So, you're contemplating about ditching your land line in favor of Voice over IP? So far so good... but how 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 machine basically requires several 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 issue(s) with Faxing over VoIP are exactly 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 machine with enough bandwidth to transmit over.
3. With Voice over IP it is possible and likely that some packets will get "lost" on their way. When carrying a regular call this is not a problem 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 mortals. If they don't get their data exactly as they like it, they throw a fit and quit the transmission.

For these reasons, faxing over IP is a daunting task indeed. Even with the best of internet pipes you're still looking at a 70% success rate at best.

Alright, enough with the sad news, let's go back to what this piece 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 Analog Telephone Adapters. Some of the steps 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 simulate 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 voice packet. You are going to lose voice packets - that is a given - but the smaller the lost packets are, the better the chance is that your fax will be able to recover from the error.
3. Stop all uploads and downloads. You have the latest 15/5mbit connection? great, now stop that bittorent client - and don't even think of the word "video" until your transmission is completed. 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 land line, 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 transmission 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 machine and your Analog Telephone Adapter.
7. Try to avoid faxing multi-page 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 chance 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 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 usage lowered. 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 pray a little, you are guaranteed... well, you're not guaranteed anything. Faxing over IP will still be totally hit-or-miss, 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!