H.323 | Signaling Protocols in Detail

H.323 is an International Telecommunication Union (ITU) specification that defines how to establish both the signaling and the bearer channels. The goal was to use the call signaling from ISDN, a call control protocol for renegotiating calls as they are ongoing, and a registration protocol, to provide for an all-encompassing solution. (Compare how SIP defines only registration, call signaling, and merely basic call control.) However, the process was different, and the H.323 technology suffers from the typical emphasis on layering and precise botanical definitions of technologies that haunts the world of telecommunications.

H.323's major advantage, compared to SIP, is that it contains the complete protocol definition for the application, covering features such as media reservation and conference negotiation that SIP leaves alone. H.323 is also able to pull together a number of other ITU definitions and technologies, into one larger umbrella. Because of the ITU protocols' amount of definition for media applications, H.323 is still the signaling protocol of choice for many videoconferencing applications.

That being said, H.323's relevance for voice is waning. For that reason, we will stick to H.323 at a higher level than we did with SIP.

H.323 Architecture

H.323 has a somewhat similar architecture to SIP. Figure q shows this architecture.

Figure 1: H.323 Architecture

The endpoints are known as terminals. The terminals must register with the registrar, which is now in a function known as the gatekeeper. The gatekeeper is the PBX, and has complete responsibility for administration, user definitions, registration, and routing. Gateways are now special devices that are specifically called out for bridging signaling and media between two different networks.

H.323 uses a protocol known as H.225.0 for call setup signaling. H.225.0 itself is a package that refers to Q.931 for call signaling definitions. (This sort of nesting is typical for telecommunications definitions). The good news is that it stops at Q.931, and we can identify it. ITU Q.931 is the call signaling protocol used in ISDN lines. H.225.0 also includes the Registration, Admission, and Status (RAS) protocol, used by the client to register with the network.

The registration function is, therefore, defined by RAS. When a phone comes online, its first task is to use RAS to find a gatekeeper (from a known or discovered list) that is willing to let it register. Once it does that, it then requests to register. After the registration is complete, the phone is ready to send or receive calls.

To place a call, the phone sends an admission request to the gatekeeper. The gatekeeper's job is to find out where the other endpoint is, by looking up in its extension or routing tables. The result will be an acceptance or rejection. If the result is an acceptance, the gatekeeper will also respond with the contact information for the other endpoint. Notice that the model here is based on admission control. The gatekeeper is allowed to monitor voice resources, and reject calls purely on the basis of there not being enough resources. In any event, the caller now has the contact information of the called party, so the caller contacts the called party directly to attempt to establish the call. This direct contact is done using Q.931 signaling over IP. If the called party is willing to accept the call, the called party must contact its local gatekeeper with an admission request. If that is granted, the call is ready to be finalized.

H.245 plays the role of establishing what the bearer channel will hold. H.245 was designed to provide the information necessary to set up the bearer channels over RTP, and so takes the place of SIP's SDP. H.245 exchanges the codec and bearer capabilities of each endpoint, and is used to negotiate what bearer technology to use. This can be done in a manner that works for multiple-party calls, and in this way is useful with teleconferencing.

It is still possible to find softphones and open source technology that supports H.323, especially because of the videoconferencing aspect. However, voice mobility networks are unlikely to see much of H.323.

X.25 Packet

X.25 packet is an international standard for reliable data communications through the use of a packet-data switching network. The X.25 standard specifies the protocol between the data device (such as a computer) and the network such as a public packet data network (PDN).

The X.25 system is a connection based packet switching system. X.25 packet data switches are initially programmed to create a logical path (virtual connection) from the entry point to the exit point before data transmission begins.

X.25 systems are used to ensure reliable data transmission as it uses advanced error protection and retransmission processes. To provide this reliable transmission of packets of data, each link in the packet data network receives, checks, requests retransmission if necessary, and forwards the data onto the next link.

The key components of a X.25 system are packet assembler and disassemblers (PAD) and packet nodes (packet switching points). The PAD divides or converts blocks of data (such as data files) to and from small packets of information. In the disassembly process, a PAD usually assigns sequential numbers to the packets as they are created to allow the reassembly PAD to identify the correct sequence of data packets to reproduce the original data signal. The ITU specification for a X.25 system PAD is X.3.

A packet node is a packet switch in an X.25 network. The packet node receives and forwards packets of data. The packet switch receives the packet of data, reads its address, searches in its database for its forwarding address, and sends the packet toward its next destination.

X.25 systems are public data network (PDN) or private data systems. The X.25 specification only defines the communication with the X.25 network. Communication within the X.25 network is often implementation specific (company proprietary). To interconnect X.25 systems together, the X.75 specification is used.

Because of the error checking and retransmission process used in the X.25 system, packet transmission time is generally longer than in newer packet switching systems such as frame relay. In a packet network, packet switches are networked together over a wide area (normally a country or continent). Packet switches are connected to each other via dedicated high-speed communication lines. Each switch is configured to have at least two leased circuits to at least two different switches. The local switch is in turn connected to local hosts via dedicated, leased lines and to multiple modems (modem banks) to allow local dial up access. The switches are constantly programmed with remote host addresses and the least cost routes to those devices.

Figure 1 shows a X.25 packet data system. This diagram shows bank teller machine in Rome is connected to a bank processing system in London. The X.25 system is setup so a virtual path is created through the X.25 network so data can reliably pass through each packet node to reach its previously established destination. This diagram shows that a virtual connection is made through a packet node in Paris. Each packet that is sent is validated over each link until it reaches its destination.


Figure 1: X.25 Packet Data System

Telecom : Protocols

Protocols are the precise set of rules and a syntax that govern the accurate transfer of information within a communications network. Protocols are used within a communication system to establish, carry out, and terminate communication circuits. Protocols are also used to coordinate billing and customer care systems, manage network devices, and any other process that requires coordinated communication and control.

There are thousands of different protocols used in communications systems. Usually, protocols are grouped into families of protocols so they can serve specific types of networks and services. When interconnecting different networks, protocols need to be converted.

Protocol conversion involves the translation of the protocols of one system to those of another to enable different types of equipment, such as data terminals and computers, to communicate. This is done by an inter-working function (IWF). An IWF system (such as a data bridge) adapts the communications between two different types of networks. Protocol conversion may be used to interconnect circuit switched or packet switched networks.