Q-Lite Satellite Modem Installation and Operating Handbook
7-20
•
You can send M&C packets to port 6701 on the local modem at any time when
using the ESC channel in IP mode, without having to set the ‘
Remote M&C
interface
’. If you just want to bridge IP packets then you can ignore the remote
M&C interface setting.
To summarise the ESC bridging options:
•
IP can be used over the ESC and the main data channel at the same time.
•
With the M&C port in the bridge and IP selected for both the ESC and main data
channel, the M&C port is now bridged to the ESC channel
not the IP traffic port.
The IP traffic port is still bridged, but only to the satellite port (when the ESC is not
being used, bridging the M&C port bridges it to the IP traffic port and the satellite
port)
Note that the ESC channel is used to support the AUPC feature but this does not prevent it
being used for other purposes at the same time (the AUPC messages are just multiplexed
in with the other data).
7.8.7 IP Interoperability
Over satellite, the modem uses HDLC to encapsulate the Ethernet frames or IP packets
(unless DVB-S2 is being used, in which case another form of encapsulation is used such
as MPE or ULE). If Cisco HDLC is selected on the modem then the received IP stream can
be forwarded out of a serial interface to a serial router that supports Cisco HDLC, which
can convert it back to IP.
In general different modem manufacturer’s IP, even when using HDLC, is not interoperable
as there is no standard defined in this area. If instead DVB-S2 is used, then generally IP is
interoperable between different manufacturer’s modems, so long as incompatible IP
features are not being used such as compression or acceleration.
7.8.8 IP Connectivity Modes
The modem software supports unidirectional and bidirectional point-to-point IP operation.
7.8.9 TCP Acceleration
TCP traffic, as opposed to UDP, requires acknowledgements to be returned to the sender
as part of the protocol flow control process. TCP was never intended for systems with long
delays (such as satellite). With no acceleration, TCP traffic over satellite would limit itself to
a few hundred kbit/s (depending on the PC TCP window size),
regardless of the actual
traffic bandwidth available over satellite.
Because the satellite delay is taken as evidence of
link congestion, TCP throttles back the amount of data it sends. Acceleration allows approx
90% utilisation of whatever the available traffic bandwidth is over satellite. Acceleration can
be used in point-to-point and point-to-multipoint bridging modes, as well as in routing mode.
When acceleration is used in bridging mode, all UDP packets are bridged. VPN packets,
although they use TCP, cannot be accelerated because acceleration relies on making
changes to the addresses in the original IP packet which is encrypted by the VPN as the
