IPPBX network core protocols-SIP protocol Xiangjie

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IPPBX network core protocols-SIP protocol Xiangjie


Abstract: SIP protocol is an important agreement in the NGN has been more emphasis on the industry. In this paper* the background of SIP protocol* function* network elements* implementation mechanism* as well as the composition of the SIP message to the SIP protocol* and several other areas have done a comprehensive summary of introduction to make the reader there is a preliminary concept of SIP and awareness.

Keywords: SIP NGN Proxy Server

1* SIP protocol and functions of the background


SIP (Session Initiation Protocol) was developed to be used to help provide advanced telephone services across the Internet. Internet Telephony (IP Phone) is to a formal model of the evolution of the business phone* SIP is used to ensure the realization of this evolving needs of NGN (Next Generation Network) series of protocols an important one.

SIP is the IETF standard part of the process* which is such as SMTP (Simple Mail Transfer Protocol) and HTTP (Hypertext Transfer Protocol) based on the set up. It is used to create* change and termination of IP-based calls between users. In order to provide telephone service also requires a combination of different standards and protocols: in particular the need to ensure that transmission (RTP)* with the current telephone network signaling interconnection to ensure voice quality (RSVP)* can provide a directory (LDAP)* to Kam power user (RADIUS) and so on.
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SIP has been described as used to generate* modify* and the end of one or more of the conversation between the participants. These sessions include Internet multimedia conferences* Internet (or any IP network) telephone calls and multimedia distribution. The members of the conversation through multicast or unicast network communication link. SIP supports session descriptions* which allows participants in a group of compatible media types agreed. It also through a proxy and redirect requests to the user's current location to support user mobility. SIP is not associated with any particular conference control protocol binding.


Essentially* SIP provides the following features:

Name Translation and User Location: No matter where the callee is to ensure that the call to the called party. Implementation of any description of the mapping information to locate information. Ensure that the call (session) the nature of the details are supported.

Features consultations: it allows with the call-related groups (this can be multi-party call) in support of the characteristics of the agreement (Note: not all parties are able to support the same level of features). For example video may or may not be supported. In short* there is plenty of room for the scope of consultations.

Participants Call Management: Call the participants to the introduction of other users to join the call or cancel connections to other users. In addition* users can be transferred or set to call hold.

Call feature changes: the user should be able to change the call in the process of calling features. For example* a CRM can be set to "voice-only"* but in the call process* the user may need to open the video capabilities. That is a third party in order to join the call to join the call can be opened in different characteristics. China Network Alliance www_bitscn_com

2* SIP network elements

SIP has two elements. SIP user agent and SIP network servers. User agent is calling end-system elements* while the SIP server is dealing with multiple call signaling associated with network equipment.

User agent client itself has an element (User Agent Client UAC) and a server element (User Agent Server UAS). Elements of the initial call the client and the server element answers the call. This allows point-to-call by a client - server protocol to complete.

SIP Server Elements offers a variety of types of server. There are three forms exist in the network server - SIP stateful proxy server* SIP stateless proxy servers and SIP redirect server. As the caller may not know that the callee's IP address or host name* SIP server's main function is to provide name resolution and user location. Access to the email address or the form of being associated with the caller's phone number. The use of this information* the caller's user agent to determine a specific server to resolve address information - which may involve a lot of the network server.

SIP proxy server receives the request* decided where to send those requests* and send them to the next server (using next-hop routing principle). In the network can have more jump.

Stateful and stateless proxy server is the difference between stateful proxy server* remember that it receives into the request and send back the response and its transfer to a request. Stateless proxy server once the transfer request and forget all of the information. This allows stateful proxy generate requests in parallel over a possible attempt to return location of the user* and the best response. Stateless proxy server may be the fastest and is the backbone of SIP structures. Stateful proxy server from the user agent may be the latest local device* which controls the user domain and is the main platform for application services.
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Redirect server receives the request* but not to those requests passed to the next server* but to send a response to indicate that the caller was calling the user's address. This allows the caller can be directly linked to the next server is the caller's address.

3* SIP protocol implementation mechanism

SIP is a hierarchical structure of the agreement* which means that its behavior according to a group of independent processing stages of equality described in only a loose coupling between each stage. Hierarchical description is intended to express the agreement* allowing the description of functions can be a part of the span of several elements. It does not specify any way to achieve. When we say that a certain element contains a layer* we mean that it obey the rules defined in this layer set.

Not stipulated in the agreement contains the layers of each element. Moreover* the elements provided by the SIP is the logical element* not the physical element. A physical implementation can be selected as different logical elements* perhaps even a one transaction basis.

SIP is the lowest level of the syntax and coding. It is encoded using the Enhanced Backus-Nayr formal grammar (BNF) Law.

