Prague
April 01, 1998
Dear Colleagues,
Mr. Fairhurst has written of an interesting point in suggesting the impact
of a small but friendly mass (ie., bird) perching & partially obscuring a
link such as, notably, a low power VSAT terminal(my addition), particularly
one of less than 2Mbps, and thereby signifcantly lowering the BER of that
VSAT link, albeit not for long. Hence the problem set instantly develops of
rapidly restoring TCP throughput over the degraded satellite link. Other
things, including passing aircraft or even a passing basketball, can lower
BER, too. Given the fact that international distance education applications
by satellite throughout the 2/3 of the planet not yet (nor soon to be)
'adequately' cabled, that they will undoubtedly be long done with RF links
of less than 2Mbps to geostationary satellites, obviously(!), and
increasingly with the new low power/low cost VSAT terminals, it would be a
significant addition to the practical results of this group's effort, I
believe, to study and determine how best to manage this kind of general
transient situation inexpensively.
This problem will become even more critical than it is today, as people
start to routinely install TCP/IP compression on framed IP links before
transmission into the satellite RF chain. The price of space segment on a
monthly basis will drive users to compress everything for the comparably
low cost of a one-time daughter-card-level hardware type purchase as an add
on to a framing unit. While it is very important to determine
tcp-over-satellite solutions for larger links, such as >4Mbps, tremendous
numbers of schools around the planet will long depend on small, low powered
VSAT's with increasingly compressed IP links for voice, WWW, interactive
desktop video and database access, and we should today recognize that fact
...now... while we are just beginning to implement and design changes which
incorporate these diverse user-driven traffic loading factors. Mr.
Fairhurst is right in suggesting that there will be several ways to do such
work and I believe it would be useful to develop a series of 'guidance'
solution sets-with commentaries(even signed, perhaps), more or less
standardized and let operators chose what they want to work with on a case
by case basis. Different gear will lend itself differently and more or less
easily to known solution approaches, once explored and developped. It would
be a tremendous fallacy to assume that all implementions start with the
latest and best equipment....as many of these distance education programs
will be retrofits of older and donated equipment.
This group's potential examination of such a low power, IP transmission
environment QOS problem would ultimately do countless people a great public
service if a way could be found to study and provide workable guidance for
these types of frankly low power VSAT applications needing to be done with
low budgets in less than perfect conditions. This does not have to be
within the framework of a committee-type institutional document process,
but I feel strongly that the true obligations of such an ad hoc body would
suggest that reaseach along the lines that Mr. Fairhurst and I have
outlined for links of <2Mbps is essential to the development of globally
usable TCP and generally speaking IP-over-satellite solutions...which is
what I believe to be the raison d'etre of such a prestigeous gathering of
experts as participate in this process. While some people will complain
that such additional work dillutes the 'thrust' of the body, I would
counter that the sheer plethora of small VSAT links which can and will be
needed to be installed in coming years to the World's regions in rapid
development -often almost exclusively for educational purposes, though
increasingly I hope for mixed use in support of the entire rural community
environment, should indicate to any observant that the moral implications
of practicing professionally in this industry should suggest also providing
some attention to this 'less commercially attractive' sized link: that
these implications are tremendous and hark to the now almost 'ancient'
communications concept of universal, ubiquitous and I would add <locally
affordable telecommunications service>. Business users and rural health
care will benefit too, as their links are also of this size-and they will
use compression: guaranteed!
I believe strongly that a rigid standard-without guidance, and only
looking at larger links >4Mbps will not be very useful in the real world of
practically building the essential and usually <2Mbps satellite links
connecting to the places which the 'other 2/3' of this planet's population
call "home", too, in their own local language. After all, they are our
neighbors on Earth and they need these links to work reliably at what they
consider <locally affordable costs>. Solution sets which count on
Washington/New York, London, Paris, Berlin, and Tokyo ideas of 'what's
affordable' won't be rapidly deployed, or at least not as rapidly as
solutions where affordable basic connectivity is really what is needed now.
This kind of work will also be supportive of the ITU's thrust for true
universal service (and beyond telephony!) and will be welcomed in such
circles, particularly among nation-members in the target populations for
increased basic connectivity such as I am discussing here, and notably for
the entire range of Voice and Video Over IP (VoIP) applications being
developped for which TCP over satellite will become the long-haul transport
mode of choice for many regions, now with the ITU H.323 standard being
accepted by equipment manufacturers everywhere.
Once again, I would like to thank Mr. Fairhurst for his provocative and
constructive comments and examples.
Respectfully,
Charles A. Ross
------------------------------------
Charles A. Ross
President
ACANTHUS Corporation
&
BEEDNET Group
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------------------------------------
At 11:00 AM 3/31/98 +0000, you wrote:
>I read with interest the assertion that satellite links may be
>"engineered" to give a BER of 10-9... Sure they can, the ways of
>doing this are either lowering the bit rate (through FEC) or/and by
>increasing the received energy per bit (i.e. utilising more
>satellite transmit power). Both of these approaches are valid for
>high bit rate systems > 4 Mbps, say, but for lower bit rate systems,
>it may be the wrong solution, and providing error control techniques
>in some way from the protocol is a very sound approach, providing it
>is transparent enougth to TCP (Hmmmm.... this is tricky)
>
>Consider for example, the power budget required to overcome
>interference from a noise source close to a satellite terminal, or
>the potential for the satellite terminal to be obscured partially
>(a friendly bird perched on the antenna feed?). If one chooses
>to only optimise the bearer network by providing an error-free physical
>transmission path, then that's one way of building of network,
>it is certainly NOT the only way.
>
>Is the group interested in 2 Mbps and lower data rates too?
>
>Gorry Fairhurst
>Communication Group,
>Dept Engineering
>University of Aberdeen.
>
>
------------------------------------
Charles A. Ross
President
ACANTHUS Corporation
&
BEEDNET Group
replies to:
Prague, Representative Office
GSM mobile +420.(0)603.500.000 (voicemail)
GSM mobile +420.(0)603.49.1000 (voicemail)
+420.2.2421.9525 tel/fax/voicemail
------------------------------------
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