(and What to do About it)

As the world’s leading provider of cloud fax services for midsized to large businesses, we receive a lot of questions from IT professionals about faxing and VoIP. “Can we fax over a VoIP line?” many ask us.  Because most of these companies have already migrated to a VoIP infrastructure (which we have written about in previous blog posts) for their voice communications, they are obviously hoping we’ll say yes.

But not before we offer them some serious warnings.

“You can try, and it may work just fine,” we would say.  “But it might not work consistently, meaning some faxes may go through but not others, especially longer ones more than a few pages. Or you may be able to send faxes but not receive, or visa versa.”  

In fact, faxing over VoIP can be so problematic that many VoIP service providers recommend keeping a plain old telephone service (POTS) line or two just to be on the safe side with analog applications like fax, postage machines and alarm systems, not to mention as a backup for when the VoIP network goes down, which it invariably will from time to time.  That advice gets the provider off the hook when problems pop up and brings in additional revenue, as traditional business phone lines typically cost over $50/month.

Can You Fax Over VoIP?

Technically speaking, yes, a business can send and receive faxes over a VoIP network. But the more you know about VoIP, the less confident you will be entrusting it with your company’s important fax transmissions, especially if you are doing a high volume of faxing.

And in case you aren’t familiar with VoIP, here’s a very brief overview of what it is and how it works.


VoIP, or Voice over Internet Protocol, is a communication technique used for sending voice over what used to data-only networks.  Rather than transmitting a conversation over the traditional circuit-based telephone network, VoIP takes the sounds in your phone call — the voices of the speakers and any background noises — and converts all of that into a series of data packets.  These packets are like envelopes containing the bits that comprise the voice call.  

The VoIP packets travel across your local area network (LAN) and/or wide area network (WAN), and may also be sent across the Internet, mixed in with many other packets containing email messages, word documents, spreadsheets, images, etc. At the receiving end, the voice packets are separated from the other ‘data’ packets and reassembled to recreate the words that were just spoken.  

Naturally this all has to happen very fast, in a fraction of a second, so VoIP packets are considered to be very time-sensitive; if a packet containing a snippet of a word is delayed or arrives out of order, it is useless and must be discarded.  That leads to the occasional blips and dropouts that one hears in VoIP phone calls, especially if they happen to travel over the public Internet where network congestion can cause packets to be delayed or lost along the way.

Converting voice to packets using VoIP technology makes sense for several reasons, but the first advantage is the tremendous cost savings that can be achieved by converging multiple types of business communications, that used to require multiple dedicated networks, over a single connection.

A related benefit is compression to reduce the amount of bandwidth required for phone calls. VoIP doesn’t just convert analog voice calls into digital format — the technology can compress that data considerably. A typical phone call, when it is digitized, requires 64kilobits per second (kbps) of bandwidth per call.

VoIP services, using compression protocols, can squeeze the number of bits in a voice call down to as little as 32, 16, 8 or even 4kbps (with corresponding reductions in sound quality), before sending that call across the Internet. For a large company or call center, whose employees make hundreds or even thousands of calls a day, this adds up to considerable savings.

But here’s the problem. While many forms of data can handle and even benefit from compression — including voice, documents and video — the analog fax tones cannot be compressed.

And this is where fax’s problems with VoIP begin.

How Fax Works In A VoIP Environment — And Why It Can Fail To Work…

There are two primary difficulties in transmitting fax over an IP network or VoIP service.

The first problem:  Fax cannot be compressed so it must be digitized for transmission over IP as a full rate 64Kbps data stream.  That may not matter for occasional use, but it adds up in a high volume faxing environment, especially at peak hours when everyone else is trying to send their documents or make calls at the same time. Most VoIP calls are compressed to 32kbps or less, so fax consumes at least twice the bandwidth of a compressed VoIP call.  In addition, there is the IP packet overhead, which increases the required bandwidth to around 88kbps, or at least 175% more bandwidth than a VoIP call.

Second, Fax has little tolerance for packet delay and packet loss. One of the virtues of IP is that large data files can be compressed and “packetized,” broken down into smaller discrete packets of information and then transmitted over the Internet. This technique attaches a ‘header’ containing destination and source IP addresses to each individual packet (like the “to” and “from” address on an postal envelope) — and including information about the packet’s place in the larger sequence of the data (“I’m the fourth piece in a seven-piece series that makes up part of this fax”), and where it’s going (“Here’s the IP address of the computer where I’m supposed to be delivered”).

What this means is that IP allows the network to find the fastest, clearest route for each individual packet to reach its destination. This sometimes means that pieces of the transmission — such as an email message — arrive out of sequence. The process still works reliably, though, because the header information helps the system put the document back together almost immediately upon arrival at the recipient’s end. If some packets get lost along the way, they can be retransmitted until the full message is assembled.  

This works well for documents and email, where a few seconds of delay is not noticed, but not so well for ‘real-time’ communications, and for Fax the delay could be deadly.

You’ve no doubt been on a phone call where someone cuts out momentarily and you miss a part of a word or two. That’s a packet(s) not reaching the other end of its VoIP journey to your phone or arriving too late, in which case the packet will be discarded. In those cases, all you have to do is ask the other person to repeat their last sentence. And believe it or not, in other cases a word or two is dropped and your brain is able to interpolate the missing information without your even realizing it consciously. This is why voice managed to make the transition to IP despite the imperfections.

A fax, by contrast, cannot be compressed and cannot tolerate even a tiny percentage of packet loss — even a 1% packet loss, and more than a couple seconds of delay, can cause the connection to time-out and the fax to fail. It also cannot tolerate a break in the packet sequence which could result in more delay. The recipient’s fax machine might very well read any of these issues as a problem with the inbound fax, and kill the entire transmission.

