Cisco MWR 1941-DC User Manual page 4

Feature Overview
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PPP Multiplexing/Demultiplexing
Encapsulated PPP frames contain several bytes of header information, which adds overhead to a network
that is used to transport PPP frames.
RFC 3153 describes a way to overcome this overhead. On the sending end, a multiplexor concatenates
multiple PPP frames (subframes) into a single, multiplexed frame (superframe). One header is included
in the superframe and the individual PPP subframes are separated by delimiters. On the receiving end, a
demultiplexor uses the delimiters to separate the individual PPP subframes.
The MWR 1914-DC router network processor software conforms to this specification and acts as both a
multiplexor and a demultiplexor.
RTP/UDP Header Compression
RTP is a protocol used for carrying packetized audio and video traffic over an IP network. RTP, described
in RFC 1889, is not intended for data traffic, which uses TCP or UDP. Instead, RTP provides end-to-end
network transport functions intended for applications with real-time requirements (such as audio, video,
or simulation data) over multicast or unicast network services.
In an RTP frame, there is a minimum 12 bytes of the RTP header, combined with 20 bytes of IP header,
and 8 bytes of UDP header. This creates a 40-byte IP/UDP/RTP header. By comparison, the RTP packet
has a payload of approximately 20 to 160 bytes for audio applications that use compressed payloads.
Given this ratio, it is very inefficient to transmit the IP/UDP/RTP header without compressing it.
Figure 2
Before RTP header compression:
20 bytes
After RTP header compression:
IP/UDP/RTP header
4
cRTP/cUDP
Link Noise Monitoring (LNM) provides configuration monitoring of individual T1/E1 circuit
quality
RTP Header Compression
8 bytes 12 bytes
IP UDP RTP
Header
2 to 4 bytes
Cisco MWR 1941-DC Router with the Cisco IOS IP-RAN Feature Set
Payload
20 to 160 bytes
Payload
20 to 160 bytes