Voice over IP H323 and SIP Protocol

Voice over IP has been implemented in various ways using both proprietary and open protocols and standards. Examples of the network protocols used to implement VoIP include:

§  H.323

§  Media Gateway Control Protocol (MGCP)

§  Session Initiation Protocol (SIP)

§  Real-time Transport Protocol (RTP)

§  Session Description Protocol (SDP)

§  Inter-Asterisk eXchange (IAX)

Voice over IP (VoIP, or Voice over Internet Protocol) commonly refers to the communication protocols, technologies, methodologies, and transmission techniques involved in the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks

Internet telephony refers to communications services —voice, fax, SMS, and/or voice-messaging applications— that are transported via the Internet, rather than the public switched telephone network (PSTN). 

Steps:

Sendind Side:

1.      Digitization of the analog voice signal

2.      Encoding

3.       Packetization

4.       Transmission as Internet Protocol (IP) packets over a packet-switched network. 

Receiving Side:

1.      reception of the IP packets,

2.      decoding of the packets

3.      digital-to-analog conversion reproduce the original voice stream

Advantages:

The biggest single advantage VoIP has over standard telephone systems is cost. In addition, international calls using VoIP are usually very inexpensive. One other advantage, which will become much more pronounced as VoIP use climbs, calls between VoIP users are usually free. Using services such as TrueVoIP, subscribers can call one another at no cost to either party.

§  Routing phone calls over existing data networks to avoid the need for separate voice and data networks.

§  The ability to transmit more than one telephone call over a single broadband connection.

§  Secure calls using standardized protocols (such as Secure Real-time Transport Protocol)

H.323:

H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols to provide audio-visual communication sessions on any packet network. The H.323 standard addresses call signaling and control, multimedia transport and control, and bandwidth control for point-to-point and multi-point conferences.

It is widely implemented^[2] by voice and videoconferencing equipment manufacturers, 

Architecture:

The H.323 system defines several network elements that work together in order to deliver rich multimedia communication capabilities. Those elements are Terminals, Multipoint Control Units (MCUs),Gateways, Gatekeepers, and Border Elements. Collectively, terminals, multipoint control units and gateways are often referred to as endpoints.

Terminals

Terminals in an H.323 network are the most fundamental elements in any H.323 system, as those are the devices that users would normally encounter. They might exist in the form of a simple IP phone or a powerful high-definition videoconferencing system.

Protocol Stack:

Inside an H.323 terminal is something referred to as a Protocol stack, which implements the functionality defined by the H.323 system

Multipoint Control Units

A Multipoint Control Unit (MCU) is responsible for managing multipoint conferences and is composed of two logical entities referred to as the Multipoint Controller (MC) and the Multipoint Processor (MP). In more practical terms, an MCU is a conference bridge not unlike the conference bridges used in the PSTN today. The most significant difference, however, is that H.323 MCUs might be capable of mixing or switching video, in addition to the normal audio mixing done by a traditional conference bridge

Gateways

Gateways are devices that enable communication between H.323 networks and other networks, such as PSTN or ISDN networks. If one party in a conversation is utilizing a terminal that is not an H.323 terminal, then the call must pass through a gateway in order to enable both parties to communicate.

Gateways are also used in order to enable videoconferencing devices based on H.320 and H.324 to communicate with H.323 systems. Most of the third generation (3G) mobile networks deployed today utilize the H.324 protocol and are able to communicate with H.323-based terminals in corporate networks through such gateway devices.

Gatekeepers

A Gatekeeper is an optional component in the H.323 network that provides a number of services to terminals, gateways, and MCU devices. Those services include endpoint registration, address resolution, admission control, user authentication, and so forth. O

Gatekeepers may be designed to operate in one of two signaling modes, namely "direct routed" and "gatekeeper routed" mode. Direct routed mode is the most efficient and most widely deployed mode. In this mode, endpoints utilize the RAS protocol in order to learn the IP address of the remote endpoint and a call is established directly with the remote device. In the gatekeeper routed mode, call signaling always passes through the gatekeeper. While the latter requires the gatekeeper to have more processing power, it also gives the gatekeeper complete control over the call and the ability to provide supplementary services on behalf of the endpoints.

H.323 endpoints use the RAS protocol to communicate with a gatekeeper. Likewise, gatekeepers use RAS to communicate with other gatekeepers.

