So, you're contemplating about dumping that land line in favor of Voice over IP? So far so good... but how about the fax machine that's sitting there, lonely in the corner? is it going to play nice with the new technology? The short answer is "No". The long answer is "it's complicated"...
To put things into perspective, a fax basically needs a few things to operate properly:
1. An analog telephone signal.
2. Enough bandwidth to transmit the data on top of.
3. No broken signals or interupptions
The problem(s) with Faxing over IP are exactly those:
1. Voice over IP carries a digital signal - not an analog one.
2. More often than not, the codecs used in Voice over IP do not supply the fax with enough bandwidth to transmit over.
3. With Voice over IP it is possible and likely that some packets will get "lost" before reaching the remote machine. During a voice conversation this is not a problem since we (humans) do not hear the subtle difference that one lost packet represents. Faxes however, are far more senstive (one could say grouchy) than us mortals. If they don't get their data exactly as they like it, they throw a temper tantrum and abort the transmission.
For these reasons, faxing over VoIP is a challenging task indeed. Even with the best of connections you're still looking at a 70% success rate at best.
Alright, enough with the sad news, let's go back to what this article is about - helping you achieve the best results you can with your fax machine and Voice over IP connection! For purposes of this example, we're going to describe settings for Linksys adapters. Some of the steps described below will work with other makes too however. Alright, let's get started!
1. Set your codec to G711u (ulaw). Why? because G711 is the only codec with enough bandwidth to simulate a land line's bandwidth.
2. Set your RTP Packet Size to 0.020, or even 0.010. This value is in essence a measurement of how much time in ms will be delivered in each voice packet. You are going to lose voice packets - that is a given - but the smaller the lost packets are, the higher the chances are that your fax will be able to recover from the error.
3. Stop all uploads. You have the latest 15/5mbit connection? good for you, now stop that p2p client - and don't even think of the word "YouTube" until your fax transmission is completed. Even the most subtle interference will cause you problems, so do yourself a favor and just stop all traffic ahead of time.
4. This is a bit counter-intuitive, but disable ECM (error correction) on your fax. ECM while great for error correction on POTS, doesn't work well over IP.
5. Decrease the transfer speed on your fax. Some machines will call this "overseas mode", and some will just have an option to decrease the speed. Either way you want to use the lowest speed possible and definitely below 9600 bps. Why? we go back to our first point - limited bandwidth. The best way to counter limited bandwidth is to, well, not use bandwidth.
6. Put a DSL filter between your fax and your ATA.
7. Try to avoid faxing long documents. Try to fax pages in bunches of 2-3 at most to maximize chances of success. The longer the fax session is, the higher the likelihood you're going to run into a problem.
8. Disable echo cancelation in your ATA. You probably want to keep echo cancelation on when you're making voice calls, but your fax cares less about echo and more about getting an accurate signal. A good way to go about this is to use echo cancelation on Line 1 - but disable it on Line 2.
9. Disable silence suppression. Silence suppression is one of those catch phrases that had a lot of hype behind them due to bandwidth being saved. But in order to save the little bandwidth that you do, you sacrifice both voice quality and signal clarity. Your faxes are definitely not going to approve.
10. Set Codec Symmetric to No.
11. Set Passthrough Method to ReINVITE.
12. Set FXS Port Input Gain and FXS Port Output Gain to -3. (this is the default for Linksys adapters)
If you follow these steps, and cross your fingers, you are guaranteed... well, you're not guaranteed anything. Using fax over IP will still be totally arbitrary, but your chances will go from 0% to something much higher than that. My advice is that if you need serious faxing for your business, and especially if your business depends on it - do yourself a favor and keep that land line. After all- if you've read one of my other articles, you already know that you should keep at least one land line for proper 911 availability!
Source: http://www.ArticlePros.com/author.php?Nitzan Kon
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