access a restricted gateway, or lack of available bandwidth.

Gatekeepers

Multipoint Control Unit (MCUs)

• MCUs provide conference support for three or more endpoints.

• All terminals participating in the conference establish a connection with the MCU.

• It manages conference resources and negotiations between endpoints to determine which audio or video codec to use.

MCU

H.323 Protocol Stack

H.323 Protocols Overview:RAS

• RAS stands for Registration, Admission, and Status. RAS is also known as H.225 protocol.

• RAS messages are used for:– Gatekeeper discovery (often done manually)– Gatekeeper registration– Name resolution (H.323 alias Æ IP Address)– Admission control (to restrict the entry of an

endpoint into a zone)– Bandwidth control– Status requests

• RAS messages use UDP protocol with ports 1718 (GK discovery) / 1719

RAS

H.323 Protocols Overview:Q.931, H.245

• Q.931- Call Signaling– call signaling protocol used in the ISDN D

Channel– Q.931: TCP, port 1720

• H.245- Control Signaling– Flow control messages– Opening and closing logical channels used to

carry media streams– H.245: TCP, dynamic port (>1024)

Q.931, H.245

H.323 Protocols Overview:Video Codecs

• H.261 and H.263 are ITU series of video codecs. • A video codec is used to encodes video from the camera

for transmission on the transmitting terminal and decode the received video code that is sent to the video display on the receiving terminal.

• H.263– successor of H.261– same video quality as H.261 but lower bitrate

• Future: H.264 = MPEG-4• Real Time Transport Protocol (RTP)

– IETF RFC 1889– end-to-end media stream transport protocol– payload type, sequence number, timestamp

• UDP, dynamic port (>1024)– (RTP: even, RTCP: odd numbered port)

Video Codecs

H.323 Protocols Overview:Audio Codecs

• G.711, G.723, G.729 are series of ITU audio codecs.• An audio codec encodes the audio signals from the

microphone for the transmission on the transmitting terminal and decode the receive audio code that is sent to the speaker on the receiving terminal.

• G.711– PCM, 64 kbits, voice quality: good

• G.729– 8 kbits, voice quality: good

• RTP Control Protocol (RTCP)– quality feedback, RTP session control

• UDP, dynamic port (>1024)– (RTP: even, RTCP: odd numbered port)

Audio Codecs

H.323 Protocols Overview:T.120

• T.120– Used for “data conferencing”

• File transfer (T.128)• Text chat (T.134)• TCP, port 1503

T.120

Reference Material

• 3G Wireless Networks (chapter 8) by CLINT SMITH and DANIEL COLLINS• CISCO Voice Over IP

by ELLIOT LEWIS• http://electronics.howstuffworks.com/ip-

telephony1.htm

• http://www.voip-info.org/wiki/view/What+is+VOIP