ND SatCom SKYWAN IDU 7000, Manual

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30 Network Design and Engineering Guide 2010-10-26
General Carrier Design
Essential SkyWAN
®
Satellite Link Layer Features
TDMA Superframe
Another TDMA frame option is the definition of a superframe size. By default (superframe
size=1) every station has to transmit a request burst in every frame. In a network with many
stations this could consume many base slots on the respective carrier leaving few slots left for
data bursts. This effect can be reduced by the superframe mechanism. This mechanism allows
to split and distribute the request bursts, normally transmitted in one frame, to several frames
of one superframe (sizes from 1 to 16 are applicable).
- Advantage: The overhead caused by request bursts is significantly reduced.
- Disadvantage: In the event of a sudden burst of data traffic, the time for the next transmis-
sion of request bursts to the master station needs more time than without superframing.
Figure 2-11 TDMA Superframes
In figure 2-11 the frame structure for superframe sizes of 1 and 6 is presented. Choosing a su-
perframe size of 6 in this example will provide 5 additional data slots. A larger super frame size
would not make sense here because 1 base slot is always needed for the request bursts.
2.3.4 Downlink and Uplink Populations
Sky WAN
®
stations within a network can be grouped according to the carrier on which they re-
ceive the reference burst from the master and on the carrier on which they transmit request
bursts to the master.
- ’Downlink Population <N>’ : Set of all stations which receive the reference burst on carrier
number <N>, i.e. all stations with Home Channel One configured to carrier <N>. The mas-
ter station(s) must belong to Downlink Population 1. The maximum number of
Sky WAN
®
stations in one downlink population is 255.
- ’Uplink Population <N>’ : Set of all stations which use carrier number <N> to transmit re-
quest bursts to the master. By default all stations are using carrier number 1 to transmit
request bursts to the master, i.e. they all belong to Uplink Population 1. The maximum
number of stations in one uplink population is 255.
In ’Dual Uplink Beam’ (DUB) mode there is an Uplink Population 2 which comprises all stations
which cannot use carrier 1 because they have no access to the carriers on this (looped) satellite
transponder. For these stations a carrier 2 on a different (cross-strapped) transponder will be
allocated. This carrier will be received by a second demodulator on the master station(s). The
master station(s) will always be members of Uplink Population 1. The maximum number of sta-
tions in a
Sky WAN
®
network using MRB-DUB mode is 510.