4.1 Overview of H.323
4.1.1 What is H.323
H.323 is an ITU-T Recommendation which covers the technical requirements for multimedia communications systems in a packet based network (PBN). H.323 entities (call control for instance) might be used in point-to-point sessions and multipoint conferences. The latest version of H.323 is Version 4 (V4).
H.323 describes the components of an H.323 system, including gateway (GW) between switched circuit network (SCN) and PBN, gatekeeper (GK) for address translation and access control, multipoint controller (MC) for controlling participation of terminals in multipoint conferences, multipoint processor (MP) for the centralized processing of audio, video, and/or data streams in a multipoint conference, and multipoint control unit (MCU) for providing the capability for terminals and gateways to participate multipoint conferences.
4.1.2 Definition of Terms
I. AAA
AAA stands for authentication, authorization, and accounting. Authentication is the process of checking whether the user has certain permission. Authorization is the process of granting the permission to the user, and allowing the user to access certain network resources. Accounting is the process of recording information when the subscriber is using a certain service. Such information is used to generate bills.
II. H.323 entity
An H.323 entity is any H.323 component, including terminals, gateways, gatekeepers, MCs, MPs, and MCUs. Terminals, gateways and MCU are known as endpoints. An endpoint can generate and terminate calls. It can generate and terminates media streams.
III. H.323 terminal
An H.323 terminal is an endpoint on the network which provides real-time, two-way communications with another H.323 terminal, gateway, or MCU. It can be integrated in a personal computer, or can be an independent device, such as Ethernet-phone or video phone.
An H.323 entity interacts with an end user directly. It can call and be called. It can also process media streams.
IV. Gatekeeper
The gatekeeper (GK) is an H.323 entity on the network that manages the H.323 terminals, gateways, and MCUs in a zone. A zone includes at least one terminal, and may or may not include GWs or MCUs. A zone has one and only one GK.
The GK provides the following services and features for the H.323 endpoints:
l Access control: Permitting the use of network resources after identity check.
l Address translation: Translating between alias and network address.
l Bandwidth management: Applying for initial bandwidth; controlling bandwidth variation.
l Charging management: Providing basic charging data to the billing center.
l Zone management: Managing terminals, GWs and MCUs in the zone.
l Call control: Providing supplementary services.
V. Gateway
An H.323 Gateway (GW) is an endpoint on the network which provides for real‑time, two-way communications between H.323 terminals on PBN and other ITU Terminals on an SCN or to another H.323 gateway. Conceptually, a GW performs conversions of media streams and signaling. The latter means that the GW serves as a terminal to convert the between user signaling and H.323 control signaling. If the GW connects two different networks, PBN and SCN for instance, it needs to convert between H.323 control signaling and signaling of the other type of network.
An H.323 GW interconnects different types of network, converts the format and content of signaling messages, and also the communication protocol procedures and media stream format.
VI. Multipoint conference
The components of multipoint conference are MC, MP, and MCU. They all serve for conferencing purpose.
The multipoint controller (MC) provides for the control of three or more terminals participating in a multipoint conference. The MC provides for capability negotiation with all terminals to achieve common levels of communications. It may also control conference resources such as who is multicasting video. When a terminal joins or leaves the conference, the MC will adjust the capability set sent to all terminals.
The multipoint processor (MP) processes the audio, video, and data streams in a multipoint conference, and returns the processed streams to each terminal. Therefore, the MP can implement the codec algorithms of all types of media streams.
The MC and MP are functional entities rather than physical ones.
The multipoint control unit (MCU) is an endpoint on the network which provides the capability for three or more terminals and GW to participate in a multipoint conference. It controls and manages the multipoint conference and all attended terminals, and performs mixing or switching of audio, video and data. The MCU consists of two parts: a mandatory MC and optional MPs.
There are two types of multipoint conferences:
l Centralized multipoint conference
A centralized multipoint conference is one in which all participating terminals communicate in a point-to-point fashion with an MCU. The terminals transmit their control, audio, video, and/or data streams to the MCU. The MC within the MCU centrally manages the conference. The MP within the MCU processes the audio, video, and/or data streams, and returns the processed streams to each terminal. Figure 4-1 shows a typical networking model of centralized multipoint conference.
Figure 4-1 A networking model of centralized multipoint conference
l Decentralized multipoint conference
A decentralized multipoint conference is one in which the participating terminals multicast or multi-unicast their audio and video to all other participating terminals without using an MCU. The MCU manages the control messages and data messages only. Figure 4-2 shows a typical networking model of decentralized multipoint conference.
