Fax over IP - What works best?

16 June 15

Why retaining a single PSTN line for the fax in your warehouse may be a better approach than Fax over IP for the rest of the office.


Where you can do away with the old fashioned fax machines and make people comfortable with email to fax and fax to email you are definitely going to achieve better cost savings in terms of delivery of information, removal of human error and better distribution to dedicated groups where virtual fax numbers are used. Better security and more options generally translate to better business.

However in certain situations, like a warehouse or similar, your company might often have orders received by fax that form part of the standard workflow and confirmation occurring by way of return fax. This approach of pulling the fax off the machine as it comes in means no upskilling of staff in terms of how things are done and adds no overhead or PC literacy requirements. Perhaps here and in other similar situations a fax machine still has its place, and given how fax over IP works means the need for a hybrid solution.

Our advice, for this type of environment, is to keep your fax machine and a PSTN line. When you use an analogue adaptor to connect your fax machine to an IP network you will be asking a lot of different providers to interconnect and make up the communication path that needs to be established to make a fax transmission in this medium possible.

Here’s our thoughts on the subject and why you might, after reading this article, also take the hybrid route.


In the nineties if you worked in the telco space then “fax” and “T.30” basically meant the same thing. T.30, as the ITU recommendation for transmitting faxes over the general switched telephone network, laid the path for handling background noise and spikes of interference on the telephone line. Transmission speed was dropped to handle poor signal to noise ratios so that the tones from the transmitting end were interpreted correctly.

However this does not work in an IP network, as impairments such as packet loss, which can result in large gaps in fax data, cannot be eliminated by a lower transmission speed.

Generally speaking lower speeds actually make the performance worse by generating additional network traffic that will be exposed to packet loss. Retries don’t always recover lost data when packet loss is frequent, because the retries can suffer packet loss as well. Even the retry requests themselves may be lost.

FoIP Transport Methods

In most instances when you build a Fax over IP solution, the transport method used is either one called T38 or based on tones G711. A number of things such as fax speed, redundancy, and error correction, can have a significant impact on FoIP performance.

T.38 versus G.711

T.38 allows for fax data to be carried over IP networks. Data is transmitted directly in T.38 without being converted to an audio stream, and results in a significant reduction in the bandwidth needed. T.38 also supports data and controls redundancy to mitigate the effects of packet loss.

Some disadvantages of T.38 are that gateway support for fax parameters are not standardised everywhere. As an example, certain modem specifications like V34.  Also, in the current mixed network environment of packet-based and circuit-switched (that is, PSTN) connections, T.38 often has a transcoding overhead (conversion from PSTN to IP and back). That can add latency which is death to the fax at the remote end.

G.711 is a commonly used form of Pulse Code Modulation (PCM) for voice frequencies. It uses an uncompressed format and requires bandwidth of about 64 kbps and in the PCM world comprises a single channel for voice. Using G.711 as the transport method for FoIP is an extension of traditional PSTN audio-based faxing. The digital fax data is converted to a PCM audio stream and then sent as G.711 Real-time Transport Protocol (RTP) packets. Its important to know this protocol was designed for a synchronous network which is why it was still effective for fax transmission. IP networks are asynchronous and as such the same robustness does not exist.

G.711 is therefore not optimized for fax transport over IP networks, and does not typically support packet redundancy. Having been developed for voice, G.711 allows the transmission of missing audio because any gaps would be filled in by a human listener. But when used to transmit modem data, any loss of packets is almost fatal as the receiver has no way to recreate the missing data.

Many IP based carrier networks may not be fax-aware, and therefore optimize the G.711 stream for voice with the use of silence suppression and echo cancellation. This can and will in most instances cause a loss of data and prevent FoIP from operating.

Different types of Modems

Two commonly used modem specifications are V17 and V34 that were developed in the PSTN era of faxing, Neither was designed with IP network impairments in mind. Both use training cycles at the beginning to detect what speed is achievable with V34 capable of higher speeds but as a result also more likely to train down when it encounters packet loss which translates to silence.

What redundancy and why?

T.38 generally supports two types of redundancy:

• Control—Refers to IP packets that contain fax control commands. This is sometimes referred to as “low speed” redundancy because in traditional analogue fax, these commands are transmitted at a low data rate of 300 or 1200 bps.

