H.323 is a set of standards to provide audio-visual communication sessions on any packet network such as IP networks.
It works in the OSI application (level 7) layer.
H.323 Components
H.323 components:
- Audio codecs
- Video codecs
- System control
- Multimedia control
- Call Signaling
- RAS
Audio coding formats used by H.323 audio codecs are G.711, G.722, G.723, G.728 and G.729, among others. You can read a post about audio coding formats.
Video coding formats used by H.323 video codecs are H.261, H.263 and H.264, among others. You can read a post about video coding formats.
Multimedia control is performed through H.245 recommendation.
The base standards for H.323 are H.225, H245 and H.323.
Call signaling and Registration Admission and Status (RAS) is performed through H.225.0 recommendation.
The recommendation H.225.0 is used for message definition. It is part of H.323.
H.245 is a network protocol for channel control, and it is also part of H.323.
H.323 Network Elements
An H.323 network element is a hardware device that has a function in a H.323 network.
H.323 network elements:
- Endpoints
- Terminal
- MCU
- Gateway
- Gatekeeper
- Border element
A Multipoint Control Unit (MCU) is a device commonly used to bridge videoconferencing connections. Its purpose within a H.323 network is to coordinate and manage audio and video conferences between multiple participants.
An H.323 gateway provides interoperability between different calling protocols. For example, it may connect H.320-H.323.
An H.323 gateway may provide access to:
- H.320
- H.321
- H.322
- H.324
- H.324M
- V.70
- H.323 (proxy mode)
A gateway can have media gateway controller (MGC) and media gateway (MG). They may also exist independently.
MGC and MG communicate each other through ITU-T’s H.248, also known as IETF’s MEGACO.
MGC’s communicate to each other using H.323 or SIP.
A gatekeeper provides control access and network management to the H.323 network for terminals, gateways and MCUs. These services encompass address conversion, admission control, bandwidth control, zone management or call control signaling.
A border element example could be a transversal firewall, that allows establishing videoconferencing from/to external IPs.
A Multipoint Controller (MC) exchange capacities between each endpoint. It can be part of an MCU, gatekeeper, gateway or terminal.
A Multipoint Processor (MP) receives audio, video or data flows.
H.323 Protocols
H.323 control protocols are defined using ASN.1 notation with binary codification PER. You can read this post about ASN.1.
RAS
Registration, Admission and Status (RAS) allows the gatekeeper to perform different actions on the terminals.
RAS uses UDP protocol.
Its standard is H.225.0, and it is a modification of Q.931.
Types of RAS messages:
- Request (xRQ)
- Reject (xRJ)
- Confirmation (xCF)
The x could be, among others:
- G: gatekeeper
- R: register
- A: admission
- L: location
- B: bandwidth
H.245
H.245 is a recommendation for message definition.
Signaling System
RAS and H.245 considers two types of signaling:
- Direct call signaling
- Gatekeeper Routed Call Signaling (GKRCS)
IP Network Multimedia Traffic
IP Network Multimedia Traffic is done through the Real-time Transport Protocol (RTP) and the Real-time Transport Control Protocol (RTCP). They both work on UDP.
H.323 Security
This section is about security on H.323
H.235
H.235 is a family of security recommendations on videoconferencing that are used with H.323.
These recommendations range from H.235.0 to H.235.9.
Bypassing Firewalls and NAT in H.323
There are different solutions to bypass firewalls and NAT:
- VPN
- H.323-H.320 Gateway
- H.460
A common solution was using H.323-H.320 gateways inside the private network to convert traffic to ISDN.
A more evolved solution is following H.460 recommendations. They consist of:
- H.460.17
- H.460.18
- H.460.19