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Modems, Faxes and VoIP

Floyd Kling 



Note:... Please be clear here, there is a difference between "VoIP" and "Digital Phone Service".  These are two different methods of sending voice over a digital medium.  This page talks to the VoIP method


Will my trusty old V.34 or V.90/92 modem or fax operate on my new VoIP service?

VoIP (Voice Over IP) is a general term that methodizes making conventional telephone calls over the Internet Protocol... a.k.a. IP)

So you have your new VoIP equipment installed and your gloating over how you've shafted old Ma Bell.  It was simple enough; you pay a skimpy $17 to $25(+) per month for unlimited calls and maybe paid that one time charge for their VoIP Converter Box. This box has two types of jacks:  RJ45 Ethernet that plugs into your broadband (DSL or Cable) and one or more RJ-11(s) that your Telephone(s) plugs into.  

But wait, you have a fax machine and sometimes use your dial-up modem (why you want to use a dial-up modem if you have broadband escapes me, but hey, stay with me here). 

You plug in your telephone and make a voice call.  You get dial tone, you dial out, terrific sound... works just like your land line... wow, this works great! 

Then you plug in your modem or fax machine to make some test calls and you get mixed results <darn!>.  Those baby's don't <quite> work on your new VoIP connection.

Why not?... well, I will be very basic here... fundamentally your clashing two technologies... Contiguous Analogue and Digital Packetizing:

Modems on VoIP:

Analogue dial-up modems require a bi-directional continuous (uninterrupted) analogue signals.  Their design incorporates several types of signal coding including PCM, PSK, DPSK, QAM and others.  These special modulations require careful consideration of amplitude, phase jitter, signal delay, echo cancellation and other signaling attributes to function properly.

VoIP on the other hand, immediately digitizes and packetizes the analogue signals and puts these packets through the internet digital network.  Since the telephone network is designed for voice, their network allows some data loss and packet delays and are of little concern because voice signaling is very forgiving.  Not so with Modems, especially the higher speed 33.6kbps and faster.

Bottom line with modems, to operate over VoIP the modem must change, or the VoIP Converter Box (aka 'gizmos') must change to adapt to the analogue modems signaling requirements.... not an easy task considering the enormous installed base of legacy equipment. 

"Dial-up modems over VoIP are unreliable and <at this time> should be avoided"

However, if you must, then limit your speeds to 0-300bps<Bell 103/V21> or V.22-<1200bps> and V.22bis<2400bps> which will have be best reliability.  Higher speed attempts will proportionally and dramatically degrade.

~  Floyd  "The Modem Guy" ~                 

Faxes over VoIP:

Faxes have a better chance operating over VoIP but it is not guaranteed.  Why?, The older fax modulation standards are 'half duplex'  This means the analogue signals transmit in one direction at a time.  This operation is more tolerant to VoIP packet delays and most of the time you may be fine.

There are a few types of FAX modulations out there.  Originally, faxes started slow with 100bps to 300bps, then settled for many years with V.29 (half duplex 9600bps)... then was enhanced to 14.4kbps.  The two <friendly> standards that evolved are T.38 and T.711 which are universally supported and could be fine (but not necessarily) with your VoIP.  Almost all fax machines support these two schemes/speeds.

What will my VoIP provider say?  They will be hesitant to give a definitive answer on FAX reliability.  When your having FAX problems on VoIP, your provider will give some suggestions such as lowering your FAX baud rate, this may help a little. (If asked about modems,... they'll probably put their hands in their pockets, shrug their shoulders and make circles in the sand with their toes - and will definitely not commit.) You should note that many VoIP providers are now offering a fax service that basically uses your email to send/receive faxes. A bit more inconvenient, but to eliminate your POTS you should look into a fax service.

Look out, there is an enhanced fax standard called "Super" G3 modulation that boosts the fax speeds to 33.6kbps.  Used over VoIP could cause your Faxes to pass a gall stone and be even more unreliable or simply not function.

