H.323

H.323 gathers a whole of communication protocols of the voice, image and data on IP. It is a protocol developed by UIT-T.
Il is derived from the protocol H.320 used on ISDN.

Secondary or associated protocols

More than one protocol, H.323 resembles more an association of several different protocols and which can be gathered in three categories: indication, the negotiation of codec, and the transport of information.

The messages of indication are those which one sends to ask to be put in relation to another person, which indicates that the line is occupied, that the telephone sounds… That includes/understands as the messages as one sends to announce that such telephone is connected to the network and can be joined in such manner. In H.323, indication is based on the protocol CLOSE-CROPPED ( Registration Admission Status ) for the recording and the authentification, and the protocol Q.931 for the initialization and the control of call. The negotiation is used to agree on the way of coding information which one will exchange. It is important that the telephones (or systems) speak a common language if they want to be included/understood. It would be also preferable, if they have several alternatives of languages which they use more adapted. It can be a question of the least greedy codec in band-width or of that which offers best quality. The protocol used for the negotiation of codec is the H.245

The transport of information is based on the protocol RTP which transports the voice, it video or the data digitized by codec S. One can also use the messages RTCP to make quality control, to even ask to renegotiate the codecs if, for example, the band-width decreases.

For control and indication: H.225 H.245 Q.931 RTCP

For the voice: G.711 G.722 G.723 G.726 G.728 G.729

For the video: H.261 H.263 H.263+ H.264

For the data: T.123 T.124 T.125

Principal applications of the H.323 protocol

  • Certaines Telephone offers on IP general public rests on H.323
  • the customers (NetMeeting, orange, Ekiga, ex- GnomeMeeting, Ophone, IChat, etc).
  • the Gatekeeper, or guards of door, being used with the administration of the communications and the translation of identifiers as connection (ID H323).
  • MCU, of the multipoint bridges.
  • Gateway, of the footbridge S H.320 <=> H.323

Simplified operation

There exist several manners of implementing an architecture H.323

  • Of many messages are optional and in practice one can choose not to use them all. For example, if the authentification is not a concern, one can do without the CLOSE-CROPPED messages.
  • It is also possible to connect the messages in several different ways. One can for example open channels RTP without awaiting the message “connect” which indicates that the person called took down well (Early h.245).
  • One can decide that the messages H.225, H.245 and RTP pass by different ways: For example indication will cross several Gatekeeper which takes part in the control and the routing of the call whereas flow RTP passes directly from a station to the other.

Case 1: “point-to-point” communication of two simple customers

  • calling It between address IP of the recipient in the field of the software reserved for this purpose.

  • the protocols of indication propose with the software of the recipient to establish the communication and transmits its ID H323.
  • the software of the recipient answers either “occupied” or “free”.
  • So “free”, calling it enumerates its possibilities of codec S audio and video (so available).
  • the recipient enumerates the compatible codecs with appealing for agreement.
  • If agreement, other ports TCP and UDP are negotiated for audio (UDP), it video (UDP) and data (TCP).

  • Each flow is then transmitted independently of the others.

  • With the closing of a session, the ports are released and the stopped transmissions of control.

Case 2: “point-to-point” communication between two customers recorded near a Gatekeeper

The Gatekeeper intervenes on indication.

  • With the opening of the software, the customer has is recorded at Gatekeeper by transmitting to him its ID H323 and its address IP.

  • the customer B makes in the same way.
  • the customer has between the ID of connection of the customer B in the field of the software reserved for this purpose.
  • the software of the customer has request the authorization in Gatekeeper to connect to the customer B.
  • If Gatekeeper accepts, this one requires of the customer B his state (already in conversation or not).
  • If compatible state, Gatekeeper transmits address IP of the customer B to customer A.
  • Gatekeeper informs the customer B whom a communication will have place with customer A.
  • the customer has between directly in negotiation with the customer B with the protocols of control of communication.
  • the customer has enumerates his possibilities of codec S audio and video (so available).
  • called enumerates the compatible codecs with appealing for agreement.
  • If agreement, other ports TCP and UDP are negotiated for audio (UDP), it video (UDP) and data (TCP).
  • All flows are then transmitted independently from/to each other without passing by Gatekeeper but directly between the customers.
  • With the closing of a session, Gatekeeper is informed of the end of connection, the ports are released and the stopped transmissions of control.

Case 3: communication “Multipoint” between several customers (MCU necessary)

MCU have capacities for treatment of the signal (diffusion, recording, mixing,…) they are used for:

  • to allow the conference by mixing the audio streams

  • to diffuse messages network like the tonality, the beep of setting on standby
  • to see fulfilling elementary functions of voice mail

  • the MCU is announced at Gatekeeper and states its possibilities to him:

    • Many possible customers.
    • Flows (in bytes/seconds) possible by customer or maximum total flow.
    • ID H323 of connection.
The communications will be then treated as with case 2, the MCU then becoming a “simple customer” within sight of the appealing ones; the difference being simply in the number of communications accepted before transmission of the “occupied” message.

The main ports used by the H.323 protocol are 1720 TCP and following, the others is negotiated dynamically.

Notes: The H.323 protocol does not follow the ISO recommendations on separations of layers of network model. One finds data on the layers IP in the layer “Application” of the model. In addition, the protocol tends to open “ports” with flown for the communication, with the result that the passage of a Pare-feu ( firewall ) where NAT is often problematic.

Nevertheless there exist today applicatives solutions (NAT ALG) which make it possible to thwart the presence of data IP (often private) in the application part of the model.

One however finds more and more equipment network knowing to manage the protocols associated with H.323, in particular because of telephony on IP (VoIP).

Case 4: 3 gatekeeper

In this example each terminal is attached to a Gatekeeper of proximity. All the Gatekeeper of proximity are attached to a Gatekeeper which has a general knowledge of the network and which carries out the routing.

One chose to make pass flow h.245 by the Gatekeeper of proximity and flow RTP by one of the Gatekeeper of proximity.

Case 5: others

In a real case, it is probable that architecture includes/understands the following elements:

  • One or more footbridges towards the STN or worms of other networks of ToIP.
  • Of the mail servers vocal (MCU with capacity of recording)
  • Of the waiters (MCU) able to diffuse messages network (setting, busy tones on standby,…)

General note:

The H323 protocol, although implemented in number of commercial software and the majority of the solutions of videoconference " all in un" , passes for a " bad protocole". This is in fact due to the freedom which the manufacturers in the implementation of the various standards of the protocol took. With the result that a protocol which was to be interopérable it is not really any more. So much so that it is relatively difficult to make function two solutions different owners between them.

For Firewall, H323 poses true problems because it is a protocol which asks for the opening of a panel of ports TCP and dynamic UDP of manner and some can " aléatoire" , incompatible with the logic of the rules " strictes" imposed by the safety of a site or an Intranet exposed to Internet. Firewall modern make it very well, but at the price of a complexification which would have been avoided if the protocol as of its origin had been thought better. And more one thing is complex, plus it with chance to be buguée or to have holes of safety.

The protocol SIP takes again the best aspects of H323 for the VoIP, and becomes more and more a standard in the exchanges between PABX. Substitute in fact the old H323 protocol, but not solving completely certain problems involved in the NAT and Firewall.

See too

External bonds

  • H.323
  • H.323 More project
  • VoxGratia (Contains the recent sources of OpenH323)
  • OpenH323 Gatekeeper
  • H.323 Protocol Overview
  • Gatekeeper H323, MCU and PABX [[open sources]

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