![]() ![]() You might better know and recognize them as rotary phones or dial-pad phones.Īs technology has developed, we’ve seen new technology and therefore new terminology appears faster than anyone can keep up - and Session Initiation Protocol (SIP) calling is just one of many. This was the case with analog or on-premise phones. Or the hours of manual installation time required to get those systems up and running. You pick up a handset, dial a phone number, and you are talking to anyone in any corner of the world.īut what they don’t see are the huge lengths of copper cabling that lie underneath making the connectivity possible. Each of the SIP entities, except the Stateless proxies, is a transaction user.For a layperson, phone systems might seem to be so straightforward. The layer above the transaction layer is called the transaction user. Stateless proxies do not contain a transaction layer. ![]() Any task that a user agent client (UAC) accomplishes takes place using a series of transactions. A transaction is a request sent by a Client transaction (using the transport layer) to a Server transaction, along with all responses to that request sent from the server transaction back to the client. All SIP elements contain a transport layer. It defines how a Client sends requests and receives responses and how a Server receives requests and sends responses over the network. Its encoding is specified using an augmented Backus-Naur Form grammar (BNF).Īt the second level is the transport layer. The lowest layer of SIP is its syntax and encoding. SIP is structured as a layered protocol, which means its behavior is described in terms of a set of fairly independent processing stages with only a loose coupling between each stage. The following figure depicts the roles played by each of the network elements in establishing a session. Only a proxy server or a redirect server can contact a location server. The location server provides information about a caller's possible locations to the redirect and proxy servers. We will discuss response codes later in this tutorial. The redirect server uses the database for getting location information and responds with 3xx (Redirect response) to the user. The redirect server receives requests and looks up the intended recipient of the request in the location database created by the registrar. So it sends a REGISTER request to the TMC’s Registrar server and the server returns a 200 OK response as it authorized the client. Here the caller wants to register with the TMC domain. Take a look at the following example that shows the process of a SIP Registration. It stores the URI and the location of users in a database to help other SIP servers within the same domain. It helps users to authenticate themselves within the network. The registrar server accepts registration requests from user agents. It can retransmit the request, if there is no response from the other side in time. Stateful Proxy Server − This type of proxy server keeps track of every request and response received and can use it in future if required. This type of server does not store any information of a call or a transaction. Stateless Proxy Server − It simply forwards the message received. There can be a maximum of 70 proxy servers in between a source and a destination. ![]() It has some intelligence to understand a SIP request and send it ahead with the help of URI.Ī proxy server sits in between two user agents. It is the network element that takes a request from a user agent and forwards it to another user.īasically the role of a proxy server is much like a router. SIP is based on client-server architecture where the caller’s phone acts as a client which initiates a call and the callee’s phone acts as a server which responds the call. User Agent Server (UAS) − The entity that receives a request and sends a response. User Agent Client (UAC) − The entity that sends a request and receives a response. User agents are logically divided into two parts − It could be a softphone, a mobile, or a laptop. User agents are the most intelligent device or network element of a SIP network. An endpoint can initiate, modify, or terminate a session. It is the endpoint and one of the most important network elements of a SIP network. In SIP, every network element is identified by a SIP URI (Uniform Resource Identifier) which is like an address. There are some entities that help SIP in creating its network. ![]()
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