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Q82642: Serial Communications Speeds in Windows 3.1

Article: Q82642
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.1,3.11
Operating System(s): 
Keyword(s): 
Last Modified: 27-SEP-1999

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The information in this article applies to:

- Microsoft Windows versions 3.1, 3.11 
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SUMMARY
=======

Microsoft Windows version 3.1 allows applications to select bit rates as high as
57.6 kilobits per second (Kbps). However, how reliably you can transmit and
receive data at high speeds depends on many variables, which are discussed
below. With Windows 3.1 in a reasonably busy system, a 19.2 Kbps XModem transfer
can run well on a 20 megahertz 80386 machine.

MORE INFORMATION
================

Maximum Bits-Per-Second (BPS) Rate for Windows Applications
-----------------------------------------------------------

Factors affecting top BPS rate in Windows applications include:

- CPU Speed: Faster CPUs allow faster data throughput with fewer errors. This
  is because they allow the communication driver to service character
  interrupts faster (preventing UART overflow), and allow applications to
  handle transfer protocols better.

  If you are experiencing slow serial performance or are losing characters on an
  80286 based PC, do the following to correct the problem:

  1. Reduce the number of MS-DOS drivers and terminate-and-stay- resident (TSR)
     programs.

  2. Do not load MS-DOS 5.0 into upper memory.

  3. Add the following line to the SYSTEM.INI [standard] section:

  FasterModeSwitch=1

  For more information on FasterModeSwitch and other SYSTEM.INI file switches,
  see the SYSINI.WRI file in your Windows directory.

- Transfer Protocols: Data transmission protocols (for example, Xmodem, Ymodem,
  Zmodem, and so on) vary greatly in robustness in a multitasking environment.
  Some are designed to be very fast, with little error checking, while others
  trade throughput for error correction. Usually, XModem and ZModem work well
  with Windows, even at high speeds.

- Serial Port Hardware: Newer serial ports use the 16550 UART, which contains a
  buffer to reduce overhead and prevent overflow. See additional discussion on
  this below.

- System Overhead: The more applications, TSRs, and device drivers that are
  running, the busier the operating system is. This reduces overall execution
  time for everything in the system. If you have many programs and devices
  active while trying to transmit data at high speeds, you are likely to
  encounter reduced throughput and transmission errors.

- Application Quality: Like most other software, not all serial communications
  applications are created equal. These vary in quality and performance, so
  comparison shopping is valuable before purchasing. In general, choose a
  program that is well-tested and supported by an experienced vendor.

Maximum Possible Bit Rate for MS-DOS Apps Running in Windows
------------------------------------------------------------

MS-DOS serial communications applications are designed to run in a non-
multitasking environment, and make assumptions about the operating system and
devices that are not always true in a multitasking operating system. These
applications may allow you to select speeds as high as 115 Kbps and beyond.
While these speeds may work very well in MS-DOS alone, there will most likely be
reduced data throughput when running these applications in a multitasking
operating system such as Windows or OS/2.

The guidelines listed above for Windows serial communications applications also
apply to MS-DOS based applications. However, the following guidelines are
important for MS-DOS applications running within Windows enhanced mode:

- Full-screen vs. Windowed vs. Minimized: MS-DOS applications run best in
  full-screen mode. They receive their foreground priority and avoid the
  overhead of display virtualization. Although MS-DOS applications running in a
  foreground window receive their foreground execution priority, the overhead
  of updating the window can seriously reduce application performance. If you
  wish to run an MS-DOS communications application in the background during a
  data transfer, run it minimized as an icon rather than in a window.

- Foreground & Background Execution Priority: Windows allocates execution
  time for MS-DOS applications according to foreground and background
  priorities that are in an application's program information file (PIF). If
  the application is not getting the time slices it needs to process incoming
  data, it will encounter errors during data transfer. Increase the MS-DOS
  application's background execution priority in the PIF to prevent errors when
  receiving data in the background. And of course, make sure that the
  "Background" execution is marked in the PIF.

- "Lock Application Memory:" In order to allow more MS-DOS applications to run
  simultaneously, Windows 3.1 386 enhanced mode swaps application memory to
  disk. However, this can slow the execution of the MS-DOS application, and
  significantly reduce communications performance. Disable this swapping of
  application memory in the MS-DOS communications program's .PIF by marking the
  "Lock Application Memory" check box in the Advanced Options dialog box of the
  PIF Editor. Note that this PIF setting only applies if you have a permanent
  swap file and 32-bit disk access is enabled.


Additional query words: 3.10 3.11 modem

======================================================================
Keywords          :  
Technology        : kbWin3xSearch kbZNotKeyword3 kbWin310 kbWin311
Version           : WINDOWS:3.1,3.11

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