2. Traditional telephony challenges
´ Capacity, 1xE1=2Mbps=30-31 calls (depending on signaling)
´ Lower diversity, a need to have end-to-end links. Low portability for end-
users
´ Cost
E1 E1
3. What is IP telephony?
Transfer of audio and/or video over IP from one endpoint to another
´ Capacity, 1xE1=2Mbps=124 calls (depending on
settings)
´ Full diversity of IP networks
´ Lower cost
4. SIP architecture
´ SIP signaling
´ RTP (real-time transport protocol) – transports the voice and video over IP
networks.
´ RTCP (real-time control protocol) – monitors transmission statistics and
quality of service (QoS). Uses ~5% of RTP
´ SDP (session description protocol) – negotiates technical data for RTP
6. RTP (real-time transport protocol)
´ Audio and video runs separately
´ Mostly use unprivileged UDP ports (1024 to 65535)
´ The audio payload formats include G.711, G.723, G.726, G.729, GSM and
other codecs
´ The video payload formats include H.261, H.263, H.264, MPEG-4 etc
7. SDP (session description protocol)
It’s a protocol that describes the media of a session.
Session description
v= (protocol version number, currently only 0)
o= (originator and session identifier : username, id, version number, network address)
s= (session name : mandatory with at least one UTF-8-encoded character)
c=* (connection information—not required if included in all media)
b=* (zero or more bandwidth information lines)
Media description (if present)
m= (media name and transport address, and set of codecs)
i=* (media title or information field)
c=* (connection information — optional if included at session level)
b=* (zero or more bandwidth information lines)
k=* (encryption key)
a=* (zero or more media attribute lines — overriding the Session attribute lines, with codec and sampling rate)
[Request]
v=0
o=Makara 2890844526 2890844526 IN IP4 10.120.42.3
s= KHNOG
c=IN IP4 10.120.42.3
m=audio 49170 RTP/AVP 0 8 97 (Real-time protocol, Audio-video profile)
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 51372 RTP/AVP 31 32
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
[Answer]
v=0
o=Vibol 2808844564 2808844564 IN IP4 10.120.32.12
s=KHNOG
c=IN IP4 10.120.32.12
m=audio 49174 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 49170 RTP/AVP 32
a=rtpmap:32 MPV/90000
13. VoIP related features
´ VAD (voice activity detection) to not transfer packets if no voice
´ CNG (comfort noise generation) to create a background noise in the
phone
´ ALG (application layer gateway) to support NAT
´ RTP header compression