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The Dust Has Settled


 

The Dust Has Finally Settled


by Stuart Baker
Stuart Baker Software
9370 Golondrina Drive
La Mesa, CA 91941-5654
(619) 466-8811

In November of 1987 I wrote an article for the Interchange news letter entitled "When the Dust Settles." This article addressed the disparity between different manufacturers 9600 bps modems and the lack of formalized standards. At that time I did not recommend choosing these high speed modems unless you could pay back your investment within one year. Well a lot has changed in 3 years.

The formal standards are in place and manufacturers are bringing out new implementations at an incredible rate. The standards I am referring to are CCITT V.32 and CCITT V.42 bis. The V.32 standard defines the full duplex protocol used by the modem to communicate over the telephone line. A complete implementation of the V.32 standard gives you true full duplex communications at 9600 bps. The V.42 bis standard is the icing on the cake. This standard defines the error correcting and data compression techniques. Under ideal line and data conditions the throughput of some 9600 bps modem can approach 38,400 bps! Try hooking that up to a 16 line comm mux and see what happens!

The V.42 bis standard is usable at line speeds other than 9600 bps. For example, it can be used with 1200 and 2400 (V.22 bis) modems. Part of the V.42 bis standard supports the MNP Level 4 protocol. This gives you the ability to communicate with existing MNP modems error free.

If you purchase modems that comply with the two standards mentioned above you will not be on an island all by yourself. Now you can communicate with modems made by many manufacturers. If you intend to communicate with many different systems, be sure that your modem also supports 300/1200/2400 bps as well. With this combination you will be able to communicate with virtually anyone.

As usual, dramatic price reductions resulted from large demand and standardization. When I wrote my article 3 years ago, the cost of a 9600 bps modem was about $1500. You can now buy full V.32 and V.42 bis implementations for under $600!

Finally, a solution to the clock rate dilemma! One of CCUR's 8-line mux "design features" is that you must live with pre-defined clock groups. The groups do not address the choices needed to handle today's modems. The modems supports 300, 1200, 2400 and 19200 bps, unfortunately neither the 8-line comm mux or the MPC is flexible enough to allow you to group these speeds together. In the past, if you had a CCUR 8-line comm mux, the only solution to the problem was to cut foils and add jumpers. With the addition of data compression, today's modems must buffer data to perform the data compression effectively. Many modems can use their buffer even when data compression is not being used. A feature known as "Speed Conversion" allows these modems to communicate at one speed over the phone line and at another speed to the host computer. You will no longer have problems with autobaud adjust if you select a modem that supports this feature.

Gee -- this all sounds really neat. Do you mean to say that all problems have vanished and modems are easy to use and install? NO! Sorry to say, this is not so. Progress very seldom comes without a price, but luckily you only have to pay the price once. Once you get it all working you can set back and enjoy clean communications for many years.

The first thing you will notice when you unpack your new modem is a thick manual. Inside, a bewildering number of commands await your perusal. To the uninitiated, the sheer number of commands is intimidating. In addition, only telecommunications experts understand the topics covered in the manual! Gone are the days of setting 8 switches, plugging in the cable and using the modem. Most of the new modems have no switches to set, instead you perform the configuration with commands. That presents you with your first problem, how to configure this beast! First, connect your new modem directly to a PC or terminal. Set the terminal speed to the highest line speed you intend to use, typically 19,200 or 38,400 bps. This step is important because some modems will not operate at speeds greater than the speed used when you configured it! If you can find someone that has already hooked the same type of modem to a CCUR system, try using their configuration as a starting point.

You need to select the flow control configuration parameters carefully. For compression to work properly, the CPU must supply data to the modem fast enough to allow buffering. Obviously, since the modem does not have infinite data storage, eventually the CPU must stop sending to prevent data loss. Most modems offers three choices for flow control, RTS/CTS, XON/XOFF or local flow control disabled.

Since XMODEM file transfers will not function if you select XON/XOFF, you should choose RTS/CTS flow control if you intend to use the XMODEM protocol. If you select RTS/CTS, the modem communicates properly using the BIOC driver. If you need to access RELIANCE using the ITAM driver, you now have problems. The ITAM driver generates an I/O error when CTS drops! If there is a way for ITAM not to report loss of CTS as an error, I am not aware of it. If there is, will someone please let me know. I have not tested CTD with RELIANCE using RTS/CTS, maybe the solution already exists.

Once you have the modem configured, you need to hook it up to the comm mux. Hooking up a modem to a comm mux and getting it working should be simple, and it is after you have succeeded. The switch settings and cable wiring recommended below allow the modem to be used for both inbound and outbound calls.

You must first insure the comm mux or MPC switches are set correctly. Note, when you set a switch to On, you disable the sensing of that signal. In some modems CTS is absent until they connect, so you may need to set that switch to On also. The comm mux should be set as follows:

SIG

RING
CARR
DSR
CTS

SW

OFF
OFF
ON
OFF

Meaning of setting

Accept RING from MODEM
Accept CARRIER from MODEM
Ignore DSR from MODEM/Force on
Accept CTS from MODEM

You must perform these steps, even if you are currently using modems on your system. Just because a modem functions for inbound calls does not mean that the configuration will work for outbound calls as well. To perform auto dialing, you must set the comm mux or MPC switch to force the DSR signal On, because the modem does not supply this signal until it is connected.

The following cable must be used if auto dialing is to be performed! The required modem cable is:

15 Pin
Female
  25 Pin
Male
Signal
2
3
4
9
11
12
13
14
15
>
<
<
<
<
<
>
>
> <
20
5
8
22
3
6
4
2
1 & 7
DTR
CTS
CARR
RING
RD
DSR
RQ2S
TD
GND

This configuration requires a 9 conductor cable. Note, you must connect pins 1 and 7 together on the 25 pin connector. This cable includes the DSR signal even though the comm mux is not set up to sense it. This increases the general purpose characteristics of the cable.

Well that about does it for this issue. I hope the information provided will ease the installation process and encourage you to join the era of high speed modems. I think the dust has really settled and now is the time to move up to truly high performance modems. Things have really come a long way since 1968 when I was having trouble getting 300 bps modems to work cleanly. I am looking forward to seeing everyone at Interchange 91 in Caesar's Palace.

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Article Copyright 1991 Stuart J. Baker

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