Share And Share Alike

by Wayne M. Krakau - Chicago Computer Guide, May, 2000
Sad, but true, whether we like it or not, clunky old modems attached to POTS (Plain Old Telephone System) lines are often still appropriate, even on a modern LAN. (How about clunky old clich�s?) In those cases, following your parents’ advice and sharing your toys (modems) may be the best policy.

There are still a lot of companies out there without dedicated internet connections. They are still using dial-up connections. For those with dedicated connections, having one or more dial-ups lines as a backup is quite useful, especially since the ever-more-common DSL lines are still having teething pains.

Remote control applications are frequently used, and, until Virtual Private Networks (VPNs - a connection to a LAN via the Internet) over dedicated lines become more common, require a phone and modem for each connection. Remote control can be used to connect with technical support personnel, for employees calling in from their home computers, or for employees calling in from on the road.

Remote node applications, in which the phone line becomes, in effect, a long extension cord for the network. Your computer acts like it was plugged directly into the LAN cabling system, though typically a lot slower. Again, an individual phone line (probably an ISDN line to be practical) is needed for each connection unless a VPN over dedicated lines is used.

Using individual modems for these applications is one alternative. However, while modern internal modems are relatively inexpensive, they aren’t free. In addition, they tend to be a lot more temperamental than their more expensive external brethren, especially in these days of Plug and Play (or Plug and Pray, as it is often called). This is especially true if you use no-name or off-brand internal modems. External modems cost a little more but are less likely to have problems and are much easier to debug. Whichever you choose, at some point buying individual modems does add up.

Once you’ve installed the modems, you need a phone line for each one. That involves an up front wiring cost to get a socket next to each computer plus the monthly tariff on the phone line. That can also ad up to serious bucks.

A better solution is to use shared modems controlled by the software that comes free with all of the major Network Operating Systems. To implement these shared modems, you need a multiport serial device along with the appropriate adapter cables. You can also use multi-modem devices, but then you run into the same manageability and debugging problems that you get with internal modems, with the added potential of obsolescence (unless you are absolutely sure that modem standards will never change).

If you use a single, non-redundant server, an internal multiport serial board is appropriate. The smaller boards will have either separate sockets directly on the boards, each with its own cable, or, more often, a fanout cable to attach to modems. Boards with a lot of ports, or at least the potential to expand to handle a lot of ports, use a single cable connected to a little black box, which, in turn, has individual sockets for the modem cables. Those modem cables are frequently custom designed. If expansion is supported, the little black boxes are daisy-chained with custom cables.

Note that you can get "smart" and "dumb" boards. The smart ones have one or more CPUs and are capable of taking almost all of the considerable overhead of serial communications away from the server. The dumb boards have just the bare minimum amount intelligence to communicate with the modems. They force the server to do almost all of the work. That can make a real dent in the overall performance of the server, so you should stick with the smart boards.

External multiport serial devices are all little (and occasionally not so little) black boxes with a network cable socket and a bunch of serial ports that frequently require custom cables. Being stand-alone devices, they are all essentially self-contained smart boards, controlled by the file server.

While they are more expensive than internal boards, they do have some advantages. The first is that they can be placed anywhere where there is a network socket, so you can put them wherever the phone lines terminate.

The second advantage only applies to those who have either true redundant servers, or who implement something similar to the concept that I call twin servers (interchangeable server and master workstation with external hard disks and tape drives). If you use internal boards with along with these designs, you have to buy two boards, one for each server. Also, when one server fails, you have to reconnect one additional large, and quite awkward, connector, with a bunch of modems dangling, to the reserve server.

However, if you use an external multiport device, that device is set to talk to one particular server, as designated by that server’s internal ID. Since that ID is on the external disk drive, the reserve server comes up with the same ID as the primary server. The little black serial box doesn’t know the difference. It thinks it’s talking to the same server, so it just keeps working.

And now for the computer trivia question of the day. What’s the other name for the fanout cable mentioned above? An octopus cable. Just don’t serve it over pasta!

�2000, Wayne M. Krakau