Specifications
UHP-1000 SATELLITE ROUTER
INSTALLATION AND OPERATION MANUAL, V. 2.5
(SW VERSION 2.5.X)
© ROMANTIS 2012 47 www.romantis.com
2.10.10 Compressing RTP packet headers
When transmitting VoIP traffic by IP-protocol the size of headers of IP, UDP and RTP protocols (in total not less
than 42 bytes) can exceed the size of the voice portion of the packet.
Transmission of such packets via a satellite is extremely inefficient with respect to bandwidth usage, hence use is
made of compression of protocols headers based on the fact that only few fields change from placket to packet,
and furthermore this change can often be predicted.
In most cases the router compresses packet headers to up to two bytes. To make compression work, it is essential
that at the transmitting side (or at both sides) the ranges of UDP (destination) port packets that the router will try
to compress be adjusted accordingly. Compression will work also in simplex mode (in one direction only). Packets
passing successful compression will always be sent with high priority.
Statistics of the compression functioning can be made available using ”show rtp” command.
UHP# sh rt
Transmit --------------------------------------------------
Source_IP Destination_IP Plt Errors
10.0.0.17 10.1.0.1 11 0
Receive --------------------------------------------------
Source_IP Port Destination_IP Port TOS Plt Packets Errors
10.1.0.1 4001 10.0.0.17 16011 0 11 1171 3
Statistics show addresses, UDP ports, Payload Type and packets TOS. Errors counter increases as packets are lost
also in other cases when it is impossible to decode the packet compressed heading. The appearance of many
unexplainable entries in the statistics may mean that these ports are transmitting non-RTP traffic.
2.11 Traffic Shaper
Traffic shaper (TS) is intended for adjustment of the bandwidth occupied by data streams in transmitting via a
satellite. TS is based on the streams concept – controlled “pipes” for traffic between router and modulator. Each
stream is provided with three priority queues and a “hard” limitation in the traffic rate at the output, i.e. the
stream behaves practically in the same way as a modulator. Prioritization of the stream queues is also adhered to.
Packets from output streams arrive in the modulator queues with the same priorities (packets from LOW queue of
TS arrive in the LOW queue of the modulator, etc.). Altogether there are 500 streams.
Each map-record in the route Table can allow traffic via some of the TS streams. One stream can accept traffic
from different networks using several records including those with different SVLANs. If stream number is not
specified on map-records, then their traffic goes via stream 0. By default, bandwidth of this stream is set to
maximum. If the bandwidth is set to a lower value and it will be forgotten, a rather strange, at first sight,
modulator behavior may be observed – when the channel is not loaded but traffic cannot fully occupy it.
If the rate of traffic entering the stream is greater than the stream bandwidth at the given moment then traffic is
buffered to create delay during transmission.
Figure 21 Principle of operation of Traffic Shaper
IP Router
Modulator
LOW
ip map …
MED
HIGH
Bandwidth calculator
STREAM 1
STREAM 2
STREAM 3
STREAM N
delay
load