Tips for Using Fax Better over IP

So, you're contemplating about dumping your land line in favor of Voice over IP? So far so good... but what about that fax machine that's over there, lonely in the corner? is it going to work well 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 several things to operate successfuly:
1. An analog telephone signal.
2. Enough bandwidth to transmit the data on top of.
3. No interupptions, broken signals, etc.

The problem(s) with Faxing over IP are precisely those:
1. Voice over IP carries a digital signal - not an analog one.
2. Typically, the codecs used in Voice over IP do not provide the fax with enough bandwidth to transmit over.
3. With Voice over IP it is possible and even likely that some data signals will get "lost" on their way. During a voice call this is not an issue since we (humans) do not hear the subtle difference that one lost packet represents. Fax machines, are far more senstive (one could say grouchy) than us humans. If they don't get their data exactly the way they like it, they throw a fit and quit the transmission.

For these reasons, trying to fax over IP is a challenging task indeed. Even with the best of connections you're still expecting a 70% success rate - if you're lucky.

Alright, enough with the sad news, let's get back to what this article is about - helping you get the best results you can with your fax machine and VoIP connection! For purposes of illustration, we're going to describe settings for Linksys adapters. Some of the tips described below will work with other brands too though. Alright, let's get started!

1. Set your codec to G711u (ulaw). Why? because G711 is the only codec with enough bandwidth to equate a "real" phone line's bandwidth.
2. Set your RTP Packet Size to 0.020, or even 0.010. This value is essentially a measurement of how much time in ms will be contained in each date packet. You are going to lose date 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 a 15/5mbit connection? good for you, now stop that bittorent client - and don't even think of the word "video" until your transmission is done. Even the most subtle interference may cause you problems, so do yourself a favor and just stop all traffic ahead of time.
4. This is a bit counter-intuitive, but turn off 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 faxes will name 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 lack of bandwidth is to, well, not use bandwidth.
6. Put a DSL filter between your fax and your Analog Telephone Adapter.
7. Try to avoid faxing long documents. Try to fax pages in bunches of 2-3 at most to maximize chances of success. The more pages the fax session is, the higher the likelihood you're going to run into an error.
8. Disable echo cancelation in your Analog Telephone Adapter. 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 recommended approach to handle 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 features that had a lot of hype behind them due to bandwidth usage lowered. Problem is, 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 like it.
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 tips, and pray a little, 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 particularly if your business depends on it - do yourself a favor and keep that land line. After all- if you've read some of my other articles, you already know that you should keep at least one land line for proper 911 availability!

Nitzan is the CEO of Future Nine Corp - a leading VoIP Service Provider, who also offers International Calling Cards utilizing VoIP.