RE: Satellite Bandwidth Questions

["Fritz, David A." <[e-mail]>]

Chris,

Unfortunately, your sample table cannot be filled in.  There is no fixed
relationship between the "band" and the "bps".  The different bands in
satellite communications can be thought of like the different bands for your
car radio, which are AM and FM.  Once you pick a band to operate in, your
are allocated a limited amount of bandwidth to send your signal.  This
allows many users to operate in the same band, just like many radio stations
operate in the AM and FM bands.  Here the analogy begins to fail.  In
satellite communications, one user may have more bandwidth than another.
Once a user has their bandwidth allocation, they are also allowed a certain
amount of power on the satellite.  Within the power and bandwidth
allocations, the user chooses a modulation scheme that maximizes their
"bps."  If a user has a lot of power to play with they might choose a
complex modulation that lets them send a high number of "bps."  If they are
power limited, the might choose a more power efficient modulation at a lower
bit rate.  So, in the end the resulting bps is a balance between power and
bandwidth allocations for a user.  

If you simply want to get in the ballpark, using Binary Phase Shift Keying,
a common type of modulation for satellite communications, you get roughly
one bps per Hz of bandwidth allocation.  So, if a user is allocated 1 MHz of
bandwidth and they use BPSK, then they can send roughly 1 Mbps.  

In closing, think about computer modems.  They all use the same amount of
bandwidth.  As technical advances became available, the modulation of the
computer modem has moved from a very simple scheme providing 2400 bps to a
dramatically more complex scheme used in 56 kbps modems.  Just as there is
no one data rate for computer modems, there is also no one data rate for
satellite modems.  

There are other factors considered beyond simple power and bandwidth
constraints, (bit error rate for instance)  but this gives you the idea.
Hope this helped.  (I gotta do more reading on this stuff!)

David

	-----Original Message-----
	From:	Chris Metz [SMTP:[e-mail]]
	Sent:	Tuesday, December 29, 1998 11:49 AM
	To:	[e-mail]
	Subject:	Satellite Bandwidth Questions

	Hi-
	I am studying TCP over Satellite considerations and have reviewed
	draft-ietf-tcpsat-stand-mech-06.txt. I live in a "bits per second"
world in
	terms of bandwidth I would like to understand how bps relates to the
terms
	used to describe satellite bandwidth. So is there a reference
somewhere or
	good book on satellite basics that can fill in the following terms:

	Band     Uplink (GHZ)    Downlink (GHz)    Uplink (bps)    Downlink
(bps)

	C            6              4                 ??               ??


	Ku           14             12                ??               ??

	Ka           30             20                ??               ??


	I suppose I am asking how does one convert MHz and GHz into
conventional
	bandwidth (bps) terminology. This for clearing this up for me and my
	apologies for the "simpleton" question.



	Chris Metz
	Consulting Systems Engineer
	Cisco Systems
	email: [e-mail]
	phone: [phone]
	pager: 800-365-4578