Subject: RE: Satellite Bandwidth Questions

From: "Fritz, David A." <[email-masked]>
Date: Tue, 29 Dec 1998 13:40:29 -0500

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:[email-masked]]
	Sent:	Tuesday, December 29, 1998 11:49 AM
	To:	[email-masked]
	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: [email-masked]
	phone: 212-714-4207
	pager: 800-365-4578

---

From: "Schram, Chris" <[email-masked]>
Date: Tue, 29 Dec 1998 14:10:33 -0500

Chris - 

As evidenced by the initial responses,  often the simplest questions are the
best.  

The bits per MHz, or the useful information rate on a satellite carrier
depends primarily on the coding rate (use of forward error correction, or
'FEC'), and modulation scheme (BPSK, QPSK, 8PSK, 16QAM).  Some web browsing
can get you to different modem manufaturers or satellite operators which
specify occupied and allocated bandwidths for different carrier types.

For practical purposes, the majority of small satellite circuits are QPSK
with R1/2 forward error correction (called 'rate 1/2', meaning two bits are
transmitted for every bit of useful information, with the additional bits
used to correct errors after reception) , or R3/4 FEC (4 bits for every
three of information).  Larger links may use R7/8, which has very litte
forward error correction.

Typical R1/2 small links use 100 kHz for 64 kbps or 200 kHz for 128 kbps,
which includes some bandwidth to allow for spacing of adjacent carriers. The
R3/4 carriers require either more of the available transponder power or
larger receive earth stations compared with the R1/2 carriers, but the
bandwidth requirements shrink to 75 kHz for a 64 kbps carrier or 150 kHz for
128 kbps.   We have anumber of new or smaller ISP customers who run 64 or
128 kbps carriers (but never for long, as they grow like weeds).  

The mid-range carrier, a T1, is allocated 2.175 Mhz for 1.544 Mbps for a
R1/2 carrier or 1.450 MHz for a R3/4 carrier.  

The next class of carriers are in the 9 MHz range, which is also favored by
some of our ISP customers.  They typically run at R3/4 and permit around
9.15 Mbps in the 9 MHz allocation.

We run a different modulation scheme for larger ISP backbone carriers out of
our teleports (see www.panamsat.com to spare me sounding like an
infomercial).  The 8PSK carriers operate at DS-3 speed (45 Mbps) and occupy
27 MHz.  Larger carriers can often be run with more efficient use of
transponder power to get into the same receive earth station universe.  

Hope these practical examples (applicable to C or Ku-band, with minor
differences at Ka) are useful, and you should be able to find a wealth of
additional information on the web.

- Chris

> 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
> 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

---

From: Heidi E Anderson <[email-masked]>
Date: Tue, 29 Dec 1998 13:16:44 -0600

conventional
> apologies for the "simpleton" question.
Chris-

Your best bet is a good introductory text to digital communications either 
at a good bookstore or local university bookstore.  Satellite 
communciations like many other communication systems employs modulation. 
 Simply put, the digital data (bps) is modulated onto a carrier frequency. 
 In the satellite world, this is typically done in the baseband to an 
intermediate frequency (IF) such as 70 or 140 MHz and again 
upconverted/downconverted to the RF carrier band.  It's this RF band that 
is referred to by the 4/6 GHZ and 12/14 GHz in your message.  The satellite 
equipment and associated baseband equipment can be collectively thought of 
as a modem in terms of the downstream communications devices such as 
routers, servers, etc.  Besides the modulation, there are also the coding 
techniques which alter the desired information bps, but this can be 
compared to any other type of communications coding.

Example:  information rate 512 kbps
	    FEC coding rate 1/2         -------> 1024 kbps
	    BPSK modulation            --------> 1024 ksps (symbols)
	    (or QPSK mod)                --------> 512 ksps	

	 	therefore, this carrier would occupy 1024 kHz (+ rolloff) of bandwidth
		(or 512 kHz, QPSK case) and then be converted to it's assigned center 
frequency
		in the RF band

Typical Satellite Config:  500 MHz wide spread over several tranponders
			Typical transponder bw: 27 MHz, 36 MHz, 54MHz, ....
			Receive carriers centered between 14.0-14.5 GHz
			Transmit carriers centered between 11.7-12.2 GHz

Hope this helps initially, but as I mentioned, you probably want to acquire 
a basic text on digital communication systems.  Most of them address 
satellite as well.

Heidi E. Anderson			Enterprise Rent-A-Car
WAN Engineering				600 Corporate Park Drive
01.314.512.2057				Clayton, MO 63105  USA
01.314.512.4057 FAX
[email-masked]

---

From: "William A. Kissick" <[email-masked]>
Date: Tue, 29 Dec 1998 12:53:00 -0700 (MST)

Hello Chris:

I believe you are asking about how many bits per second (bit rate) one can
achieve for a given bandwidth (channel width, not uplink or downlink
frequencies as in your table). The ratio of bit rate to channel width is
called spectral efficiency. Sorry, there is no simple answer to your simple
question. Look for any book with the approximate title "Satellite
Communications."

The information capacity of any communication system operting over a noisy
channel is limited. This limit is given by the Shannon-Hartley law which
gives the information capacity in terms of the channel width in Hertz and
the received signal to noise ratio (for Gaussian noise). In digital terms,
regardless of the bandwidth used, the Eb/N0 (energy per bit to noise density
ratio) cannot go below -1.6 dB. This is the Shannon limit below which NO
information gets through.

