Multiservice Networking, IPX Networking, and Security - CCIE Routing and Switching Exam Certification Guide - page 550

particular IP precedence. You can also use RTP header compression on WAN links to reduce
the size of voice packets. It is also referred as Compressed RTP (CRTP). RTP header compres-
sion is enabled in serial interfaces with the ip rtp header-compression command. The maxi-
mum number of compression sessions is controlled with the ip rtp compression-connections
number command. As shown in Figure 12-2, CRTP reduces the IP/UDP/RTP header from 40
bytes to up to 4 bytes in length—a significant decrease in overhead. CRTP is done on a hop-by-
hop basis with compression and decompression occurring on every link.

Compressed RTP (CRTP) Figure 12-2

Before CRTP

40 bytes of headers

IP header
(20 bytes)

UDP
(8 bytes)

RTP
(12 bytes)

Payload
(variable)

Header
(2 or 4 bytes)

Payload
(variable)

After CRTP

Codecs

Codecs transform analog signals into a digital bit stream and digital signals back into analog
signals. In this case, it specifies the voice coder rate of speech for a dial peer. Figure 12-3 shows
an analog signal that is digitized with a coder for digital transport. The decoder converts the
digital signal into analog form. Codecs are presentation layer protocols.

Codec

Figure 12-3

Analog input
signal

Digital coded
signal

Analog output
signal

Coder Decoder

The default codec setting for VoIP is G.729 ( g729r8 ). Some other codec standards are listed in
Table 12-2.