• Data—Refers to IP packets that contain fax image data, and is sometimes referred to as “high-speed” redundancy. G.711 can also support redundancy through the use of redundant RTP, but support for this is not currently widespread.


Error Correction Mode (ECM) is a traditional check summing method applied to blocks of fax data. Note not all fax devices support this way of protecting the integrity of the reproduced image on the remote fax, where a segment of fax data is sent with a checksum it is verified at the receiving end. When data is corrupted or missing, the receiving fax machine will request that it be re-transmitted.

When this is not used, missing data will simply be omitted from the received fax image, causing some degradation in image quality. Because network impairments almost always mean a loss of data, ECM can help to preserve image quality in these situations. However there is a trade-off here in that ECM usage can cause an increase in both the transmission time and the number of unsuccessful faxes (fax attempts that do not complete). The transmission time increases because missing data needs to be requested and retransmitted.

The number of unsuccessful faxes can increase because impairments may corrupt re-transmissions, causing the fax to be aborted when the maximum number of retries is reached

What do we mean by Network Impairments


Latency is a term used to define the amount of time it takes transmitted data to reach its destination. Round trip latency refers to the amount of time it takes transmitted data to reach its destination plus the time for the remote ends response to be returned.

Let’s look at a fax that needs to be sent out using an IP network. IP data is sent from a fax endpoint to a receiving fax machine, the data packets are relayed through a number of network elements. The first would normally be a series of Ethernet routers. Then a gateway may receive the data and transcode it into another format. For example, a gateway may transcode a T.38 digital fax into a PSTN audio stream. If the fax is being sent to an international or national location then it may go through more than one gateway. Every network device adds some amount of latency to the data. Some add a very small amount in the order of milliseconds, whereas others can add a whole lot more time. Think IP to PSTN then PSTN to IP then back to IP to PSTN and you start to get the idea.

If the data travels over the traditional PSTN network, it may also add delays depending on what type of transmission devices are used to reach the destination hence the use of ECM and the speed of transmission varying to take this into account. Even in a PSTN network it’s still possible for a fax not to be successful.

In general, the amount of latency for a given fax call can vary widely, depending on such factors as the source and destination locations, as well as what network elements are between them.

Packet Loss

Packet loss can occur when network congestion is high and a network device is unable to relay all the packets it receives. Typically these congestion periods are not long in duration, typically this happens when a large data file is being transferred over a network. This can also occur when high priority data, such as real time voice data, is given priority over other network traffic. In addition hardware causes, such as poor signal quality on a cable, can also lead to packet loss.

Burst Packet Loss

Burst packet loss refers to a series of consecutive IP packets from a stream not reaching its destination. The length of the burst often determines how negatively it will affect a FoIP call.

Single Packet Loss

Single packet loss refers to the occasional loss of single packets (that is, non-consecutive packets) from a data stream. This type of loss is not as significant as burst losses, but can cause serious issues if it occurs frequently.


Jitter involves elements of both latency and packet loss. Jitter refers to variations in latency the network adds to transmitted packets. Because faxes are sent in real time (remember that word synchronous), they require a continuous stream of data to transmit successfully, particularly if the data is being carried as an audio stream (ie G711). If a packet of data has not been received when it is needed, then the net effect is the same as a lost packet. If several packets are received too late to be used, then that has the same net effect as a burst of lost packets. Even if the late packets eventually arrive, it will be too late for the receiving side to use them, and thus they will be discarded. For FoIP, packet loss due to jitter can be well controlled with the use of jitter buffers. This process buffers several packets before they are read so that the receiver will not run out of packets if/when some are delayed. But a drawback of jitter buffers is that they add latency. And when multiple network elements are in a connection employing jitter buffers, then the latency effect can mean the commulative effect means the fax machine at the remote end interprets this as lost connection.

Another source of packet loss related to jitter can occur when G.711 faxes experience clock skews. This happens when the sender and receiver are using unsynchronized clock speeds for the audio stream. If the sender is creating packets slightly faster than the receiver is reading them, eventually the jitter buffer of the receiver will overflow and a packet will be lost. Or if the receiver is reading packets faster than they are being sent, eventually the jitter buffer will run dry, and the receiver will not have a packet to read when it needs it. So although jitter is a form of latency, its impact on fax performance is usually related to how much packet loss it causes, rather than the latency itself. Remember IP networks are asynchronous and unlike an ISDN environment do not use a common clock source.

