Re: [hylafax-users] large multiport systems

["Darren Nickerson" <darren.nickerson@xxxxxxxx>]

Lee,

 

Your recent in-depth work on ECM, and discussion/conclusions on ECM vs. non-ECM throughput offers some very interesting food for thought, thanks! We'll study your comments, and get back to you in due course. We're also going to spend some time doing real-world measurements in collaboration with several of our fax broadcast customers, and see how our practice matches up with your theory.

 

There are a couple of V.34 advantages I'd like to highlight/excerpt from that body of "propaganda" you love to hate that you might want to take into account, since some of them are at least partially relevant to even single-page faxing:

 

1. V.8 fast handshaking only occurs when both sender and receiver support it, so there's no time lost trying to negotiate this capability.

 

2. V.8 fast handshaking  really is (theoretically at least) significantly faster. With a 9.6 Kbps or a V.17 modem, the handshaking is done at 300 bps. With V.8 (ie: when sender and receiver support V.34) the handshaking is done at a much faster rate of 1,200 bps. The result is that handshaking time is reduced from approximately 16 seconds with 9.6 Kbps and V.17 to 7 seconds with V.34.

3. V.34 implements a "line probing" procedure that is much more efficient at retraining on noisy calls. Immediately following the handshake stage, a signal exchange allows the V.34 receiver to analyze the characteristics of the connection before beginning the data transmission stage, and choose several key session parameters. This isn't only performed on every new connection, but can occur at any time during the connection as part of the retraining process, which can be a huge win on multi-page faxes.

 

Consider the following table of average fax transmission times. I think it's safe to assume the author used ECM throughout, and that the numbers are for relatively healthy line conditions (Source: Davidson Consulting, 2003):

 

                9.6 kBps 4-Page Fax   V.17 4-Page Fax    V.34 4-Page Fax    

                   (seconds)           (seconds)           (seconds)

 

Handshake             16                   16                  7

Page 1                18                   12                  5

Retraining             6                    6                 0.25

Page 2                27                   18                  7

Retraining             6                    6                 0.25

Page 3                27                   18                  7

Retraining             6                    6                 0.25

Page 4                54                   36                  14

Retraining             6                    6                 0.25

==========================================================================  

Total                166                  124                  41

 

 

Clearly V.34, with the quick handshake, faster data transfer and super-quick line-probe retrains is very relevant to multi-page faxing.

 

One strategy we recommend to our broadcast customers who aren't too worried about 100% optimum quality on the receiving end is to try doing V.34 with all capable receivers, but to do non-ECM V.17 or 9.6 KBps to the destinations that don't support it. In other words only do ECM in V.34 calls that are so quick that it essentially comes for free ;-)

 

One interesting wrinkle whenever one speaks of ECM is how hard the ECM implementation actually tries to recover from poor line conditions (using a sequence of PPR/CTC cycles combined with bitrate reduction). Clever manipulation of this "persistence" can make a large difference in some ECM calls, as you have learned. If you're not careful, you can find 1-page ECM faxes occasionally taking 15 to 20 minutes and still never get that perfect fax sent! Some times it's just best to drop the call and move to the next call, as you quite rightly point out.

 

Anyway, all very interesting and it bears careful study & measurement. Fun stuff!

 

-Darren

 

--
Darren Nickerson
Senior Sales & Support Engineer
iFax Solutions, Inc. www.ifax.com
darren.nickerson@xxxxxxxx
+1.215.438.4638 office
+1.215.243.8335 fax

 

 

----- Original Message -----

From: "Lee Howard" <faxguy@xxxxxxxxxxxxxxxx>

To: "Darren Nickerson" <darren.nickerson@xxxxxxxx>

Cc: <jlewis@xxxxxxxxx>; <hylafax-users@xxxxxxxxxxx>

Sent: Wednesday, September 10, 2003 4:31 PM

Subject: Re: [hylafax-users] large multiport systems

> On 2003.08.26 10:24 Darren Nickerson wrote:
> 
> > I 
may be wrong here Lee, but my research tells me you should look for
> > 
another culrprit if you're looking to improve your V.34 broadcast
> > 
performance.
> 
> I have suspicions that V.8 handshaking functions 
slightly different 
> when using digital equipment than when using analog 
equipment.  The 
> only equipment of which I am aware that supports 
V.34 faxing with 
> analog equipment comes from either MultiTech or 
BrookTrout.  As HylaFAX 
> (open source) only supports the former, I 
have no reason to blame the 
> increase in handshaking time when V.8 is 
attempted (either by the 
> receiver, by the sender, or by both) on the 
manufacturer's 
> implementation.  When HylaFAX truly supports Class 
1.0 then maybe I'll 
> be in a position to draw conclusions that 
way.  In any case, the 
> increase in handshaking time of which I 
have spoken is "only" on the 
> order of 1-3 seconds per call.  
Almost any significant amount of image 
> data will easily offset this 
delay.  I was wrong in concluding 
> otherwise.
> 
> The 
other culprit of which you speak is ECM protocol.
> 
> Usage of V.34 
requires the usage of ECM protocol.  ECM protocol 
> actively strives 
for perfect image quality.  Non-ECM protocol only 
> strives for 
near-perfect quality, depending upong the copy-quality 
> settings of the 
receiver.  So the likelihood that retransmission of 
> image data 
(due to PPR) will occur in an ECM fax is much greater than 
> the 
likelihood that retransmission of image data (due to RTN) will 
> occur in 
a non-ECM fax.
> 
> It is inaccurate and misleading to compare ECM 
faxing against non-ECM 
> faxing (or vice-versa) due to this difference in 
goals.
> 
> I've seen lots of marketing material out there 
(propaganda?) that tries 
> to sell people on purchasing new hardware, 
software, or both because 
> making that investment into V.34 and 
MMR-supporting equipment will "pay 
> for itself over time in toll savings 
and necessary lines".  This is 
> only always true if we are only 
considering ECM faxing.  It is only 
> sometimes true (and in my 
experience, more often it is untrue) when 
> also considering non-ECM 
faxing.
> 
> A fax sent without ECM at 14400 bps with MR compression 
can, when 
> enough noise exists to put image quality less than perfect 
but still 
> "near-perfect" (and this is a very common situation), be sent 
faster 
> than a fax sent at 33600 bps with MMR compression (with 
requisite ECM, 
> of course) because with ECM faxing image data had to be 
resent, 
> sometimes repeatedly, to acheive perfect image quality while 
non-ECM 
> faxing completed successfully without image data 
retransmission.
> 
> I suspect that most people are going to find 
the value in ECM faxing to 
> be worth it.  However, I also suspect 
that there are going to be some 
> people, particularly bulk faxers, which 
will think otherwise - because 
> in non-ECM faxing if the receiver sends 
MCF rather than RTN, that means 
> that the image quality was "good 
enough" for them... and the "savings" 
> of not using ECM is better spent 
sending more faxes out.
> 
> Lee.
>