RE: Satellite Bandwidth Questions

["Zwart, R." <[e-mail]>]

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:[e-mail]]
> Sent: 	dinsdag 29 december 1998 17:49
> 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  
>