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Sending & Receiving Faxes

We support inbound fax for numbers in the US, Canada and the UK. We may well support it for numbers in other countries as well, but we'd like to try it first. So, if you'd like fax numbers in another country, please tell us!

Aculab Cloud offers fax sending and receiving from UAS or REST applications. To enable fax for an application you must first enable it on the inbound or outbound service that will activate the application. Simply enabling fax in this way will make faxing possible so, if basic faxing is all you need to do, you can skip the following sections. However, some fax users may want to have more advanced control over the way fax behaves, and for these users we provide a number of configuration options. These are described below.

Service settings can be chosen using the options in the inbound and outbound service fax tabs. For REST applications, these settings are used always. For UAS applications, these settings serve as default and can be overridden by an application instance through the UAS fax API. The service settings are used during the fax negotiation phase. During the negotiation phase the two fax endpoints will determine which settings are suitable; this depends on the endpoints' capabilities and on the line conditions. The UAS fax API allows you to examine the negotiated settings which are provided as key:value pairs. The REST API allows you to examine a subset of these. See Settings reported by the fax negotiation.

Achieving a high success rate when sending and receiving faxes on the Cloud can be a challenge, not due to the Cloud itself, but due to the nature of global telecoms networks. It's more reliable in some countries than others and, even within one country, more reliable in some areas than others. As such, we strongly recommend that you thoroughly test your service before deploying it. We would also recommend that you use our PSTN providers for fax. That way, if there are problems we're in the best position to sort them out. If you are having problems sending or receiving faxes reliably, please tell us well before you want to deploy your service and we'll work to fix them.

T.30 or T.38

T.30 is an International Telecommunications Union (ITU) recommendation that defines the procedures for transmitting a document between two devices over the public switched telephone network (PSTN). T.30 dates from before the advent of the internet. More recently, Voice over IP (VoIP) has become a popular alternative to the conventional PSTN, and this has given rise to fax over IP.

Conventional fax machines were not designed to deal with the vagaries of IP networks that impair the data stream, such as latency, jitter and packet loss (see below). And so the T.38 recommendation was devised as a way to transport fax data across IP networks and overcome the problems that IP presents. However, in practice, it seems that the vagaries of IP are not as bad as they might once have been, and T.30 faxes are routinely sent over IP with very good results.

Choosing between T.30 and T.38 can come down to experimentation. Which of the two will work best for you might simply depend on the quality of your telephony provider's IP link. Also, find out whether your telephony provider supports T.38, not all of them do.

Network Impairments

Latency refers to the amount of time it takes a data packet to reach its destination. Data packets can be relayed through a large number of network elements, and each element will add to the latency. Too much latency can cause a fax to fail.

Packet loss happens when a network element is unable to relay all the data packets it receives. This can occur due to high network congestion or bad signal quality. A small amount of packet loss can usually be tolerated, but too much will cause a fax to fail.

Jitter is the term given to variations in latency on the network. Because faxes are sent in real time, they need a continuous stream of data. If a packet of data is not received when it is needed then the effect is the same as packet loss. Large variations in latency (lots of jitter) will cause a fax to fail.

The Subscriber ID

Both the fax sender and the fax receiver have a subscriber ID. These are often referred to as the called subscriber identification (CSID) and the transmitting subscriber identification (TSID).

The CSID is a string that is sent to the fax transmitter to identify a fax machine as the recipient of a fax. This string might be the fax machine's telephone number. When a fax machine receives a fax, it sends its CSID to the transmitting machine. The CSID is displayed on the sending fax machine and is usually recorded in its log of sent faxes and printed on a report or confirmation of the fax transmission. This helps to confirm that the fax is being sent to the correct recipient.

The TSID is a string that identifies a fax machine as the sender of a fax. This string might be the fax machine's telephone number. The transmitting machine sends its TSID to the receiving machine. The receiving machine typically prints the TSID at the top or bottom of the received fax to help the recipient determine the fax's origin.

SubscriberId

OPTIONS: string with a maximum of twenty characters, only 0-9 and '+' allowed

A string to be sent to the fax transmitter to identify a fax machine as the recipient of a fax.

Modems

Aculab Cloud supports three modems, V17, V29 and V27.

v17

OPTIONS: YES or NO

ITU-T V.17 modem, 7.2, 9.6, 12 and 14.4 kbit/s.

V.17 offers the highest speed, and thus the shortest transmission time, we support. However, if the line quality is not suitable for the higher speeds, this modem will drop the rate as low as 7.2 kbit/s.

v29

OPTIONS: YES or NO

ITU-T V.29 modem, 7.2 and 9.6 kbit/s.

V.29 is a simpler modem than V.17 and can be more robust against bad line conditions. It's maximum rate is 9.6 kbit/s. If the fax endpoints negotiate a drop in modem from V.17 they will skip this modem and go straight to V.27, this would represent a drop in speed from 7.2 to 4.8.

v27ter

OPTIONS: YES or NO

ITU-T V.27 modem, 2.4 and 4.8 kbit/s.

