Q-Flex Satellite Modem Installation and Operating Handbook
7-33
Note that with DVB-S2 ACM, pilots are automatically switched on at operating points
where their use achieves the maximum throughput for a particular Es/No. For example,
for an available Es/No of 6.5dB, it is better to use short-block 8PSK 3/5 with pilots on
(spectral efficiency 1.69bits/s/Hz) than to use the most suitable short-block modcod with
pilots off, i.e. QPSK 5/6 (spectral efficiency 1.6bits/s/Hz). Pilots are sometimes viewed as
causing an undesirable increase in overhead but it should be remembered that they also
reduce the Es/No at which the modem can lock – this trade-off means that optimal
throughput is sometimes achieved only with the active use of pilots.
Note that ACM can be used with AUPC at the same time (providing automatic uplink
power control helps maintain the remote Eb/No at the target level).
Modcod changes are totally transparent at the receiver - it does not involve the
demodulator having to reacquire the signal. Modcod changes can be made instantly
whenever required, maximising throughput at all times.
Because the symbol rate is kept constant, any change in modcod will change the
terrestrial data rate. An increase in data rate does not need to be explicitly signalled to the
terrestrial network. A decrease does, because it could result in a buffer overflow in the
transmit modem if it receives more data than it can transmit. Upstream buffer control is
achieved by sending Ethernet Pause frames that define a period of time for which
transmission should be suspended. In principle, Pause frames are supported only for full-
duplex links and support for them in vendor equipment is optional, therefore the modem
auto-negotiates this capability with the attached equipment.
Paradise ACM is intrinsically tied to an IP traffic shaper and TCP accelerator, allowing
data grooming to match data priority to the available bandwidth. Without this, there would
be no effective data management of a dynamically varying channel, which would become
a free-for-all where you would not be sure which packets will be passed over satellite and
which will be dropped. Note that an external traffic shaper cannot be aware of the
instantaneous data rate in the modem and is therefore restricted to shaping for the
nominal data rate of the link (i.e. for a fixed modcod). (The Ethernet Pause frame does
not help in any way with external traffic shaping – its purpose is purely to minimize any
packet loss at the point in time when a modcod change occurs. It is therefore strongly
recommended to use the modem’s internal traffic shaping feature in conjunction with
ACM.)
Note that ACM works to the full symbol rate of the modem.
ACM menu on/off options are accessible once DVB-S2 is selected as the Tx or Rx
service (see the
Edit->Tx-Rx->Service
screen). ACM status can be viewed from the
Status
screen).
ACM is built into the modem and does not require the use of a separate controller box.
ACM is compatible with the use of Paradise’s
Paired Carrier
, allowing both carriers to use
the same space segment, further reducing bandwidth requirements.
