ND SatCom SKYWAN IDU 7000, Manual

Page: 127 / 154

2010-10-26 Network Design and Engineering Guide 123
Data Networking
SkyWAN
®
Internet Protocol Features
The following figure 4-8 gives an overview of the necessary steps for header compression.
Figure 4-8 RoHC Feature Overview
If the compressor station detects that the IP flow mapped to a Platinum Dynamic aggregate can
be compressed it will prepend the IP packet for the next 2 packets with a RoHC header. The
receiving station will evaluate both the RoHC header and the IP/UDP/RTP header to establish
a context between the RoHC Call ID and the original header information. Now the compressing
station will not include anymore the IP/UDP/RTP header but will only transmit the RoHC head-
er. The decompressing station will use its stored context to recreate the original header before
forwarding the packet to its Ethernet port. The compressing station will regularly retransmit the
IP header to avoid context losses at the receiving station. This refresh interval may be config-
ured at each station.
4.2.10 Transmission Control Protocol Acceleration (TCP-A)
The throughput of an individual TCP connection over long delay links is limited by the fact, that
the sender has to wait a long time for acknowledgment from the receiver, before it can resume
sending further data. To overcome this limitation
Sky WAN
®
IDU supports a TCP acceleration
functionality.
Theoretically the maximum throughput of one TCP connection is given by:
TCP Throughput = window size / round trip delay
where the window size denotes the maximum amount of unacknowledged date which the send-
er is allowed to transmit. The round trip delay over geostationary satellite links is larger than
0.55 sec. The window size used by most computer operation systems is 16-64 kB. Therefore
a typical TCP session will have a maximum throughput in the order of 100-800 kbps even if the
channel bandwidth is substantially larger.
Another drawback of standard TCP is the use of cumulative acknowledgements. If a packet
gets lost over the satellite link not only multiple packets have to be retransmitted but also a rel-
atively large timeout has to expire before that retransmission starts. As packet losses due to bit