At higher (more realistic and typical for satellite channels) values of
Eb/N0, like 5 to 25 dB, the theoretical spectral efficiency is about 16
bits/Hz. (A channel width of 100 kHz would theoretically support 1,600
kB/s). In practice, links using PSK (phase shift keying) do not achieve
capacities anywhere near the Shannon limit because the theory assumes zero
bit errors. For a practical BER (bit error rate) of about 10**-10, in a QPSK
(quadrature phase shift keying) the spectral efficiency will be about 2
bits/Hertz. So, a 100 kHz channel will support about 200 kB/s.

Now, you should also know that satellite channels fade, so FEC may be added
during times of rain attenuation. This will reduce the information bit rate
to one-half, for example, to tolerate a 3-dB fade and keep the BER the same
in the same bandwidth.

Regards,
Bill

>draft-ietf-tcpsat-stand-mech-06.txt. I live in a "bits per second" world in

>Consulting Systems Engineer

-----------------------------------------------------------
  Dr. William A. Kissick
  Institute for Telecommunication Sciences
  National Telecommunications and Information Administration
  325 Broadway
  Boulder, Colorado 80303

  Phone: 303-497-7410
  FAX:   303-497-5351
-----------------------------------------------------------

---

From: "Brooker, Ralph" <[email-masked]>
Date: Tue, 29 Dec 1998 14:41:00 -0600

Chris,
     The "GHZ" numbers you mention are shorthand for the allocated band 
     pair (up/down) used by the satellite, and have no direct 
     relationship to the bandwidth of the channel.
     
     On "conventional" (geostationary, bent-pipe) satellites, one rents 
     the amount of bandwidth needed to transmit the desired signal(s), 
     typically from 100 kHz up to 36-72 MHz.
     
     The bandwidth in kHz of a signal is proportional to the data rate 
     in bps and depends on the type of modulation (BPSK/QPSK) and the 
     parameters of the forward error correction.  Signals using 
     band-limited QPSK modulation and Rate 3/4 convolutional encoding 
     plus Reed-Solomon outer block coding require approximately 1.1 Hz 
     per bps.  This is usually the best selection of parameters for low 
     BER links (as required for TCP), but in some circumstances, to 
     minimize satellite cost, reduce modem cost, and/or to enable the 
     use of smaller antennas, other parameters might be better, 
     resulting in anywhere from 0.5 to 2 Hz per bps.  For a symmetric 
     two-way point-to-point link, multiply by two; for other carrier 
     configurations, add up all the signals needed to form the network.
     
     Ralph Brooker
     Andrew Corp.

______________________________ Reply Separator
_________________________________
Subject: Satellite Bandwidth Questions
Author:  "Chris Metz" [SMTP:[email-masked]] at AOP
Date:    12/29/98 10:49 AM

Hi-

C            6              4                 ??               ??
     
Ku           14             12                ??               ??
     
Ka           30             20                ??               ??

     
Chris Metz

---

From: "Zwart, R." <[email-masked]>
Date: Wed, 30 Dec 1998 16:43:07 +0100

Hi Chris, 

I saw you already got a lot of good reactions upon your question, including
some definitions, and some book advices. 
I can give you a daily user practice example on how we normally convert from
user bitrate (Mbps) via telco symbolrate (MBaud or shortly MBd) to allocated
bandwidth (MHz) on satellite, for commercial use (= good quality for as many
simultaneous customers as possible). The example I use is Digital Satellite
News Gathering (DSNG) via the MPEG-2 DVB standard, on a part of a Eutelsat
transponder, that KPN Telecom (the Netherlands) provides. It's for your
telly news feeds, but the calculation can be used generally for an arbitrary
bitrate satellite transmission. 

Audio and video is compressed into 8.448 (Mbps). 
Added is an overhead of 16 bytes upon each packet of 188 bytes. 
Forward Error Correction adds a bit onto each 3, and then convolves (=
mingles) those bits, so that's noted as FEC = 3/4. 
The modulation key (M-ary Phase Shift Keying) is nowadays mostly QPSK (M=4),
which puts a di-bit on each transmitted symbol. 
Multiplying this all, you get a symbolrate: 

symbolrate = (bitrate + overhead) x (1 / FEC) : (M/2) 

So, symbolrate = (8.448 x 204/188) x 4/3 : 2 = 6.111 (MBd). 

Now, the bandwidth and power for digital (bell shaped) carriers on satellite
is a matter of optimal use of the transponder resource; it's determined such
that receivers hooked onto the dishes at home etc. get enough C/N, and not
too much intermodulation (caused by the non linear transponder tube). A
roll-off factor of roughly 1.5 times the symbolrate is assigned to them.  
In the example, this leads to a commercial lease bandwidth of 9.5 (MHz). 
For thin route traffic (= well below 1 (Mbps)) the factor could be
diminished to 1.25, because they also get less power and thus less burden
the transponder. 
 
If you need to know more, just let me know. 

Success & regards, 

Ronald Zwart 
KPN Research 
Netherlands. 

> ----------
> From: 	Chris Metz[SMTP:[email-masked]]
> Sent: 	dinsdag 29 december 1998 17:49
> To: 	[email-masked]