SatX/SE300—
ACM - Adaptive Coding and Modulation
Proprietary and Confidential
121
common modulation level for that group, so its transmission will be received by all the remotes in the
group.
When using ACM over L2 VLAN switching, careful planning of IP addressing, VLANs and routing is
require to allow ACM messages to be send from the remotes to the hub.
6.2.3
ACM in L3 IP Routing Forwarding Mode
In L3 IP routing forwarding mode, when activating the ACM, each remote can have a separate
modulation level based on that remote’s SNR value as reported back to the hub. Remotes can also be
divided into groups of remotes, with each remote reporting its SNR value, and the hub employing the
highest common modulation level for that remote or group, so its transmission will be received by all
the remotes in the group.
6.3
ACM Additional Parameters
ACM Mode is an Adaptive Coding and Modulation mode used to automatically adjust modulation
parameters to changing link conditions. The ACM Mode supports multi-receivers and changes the
modulation per remote, according to each remote’s link conditions. Remotes can also be grouped, so
the remote with the worst link condition within the group will be used to define the modulation for the
group, in order to prevent loss of data in all receivers.
6.3.1
Flow Control
The transmitter has an internal buffer and also uses Ethernet Flow Control (IEEE 802.3) to regulate the
transmit rate of the device (e.g. switch, router) connected to the modulator. When using ACM, the line
rate can decrease if the modulation parameters have modified because the link conditions deteriorate.
If that occurs, some Ethernet packets could be lost because of the congestion caused by the decreasing
line rate. Using flow control, the modulator can ensure no packet loss when the ACM changes
modulation parameters due to changes in the link conditions.
6.3.2
Communication between Hub and Remotes
For ACM to work properly, each remote unit needs to alert the transmitter about its SNR value. Based
on this value, the transmitted will use the optimal modulation for that remote. ACM Mode can work in
“point-to-point” mode (Figure 96 ) as well as “point-to-multipoint” mode (Figure 97). The
communication can be done over the air or via IP link, at the expense of a data overhead taken by the
ACM messages.