Q-Lite Satellite Modem Installation and Operating Handbook
6-65
performance at the application level suffers due to excessive buffering of packets during
periods of congestion, leading to extremely high latency levels with old data being kept
almost indefinitely in the hope that extra bandwidth will become available and any
overload will ease.
Most buffer management in TCP/IP devices is passive and relies on the user setting
internal buffer sizes to be consistent with the needs of the application and its data rates.
However, TCP/IP is bursty by nature and if the rate of arrival of packets at the satellite
modem exceeds its transmission capabilities then packets start to back up and will
eventually get dropped if the overload continues. The problem with this is that the end-to-
end packet delay can vary greatly and data becomes increasingly stale as the backlog of
packets to be transmitted builds up.
Active Queue Management continually measures the packet delay through the modem
and rather than let the backlog of packets build up, it ensures that the delay through the
modem is kept constant by dropping packets early if required. (The modem implements a
form of active queue management called CoDel, which stands for Constant Delay. The
delay has been preset to 5ms, from packet ingress to egress, which is suitable for most
applications. It can be changed ‘under the hood’ – please contact Customer Support for
details.) The effect of this is that transit times through the network typically continue to be
constant even in an overload situation.
The use of AQM can be combined with traffic shaping to ensure that high priority traffic is
unaffected when demands on bandwidth are exceeded.
The use of AQM is especially important for latency sensitive applications. It needs to be
enabled on the modems at both ends of the satellite link in order to be effective.
DHCP and NAT
The modem supports the automatic allocation of IP addresses to
network devices using DHCP, combined with the ability to hide all
of these addresses behind an optional NAT firewall. With DHCP
and NAT enabled, this gives the ability for all of the near-side
network devices to appear as a single IP address when viewed
from the far end of the satellite link.
This is supplemented by the use of a caching DNS proxy server in
the modem that can reduce the number of satellite round trips
associated with connecting to the actual DNS server.
When DHCP is active, the modem’s M&C port has a separate IP
address to the modem’s IP traffic address. The modem IP traffic
address effectively doubles up as the DHCP server address and
proxy DNS server address in this situation.
