RAS 1.0 and 1.1 Data Compression, Correction, Flow Control

ID: Q98594

The information in this article applies to:

Microsoft LAN Manager, versions 2.1, 2.2

SUMMARY

Remote Access Service (RAS) version 1.0 and 1.1 do not support flow control of any kind. Almost all modems require flow control for modem data compression and error correction. This article examines these and other aspects of modem communications.

NOTE: At the same time Microsoft released the graphical version of RAS in Windows NT version 3.1, the RAS 1.1 client for MS-DOS was upgraded to RAS 1.1a and includes hardware flow control. This allows RAS 1.1a to communicate with Windows NT RAS servers at baud rates of 19200 bps and support modem data compression, However, the RAS 1.1a client does not support the new and more efficient compression scheme used by Windows NT RAS. To take advantage of the new software compression, you must use the Windows for Workgroups version 3.11 RAS client or Windows NT RAS client. RAS 1.1a is a free upgrade to the previous versions of RAS.

MORE INFORMATION

Flow Control

Flow control is a way for a modem to tell the computer to stop sending data to it through the RS-232 cable. (The reverse is also true: the computer also tells the modem when to stop sending. We are looking only at one side here.) For example:


     DTE         DCE                DCE            DTE
Terminal---------Modem------Z-------Modem----------Terminal
          9600             2400             9600

Assume you set your data terminal equipment (DTE) to 9600 bps so you can get better performance from the modem (that is, the modem is always busy and doesn't wait on the terminal for more data), but the modem is capable of only 2400 bps over the phone line (the data communication equipment, or DCE, rate or link rate). Most modems have some small internal buffer, so when the terminal starts sending data to the modem at 9600 bps the modem puts the data in a FIFO buffer and begins transmitting data over the phone line. Because of the difference in DTE and DCE rates, the modem buffer begins to fill. When it reaches a certain level, the modem uses flow control to tell the terminal to stop sending. When the buffer reaches a certain low level, the modem tells the DTE to start sending again.

Hardware Flow Control

Hardware flow control (or RTS\CTS) uses an "out of band" technique, meaning that the control signal has a special line outside the data channel. With full duplex, the modem drops CTS (clear to send) to tell the terminal to stop sending. The modem sets CTS high again when its buffers reach the set low level. RTS (request to send) is used by terminal (DTE) when receiving data. RAS ignores hardware flow control; so if the modem has it turned on, the modem could turn off CTS and RAS will just keep sending data, which will be dropped when the modem buffer fills.

Software Flow Control

Software flow control (or XON\XOFF) uses an "in band" technique, meaning that the control signal\command is sent along with the data stream. The modem sends an XOFF character to tell the DTE to stop sending. XON tells the DTE start again. These characters are mixed in with the data. RAS is capable of sending and receiving data at the same time, but it doesn't filter for the XON\XOFF characters. So if a modem is looking for these characters, RAS might accidentally tell the modem to stop sending, then never tell the modem to re-start (with XON) and communications fail.


   XON  - [CTRL Q] - ASCII 17
   XOFF - [CTRL S] - ASCII 19

As stated above, RAS 1.0 and 1.1 do not support flow control. Do not configure your modems to use flow control; it could cause RAS to fail.

If you cannot have flow control, set up the DTE-to-DCE rate to match the DCE-to-DCE rate. This minimizes the chances of modem overflow, and means that your high speed modems should be configured to autobaud, that is, the DTE rate should be adjusted by the incoming DCE rate. This is configured by default in RAS with all the supported high speed modems.

Almost all modems need flow control for modem data compression and error correction.

Error Correction

Error correction is a modem technique used to ensure that blocks of data are accurately received by the remote modem. Common error correction schemes are MNP 4 and V.42 (preferred).

The process works this way: First, data to be transmitted is gathered into a block of characters. Then, an algorithm generates a number based on the block (a checksum) and attaches it to the end of the block. When the other modem received the block, it performs the same algorithm and compares the result with the sent checksum. If these values do not match, the receiving modem tells the sender to retransmit the block.

Because a block of data may have to be retransmitted, modems store blocks in a buffer until sure that the receiving modem has correctly processed it. The modem must use flow control to keep the DTE from overwriting the frame with new data. Because RAS does not respond to flow control, it may overwrite a block that has not yet been acknowledged.

AsyBEUI, which is almost identical to NetBEUI, implements its own error control. All LLC type 2 (connection oriented) frames must be explicitly or implicitly acknowledged within a specified amount of time or AsyBEUI retransmits them. If modem error correction retransmits a frame, then AsyBEUI times out and retransmits the frame again, the unnecessary traffic duplication slows things down, and generally confuses RAS.