The second problem:  Fax transmissions have low tolerance for interoperability issues. The hundreds of millions of active fax machines in the world use several different fax protocols — T.30, T.38 and G.711 being the primary ones, and speeds like V.14 or V.34, while VoIP typically uses G.729 to compress calls and save bandwidth.

When a fax is sent over an analog network like the phone system, the two fax machines communicate with each other and agree on the type and speed of transmission. But when the fax is being transmitted over VoIP, any gaps in the tones create the same problems for the recipient’s fax machine.

If a fax travels over a VoIP network from a machine using one protocol and arrives at a machine that expects another, this can cause gaps in the fax’s analog tones as the system tries to work out the protocol issues. The fax machines misinterpret the gaps and lose synchronization with each other.

For example, when the VoIP network is set to use G.729 compression, it has to switch to G.711 for uncompressed transmission when a fax is sent.  The brief breaks on the fax tones that occur as the VoIP system tries to negotiate between the two protocols can cause the fax to fail.  And the longer the fax, the less likely it is to make it through.

T.38 may save the day, someday. The newer T.38 protocol was intended to transmit faxes directly over IP (FoIP), so the fax doesn’t need to be converted to an audio stream first. In theory, two T.38 capable fax machines should be able to communicate over VoIP.

But T.38 must be on both ends of a network to work, and many service providers never implemented the protocol. If the fax has to traverse networks that do not support T.38, it will need to be transcoded, which can add latency, increase cost, and may cause the call to disconnect. In addition, the spec has been implemented in various ways by manufacturers, so that one machine’s T.38 may be incompatible with another vendor’s equipment. The result is a failure to communicate.

Fax Can’t Share the Information Highway

An intuitive way to understand the unique challenges that Internet Protocol creates for faxing is by thinking of a standard analog fax transmission as a presidential motorcade. Fax was designed to enjoy a dedicated and direct path from sender to recipient.  On the old telephone network, fax traveled over a dedicated circuit it didn’t have to share with anybody.  Returning to our motorcade analogy, this is where all cross-traffic is blocked to keep the motorcade’s speed high and consistent, and in which all of the cars in the motorcade can remain in their original sequence for the entire journey. Put simply, all lanes for the fax are cleared from start to finish so there is never any delay.

A VoIP or other IP-based network, on the other hand, was designed for complex and ever-changing traffic patterns — more like a 12-lane highway where a mixture of real-time and non-real time data packets (cars) are frenetically traversing the path and jumping in and out of lanes at all times. Some of these pieces of data share a lane for part of their journey; some data packets arrive in a different order than they were sent; still others might get re-routed or even stuck on the road for a few moments, forcing the finished data transmission to wait at the recipient’s end until they arrive and can be pieced back together in order.

Fax is a road-hog of a technology, not designed to share its lane with anyone else. So when confronted with delayed or dropped packets, fax simply shuts down.

Which is why we at eFax Corporate explain to the IT professionals who ask us that, “yes, technically you can send or receive a business fax over a VoIP network — but doing so may create more problems for your organization than it solves.”

So What Can You Do About Fax After You’ve Migrated to an IP Environment

It’s tempting to look for a way to migrate your company’s legacy fax infrastructure to your new IP environment. After all, IP creates efficiencies, it helps your organization save money, and it can centralize many of the communications technologies that your IT department once had to manage and troubleshoot separately.

But if we’ve convinced you that fax won’t enjoy the many benefits of IP that your other data communications are enjoying, then the question is: What can you do to modernize, streamline and improve the efficiency of your legacy fax infrastructure?

The way we see it, you have the following four options:

1.  You can leave your existing fax infrastructure in place, and continue to pay for dedicated telecom services.  This is relatively safe, at least for now, but it fails to address many of your existing issues with faxing and creates new ones of it own. Caring for an aging in-house fax infrastructure is costly and time-consuming for your business or IT department.  Also, as you know because you’re managing it, a legacy fax environment is inefficient.

2.  If you’ve migrated to IP and now are having trouble with faxing, you can roll-back to costly analog lines for every fax number (or to a full onsite network of fax servers and fax machines that also require their own numbers over digital T1 lines). A big step backward for the IT team — but a happy turn of events for your telco provider.

3.  Wait for the standards bodies to introduce a new protocol that fixes the fax-over-IP problems such as delay, jitter, packet loss and other reliability issues. Keep in mind that G.711, T.37, T.38 and other protocols are still in operation decades after they were introduced. So you might be waiting a long time for the perfect, standards-body solution.

4.  Move to a cloud fax model. The cloud faxing solutions from eFax Corporate provides the ideal platform for delivering faxes over IP networks because they convert the fax into an email attachment which is independent of the underlying network technology.  The fax is now a series of data packets riding on a data network. Voila! Problem solved, and users can now send and receive faxes directly from their desktop, with a complete audit trail of very fax sent and received.

For Cloud Faxing, You Can Trust Industry Leader eFax Corporate

For the confidential documents you need to send or receive by fax, your enterprise can check all of the important boxes by upgrading your VoIP fax infrastructure to the cloud fax leader, eFax Corporate.

Our fax by email service is built on a highly secure, redundant global network — which can enhance your organization’s document security and regulatory compliance while at the same allowing your IT teams to outsource their entire fax infrastructure to a trusted cloud provider.

To learn more about the challenges of faxing over VoIP, register to attend our upcoming Webinar here:  Unify your Business Fax & VoIP in the Cloud

For more about Cloud Fax, you can also download our free white paper:

The IT Manager’s Survival Guide: Outsource Your Fax Infrastructure to the Cloud