Cisco MWR 1941-DC - 1941 Mobile Wireless Router Software Configuration Manual page 24

Cisco IOS Software Features
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 1-3
Before RTP header compression:
20 bytes
After RTP header compression:
IP/UDP/RTP header
RFCs 2508 and 2509 describe a method for compressing not only the RTP header, but also the associated
UDP and IP headers. Using this method, the 40 bytes of header information is compressed into
approximately 2 to 4 bytes, as shown in
basis, the delay and loss rate are lower, resulting in improved performance.
The MWR 1914-DC router network processor offloads both the compression and decompression of RTP
frames from the Cisco IOS software.
The MWR 1941-DC router can be configured to perform only IP/UDP compression, in which case the
Note
header is reduced from 28 bytes to 2 to 4 bytes.
Redundancy Support
In an IP-RAN application, to ensure availability, the backhaul links to an MWR 1941-DC router are
redundantly cabled to the VWIC-2MFT-T1-DIR/ VWIC-2MFT-E1-DIR cards. This card, designed
specifically for the MWR 1941-DC router, is a modified 2-port T1/E1 Multiflex VWIC with Drop and
Insert.The modifications include the addition of relays to activate the T1/E1 ports. The relays allow "Y"
cabling for router redundancy where the T1/E1 link is not redundant and default to open. The relays are
controlled by HSRP/redundancy protocol between the two routers connected to the same T1/E1.
Note If you choose to use the MWR 1941-DC router in a non-redundant configuration, you must close the
relays on the card using the standalone subcommand. Also, redundancy parameters are processed when
the router is booted up. These parameters cannot be changed "on the fly."
Cisco MWR 1941-DC Mobile Wireless Edge Router Software Configuration Guide
1-6
RTP Header Compression
8 bytes 12 bytes
IP UDP RTP
Header 20 to 160 bytes
2 to 4 bytes
20 to 160 bytes
Chapter 1
Payload
Payload
Figure 1-3. Because the frames are compressed on a link-by-link
Overview of the Cisco MWR 1941-DC Router
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