Newsgroup: it.comp.hardware.modem
Titolo: Fax di classe 1 e 2

Chi sa dirmi quale è la differenza tra fax di classe 1 e fax di classe 2 
? Quale è il migliore dei due ?

? Quale è il migliore dei due ?
Forse può essere utile il seguente documento:
From "The Modem Report", Rockwell International - Digital Communications
Modem Technology News. Issue No. 3, May 1993.
CLASS 1 OR CLASS 2, DOES IT MATTER?
By Mark Chapman, Chris Sneed and Tony Zuccarino
The widespread availability of low cost PC Fax modem boards has resulted in
explosive growth in PC fax communication. This in turn has spawned a range
of good PC fax applications software offering a variety of user friendly
features.
To the user, the real questions are "how easy is it to send a fax" and
"will the software work with my hardware".
There are many issues of standardization and much discussion in the
industry over just what overhead a real time task such as receiving a fax
will place on the PC.
Standards
International standards have facilitated reliable inter-working of modems
and software by creating command interfaces for modem hardware and software.
As long as any modem board and any communications software package comply
with the same standards, they will work together.
Compatibility
To the end user, the interaction of the communication software and modem is
irrelevant as long as the system works. But the question, "are there
advantages to buying a modem complying with one standard versus a different
standard?", is often raised.
The current industry debate centers on the relative merits of the two major
standards for PC fax , the Class 1 and the Class 2 specifications.
About fax machines...
To understand the issues, let's review fax operation and how this affects
the PC. A typical fax transaction consists of five phases:
   * Phase A. Call Establishment (e.g., dialing or answering the phone).
   * Phase B. Pre-message Procedure -- selecting parameters acceptable to
     the calling and the called station (i.e., speed, fine resolution,
     modulation etc.).
   * Phase C. Message Transmission -- usually image data for one page.
   * Phase D. Post-message procedure -- page break marker, and opportunity
     to change settings (e.g., drop to a lower speed because of too many
     errors detected during Phase C).
   * Phase E. Call clear down (i.e. hanging up the phone).
The actual operation of the five phases is specified by two CCITT standard
protocols, T.4, and T.30.
T.4 defines the image transfer portion of the session used in phase C.
T.30 defines the negotiation and inter page phases (ABD&E). A normal
multi-page transfer uses T.30 only at the beginning of the call, between
pages, and after the last page.
Class 1 and Class 2:
Class 1 and Class 2 standards specify command sets for modems and software
packages that allow modem manufacturers and software developers to build
products independent of each other. As long as both of the vendors support
either Class 1 or Class 2, their products will work well together. Class 2
is complex, supporting about 30 commands, while Class 1 supports less than
10. However, the main difference is that Class 1 requires the PC to handle
both T.30 and T.4 protocols, while in Class 2 only T.4 is done in the PC.
                    Class 1        Class 2
   T.30 Protocol    PC software    PC fax modem
   T.4 Protocol     PC software    PC software
So how much does this additional processing mode affect the PC, especially
one running Windows?
Significantly, Phase C, using T.4, takes up about 95% of the time and CPU
effort during a typical facsimile transfer. This is because most of the
time is spent scanning, compressing (or decompressing if receiving) and
sending the pages, and very little time is spent by T.30 negotiating at
the start and end of the call as well as between pages.
During the message transfer Phase C, Class 1 and Class 2 have the same
performance. The two situations where there could be a difference between
Class 1 and Class 2 in Phase C are:
   * (1) receive line quality checking (to determine if the received line
         is bad)
   * (2) padding the transmitted line (to meet minimum line transmit times)
Line quality checking is much more strenuous than line padding.
Due to the many methods of line quality checking, and the vagueness of the
specifications, many, if not all, software packages perform this function
in any case irrespective of the modem type. This means that line quality
checking gets done in the PC irrespective of whether the modem supports
Class 1 or Class 2.
As message transfer dominates fax transmission, most of the CPU intensive
tasks are incurred by Class 1 and 2 alike. In fact as the number of pages
increases, T.4 becomes by far the dominant factor in fax transmission and
the difference between Class 1 & Class 2 becomes negligible.