Figure 4-2 A networking model of decentralized multipoint conference
l Hybrid multipoint conference
A hybrid multipoint conference is a mixture of centralized and decentralized multipoint conference. Figure 4-3 shows a typical networking model of hybrid multipoint conference.
Figure 4-3 A networking model of hybrid multipoint conference
VII. Radius
Radius stands for Remote Authentication Dial-In User Service. It is an AAA protocol widely in use. Radius specifies the format of the authentication, authorization and accounting messages interacting at Radius server and client.
4.1.3 Structure of H.323 Protocol Stack
Figure 4-4 shows the structure of H.323 protocol stack. The three layers at the bottom of the stack are bottom-layer protocols of PBN. In LAN, the physical layer is MAC-IPX. In IP network, the network layer is IP. There are two types of transport-layer protocols. One is unreliable transmission protocol like User Data Protocol (UDP), which transmits audio-visual signals in real time, and sends registration messages to GK. The other is reliable transmission protocol like Transmission Control Protocol (TCP), which transmits data signals, call signaling and media control messages.
Figure 4-4 H.323 protocol stack
The H.323 protocol processing software includes the following parts:
l All H.323 terminals shall have an audio codec. All H.323 terminals shall be capable of encoding and decoding speech according to Recommendation G.711. G.711 for PCM is mandatory and other G series Recommendations are optional. The most frequently-used Recommendations in IP telephony are G.729A and G.723.1.
l H.260 Recommendation series, such as H.261 and H.263 specify the video codec.
l The real-time audio and video encoded signals are all encapsulated in Real-time Transport Protocol (RTP) packets to provide timing information and datagram serial number for the receiving end to re-organize the signal. RTCP (Real-time Transport Control Real-time Transport Control Protocol (RTCP) is a part of RTP, and provides QoS monitoring feature.
l Recommendation T.120 is the default basis of data interoperability in multimedia conferences.
l H.225.0 is the core of H.323 protocol stack. It defines call signaling protocols and media stream packetization for packet-based multimedia communication systems. H.225.0 serves for call control. The principal function of H.225 is to establish call connections and H.245 control channel between H.323 endpoints before starting a call. H.225.0 also covers two other features: specifying the use of RTP/RTCP for media stream packetization and synchronization; defining RAS.
l RAS, which stands for Registration, Admission and Status, specifies a type of message used between endpoint and GK. The RAS signaling function uses H.225.0 messages to perform registration, admissions, bandwidth changes, status, and disengage procedures between endpoints and GKs.
l Recommendation H.225.0 is drafted based on Q.931 and Q.932. ITU-T Recommendation Q.931 is ISDN user-network interface layer-3 specification for basic call control.
l Recommendation H.245 is the control protocol for multimedia communication. It is designed for conference communication. H.245 is the control protocol in H.323 stack, and controls the establishment, maintenance and release of channels.
l RAS, H.225.0 and H.245 are used in SoftX3000. The network protocol is IP, and the transport protocol is UDP and TCP. RAS is borne over UDP; while H.225.0 and H.245 are borne over TCP.
RAS, H.225.0 and H.245 will be detailed in the following sections.
4.1.4 H.323 Applications in SoftX3000
SoftX3000 implements multimedia communication services in H.323. Figure 4-5 shows the H.323 applications in SoftX3000. SoftX3000 provides two H.323 interfaces.
1) One is called SoftX3000 H.323 domain, which is the interface of H.323 terminals under direct control of SoftX3000. SoftX3000 can serve as an H.323 GW and an H.323 GK at the same time.
2) The other is called H.323 domain, which interfaces the external H.323 network. SoftX3000 serves as an H.323 GW.
Figure 4-5 H.323 Applications in SoftX3000
I. SoftX3000 H.323 domain
l SoftX3000 services as an H.323 GK in the domain. All H.323 terminals in the domain must register at SoftX3000 before using any services provided by SoftX3000.
l SoftX3000 serves as an H.323 GW, and provides SIP, ISUP and MGCP interfaces to other networks.
l The H.323 interfaces support signaling and protocol of RAS, Q.931 and H.245.
l SoftX3000 verifies the username and password retrieved from the H.323 terminal.
II. H.323 domain
l SoftX3000, as an H.323 GW, must register at an external H.323 GK before using the services provided in the H.323 domain.
l The H.323 interfaces support signaling and protocol of RAS, Q.931 and H.245.
l SoftX3000 registers the addresses of all terminals connecting through itself to the external GK, including all H.323 terminals in the domain.
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