D-Link DI-1750 Reference Manual page 409

Note: You must enable CRTP on both ends of a serial connection, otherwise, it's unavailable.
9.3.2 Brief Introduction Of CRTP
The Router CRTP is only supported on serial lines using PPP encapsulation currently, and we will
support on Frame Relay, HDLC encapsulation and over ISDN interfaces in the future.
You should configure CRTP if the following conditions exist in your network:
♦ Slow links
♦ The need to save bandwidth
CRTP is a very effective way to the above situations, you can see the advantage from the figure below:
In the best condition, the IP header without options is 20 bytes and the minimum size of the RTP
header is 12 bytes, adding 8 bytes of the UDP header, the total bytes of these headers is 40 bytes;
when sends the RTP payload with two frames every time in G.729 call, then the payload is 20 bytes;
adding 4 bytes PPP link layer encapsulation, the payload ratio is only 31.25 percents.
Using RTP Header-Compression (cRTP) and ignoring the UDP check sum these headers can be
compressed to 2 bytes, adding 4 bytes PPP link layer encapsulation, the payload ratio is 76.92
percents, there is a very big gap between them.
Note: CRTP should not be used on links greater than 2 Mbps.
9.3.3 Enable CRTP On A Serial Interface
To enable CRTP header for serial encapsulations PPP, use the following command in interface
configuration mode: (You must enable compression on both ends of a serial connection.)
Command
ip rtp header-compression
[{ cisco-format | iphc-format |
passive}]
The optional parameter cisco-format, which specifies IPCP to adopt packets with D-Link format when
CRTP is applied to PPP links, is a default value; If you include the passive keyword which specifies
IPCP to adopt packets with D-Link format when CRTP is applied to PPP links, the software compresses
outgoing RTP packets only if incoming RTP packets on the same interface are compressed; the key
word iphc-format specifies IPCP to adopt packets with RFC2509 format when CRTP is applied to PPP
links
[DEFAULT@Router /config/]#interface
Key Word:
U(undo) D(default) Q(quit)
(00)FastEthernet FastEthernet interface
Model Name
Purpose
Enable CRTP
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