Q-Lite Satellite Modem Installation and Operating Handbook
6-73
6.2.4.25 Stream Tx Frame Size
Short
This represents a frame size of 16,200 bits per frame.
Normal
This represents a frame size of 64,800 bits per frame. This is more
bandwidth efficient than short frames but has four times the
latency. As a guideline, short frames have a latency of around
25ms at 1Mbps, whereas the latency for normal frames is around
100ms at 1Mbps. The latency will halve as data rate doubles.
Very short
This represents a frame size of 5,400 bits, reducing latency to 33%
of the standard DVB-S2 Short frame.
Ultra short
This represents a frame size of 3,240 bits, reducing latency to 20%
of the standard DVB-S2 Short frame.
Table 6-67 Stream Tx Frame Size
6.2.4.26 Tx/Rx Symbol Rate
The calculated transmit and receive symbol rates, aggregated for all streams, are
displayed in order to allow the user to verify that the streams have been set up correctly.
6.2.4.27 Point-to-multipoint Operation
By default it is assumed that the modem is being used in a point-to-point mode. It is also
possible to use the modem as a remote modem in a point-to-multipoint system involving
the
Q-MultiFlex™
. The operation of the modem with respect to packet processing is
different in each case and therefore it is necessary to explicitly set the required mode.
Point-to-multipoint multistreaming is where the Q-Flex receives a shared outbound
consisting of multiple modcods (where each stream in the outbound is using a different
modcod from the other streams). For example, one remote may receive 8PSK ¾ while
another receives QPSK 9/10 and another receives 16APSK 8/9, all as part of the same
shared outbound carrier from the hub. (The shared outbound is partitioned using traffic
shaping in the Q-MultiFlex, which allows a number of streams to be created, each
associated with a particular modcod.) To demodulate just one of the streams (modcods)
in the outbound, the
Point-to-multipoint-multistream
setting is used – see below. The Q-
Flex needs to be set to receive the entire outbound carrier (in terms of receive symbol
rate) but the normal receive modcod selection should be for just the modcod of the
stream that is to be demodulated. In addition, alongside the point-to-multipoint option on
the
Edit->IP->Advanced
menu, there is a
Stream identifier
setting, which should be set to
the required stream number (as entered via the traffic shaping feature of the Q-MultiFlex
that is generating the shared outbound carrier).
