ModemTutorial - Other Settings
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Copyright (c) 1991 Patrick Chen. All rights reserved.
Modem Tutorial Index
Other Settings for Your Communications Software
Online services use different phone numbers for different kinds of modems. To get the best throughput, make sure you dial the right phone number. Note that many bulletin board systems do not allow calling their high-speed modem lines with 2400 bps modems. You would be disconnected.
EXEC-PC, the largest BBS in the United States, has the following telephone numbers:
#1: Standard 2400 bps modems 414-789-4210
#2: US Robotics HST 9600 MNP5 414-789-4337
#3: US Robotics HST 14400 & V.32/V.42bis/MNP5 414-789-4352
#4: US Robotics V.32bis/V.42bis & HST 414-789-4360
#5: CompuCom Speedmodem 9600 MNP5 modems 414-789-4450
#6: Hayes 9600B V-series modems, NON-V.32 414-789-4315
If you have a 9600 bps HST modem, call #2.
If you have a 14400 bps HST modem, call #3 or #4. You can also call #2 but you won't be able to get the best throughput.
If you have a V.32 modem, call #3 or #4.
If you have a V.32bis modem, call #4. You can also call #3 but you won't get the best throughput.
If you have a CompuCom SpeedModem Champ, call #5
If you have a CompuCom SpeedModem Storm, call #3 or #4 (V.32), #5 (CSP)
If you have a CompuCom SpeedModem Star, call #4 (V.32bis), you can also call #3 or #5 but you can only connect at 9600 bps.
If you have a Hayes V-series Smartmodem 9600 (or 9600B) modem, call #6.
Dial String: ATDT
You don't have change the dialing string unless you use a PBX system or have call waiting service.
- If you have to dial the digit 9 to obtain an outside line, Use ATDT9, (the comma , instructs the modem to pause two seconds). This allows enough time for the dial tone to occur before the modem dials. You can use as many commas as you like.
- Call Waiting
- Call waiting service will disrupt modem sessions. If your telephone company supports the ability to disable call waiting, use the dial string ATDT*70, (make sure you add the comma ,) instead of ATDT.
Also, adding 1170 after the ATDP dial command can be used to disable call-waiting on some pulse-dialing phone systems. Check with your phone company to see if these features are supported.
8-N-1 or 7-E-1 (data bits-parity-stop bits)
In general, set the parameters to 8-N-1. If you are calling a commercial online service (such as GEnie), you may need to set the parameters to 7-E-1.
Half vs. Full Duplex: Local Echo
The only popular on-line service that uses half duplex is GEnie.
If you are using an IBM compatible, choose IBM PC or ANSI. Otherwise, try VT102, VT100, VT52, TTY.
For your computer to talk to your modem, you need to tell the software where to find the modem. If you use a PC with an external modem, you need to specify which serial port the modem is connected to. If you have an internal modem, you need to configure the modem and tell the software which COM port the modem is configured for.
If you use a Macintosh, specify whether your modem is connected to the modem port or the printer port.
File Transfer Protocols
Errors that occur during file transfer are automatically detected and corrected by file transfer protocols. If a block of data is received incorrectly, the receiving system sends a message to the sending system and requests the re-transmission. This process is automatic. When errors occur during file transfer, the communication program shows an error in the file transfer status window.
- This is designed to work with ASCII text files only. Notice that you do not have to use this protocol when transferring text files. The ASCII protocol is useful for uploading a text file when you are composing e-mail online.
- Xmodem is one of the most widely used file transfer protocols. The original Xmodem protocol uses 128-byte packets and a simple "checksum" method of error detection. A later enhancement, Xmodem-CRC, uses a more secure Cyclic Redundancy Check (CRC) method for error detection. Xmodem protocol always attempts to use CRC first. If the sender does not acknowledge the requests for CRC, the receiver shifts to the checksum mode and continues its request for transmission.
- Xmodem 1K is essentially Xmodem CRC with 1K (1024 byte) packets. On some systems and bulletin boards it may also be referred to as Ymodem. Some communication software programs, most notably Procomm Plus 1.x, also list Xmodem-1K as Ymodem. Procomm Plus 2.0 no longer refers to Xmodem-1K as Ymodem.
- Ymodem is essentially Xmodem 1K that allows multiple batch file transfer. On some systems it is listed as Ymodem Batch.
- Ymodem-g is a variant of Ymodem. It is designed to be used with modems that support error control. This protocol does not provide software error correction or recovery, but expects the modem to provide the service. It is a streaming protocol that sends and receives 1K packets in a continuous stream until instructed to stop. It does not wait for positive acknowledgement after each block is sent, but rather sends blocks in rapid succession. If any block is unsuccessfully transferred, the entire transfer is canceled.
- This is generally the best protocol to use if the electronic service you are calling supports it. Zmodem has two significant features: it is extremely efficient and it provides crash recovery.
- Like Ymodem-g, Zmodem does not wait for positive acknowledgement after each block is sent, but rather sends blocks in rapid succession. If a Zmodem transfer is canceled or interrupted for any reason, the transfer can be resurrected later and the previously transferred information need not be resent.
- Kermit was developed at Columbia University. It was designed to facilitate the exchange of data among very different types of computers (mainly minicomputers and mainframes). You probably will not need to use Kermit unless you are calling a minicomputer or mainframe at an educational institution.
- Sealink is a variant of Xmodem. It was developed to overcome the transmission delays caused by satellite relays or packet-switching networks.
- Which file transfer protocol should you use?
- In general, I recommend Zmodem. If Zmodem is not supported by the system you are calling, use Ymodem-g. (If you are connecting to a UNIX system in a university, you may have to use Kermit or Xmodem to transfer files.)
- Here are the test results obtained by downloading the files using various file transfer protocols. The number before the parentheses is the transfer speed (in cps) and the number in the parentheses is the time elapsed (in seconds).
Protocol Xmodem Xmodem-1K Ymodem Ymodem-g Zmodem
the-wave.txt 429(334) 1508(95) 1527(94) 3261(44) 3296(43)
dayrpt.arc 314(26) 758(11) 761(11) 1042(8) 1025(8)
dayrpt.wks 415(47) 1252(15) 1244(15) 2314(8) 2337(8)
sunset.arc 337(15) 771(6) 745(6) 987(5) 965(5)
sunset.pic 399(41) 1337(12) 1297(12) 2594(6) 2588(6)
text109k.arc 343(86) 817(36) 814(36) 1089(27) 1064(27)
text109k.txt 410(271) 1379(80) 1351(82) 2812(39) 2885(38)
Appendix A: Resources
Modem Tutorial Index