Subject: Bit errors, is this really an issue ?

From: Jordi Nelissen - Alcatel Bell Research <[email-masked]>
Date: Thu, 24 Sep 1998 14:55:58 +0200

Dear tcpsat,

I have a question with respect to the effect of BER on TCP performance
on satellite links. From draft-ietf-tcpsat-stand-mech-06.txt, I read :

'Typical bit error rates (BER) for a satellite link today are on the
order of 1 error per 10 million bits (1 x 10^-7) or less frequent. 
Advanced error control coding (e.g., Reed Solomon) can be added to
existing satellite services and is currently being used by many
services.  Satellite error performance approaching fiber will become
more common as advanced error control coding is used in new systems.'

As I understand this, in optimal operation, a modern satellite link can
approach fiber BER, provided adequate FEC is used. Further on I also
read :

'FEC should not be expected to fix all problems associated with noisy
satellite links.  There are some situations where FEC cannot be expected
to solve the noise problem (such as military jamming, deep space
missions, noise caused by rain fade, etc.).'

Let us take the most common situation, i.e. rain fade. Can someone
explain to me its quantitative impact on BER. Is the satellite link
completely out of order when it rains or can we still find rescue in
methods like ARQ or some TCP which is able to distinguish corruption
from congestion ? Is there information available on the noise
characteristics on modern satellite links ? (BER, distribution of bit
errors, ...)

thanks, Jordi
-- 
_____________________________________________________________________
Jordi Nelissen - [email-masked]

Alcatel Research - Network Architecture - Traffic Technology
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone: external: +32(0)3240 7192, alcanet: +6057192
Fax:   external: +32(0)3240 9932, alcanet: +6059932

Visit our webpage : 
http://www.rc.bel.alcatel.be/projects/networkarchitecture/traffic
_____________________________________________________________________

---

From: Francesco Potorti` <[email-masked]>
Date: Thu, 24 Sep 1998 16:43:30 +0200 (CEST)

Let us take the most common situation, i.e. rain fade. Can someone
   explain to me its quantitative impact on BER. 

If no countermeasure is taken, a  satellite link using QPSK and no FEC goes
from 1e-8 BER to 1e-3 with only 5 dB of signal attenuation.  With FEC, less
dB's are needed.   In general, however, you must  account for power margin,
so there is no general answer.  Also, experimental systems such as ACTS and
our old  FODA/IBEA use fade  countermeasure adaptive techniques, even  if I
don't know if any commercial system uses such methods.

-- 
Francesco Potorti` (researcher)        Voice:    +39-050-593 203 (op. 211)
Computer Networks Group                Fax:      +39-050-904052
CNUCE-CNR, Via Santa Maria 36          Email:    [email-masked]
56126 Pisa - Italy                     Web:	 http://fly.cnuce.cnr.it/

---

From: Gorry Fairhurst <[email-masked]>
Date: Thu, 24 Sep 1998 19:17:12 +0100

> characteristics on modern satellite links ? (BER, distribution of bit

You need to look at the link power budget and know the rain conditions
locally.  Both an outage (if the fade is deep) or a period of
increased BER may result. Depends upon where you are, how bad the
rain is (act of god), and how you designed the system.  The latter
part is up to the engineers building the link.  If they choose to
use lots of FEC, then that is one approach which sacrifices bandwidth
for higher availability. ARQ sacrifices complexity and delay for
better maintained throughput. TCP enhancements may also help somewhat
mitigate the effect.

There is no single answer, which keeps engineers busy. Things are
certainly worse at Ka-Band and even more so in certain locations.

Gorry Fairhurst.