What is in the future?  There is work in ITU-T (International Telecommunications Union-Telecommunications) to address this issue.  It has a few preliminary names... MoIP (Modem over IP or also called Media over IP), and AoD (Analogue over Digital).  Other names will evolve as proposals are introduced and settle into a formal standard.

T.38 Fax Relay Standard, aka FoIP (Fax over IP), has actually been around since 1998, and proponents argue its reliability.  Is it reliable???... In theory yes, but practically, you should test with your application to be sure before you commit to it.  The problem is, you need both fax machines (sending and receiving) to be supportive of the same standards, and there are literally millions of <older> machines out there so each fax session will most likely vary.

Any new standard must be defined, argued, ratified and then incorporated... this takes years, not months.

How does the PSTN/POTS networks interact with the IP networks for phone calls?:

The 'magic' is all about the VoIP gateway   Here is a basic drawing (opens a new window) of how the two networks pass voice (analogue) betwixt each other.  At first, this drawing may appear complicated but give it a chance and it will make sense.

Summary on VoIP and Dial-up Modems/Faxes: 

IMHO  <<---that's a disclaimer!> ... Until a modem is produced that will have the ability to <automatically> switch between conventional PSTN and VoIP networks (with a corresponding modem at the other end) -or- the VoIP Converter Box is able to <auto> adapt, much to your chagrin, your dial-up modems should stay on Ma Bell, and if you need reliable Faxing, you should strongly consider keeping your faxes on Ma Bell's POTS network also.

Why do I <TheModemGuy> say that?... 

Today's modem and fax standards (except T.38) were designed for The Bell Telephone <TDM - contiguous>  networks... Modems were not designed for the VoIP <packet> networks.  Incompatibilities WILL exist and reliability WILL be 'dicey'.

Regarding Modems and Faxes over VoIP networks...

You can have "Reliability" or "Independence" (from Ma Bell) ... Pick One

If you want to completely plunge into VoIP in lieu of your POTS service and want reliable faxing, I suggest you look at an Internet FAX service, of which there are many... (that's what I do).


UPDATE 6/8/09

This web page was originally written to alert you of the reliability issues when attempting to operate dial-up modems and faxes over a VoIP connection (Notice the word "attempting"). For the most part, I discouraged Dial-up modems and faxes over VoIP due to serious reliability issues.


... but now, there are new kids on the block....


In 2007/2008, many cable companies began offering more <and improved> services over their cable connection.  Along with TV and Internet, they are now offering "Digital Phone Service".  This new voice service uses special adapters with enhancements to high speed digital Cable connection called EMTA - "Embedded Multimedia Terminal Adapters"  i.e.  COX, Comcast and others.  This is commonly called "Digital Phone Service" 


These special ETMA adapters are capable of significantly improved <digital> bandwidth allocation.  These devices apply this high speed digital into a more analogue friendly algorithm. The EMTA breaks the analogue signals into much higher sample rates and dramatically improves the bandwidth for analogue signals.  These digit packets are different than on IP networks. This higher resolution does a much better job of bridging the gap between 'Packet' (IP) protocol<used on the internet>, and 'Contiguous' (TDM) protocols <used by the telephone companies>.  Bottom line, this means that if your broadband provider is utilizing an "EMTA" you have a higher likelihood of reliable (and higher speed) dial-up modem and fax connections over this digital connection. 


BE CAREFUL!  There is a difference between Digital Phone Service and VoIP... just because you have Cox or Comcast cable, DOES NOT MEAN YOU automatically have DIGITAL PHONE (EMTA) SERVICE... You must check with your provider before you start using your dial-up modems and fax machines on your digital phone connections.  For instance, the popular "Vonage" is VoIP and IMHO will not reliably support Modems/Faxes.





Copyright © 1997-2011 Floyd Kling 
- All Rights Reserved -