V.27 is simpler again and offers speeds up to 4.8 kbit/s. If the fax endpoints negotiate a drop in modem from V.17 or V.29 they will come here. This is a mandatory modem that cannot be turned off.

Choose which modems to enable or disable. V27ter is compulsory and cannot be disabled. The fax endpoints will attempt to negotiate the highest speed depending on the modems available. While the fax session is in progress, if line conditions are bad, the fax endpoints can renegotiate to a lower speed and even drop to a slower modem in an attempt to get the fax through.

Further modem control

MaxModemFallback

OPTIONS: positive integer

As mentioned above, the fax endpoints can negotiate slower speeds if the line conditions are bad. This option is used to limit the number of times the fax endpoints can drop the speed before the fax fails and an error is returned. Limit the number of modem fallbacks if you do not wish the fax session to keep dropping the speed if line conditions deteriorate and, instead, you wish to abort the fax and try again later.

A value of 2 or 3 would be suitable here.

MaxDataRate

OPTIONS: 0, 2400, 4800, 7200, 9600, 12000, 14400.

The maximum data rate to use, where 0 means use the maximum for the modems enabled.

Use this to restrict the maximum speed that the fax endpoints can negotiate. Note that slower speeds mean that the fax will take longer to transmit.

MinDataRate

OPTIONS: 0, 2400, 4800, 7200, 9600, 12000, 14400.

The minimum data rate to use, where 0 means use the minimum for the modems enabled.

Use this option to restrict the minimum speed that the fax endpoints can negotiate. If the fax endpoints need to drop below this speed the fax will fail and an error will be returned. This option can be used to prevent fax negotiation if the line condition is bad and, instead, try again later. Note that restricting the data rate to 14400 might cause a large number of faxes to fail if the line quality is not near perfect. Also, please be aware that turning off V.17 and restricting the minimum data rate to 14400 will make all faxes impossible.

Fax encoding

Aculab Cloud faxing supports three encoding schemes (see also the section on Tagged Image File Formats). Choose which encoding schemes to enable or disable. MH is compulsory and cannot be disabled. MMRT6 must be used in conjunction with ECM (see the section on Error Correction).

MH

OPTIONS: YES or NO

ITU-T T.4 Modified Huffman One Dimensional encoding.

Modified Huffman, the encoding scheme that all Fax machines must support. MH is an encoding scheme optimised to efficiently compress whitespace. As most faxes consist mostly of white space, this minimises their transmission time. Each line is compressed independently of the one above and below it.

MR2D

OPTIONS: YES or NO

ITU-T T.4 Modified Read Two Dimensional encoding.

Based on MH. Modified read (MR2D) encodes the first scanned line using MH. The next line is compared to the first, the differences determined, and then the differences are encoded and transmitted. This is effective as each line will probably differ little from the one before it. This is not continued to the end of the fax transmission, but only for a limited number of lines until the process is reset and a new 'first line' encoded with MH is produced. Limiting the number of lines is to prevent errors propagating throughout the whole fax if error correction (ECM) is turned off. The limited number of lines is two for low resolution faxes, and four for high resolution faxes.

MMRT6

OPTIONS: YES or NO

ITU-T T.6 Modified Modified Read - Error Correction (ECM) is mandatory.

Modified Modified Read (MMRT6) expands on MR2D. It simply allows for a greater number of lines to be coded by MR2D. In this case, there is no maximum number of lines for which the differences are encoded. Error Correction Mode (ECM) is mandatory when using MMRT6.

Error Correction

ECM

OPTIONS: YES or NO

ITU-T T.30 Annex A and ITU-T T.4 Annex A.

Error Correction Mode (ECM) automatically detects and corrects errors in the fax transmission process caused, for example, by telephone line noise. This generally means an ECM coded fax will be more likely to succeed on a bad line. ECM is the norm rather than the exception. ECM can be used with any of the encoding schemes and it is mandatory when using MMRT6.

In ECM mode a fax page is divided into blocks (or partial pages) where each block consists of 256 frames. Once the receiver has received all the frames for a particular block, it examines them and then advises the sender of any frames that are in error. The sender then need only resend the frames that are in error rather than the whole block, this is known as a Partial Page Request (PPR). If line conditions are particularly bad, there can be several PPR requests on a single partial page before all the frames get through without error.

Further error correction control - sender side

ContinueToCorrect

OPTIONS: YES or NO

ECM Continue To Correct. Enable this option to allow more than four PPRs (see ECM, above) in a single page. If this is disabled the fax session will exit with an error after four PPRs. If it is enabled, the fax endpoints will negotiate a slower speed after four PPRs to increase the likelihood of the frames getting through without error. Disable this to prevent a fax session from continually dropping the speed when line conditions deteriorate and, instead, abandon the fax to try again later.

MaxPageRetries

OPTIONS: A positive integer.

