98-175666-C
C-1
Appendix C
Miscellaneous
C
C.1 Performance optimization for blockage
C.1.1 Encountering blockage, configured with multiple satellites.
Definition of blockage
In most VSAT installations the VSAT antenna is installed in a position with areas of
blockage. Blockage is often caused by the vessel's masts, stacks and other equipment
installed on board. During installation the blockage areas should be entered in the
web interface, see
When blockage occurs, the BDU can inform the VSAT modem (if the blocking zones
have been typed correctly into the BDU web interface). It is often seen that by not
informing the VSAT modem of blockage the VSAT system gains a higher uptime,
although the quality of the extra gained uptime is not good enough to give the user a
stable data connection. Therefore it is not of any value to the user. Another
disadvantage of not informing the VSAT modem of blockage is that the VSAT modem
does not have the option to switch to a different satellite to avoid the blockage.
VSAT modems can typically
2
only receive one signal from the BDU, which is "TX-mute"
/ "modem must not transmit", they are therefore not able to perform fast switching,
but are limited to use a simple time-out, which is configured as a fixed value in the
modem configuration.
The simple time-out means that there is a fixed delay, plus the time needed to acquire
another satellite, before there is a chance of regaining good link performance.
If the signal is not sent to the modem, the system can in some cases remain linked and
have a higher uptime, but not provide a stable data connection. Such a link is of no
value to the subscriber. Not sending the blockage (TX-Mute) signal also extends the
period of the poor-quality link, as the VSAT modem is still relying on a time-out before
switching to another satellite, and the time-out may be constantly reset by the link
coming and going.
Better blockage communication
A major disadvantage of this single signal is that if the VSAT modem has multiple
satellites to choose from, then, when selecting a new satellite, the VSAT modem is
again relying on the simple time-out. This continues until a satellite with no
obstruction in the view from the satellite terminal is selected. If the VSAT modem was
able to receive information from the BDU that the view towards the current satellite is
blocked, it would be able to choose a visible satellite much faster without the need to
wait for multiple time-outs.
2. There are VSAT modems that can interpret more detailed information about
blockage from the BDU. This allows for increased performance in the event of
blockage.
