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  • What is SIP: Explain It!
  • What is SIP: Explain It!

    March 20th, 2017

    Many good call center managers have taken on the mission of exploring telecom options for their company, only to get lost within the blizzard of acronyms and jargon that seem designed to frustrate and intimidate the non-technical professionals among us.  Even though your aims might be very straightforward (e.g. I want to reduce the amount of money I’m spending on phone services without sacrificing call quality), this foreign language can quickly knock you off course.

    SIP, or Session Initiated Protocol, is one of the terms which seems to generate the most questions these days, due to its increasing popularity within the IP telephony space.  But what exactly is SIP?  And how does it work?

    SIP Basics

    All data that is transmitted on the internet is encoded into “packets,” whether it’s video, sound, or instant messages.  In the early days, the order in which packets were sent didn’t matter, as long as each packet reached its destination.  If you were downloading a song from a website, for instance, it would be chopped up into hundreds of small packets and transmitted to your computer, where it would be reassembled once all the packets arrived.  However, conducting a phone conversation requires a higher level of order and precision, since it involves two streams of data packets passing between each caller in sequence.

    To manage this process, signaling protocols were needed to establish, maintain, and end the call.  SIP is a signaling protocol for internet telephony, instant messaging, and internet conferencing to control multimedia sessions and establish the location of each caller.  SIP can set up, modify, and end sessions.


    Imagine a pair of kindergarteners named Ethan and Olivia, two kids who live in the same neighborhood and want to have a playdate.  Since they’re not old enough to handle this on their own, Ethan needs to let his mom know that he wants to have Olivia over, while Olivia needs to let her dad know that she wants to play at Ethan’s.  In SIP, this would be considered the signaling phase, where Ethan’s mom will then talk to Olivia’s dad to extend the invitation.

    In SIP, when one caller (often referred to as an agent) initiates a call, it sends an invitation signal to the recipient’s server, which checks to see if that person is available.  If they are, an OK response signal is then sent back to the caller.  Once acknowledged, the two parties can begin speaking, instant messaging, or video conferencing.


    Of course, there is also a protocol to the playdate, information that the parents need to communicate with each other when the arrangements are made.  Olivia’s dad needs to know where Ethan’s house is located so that he knows where to take her.  And since Olivia has a peanut allergy, her dad must ensure that they have safe snack options at Ethan’s house.  Ethan has a cast on his arm, so his mom says they won’t be jumping on the trampoline in the backyard.  Basic ground rules are covered to ensure that Ethan and Olivia have a safe and successful update.

    In SIP, more technical information needs to be determined, such as:

    • How will one party locate the other party?
    • Which codec should be used? (Codecs refer to how the packets are encoded and decoded)
    • How will the IP signaling packets be created and transmitted?
    • How will the communication be managed?

    The answers to these questions are carried within the signaling invitation and sets the stage for a secure connection.

    Connection and Termination

    When Olivia’s dad drops her off at Ethan’s house, the playdate starts.  On a SIP call, the signaling is designed to establish a connection between two parties so that the exchange of media packets (i.e. the conversation) can begin.

    Olivia’s dad returns home, but he knows that Ethan’s mom is overseeing everything, even though she isn’t actively involved.  However, if one of the kids begins feeling ill or they stop getting along, he may receive a signal from Ethan’s mom that they should end the playdate.  When Olivia’s dad arrives and picks up his daughter, the playdate is considered over.

    On a SIP call, there is a steady stream of signaling packets and media packets being exchanged, with the signaling packets ensuring that the call is active and the media packets containing the voice and multimedia data.  When one caller decides to leave, an “ending now” signal is sent to the other caller’s server, which will send an OK response signal.  This ceases the transmission of both signaling and media packets.

    The innovative nature of SIP’s signaling protocol is one of the reasons behind SIP’s meteoric rise. In the next part of this explainer series, we will look at why more and more businesses are choosing SIP for their telecom needs.

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    What is SIP: Explain It!