If ECM is off, a received fax page can contain bad lines. This parameter specifies the maximum percentage of bad lines received in a single page before requesting that the page be resent.

If ECM is on, a received fax page will not contain bad lines, instead, the receiver will send PPRs. After every set of four PPRs the ContinueToCorrect parameter is checked and, if it is on, the modem speed will be reduced and another batch of four PPRs will be allowed. This parameter sets the maximum number of times we can "continue to correct" before terminating the fax. If ContinueToCorrect is disabled, this parameter will have no effect.

If line conditions are bad, a page or partial page might be attempted many times before getting through, and this could take a long time. Set this parameter to a low number to abort such a fax session and try again later.

Further error correction control - receiver side

PercentBadLines

OPTIONS: An integer from 0 to 100.

If ECM is off, a received fax page can contain bad lines. This parameter specifies the maximum percentage of bad lines received in a single page before requesting that the page is resent.

MaxConsecutiveBadLines

OPTIONS: A positive integer.

If ECM is off, a received fax page can contain bad lines. This parameter specifies the maximum number of consecutive bad lines received in a single page before requesting that the page is resent.

The Image - receiver side

HighRes

OPTIONS: YES or NO

If this is YES, 200x200 dpi resolution is enabled and any low resolution pages will be transcoded to high resolution before being transmitted, thus increasing the amount of data to be sent. If this is OFF, the resolution is restricted to 200x100 dpi and any high resolution pages will be transcoded to low resolution, thus reducing the amount of data to be sent and causing the fax to be transmitted more quickly.

MaxPageWidth

OPTIONS: A4 (1728 pixels), B4 (2048 pixels) or A3 (2432 pixels).

This specifies the maximum page width that the receiver can support. The B4 width shown above is the Japanese variant, the ISO B4 width is 250 millimetres. B4 width faxing is common in Japan. If the sender requests a page width that is bigger than the maximum supported by the receiver, the fax will fail with an error.

The Image - sender side

HighRes

OPTIONS: YES, NO, and AUTO.

If this is YES, 200x200 resolution is enabled and any low resolution pages will be transcoded to high resolution before being transmitted, thus increasing the amount of data to be sent. If this is OFF, the resolution is restricted to 200x100 and any high resolution pages will be transcoded to low resolution, thus reducing the amount of data to be sent and causing the fax to be transmitted more quickly.

This option can also be set to AUTO. AUTO will enable high resolution if any of the pages to be sent are high resolution, otherwise, it will be set to low resolution.

Note that even if a fax sender enables high resolution, the resolution that is negotiated will also depend on the capability of the receiving end. If the receiving end does not support high resolution the fax will be transmitted at the lower resolution even if high resolution is enabled.

Tagged Image File Format (TIFF)

Aculab Cloud faxing supports TIFF files only. The compression schemes supported are:

  • CCITT Group 3 1-Dimensional Modified Huffman run length encoding (RLE). Also known as MH or CCITT 1D.
  • CCITT T.4 bi-level encoding as specified in section 4, Coding, of ITU-T Recommendation T.4. Also known as MR2D, CCITT Group 3 fax encoding or CCITT Group 3 2D.
  • CCITT T.6 bi-level encoding as specified in section 2 of ITU-T Recommendation T.6. Also known as MMRT6 or CCITT Group 4 fax encoding.

These compression schemes do not support colour or greyscale. The image will be black and white only.

We support three page widths, A4 (1728 pixels, 21.95cm), B4 (2048 pixels, 26.01cm) or A3 (2432 pixels, 30.89cm).

Before sending a fax, when converting an image to TIFF from another format, e.g. Windows BMP, several options may be presented. For example, IrfanView (www.irfanview.com) offers:

  • None
  • LZW
  • Packbits
  • JPEG
  • ZIP
  • Huffman RLE
  • CCITT Fax 3
  • CCITT Fax 4

The three options that match those we support are Huffman RLE (MH), CCITT Fax 3 (MR2D) and CCITT Fax 4 (MMRT6). The option that offers the best compression is CCITT Fax 4.

Settings reported by the fax negotiation

These are the fax settings chosen by the fax endpoints during the negotiation process.

modem

VALUES: v17, v29 or v27ter

The modem to be used for page data transmission

datarate

VALUES: 2400, 4800, 7200, 9600, 12000, 14400

The data rate to be used for page data transmission. The data rate that can be chosen is limited by the modem that has been negotiated; v17 can go up to 14400, v29 can go up to 9600, v27ter is limited to 4800.

ECM

VALUES: YES or NO

Whether error correction will be applied.

encoding

VALUES: MH, MR2D, MMRT6

The page data encoding method. MMRT6 cannot be negotiated if ECM is turned off.

resolution

VALUES: HighRes (200x200), or LowRes (200x100)

The page resolution.

pagewidth

VALUES: A4, B4 or A3

The width of the page

remotesubscriberid

The remote end's subscriber ID