The dichotomy of fax protocol conformance versus interoperability becomes most pointed when it is necessary to gauge the performance of a fax terminal. A Quality of Service (QoS) standard is simply a necessity for this type of evaluation. The ITU undertook the task of creating such a standard in 1993. The ITU-T recommendations E.450 through E.457 define an objective quality measurement standard for Group 3 facsimile transmissions. This standard is implemented in QualityLogic’s FaxLab test tool in order to provide a quick means of evaluating the relative quality of a series of test calls and, thereby, facilitate an operational assessment of a test target.
Assessing Fax Terminal Performance
The E.450 series of documents form the basis of a Figure Of Merit (FOM) that is used to assess a fax terminal’s performance from the point of view of the user. Rather than wade off into the technical morass of page encoding and decoding, data rate control, message response times and the like, FOM measures fax transmissions on the basis of those call qualities that are both obvious and desirable to the user. More specifically, it breaks call quality down into three aspects:
- speed of transmission
- presence of errors in the decoded page, and
- success in transferring all the intended pages
To properly understand these aspects it is necessary to define some of their parameters:
complete – The transmission completed all 5 phases of the T.30 protocol specification ending in the originating device emitting its DCN message. This usually (though not invariably) indicates that the transmission has successfully transferred most if not all of its pages without major mishap.
error-free page – A page is considered error-free if it has no errored scan lines and contains all the scan lines of the transmitted original page. Pages delivered with error-free quality will retain all of their original information content, as though the facsimile operations were a lossless encode-transmit-decode process.
errored page – A page is considered errored if it has one or more scan line errors but less than the number required to declare the page severely-errored (see severely-errored). Pages received at this level of quality contain some degradation to the original image, but are very likely to retain their intended utility.
maximum speed – The transmission was conducted at the maximum data rate mutually supported by both facsimile terminals. This indicates that there was no fall back during either the initial phase B negotiation or between page image transmissions.
severely-errored page – A page is considered severely-errored if it has at least 1 instance of four or more consecutive scan line errors; and/or at least 12 total scan line errors in single instances; and/or at least 3 instances of consecutive errored scan lines with two and/or three consecutive scan lines in each instance. Pages received with this level of quality contain substantial deviation from the original image bit-map and, based on subjective studies, are considered unacceptable. It is likely that they no longer convey some part of their original information, or contain distortion that significantly detracts from their usefulness.
Note that the severely-errored page definition is scaled for a 200 x 100 dpi resolution. While the ITU has not specified values for higher resolutions, it may safely be assumed that a linear interpolation will be appropriate for those resolutions.
These parameters are combined to create seven levels of fax call quality as reflected in the table below:
Figure of Merit Values
|Complete||Maximum Speed||Image Quality|
|7||No||Not Applicable||Not Applicable|
Note: If the transaction is incomplete, it is categorized as type 7 irrespective of the speed and image quality of the completed pages.
Implications of Fax Figure of Merit
Type 1 transactions are those that are complete, have the highest possible speed and are error-free, i.e. these can be considered “perfect” from a transmission point of view.
- Type 1 and type 2 transactions are those that are complete, have the highest possible speed and do not have any severely-errored pages. For most facsimile applications, these transmissions maintain their utility to the customer, i.e. they can be considered “effective.”
- Types 3, 6 and 7 represent transactions that are “badly” damaged in some form and likely indicate a need for some form of corrective action.
- Types 4, 5 and 6 represent transactions that are mirrors of types 1, 2, and 3 but have speed fallbacks and result in longer connection times.
Obviously, the FOM levels lack precision and would not be what a telecom engineer would typically use to rate fax transmission quality. What they do offer is a quick, easy to understand look at how a customer will perceive the operation of his new fax machine. If he is seeing a high proportion of type 1 pages, he will be a happy camper. If there are too may instances of type 3, 6 and 7 operation, expect a warm call to tech support.
FOM is a useful yardstick for marketing and engineering to create a common measure of the new product’s ability to please its intended customer base. Or do you have a different take?
Let me know. I would like to know your views on FOM.