Cisco Systems 78-13983-04 manual PPP Multiplexing/Demultiplexing, RTP/UDP Header Compression

Chapter 1 Overview of the MWR 1900

Software Features

The following features are supported in the network processor:

•MAC Classify

•ICMP

•FIB (CEF)

•Load-balancing

•MAC Rewrite

•QoS Matching, including IP Access Lists (Input/Output Security ACLs are not supported), QoS Group, IP Precedence, IP DSCP, and Input Interface

•QoS Actions, including Set IP Precedence, Set IP DSCP, Set QoS Group, Traffic Shaping, Class Based WFQ (CB-WFQ), and Low Latency Queuing (LLQ)

•Maintenance of statistics, such as Forwarding, Drop, and Interface

•IPv4

•MLPPP

•MLP, PPP Data Path (MLP LFI is not supported)

•PPPmux

•cRTP/cUDP

PPPMultiplexing/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 1900 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.

Cisco MWR 1900 Mobile Wireless Edge Router Software Configuration Guide

78-13983-04 1-3