rbl@nitrex.UUCP (02/24/87)
Does anyone have experience using modems over a cellular telephone link? We are interested in using a terminal at a site where there are no telephone lines. Thank you, Robin Lake Standard Oil R&D Cleveland, OH 44128 decvax!cwruecmp!nitrex!rbl ihnp4!cbatt!cwruecmp!nitrex!rbl
poslfit@utgpu.UUCP (03/01/87)
In article <425@nitrex.UUCP> rbl@nitrex.UUCP ( Dr. Robin Lake ) writes: > Does anyone have experience using modems over a cellular telephone link? > We are interested in using a terminal at a site where there are no telephone > lines. There are a number of companies offering cellular modems in the North American market. Since I am doing some consulting for one (Rectronix Inc. of Toronto), I won't mention specific brand names or advertise our product beyond this. Send me mail if you want to know about our briefcase PC/cellular modem product. Here, however, is a brief list of features to look for in a cellular modem. 1. The ability to continue a transmission without carrier loss through a cell-site hand-off. If your transmitter is in motion, or your cellular system is crowded, your connection will be interrupted periodically as your call gets handed off from cell site to cell site. This isn't too annoying in a voice call, but tends to make a modem detect carrier loss. If you have control over the modems at both ends and don't mind big bursts of noise, on a Hayes-compatible modem you can set the carrier loss detection interval to something long. If you do mind noise, you have to get a cellular modem. One popular model on the market is generally Hayes-compatible, but when attempting to establish a connection will transmit a sequence of characters to see if the hand-off protection protocol should be enabled. You of course have to have the same brand of modem at each end of the connection, but (at least in Canada on Cantel) your cellular carrier may already have installed piggybacked pairs of cellular and regular Hayes-compatible modems at each cellsite in a user- transparent manner. If that's the case, all you need is one cellular modem. At any rate, in a connection between two of these modems, you will have brief pauses every now and then during handoffs, but no data should be lost, even at speeds up to 2400 bps. 2. Portability. The best I've heard is a battery-operated modem with trans- mitter that will run for eight hours between charges and fits into a custom briefcase. A cellular modem hardwired to your car is limited in its utility. 3. Durability. Goes with portability. If you carry the modem around with you, someday, you or a baggage-handler will drop it. Make sure it's tough, or comes in a well-protected case. 4. Encryption. Anyone with a scanner can listen in, though they may have to hop frequencies quite often. Be paranoid. Get DES encryption. Expensive, but you never know... 5. Error correction. The cellular environment is noisy. If you can, try out the equipment where you'll be using it before you make a final commitment. Look for a modem and/or software that can handle the noise, and I'd recommend at least 1200 bps since once you subtract error-correction overhead, your effective bps may drop as far as half of that on a bad line. The problem of course, is that no modem currently on the market has everything. I'd love to have a 4800 bps cellular modem that could handle a small EMP and a few dozen G's without dropping a character, and would run on a 9-V battery for a few days. Dream on... Hope this is of help. -- -- john j. chew (v3.0) poslfit@gpu.utcs.toronto.edu +1 416 463 5403 (300/1200 bps) poslfit@utorgpu.bitnet {cbosgd,decvax,mnetor,utai,utcsri,{allegra,linus}!utzoo}!utgpu!poslfit "Script-G for open, sub-delta for durchschnitt"