RE: Satellite Bandwidth Questions

["Schram, Chris" <[e-mail]>]

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


> -----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