>>I was confused whether this is referring to the incoming and outgoing
>>using the *same* satellite link, and the asymmetry comes from uplink/
>>downlink capacity asymmetries; or if they're using different satellite
>>links.
>
>Like with mobile phones, the forward and reverse links are discrete.
>Generally, one of these is *much* smaller, as it is/was generally expected
>to be supporting only housekeeping and control operations.
The issue of Asymmetry is a big one with current and new networks for
Internet Access via satellite.
For simply doing a WAN connection via satellite the links will usually be
symetrical, with equal bit rates in both directions (over the same
satellite,
via one or more carriers using SCPC or TDMA access).
The asymetrical case becomes more complicated where you have an
internet via satellite service offering. In these cases there is usually a
TDM outbound carrier, which is intended to be received by thousands,
or tens of thousands of users. This carrier will be a big fat one that will
probably saturate the transponder to allow the users to minimize their
antenna sizes.
The return channel for these types of systems will include:
- telephone dial up direct to the uplink site
- telephone dial up via an intermediary ISP
- satellite return link via TDMA VSAT (contention based access)
- satellite return link via Inmarsat or MSAT (very narrow band channel)
- satellite return link via Ka-Band (ala Astra, CyberStar et al), again a
TDMA access scheme with contention and reservation based access.
For the dial up cases the BW will be fairly consistent.
For the TDMA cases, the bit rate available for the return channel may vary
from
frame to frame (i.e. every few hundred milliseconds), and thus the delay on
the
return channel may vary from simply the propagation delay to several seconds
while the packet waits for a chance to access the satellite.
All these systems are available commerially now. All are in service now. We
have
heard some comments from some users and suppliers of these systems.
In general, we cannot say that all satellite TCP will be symetrical, or all
will be asymetrical. There are many variations .... and will probably have
many more interesting cases when the LEO's come on board.
Ian
This archive was generated by hypermail 2b29 : Mon Feb 14 2000 - 16:14:44 EST