The only reason to turn on error correction with RAS is to use data compression (most modems require error correction for data compression). If you want to use error correction, please see the excerpt from the README.TXT file at the end of this article. In addition, you should do the following:

On both client and server, turn on hardware flow control on your modem. Again, RAS ignores this but at least it won't interfere with data transfer. Add the flow control command to the INIT command in the MODEMS.INF file.

On both client and server, turn on error correction on the modem. Use v.42 if available. Add the AT command to the COMMAND = line in MODEMS.INF file. See the RAS manual for more information on editing MODEMS.INF.

Data Compression

Data compression allows modems to convey more information in a given block of data, thus increasing throughput and reducing transmission time. Common compression routines are MNP 5 and V.42bis (the preferred standard--it's 30 percent faster, and more commonly used).

Most modems won't let you turn on compression without error correction, and thus flow control. It is not enabled by default, and generally should not be used. If you want to try using it, please see the excerpt from the README.TXT file at the end of this article. In addition, enable compression on both the client and the server. Use V.42bis instead of MNP 5 if you can, and add the command to the MODEMS.INF INIT command for your particular modem type.

Compression can increase RAS performance, if you can avoid the pitfalls that can occur because of lack of flow control. If it does not work, there is little you can do until flow control is implemented in RAS.

If a compression routine can achieve a 2:1 ratio (2 bytes of information with every 1 byte sent over the modem link) then a modem with a link rate of 9600 bps should receive data from the DTE at 19,200 bps (giving you an effective throughput of 19,200 bps). Anytime the terminal sends data faster than the modem can transmit it over the link, you may overflow the buffers if you have no flow control (even with compression). Increase the MAXBAUD parameter (in MODEMS.INF) to the maximum throughput rate expected, then reinstall your modem through Setup to modify the baud rate in COMDEV.INI. See the README.TXT excerpt below for more information.

Compression methods are generally based on techniques that replace frequently occurring characters with some kind of token representation that is shorter than the original data. Compression techniques need to examine a group of characters in order to determine frequency, so it works better if you can keep the modems buffers relatively full. Turning on LAN Manager raw mode helps, because it transfers large files in blocks of 64K. See the README.TXT file below for more information.

Generally, if you have a large enough buffer on your modem, you can avoid overflow situations. See your modem manual for information on buffer size. A 3,000 byte buffer is a fairly safe size.

Excerpt from RAS README.TXT

Before Enabling Error Control and/or Compression, Check to See That:

Your computer can accommodate the higher baud rates required for compression. Make sure your Remote Access client workstation has a 80386 or higher processor and also has 16450 or higher serial port UART chip installed.

Your Remote Access server has a multiport adapter installed, relieving the main processor of some of the interrupt processing load.

You have identical or compatible modem types on the server and client workstations.

Your telephone line has little or no static. Static minimizes the effectiveness of redundant error control.

Enabling Modem Error Correction and/or Compression

Set the MAXBAUD parameter for your modem in the MODEMS.INF file to the maximum baud rate at which you will be communicating. Valid baud rates are 1200, 2400, 4800, 9600, 19200, and 38400.

For MS-DOS workstations running LAN Manager Enhanced, change digit 14 of the WRKHEURISTICS line in LANMAN.INI from 1 to 0. This causes the LAN Manager workstation to transfer data in 64K blocks. This large block size ensures a smooth, constant flow of data from the computer to the modem. The modem's compression buffers will fill up quickly, thereby minimizing data transmission delays.

For MS OS/2 workstations, change digit 11 of the WRKHEURISTICS line in LANMAN.INI from 0 to 1. This causes the LAN Manager workstation to transfer data in 64K blocks. This large block size ensures a smooth, constant flow of data from the computer to the modem. The modem's compression buffers will fill up quickly, thereby minimizing data transmission delays.

Increase the value of the MAXDYNMEM parameter in the LANMAN.INI file of the Remote Access server to accommodate Remote Access clients that use a large frame size for data transfer. There is a trade-off between gaining faster file transfer, and the amount of memory required on the Remote Access server. You will have to increase the amount of memory installed on your Remote Access server. For more details on tuning the Remote Access server, see Chapter 6 in the "Microsoft LAN Manager Remote Access Service Administrator's Guide."

Note: Microsoft cannot guarantee proper functionality at baud rates higher than those indicated in the default MAXBAUD = line of your MODEMS.INF file.

Windows NT

RAS for Windows NT support both hardware and software flow control, error correction, and data compression.

REFERENCES

README.TXT file on RAS 1.0 and 1.1 client or server

Additional query words: wfw wfwg 2.10 2.1a 2.20 CCITT Microcom

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