On Wed, 17 Dec 1997, hany eldeib wrote:
> In addition, I understand that some LEO systems could involve additional
> delay due to on-board processing and switching between satellites. I have
> not seen reliable estimates of this delay but I understand that it could be
> significant taking into consideration that a typical message may need to
> pass through several LEO satellites, each contributing some processing
> delay.
Teledesic:
claim to be using non-blocking switches in the satellites that will
produce delays in the microseconds range (at least, that was claimed
for the original much-publicised 840-active satellite design. I've not
seen anything technical describing the Boeing 288-active-satellite
redesign.) Apparently using own connectionless protocols; not ATM.
Celestri (Motorola):
Each satellite is an ATM switch linked to six other satellites; delays
will be in the circuit setup, not in the ATM cell switching. Described
briefly in their June FCC application that you can download from their
website.
Skybridge (Alcatel):
No intersatellite links; no onboard switching, satellite is
transparent repeater. Routing is really a problem for the ATM switches
on the ground (as single-hop tunnels IP won't even care), and
Skybridge is really a ground network with satellites added. See their
FCC application of Feb 28.
Various GEO systems (Spaceway, Astrolink, Cyberstar, etc):
ATM switching of one variety or another.
On-board processing and switching is of fixed-length cells (ATM/
wrapped ATM/own design) rather than of variable-length IP packets
requiring lookup in large routing tables to go through. Adjust your
expectations of switching times accordingly.
Orbital geometry and ISL connectivity, ground station placement and
internetworking connectivity are likely to have considerably more
effect on the delay budget than OBP switching delay, IMO. TCP over
satellite people should really be considering effects of satellites'
IP over ATM tunnelling on TCP performance...
L.
and can anyone give me a list of 'satellite operators carrying
internet backbones'? For various Pacific islands, I suppose? TIA.
<[email protected]>PGP<http://www.sat-net.com/L.Wood/>+44-1483-300800x3641
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