Information Technology Reference
In-Depth Information
CUCM
SCCP
SCCP
RTP
Figure 14-16
SCCP
RTP and RTCP
In VoIP, RTP transports audio streams. RTP is a transport layer protocol that carries digi-
tized voice in its payload. RTP was initially defined in RFC 1889, and the current RFC is
3550. RTP runs over UDP, which has lower delay than TCP. Because of the time sensitivity
of voice traffic and the delay incurred in retransmissions, UDP is used rather than TCP.
Real-time traffic is carried over UDP ports ranging from 16,384 to 32767. The only re-
quirement is that the RTP data be transported on an even port and that the RTCP data be
carried on the next odd port. RTCP is also defined in RFC 3550. RTCP is a session layer
protocol that monitors the delivery of data and provides control and identification func-
tions. Figure 14-17 shows a VoIP packet with the IP, UDP, and RTP headers. Notice that
the sum of the header lengths is 20 + 8 + 12 = 40 bytes.
IP Header
(20 bytes)
UDP Header
(8 bytes)
RTP Header
(12 bytes)
CODEC Sample
(10 bytes)
CODEC Sample
(10 bytes)
Figure 14-17
IP, UDP, and RTP Headers of a VoIP Packet
WA N l i n k s u s e R T P h e a d e r c o m p r e s s i o n t o r e d u c e t h e s i z e o f v o i c e p a c k e t s . T h i s i s a l s o
called Compressed RTP (cRTP), which is defined in RFC 2508. As shown in Figure 14-18,
cRTP reduces the IP/UDP/RTP header from 40 bytes to 2 or 4 bytes (a significant de-
crease in overhead). cRTP happens on a hop-by-hop basis, with compression and decom-
pression occurring on every link. It must be configured on both ends of the link. It is
recommended for slow links up to 768 kbps. cRTP is not used much anyone because slow
WA N l i n k b a n d w i d t h s a r e s e e n l e s s . H i g h e r s p e e d l i n k s are not recommended because of
the high CPU requirements and they reduce call quality.
40 bytes of headers
IP Header
(20 bytes)
UDP
(8 bytes)
RTP
(12 bytes)
Payload
(variable)
After CRTP
Header
(2 or 4 bytes)
Payload
(variable)
Figure 14-18
cRTP