Border Elements and Peer Elements

Border Elements and Peer Elements are optional entities similar to a Gatekeeper, but that do not manage endpoints directly and provide some services that are not described in the RAS protocol. The role of a border or peer element is understood via the definition of an "administrative domain".

An administrative domain is the collection of all zones that are under the control of a single person or organization, such as a service provider. Within a service provider network there may be hundreds or thousands of gateway devices, telephones, video terminals, or other H.323 network elements. The service provider might arrange devices into "zones" that enable the service provider to best manage all of the devices under its control, such as logical arrangement by city. Taken together, all of the zones within the service provider network would appear to another service provider as an "administrative domain".

The border element is a signaling entity that generally sits at the edge of the administrative domain and communicates with another administrative domain

H.225.0 Call Signaling

Once the address of the remote endpoint is resolved, the endpoint will use H.225.0 Call Signaling in order to establish communication with the remote entity. H.225.0 messages are:

§  Setup and Setup acknowledge

§  Call Proceeding

§  Connect

§  Alerting

§  Information

§  Release Complete

§  Facility

§  Progress

§  Status and Status Inquiry

§  Notify

EP:endpoint

RAS Signaling

Endpoints use the RAS protocol in order to communicate with a gatekeeper. Likewise, gatekeepers use RAS to communicate with peer gatekeepers. RAS is a fairly simple protocol composed of just a few messages. Namely:

§  Gatekeeper request, reject, and confirm messages (GRx)

§  Registration request, reject, and confirm messages (RRx)

§  Unregister request, reject, and confirm messages (URx)

§  Admission request, reject, and confirm messages (ARx)

§  Bandwidth request, reject, and confirm message (BRx)

§  Disengage request, reject, and confirm (DRx)

§  Location request, reject, and confirm messages (LRx)

§  Info request, ack, nack, and response (IRx)

§  Nonstandard message

§  Unknown message response

§  Request in progress (RIP)

§  Resource availability indication and confirm (RAx)

§  Service control indication and response (SCx)

§  Admission confirm sequence (ACS)

GK:Gatekepper

Master/Slave Determination

After sending a TCS message, H.323 entities (through H.245 exchanges) will attempt to determine which device is the "master" and which is the "slave." This process, referred to as Master/Slave Determination (MSD), is important, as the master in a call settles all "disputes" between the two devices. For example, if both endpoints attempt to open incompatible media flows, it is the master who takes the action to reject the incompatible flow.

Logical Channel Signaling

Once capabilities are exchanged and master/slave determination steps have completed, devices may then open "logical channels" or media flows. This is done by simply sending an Open Logical Channel (OLC) message and receiving an acknowledgement message.

 When an H.323 device initiates communication with a remote H.323 device and when H.245 communication is established between the two entities, the Terminal Capability Set (TCS) message is the first message transmitted to the other side.

 Master/Slave Determination (MSD)

Open Logical Channel (OLC) message

H.323 is a system specification that describes the use of several ITU-T and IETF protocols. The protocols that comprise the core of almost any H.323 system are:^[7]

§  H.225.0 Registration, Admission and Status (RAS), which is used between an H.323 endpoint and a Gatekeeper to provide address resolution and admission control services.

§  H.225.0 Call Signaling, which is used between any two H.323 entities in order to establish communication.

§  H.245 control protocol for multimedia communication, which describes the messages and procedures used for capability exchange, opening and closing logical channels for audio, video and data, control and indications.

§  Real-time Transport Protocol (RTP), which is used for sending or receiving multimedia information (voice, video, or text) between any two entities.

Many H.323 systems also implement other protocols that are defined in various ITU-T Recommendations to provide supplementary services support or deliver other functionality to the user. Some of those Recommendations are:^{[citation needed]}

§  H.235 series describes security within H.323, including security for both signaling and media.

§  H.239 describes dual stream use in videoconferencing, usually one for live video, the other for still images.

§  H.450 series describes various supplementary services.

§  H.460 series defines optional extensions that might be implemented by an endpoint or a Gatekeeper, including ITU-T Recommendations H.460.17, H.460.18, and H.460.19 for Network address translation(NAT) / Firewall (FW) traversal.

In addition to those ITU-T Recommendations, H.323 implements various IETF Request for Comments (RFCs) for media transport and media packetization, including the Real-time Transport Protocol (RTP).

RAS:Remote Access Service