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Quantum and Evolution Series Installation and Operating Handbook
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In point-to-multipoint operation, the modem adds VLAN tags intelligently at the
Hub and then filters specific streams at each Remote based on those tags,
discarding the VLAN tags prior to outputting the packets.
The VLAN tags are used over just the satellite portion of the link where there is a
return path and VLAN tags are not already present in the data coming into
the modems.
In this mode, each remote modem is assigned a unique VLAN tag by the user
(4094 tags are available). The Hub Tx modem automatically learns which tags
are being used by each remote. It also learns which devices are connected to
each remote. It then adds a VLAN tag to each incoming packet, labelling it with
the correct VLAN id for the particular remote it is destined for.
At each remote, all packets are inspected and rejected unless they contain the
relevant VLAN tag for that particular remote, thus filtering out unwanted data. The
remotes remove the VLAN tags for onward transmission of packets onto the local
network at the remote. VLAN tagging by the modem is supported only in point-to-
multipoint mode (selected by setting the Ethernet traffic mode to Hub or Remote
as appropriate).
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The modem can apply TCP acceleration to a specific VLAN stream.
Select TCP acceleration as normal at the hub and remote modems. Set a VLAN id
at the remote modem that represents the data stream to be pulled off. At the hub,
set the VLAN id of the VLAN that you want to accelerate (note this is the full 16-bit
VLAN field value from the IEEE 802.1q header, not just the 12-bit VLAN id
portion). The IP traffic addresses of the modems must be on the same subnet as
the selected VLAN. Only packets coming in that have a matching 16-bit VLAN
field will be accelerated.
8.12.14
Adaptive Coding and Modulation (ACM)
Adaptive Coding and Modulation (ACM) uses feedback from the receiver to the
transmitter to respond to changes in channel conditions to optimise throughput, by
providing only the level of error protection that is required. ACM is a new Paradise feature
initially available in DVB-S2 mode.
By varying the error correction strength to match atmospheric conditions, link margin is
converted into useful bandwidth. Modulation and FEC rate (modcod) is dynamically
matched to the current Es/No. Symbol rate is kept constant, changing the terrestrial data
rate up or down with Es/No.
ACM can be used on point-to-point IP links. ACM must be on in both modems and
requires a satellite return channel (either DVB-S2 or SCPC). The return channel can be
used for user data as well.
Note that ACM can be used in both directions at the same time, maximising throughput
for outbound and return. Deployed links have reported throughput gains of up to 100%.
8-52
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