Impact of Network Impairments

Network impairments can causes faxes to fail in many different ways. Several layers and phases are employed by FoIP, and each one can react differently when impacted by network impairment. The specific reaction is largely determined by the severity of the impairment and which phase of the call is in progress when the impairment happens. On an application level, fatal errors will typically be reported by the fax API function that was in progress when the impairment occurred. Non-fatal errors are not reported by all types of network elements, but can be observed by products that support the collection of network quality metrics.

Call Control

The first phase of a fax that can be impacted by network impairments is the call control phase. For FoIP, call connections are generally controlled with the Session Initiation Protocol (SIP) or H.323 protocols. For example, if packets are lost on the network and the call setup packet is dropped, then the receiving side will not know that a call request has occurred. The sender will wait for a response, eventually time out, and end with a “no answer” or “line busy” error. If the response packet to the call setup is dropped, the result would be similar. To complete the call, the fax server or user would need to make another attempt to send the fax. Other call control operations that can be impacted include the fax transport negotiations (that is, setting up T.38 or G.711) and call termination.

Fax Control

Once a call has successfully transitioned into fax mode, using either T.38 or G.711, the call enters the fax control phase. During this time, traditional T.30 fax operations take place, such as training the modem to the highest working bitrate, and negotiating to send the next page in the fax. Packet loss during this time could cause the fax speed to train to a lower speed, or cause the fax to fail altogether. Lower speeds will increase the fax transmission time. If fax control commands or their acknowledgements are dropped, it triggers several retries (usually up to three) for those commands until they succeed, or until one side gives up and ends the call. The result could be that all or just part of the fax is truncated at the receiving end.

High latency may cause fax control failures because T.30 has required timer values for completing some command sequences. For example, most commands use a three second timer for receiving a response. If a response is excessively delayed, these timers will be exceeded and the call will fail. Fax calls that end prematurely will need to be re-sent by the fax server or a user.

Image Data

During the fax data phase, the actual image data for each fax page is transmitted. Depending on whether ECM is being used, missed data will be re-requested or simply left out of the received image. If re-requested, the fax transmission time will increase. If packets for the re-transmission requests are lost, then there will be further delays and possibly failure of the fax if retry limits are reached. If ECM is not used, then lost data will result in omissions in the received fax image, usually in the form of missing or repeated horizontal strips.

Impairment Frequency

How often the network impairment occurs can also have a large impact on the overall fax completion rate. If the impairment is continuous, (for example, consistently dropping some percentage of transmitted packets) then the impact on the fax completion rate will be more pronounced. Also, with a continuous impairment, faxes with longer durations (that is, those faxes with a large number of pages), will have a greater exposure to the impairment, thus increasing the chance the fax will fail or terminate prematurely. If the impairment is infrequent and short in duration, it will have less of an overall impact on fax performance. However, it is important to note that some packets within a fax transmission are more critical and vulnerable to loss than others. Data packets, for example, can often be lost without causing the fax call to fail, but the loss of certain control packets can more readily lead to a failed call. With that in mind, even a small and infrequent network issue could result in occasional fax failures, if, by chance, it impacts a particularly important set of packets.


Imagine you go to send a fax from the warehouse to a customer located across town, by the time it reaches them the reproduction they receive may have touched three of more carriers and have had all of the issues mentioned above applied to it if there is Fax over IP used at any point in the overall path.

In the event the document is treated as a fax over PSTN end to end, it (the fax machine) will simply train down to a speed that supports the signal to noise on the line that is acceptable for transmission to occur between both parties. As this mode of communication is synchronous the only buffer required is on the remote fax machine itself. This makes transmission simple and reliable which is why it works every time. Its now eight years since companies suggested it would be possible to support Fax over IP and we are sure one day it will be possible.

However we believe all aspects of your business need to work effectively and with cost savings that are tangible and not diluted by hidden costs associated with lost productivity which is often the case when all options are not investigated

We hope you will form the same opinion.