The second layer is the transport layer. It defines a client on the network how to send requests and receive responses* as well as how a server receives the request and sends a response. Element contains all of the SIP transport layer.

The third layer is the transaction layer. Affairs are the basic elements of SIP. A transaction is a transaction sent by the client request to the server transaction (using the transport layer)* as well as requests from the server transaction should be sent back to the client component of all responses. Transaction layer handles application-layer retransmissions* matching response to the request* as well as application-layer timeouts. Any user agent client (UAC) to complete the task using a set of transaction generated. User Agent includes a transaction layer* a state agency* too. Stateless proxy does not contain a transaction layer.
Transaction layer has a client component (referred to as Client Services) and server components (referred to as a server transaction)* each representing a finite state machine* which was constructed to handle specific requests. Network Management Network bitsCN.com

Services layer above the layer called the transaction user (TU). Each SIP entity* in addition to non-state agents* are the services the user. When a TU wish to send a request* it generates a client transaction instance and passes the request to it and IP address* port* and the transport mechanism used to send the request. TU generate a client transaction can also delete it. When a client cancel a transaction* it requests the server to stop further processing* will be restored to the state before the transaction is initialized* and generates a specific error response to the transaction. This is completed by the CANCEL request* which constitutes its own affairs* but to cancel the transaction involved.

SIP address of the form by EMAIL to indicate the user's address. Each user through a hierarchical URL to identify it by* for example the user's phone number or host name and other elements to construct (for example: SIP: [email protected]). Because of its similarity with the EMAIL address* SIP URLs easy for the user associated with the EMAIL address.

SIP provides its own reliability mechanisms and thus independent of the packet layer* and the only unreliable packet data service can be. SIP can be typically used on top of UDP or TCP.

SIP protocol to provide the necessary mechanisms to ensure that end-system and the proxy server provides the following operations:

● User Location

● User capabilities

● User availability

● Call Set-up

● Call Processing

● Call Forwarding* including: (1) is equivalent to 800 types of calls* (2) No Answer Call Forwarding* (3) Busy Call Forwarding* (4) unconditional call forwarding China Network Alliance www.bitscn. com

● call number transmission* the number can be any naming scheme.

● personal mobility* for example* through a single* location-independent address to reach the callee* even if the caller changes the terminal.

● Terminal type of consultation and choice: the caller can choose how to reach the other side is given* for example* via the Internet* telephone* mobile phone or answering services* etc..

● Terminal capability negotiation

● caller and callee authentication

● type of call without the knowledge and guidance on the transfer of

● multicast session invitation

When a user wants to call another user* the caller with the initial call INVITE request* the request contains enough information to participate in the session the callee. If the client knows the location of the other party that it can directly send a request to the other party's IP address. If you do not know* the client sends requests to a locally configured SIP network server. If the server is a proxy server will parse the user's position has been called and will send the request to them. There are many ways to complete the step* for example* DNS or access a database search. The server can also be a redirect server*
It can be called to return the user's location to call the client to contact it directly with the user. In the process of positioning the user* SIP network server can certainly redirect calls to other agents or servers* until it reaches a clear understanding of what is called the user IP address of the server.

Once a user address* the request will send to the user* at this time will have several options. In the simplest case* the user's phone client to receive requests - that is* the user's phone ringing. If the user accepts the call* the client with the specified client software and the ability to respond to a request to establish a connection. If the user declines the call* the session will be redirected to a voice mail server or another user. "Designated capabilities" refer to the user want to enable the function. For example* the client software can support video conferencing* but users want to use audio conference* that will only enable audio functionality. China Network Forum bbs.bitsCN.com

SIP also has two other significant features. The first one is stateful SIP proxy server has a partition into the call or the ability to copy the incoming call* which can run several extended branches. The first branch of responses to accept the call. This feature in the user working in between the two locations (such as laboratory and office)* or the same time ringing the manager and his secretary* when it is very convenient.

The second feature is SIP's unique ability to return to the different media types. To give an example of the user contact the company. When the SIP server receives client connection requests* it can through the interactive voice response WEB page to return to the customer's client machine* the page with a branch or department available to provide the list of users. Click on the appropriate link will be sent a request to the Click to select the user in order to establish the call.

4* SIP messages the composition of the

There are two types of SIP messages:

● Request: sent to the server from the client machine

● Response: From the server sends to the client

SIP request message contains three elements: a request line* headers* message body.

SIP response message contains three elements: the status line* headers* message body.

Request line and header fields based on business* address* and protocol characteristics define the nature of the call* the message body independent of the SIP protocol and can contain any content.

SIP defines the following methods:

INVITE - inviting users to join the call.
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