SIP is the most popular voice over IP standard (VOIP). SIP enables two or more people to make phone calls to each other using the INTERNET to carry the call. By using the INTERNET, you gain some distinct advantages over the PSTN (Public switched telephone network).SIP calls on broadbandare digital quality calls across the street and the globe. SIP to SIP calls are always free and calls to old PSTN phones are very cheap with no taxes or monthly charges. Because SIP calls are part of the INTERNET you get great boasts like phone numbers from many places in the world no matter where you live and free voicemail to email.

SIP phone is a SIP service. It is the compounding of all the elements necessary to provide inexpensive, simple and high caliber experience. It works with the hardware manufacturers, the various service providers and even government agencies to make sure you have a positive experience. To use SIP phone you need to have either a hardware adapter or download free soft phone, a broadband connection to the INTERNET, to make free SIP to SIP calls two sides of the call must have a SIP phone adapter or soft phone, to make super inexpensive calls to any of the billions of non-SIP phone (PSTN) phone, you will need SIP minutes, to receive calls on your SIP phone from non-SIP phone (PSTN) phones, you need at least one virtual number.

A SIP user agent (UA) is a logical network end point. It is used to create or receive SIP messages and thereby manage a SIP session.  A SIP phone is hardware or software grounded SIP user agent which performs call functions such as answer, hold/unhold, reject and call transfer. Each source of a SIP network, such as a user agent or a voicemail box, is identified by a uniform resource identifier (URI) based on the general standard syntax also used in web services and email.

The URI scheme used for SIP is sip:. If  secure transmission is required ,the scheme sips: is used and SIP messages should be transported via Transport Layer Security(TLS).In SIP, as in HTTP, the user agent may identify itself using a message header field ‘User-Agent’, containing a text description of the hardware/software/product involved. If the User-Agent field has sent request message means that the receiving SIP server can see this information. SIP network element sometimes depot this information, and it can be helpful in analyzing SIP compatibility problems.

SIP also determines server network elements. Although two SIP end points can communicate without any intervening SIP infrastructure, hence the protocol is defined as peer to peer. It is an important concept that the difference between types of SIP servers is logical not physical. SIP is a text based protocol with syntax same as that of HTTP. There are two different types of SIP messages: requests and responses. The first line of a request consists of a method defining the nature of the request, and a request URI, denoting where request should be sent. The first line of a response has a response code.

The Session Initiation protocol for Instant Messaging and Present Leveraging Extensions (SIMPLE) is the SIP based suite of standards for presence information and instance messaging. During an instant message session, files can be transmitted using Message Session Relay Protocol (MSRP).Some attempts have been made to incorporate SIP-based VoIP with the XMPP specification. Especially, Google Talk, that extends XMPP to support voice, plans to desegregate SIP. Google’s XMPP extension is addressed as Jingle. It is similar to that SIP acts as a Session Description Protocol carrier.

Though you can enjoy numerous advantages by using SIP phone, some disadvantages also exists. SIP’s peer to peer nature does not enable network provided services. The network cannot easily support legally mandated intercede of calls. Emergency calls are difficult to route. But there is a solution to this problem that is burrowing the media packets among TCP or HTTP packets to a relay. This is now implemented in SIP phones to rectify those problems. So without any fright you can use SIP phones and get benefited.