SIP and H.323 (Part 1)

Computer networks devided into two types :
Voice networks : based on circuit swithcing. Communication is always made by the same path. Example: Public Switched Telephone Network (PSTN).
Data Netwoks : based on packet swithcing. information data is devided into packets, and the packets can travel accros different route/path. Example : internet.

Main problem about circuit swithcing is it need a lot of bandwidth for each communication. Why ?? because same channel is used when during a call (communication) and most of the phone calls have a lot of silence moments.
Data networks only transmit information when it is necessary, so it using bandwidth more efficiently. Delay and loss packets should not be a disadvantage, due the system has a capability to recover the information. However, voice and video streaming are sensitive with those parameters (Delay and loss). So, networks and protocols with high degree of QoS are required.

Voice over IP (VoIP) defines the necessary routing systems and protocols for transmit voice conversations over Internet. Internet is a packet swithcing networks based on TCP/IP protocols.

So, what is SIP and H.323??? VoIP has two architecture for the voice transmission :
SIP (Session Initiation Protocols) : SIP is a signalling protocol to establish and conferences in IP networks. Beginning of the sessions, change or term of the sessions, is independent of the type of application that it is being used in the call. (a sessions including several data types : voice, video, or many other formats.)
H.323 : standard of communications multimedia, that facilitated the convergence of voice, video and data. nitially it was thought for packet circuit networks.

SIP is used for initiating, modifying, and terminating user sessions that involves multimedia communication elements ; ex : voice, video , instant messaging, etc. Main objective of SIP is the communicating between multimedia devices. SIP using two main protocols, RTP and SDP (you can read the RFC or manual about RTP and SDP). RTP is used to transport voice data in real time; SDP is used to negotiate the participant capabilities, codification type, etc. SIP is end-to-end oriented protocols. it means that all the logic is stored in end devices. State is also stored in end-device only. SIP is an application-layer protocols, a signalling protocol for internet-telephony.
SIP has ability to establish and end multimedia sessions (ex : location, availability, resource use, etc). In order to implement these functions, SIP has different components. main components are User Agent (UA) and SIP servers.

  1. User Agent (UA): User Agent has two different parts, User Agent Client (UAC) and User Agent Server (UAS). UAC is used for sending SIP request and receive the answers for those request. UAS used to send answer to the SIP request. Both entities are in every user agent, to allow the communication between different user agents in a client-server communication.
  2. SIP servers, devided into 3 types :
  • Proxy servers : This server has a similar functionality to an HTTP Proxy. Proxy servers devided into 2 types, statefull proxy and stateless proxy. Statefull proxy keep the state of the transaction during the request processing. Stateless proxy do not keep the state of the transaction during the requests processing, They only resend messages.
  • Registrar Servers : a server which accepts register request from the users and keep the information about this request t provides a location and address translation service.
  • Redirect Servers : server which generates redirection answers to the received requests. This server routes again the requests to the next server.

All of thoose parts are conceptual, can be placed in the same machine, or may be in different machine

One of the SIP server function is user location and name resolution. Normally, user agents doesnt know the IP address of the called persons. SIP entities identify a user by SIP URI (Uniform Resource Identification)see RFC 2396. SIP URI has a format similiar with email address consists of a user and a domain delimited by one @. examples :

and, how about H.323????……….. next article. i promise it!!! ^_^


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