Understanding File Compresion
(What all this zipping and unzipping is all about)

by

Graeme E. B. Gibson, C.D.P.

Getting free software is one of the best things you can do from a Bulletin Board system, but a lot of new users are daunted by the whole matter of file compression and how to get a file to the point you can use it.

Files are compressed with tools like PKzip for several reasons but there are two primary ones The first is that online time and disk space is saved, most often 30 to 50 per cent of the time and space that would be used without compression. This is particularly important when long distance charges are involved. The other important reason is that with compression, one file can be transferred then exploded into many discreet files on the receiving end. This saves you the trial of having to select and mark (tag) many files for downloading.

Generally speaking compressed files are easy to recognize due to the file extension .zip. The zip format was developed by software author Phil Katz (his initials are where the PK in PKzip comes from). Phil has not been resting on his laurels; about a year ago the latest version of PKzip came out, version 204G. This is the only version to use now. I am telling you this because there is a Trojan horse (a kind of virus) out masquerading as the file PKZ300B. If you find this file, delete it without typing its name.

Two other file compression formats can be recognized by the file extensions .LHA and .ARJ, however we will focus on PKzip as our example because as we reduce options we increase simplicity. Basic instructions on how to use most file extraction and compression utilities are found by typing the command used to start them and following it with a /? So to get instructions on PKunzip (the extraction utility used to decompress a file compressed by PKzip) you could type PKUNZIP /? and that would give you basic instructions.

So you can get an idea of this process, visually consider a set of sponges sitting on a shelf the size of a large book. This represents the files needed. They have to come down your phone wire, so consider this wire a soda straw. What PKzip does is to squeeze this set of sponges until it will fit down the soda straw, instead of having to take it apart and putting small enough pieces down the soda straw to fit. At the other end PKunzip will unwrap the sponges and pump air back in till the sponges blow up to normal size. If you don't like sponges try a NERF ball, my other favorite visual analogy.

Ok, so lets look at a practical example. Let's say you are after the book file: How to Build a Clone Computer. This is the full text of my book by the same name. Now text files compress fairly well and it saves a lot of time if you can download this file in the compressed form. The file name is HOWBUILD.ZIP and it is in the TEXT file library on our BBS One Crossroads Place.

You log on to the BBS by calling 913-663-1100, or telnet 198.69.223.38 port 23 Binary mode. (type NEW if you have never called before) if you have already gotten an account type your FULL NAME and PASSWORD. Once logged on and passed the mail and news sections you will come to the service selection menu pick O for One Crossroads Place. Then the main menu will paint, choose F for FILE LIBRARIES. Optionally you can leap to this area by typing /GO FILES.

Once the file libraries are up, select S for Library selection and type TEXT. Once you have the TEXT library up, you type F for files then select the file HOWBUILD.ZIP. Once it is highlighted you can tag it for later downloading or just type Ctrl-D to start the download. By the way, the CTRL key is often shown using a ^. If you see this, it means hold the CTRL key and strike the letter involved. The BBS will then prompt you for a transfer protocol. While all the rules for file protocols are enough for a full column of its own, for now select Zmodem or if unavailable to you select Ymodem-G. Both will start an automatic transfer.

Once the file is down (ie: on your hard drive), try locating it by issuing the DOS command DIR HOWBUILD.ZIP /s. This will search your subdirectories (DOS 6.0 and later) until it is found. Make a temporary directory and copy the file from where it is to a workspace to unzip it. I send all my downloaded files to \Download and I made a directory called \UNZIP to expand my files in. So the command I would issue is: COPY \DOWNLOAD\HOWBUILD.ZIP \UNZIP then I would change to the unzip directory by typing CD\UNZIP and issue the following command, if I had PKUNZIP on my computer: PKUNZIP HOWBUILD.ZIP .

If I didn't have PKUNZIP on my computer, I would also have to download the file PKZ204G.EXE from the BBS. You will note the EXE extension to this file. It is what is called a self-extracting zip file. In other words, when you type its name it extracts itself. Many files are made this way by using another PK product ZIP2EXE to make files self-extracting. Once the file PKUNZIP.EXE is available, put it in your \DOS subdirectory, or some other directory in the path, not in the UNZIP subdirectory. Now the command PKUNZIP HOWBUILD.ZIP will work.

I know that this all seems like a powerful lot of steps, but once you get used to it, you can do almost any file in less than ten minutes and that is not a lot of time to spend to get something you really want. Unzipping files is not much more complicated than getting at the sponge by removing the cellophane wrapper. By using the examples here you should be able to get a file down. When you have done one, it becomes a lot easer.

For those who want to know more a file named UPDOWN.ZIP (Upload Urges & Download Delights) can be downloaded from the same text library used as the example. It will give you some more clues to the zipping business. To read it online type /go BASICS on One Crossroads Place BBS.

Graeme Gibson, C.D.P. is the Service Director of Computer Training Corporation and author of ``How to Build a Clone Computer'' and ``The Computer Repair Handbook.'' A former NASA programmer, he has worked in the computer field since 1972. His weekly TV show Computers Today is distributed locally by American Cablevision and Tele-Cable.

(c) copyright Graeme Gibson 1995

copyright Graeme Gibson, C.D.P. 1996

Home Page of Graeme Gibson

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Added to the Web 1996
Updated 03-08-98