What is SIP?

sip the session initiation protocol a
protocol to initiate a session but what
is a session and why do we need a
protocol to get one started well we're
talking about a communication session in
fact an IP communication session using
IP devices and an IP network at its
heart sip is a signaling protocol to set
up an IP communication session now when
we think about it virtually all
communications begin with signaling even
a face-to-face conversation
davia wants to talk to Nathan tent by
calling over hey Nathe
a sound wave carried through the air
recognising the invitation to talk Nathe
signals back to accept what's up with
Dave's invitation signal answered with
Nathan's acceptance signal both parties
are ready to converse they begin a
dialogue and exchange of media in this
case more sound waves traveling through
the air conversation complete Dave
issues another signal this time to end
the session and just to be polite
Nathan signals back confirming he knows
the session is ending signaling in an IP
world follows the same principles
instead of signaling with an audio way
through the air IP devices signal with
IP packets over the IP network now
there's lots of different types of IP
packet each with their own function and
content the important thing is that both
sender and receiver understand the
packet construct and what to do with its
data in IP communications there are two
types of IP packets there's signaling
packets and immediate packets back is to
establish or set up the session and
media packets to then convey the audio
the video the whiteboarding data and so
on with a voice session the analog wave
on one side is encoded into ones and
zeros that are put into media packets
and then sent out over the IP network on
the other side the ones and zeros are
unpacked and decoded the wave
reconstructed and played back to the
user n coding and decoding also applies
to video images from the
camera are encoded the binary digits
packaged in media packets and sent out
over the IP network at the other side
again the data is decoded the image
reconstructed and then displayed to the
user now every second of conversation
has many many media packets audio and
perhaps video packets that convey the
real-time conversation simple enough but
for this media exchange to take place
we're assuming two important things have
already been sorted that both parties
know each other's location they know
where to send their media packets and
that both parties are using the same
codecs for encoding and decoding the
media so here's the big question how do
we locate or find the other users IP
address and how do we decide which
codecs to use we need a process some
descriptions and rules that help us
locate each other and agree on codecs
what we need is a protocol and that's
where sip comes in the session
initiation protocol it's essentially a
rulebook that describes how to locate
the other party and which codec to use
for encoding and decoding media it also
defines how to construct and send IP
signaling packets to set up the call and
what to do to then manage the call so
now if Dave wants to use his sip phone
to call Nathe on his sip phone Dave's
phone follows all the steps defined by
sip to construct a special IP signaling
packet a sip packet populated with all
the data needed to set up the call and
send it out over the IP network on the
other side Nathe sip phone understanding
the rules of sip recognises the packet
as an invitation to talk and knows to
alert Nathe by playing the phone's
ringing sound more signaling packets are
exchanged we'll talk about those later
then with signalling complete and codecs
determined the phone's get busy
exchanging media packets that digitally
convey the conversation then sometime
later one of the parties signals the end
of the call and both stop sending
Media the call is terminated simple now
understanding the essence of sip is
really quite easy it's just signaling to
set up and manage IP communication
sessions understanding its full
potential however and knowing enough to
fully implement real-life sip solutions
and to support real IP communications
networks is something else there's much
to learn many complexities to unravel
and understand but worry not that's why
we're here that's the purpose of this
course by the time we're done you'll be
a sip expert it will all make good sip
sense