Multitasking Environment
There are certain stages in call set up and interpage negotiation during
which the modem requires the close attention of the PC. At these times, the
PC software must respond rapidly and for Windows-based software (the
application is controlling T.30), this means that the application cannot
relinquish control to other applications during these critical times. It is
this real-time constraint and associated interrupt overhead rather than any
processor load factor which affects operation under Windows.
In practice, this effect can be largely prevented by suitable design of the
applications software where the time critical T.30 portions are included at
the driver level and these days all good commercial Class 1 fax packages
work as well as Class 2 under Windows.
Class 1 Provides Flexibility
Class 2 is a "closed" standard, in that any changes to the T.30 protocol
require a revision to the firmware (EPROM) on the fax modem board.
Modem and software manufacturers occasionally develop T.30 code updates in
order to add additional features, correct errors in prior code revisions,
or resolve interoperability issues with particular fax machines (the T.30
standard leaves some room for interpretation). While a Class 2 fax modem
will require a firmware update (new EPROM) to incorporate the change, a
Class 1 fax modem user can incorporate the change simply by downloading new
software from a bulletin board, or ordering it from the software
manufacturer.
There are about five different "Class 2" versions out there with no
guarantee of compatibility (there are software packages on the market that
do support all of the variants). However, most Class 2 fax boards support an
interim working specification of Class 2 that has become the defacto Class 2
standard. Recently, the EIA-TIA approved a real class standard, but it will
take 6 months to a year before software and hardware will emerge based on
this new Class 2 standard.
Non-standard facilities (allowed under T.30) are not supported by Class 2
but can be easily added to Class 1 boards extending the life of modem
hardware as new software or operating systems features become available.
In short, the Class 2 standard is fixed, whereas Class 1 is more flexible
and can be maintained as software evolves. There are niche markets in which
unique technical constraints dictate that Class 1 is favored and other niche
markets where Class 2 is optimum but for the vast majority of the computing
public, there is no discernible difference between Class 1 and 2. Both
standards send and receive faxes reliably and the end user will neither care
nor need to get involved comparing Class 1 and Class 2 as long as the
application works.
Enough theory... is there actually a difference?
To evaluate the practical effect on CPU performance between Class 1 and
Class 2 fax, a series of tests were performed on a representative set up
using popular fax software packages. (See Figure 1). This was done by
measuring the effect of receiving a fax in background mode while the PC in
foreground is busy with a task.
(Figure 1 showwed time results of Class 1 and Class 2 using three Fax
packages: Winfax, QuickLink II, BitFax on three CPU types: 386SX/25-MHz,
386DX/33-MHz, 486DX2/50-MHz. The times for a given CPU were within 5% of
each other independant of Class or Fax Software.)
The task chosen for the series of tests was both CPU-and disk
storage-intensive, and consisted of compressing a large collection of
varied files. The received fax transmission consisted of 3 pages, including
a cover page and representative test fax pages. The tests were repeated
several times, and figures varied less than +/- 2% leading to a high degree
of confidence in the test results.
Conclusions
There is a measurable amount of time "stolen" from a PC's CPU by fax modems.
However, the difference between Class 1 and Class 2 in terms of foreground
processing load is negligible. In fact the comparison between individual
manufacturers fax packages is in some instances significantly greater than
the difference between Class 1 and Class 2. The effect diminishes
considerably as the processor power increases, and the difference between
Class 1 and Class 2 reception is further reduced as more pages are sent.
End users have consistently demonstrated their preference by choosing the
low cost, flexibility and future upgrade capability of Class 1 over the
slightly faster speed of Class 2. Which one provides more functionality? As
most, if not all, software packages support both standards with the same
user interface, the answer is that to the end user, Class 1 and 2 are
transparent and offer the same functionality. Rockwell offers modems
supporting both standards allowing users to make their own choice, safe in
the knowledge that they are buying the leading solution in either case.