COBHAM Sailor 250 FleetBroadband, Training Manual, Page: 25 / 270

15
SAILOR FleetBroadBand
Training Manual
1
The air interface can operate with a coding rate as low as 1/3 (i.e.
sending two bits of redundancy in addition to each bit of data)
and as high as nearly 1 (i.e. negligible redundancy). We can thus
operate over a wide range of link conditions from 42.0 dB C/No in
the worst case (for decoding the global beam forward bearer with
the class 14 FleetBroadband Terminal at the edge of coverage) to >
71.0 dB C/No in the best case (for decoding the narrow beam traffic
bearer with the class 8 FleetBroadband Terminal in the centre of
beam, centre of coverage). Although there is no scope to vary the
data rate delivered by a circuit switched connection such as voice
or ISDN, link adaptation ensures that circuit switched connections
consume as little capacity as possible to meet the target grade
of service. For variable bit rate packet switched connections (i.e.
background and interactive class IP), link adaptation constantly
delivers a peak bit rate to each connection as high as permitted by
its link quality. The target loss rate of 0.1% is not appropriate for
some types of traffic. Voice traffic is fine – error concealment in the
voice codec masks lost frames. ISDN needs a bit error rate of less
than 1 in a million (10
-6
), so we give ISDN data special treatment
to reduce the error rate.
Packet traffic is transmitted in “reliable mode”, (error correction).
½ i.e. any packets lost on the satellite link are automatically
retransmitted and re-ordered before being delivered. The residual
error rate after retransmission is thus negligible. Reliable mode will
incur a certain amount of random delay jitter when retransmissions
occur but this does not significantly add to the inherent jitter that
occurs in a packet data connection over a satellite link using TCP.
For users that need to minimize delay jitter on their IP traffic and
can tolerate a packet loss rate of 0.1% (e.g. certain real-time
applications or transport protocols) we offer a guaranteed bit rate
service that operates in unreliable mode. The RNC dictates what
code rate the FleetBroadband Terminal must use in the return
bursts to maintain the target packet error rate of 0.1%. Only if
a FleetBroadband Terminal has less data to send than the burst
size can it choose to lower the code rate and simultaneously
reduce the transmit power by an equivalent amount to achieve
the same target error rate. When retransmissions occur this does
not significantly add to the inherent jitter that occurs in a packet
data connection over a satellite link using TCP. For users that
need to minimize delay jitter on their IP traffic and can tolerate
a packet loss rate of 0.1% (e.g. certain real-time applications or
transport protocols) we offer a guaranteed bit rate service, (QOS)
that operates in unreliable mode (no error correction). The RNC
dictates what code rate the FleetBroadband Terminal must use in
the return bursts to maintain the target packet error rate of 0.1%.
Only if a FleetBroadband Terminal has less data to send than the
burst size can it choose to lower the code rate and simultaneously
reduce the transmit power by an equivalent amount to achieve the
same target error rate. This allows the FleetBroadband Terminal
to save some battery power.