Shareware Grab Bag/Shareware Grab Bag.iso/001/network.arc/NET.TXT

Text File | 1986-05-17 | 13.4 KB | 334 lines

- LANning With Your PC -
A NETWORKING PRIMER

Written by
Ira Chandler,
Contributing Editor

   A local area network (LAN) is a communications system that
connects computers and computer devices within a localized area.
The communications systems allows these devices to communicate
with one another, as well as to share resources among all of the users
on the network.  This article is an overview of LANS, their
capabilities and concepts.

- GENERAL FEATURES OF LANs -

* share processors, modems, printers, plotters, buffers, ports and
  floppy, RAM and hard disks

* unlimited disk storage is supported

* unlimited transmission distance by modem

* about 1 mile cable transmission, depending upon cabling system

* access protection for all files

* remote control of the shared pc provided

* multi-tasking, allowing any pc to run more than one program at a
  time using the remote execution function

* disk cacheing and internal electronic mail available

* supports pc-terminal links

* support for many brands of computers

- ARCHITECTECTURE -

   Several hardware and software designs are commonly used in LAN
systems.  These different design characteristics form the basis for
categorizing LAN systems, as well as dictating the performance and
suitability for a given application.  The primary design characteristics
of a LAN are:
      1)  Bandwith      2)  Topology      3)  Protocol

   = BANDWIDTH =

   BANDWIDTH is the data transmission capacity of the LAN.  Two
components of data transmission are speed and multiplicity.

   Data transmission speed is measured in megabits per second.
Generally speaking, microcomputer LANs range from 1 to 10
megabits/second.  These numbers refer to the maximum possible
data transmission rate - they do not guarantee that the real
throughput will be equal to the bandwidth.  The higher the
bandwidth, the more sophisticated (and expensive) the hardware.

   Multiplicity is the network's ability to transmit a single data
stream or multiple data streams simultaneously.  A network which
can only transmit a single data stream is considered a BASEBAND
network.  A network capable of transmitting multiple data streams
simultaneously is known as a BROADBAND network.

   = TOPOLOGY =

   TOPOLOGY is the physical layout of the network's connections.
Three basic network topologies are commonly used in LANs:

      1)  Star      2)  Ring      3)  Distributed Bus

   In a STAR configuration, each computer is connected directly to
the host (or central) computer.  If the host computer fails, the
network goes down.  On the other hand, adding or removing
computers to or from the network is relatively easy.  Star networks
exhibit superior performance due to the direct connection between
user and host.

   A RING configuration connects the computers and the host in a
ring arrangement.  The ring topology is less sensitive to computer
failure, but reconfiguration is generally more difficult.

   Using a DISTRIBUTED BUS configuration, the network is least
sensitive to computer failure and the easiest to configure.  For this
reason, most microcomputer LANs use this topology.

   = PROTOCOL =

   PROTOCOL is the rules and mechanisms controlling communicatons
on the network.  Without this control, the computers might send
data simultaneously.  This is the same as two people trying to talk
to each on the telephone at the same time - neither person will
understand what is being said.  The two most common protocols
employed by LANs are:

   1) Token-Passing
   2) Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

   TOKEN-PASSING protocols employ a unique message packet, or
token, that is constantly circulating from one computer to the next.
When a computer needs to send data, it must "grab" the token from
the network (thus preventing any other computer from attempting
to send data), and then send its data.  When the transmission has
been successfully completed, the computer must release the token
and allow it to once again re-circulate the network.

   The CSMA/CD protocol is based on the ability of a computer to
"listen" to the network (Carrier Sense).  If it "hears" data, it will
wait until the network is "quiet" to begin its own transmission.  It
continues to listen as it begins to transmit data - if the data is
garbled (Collision Detection) the computer will stop transmitting,
wait for a random period of time, and then retransmit the data.

   Networks using CSMA/CD technology are affordable.  These
systems allow several users to share data files, programs, hard disks,
printers, buffers, modems, other IBM-PC type computers, and
electronic mail.

   Token-passing protocols have better performance characteristics
in a large network with heavy traffic, but is more complex to
implement.  CSMA/CD protocols have good performance
characteristics and are simpler to implement.

- CABLES -

   Another feature to consider is the type of cabling used by the
network.  Two types of cables are commonly used:

      1)  Twisted Pair      2)  Coaxial

   TWISTED PAIR cables, as the name implies, are a pair of wires
twisted together.   They are lightweight, flexible, and inexpensive.
However, they are not well suited for higher bandwidths or longer
distances.  Networks with a distance no greater than 3,000 feet can
use a twisted pair cabling scheme.  Twisted pair cables are very
susceptible to electrical interference.

   COAXIAL cables, while somewhat more expensive, can support
very high bandwidths over longer distances.  Networks can span a
maximum distance of approximately 7,000 feet using coaxial cabling.
The cable is very durable due to its heavier construction, and is
almost impervious to electrical interference because it is shielded by
copper insulation.

- SPEED -

   The dependence of users on the shared resources determines the
speed of the network.

   Realistic number of users for most networks are:

      3 for network intensive usage (much communication over the
         net and file sharing).
      6 users for minimum net and resource usage (this means up to
         8 users running applications on their own pcs, using the
         network for infrequent shared disk access)
     12 users for independent applications running at each user with
         only the most occasional use of shared disk or other resources

   Test a network in a real-time simulation BEFORE suggesting it for
any particular application.  The interaction of the network software,
operating system, and application software can cause unpredictable
results.  Network installations require planning and advanced
resource management.  Network systems employing a central,
dedicated, file server are more expensive, but often perform faster.

- RECORD LOCKING -

   Voluntary file or record locking offered by software must be
enforced for all users.  Batch files from MSDOS, or the use of file or
record locking in the applications program, can control user access.
For instance:

   TEXT OF SAMPLE BATCH FILE NAMED words.bat - pw.com is the
   name of the wordprocessor file, and letter.mss is the name of the
   document file we want to protect:

      LLOCK LETTER. MSS
      PW LETTER.MSS
      UNLOCK LETTER.MSS

   This file is used by typing  words <CR>  at the system prompt.
The first line LoopLOCKs the letter file.  If the LOCK has been gained
by another user, the network software will keep trying to LOCK the
file for this user until the file is UNLOCKED by the current user, or
until the ^C, or CONTROL C, is pressed.

   The second line employs the word processor to open and edit the
letter.txt file.  This occurs only after the LOCK has been gained for
this user.  When the word processor is exited, the next line of the
file is executed.

   The final line of words.bat releases the LOCK we have on the
letter.text fiel so another user has access.

   An improvement to this batch file is to use the variable %1 to
represent the filename of the text file.  This would allow this file to
be used with any text file by entering:

A> WORDS <textfile.name>  <CR>

at the DOS prompt.  The real name that you substitute for
<textfile.name> would be used by the batch file to replace the %1 in
the commands.  This is the text of the new file words.bat:

   LLOCK %1
   PW %1
   UNLOCK %1

   This file is used by typing  words <textfile.name>  <CR> at the
system prompt.

   The sequence looks like this for the example text file, memo.mss:

   A>  WORDS  MEMO.MSS

   A>  LLOCK  MEMO.MSS

         File Locked.

   A>  PW  MEMO.MSS

   This places you editing the file memo.mss.  When you finish editing
the file, you quit from the word processor, and see:

   A>  UNLOCK  MEMO.MSS

         File UNlocked.

   This is a representation of the general method used by several
networks to protect their files.

- NETWORKS SHARE PERIPHERALS -

   Sharing expensive pritners, or hard disks, between two or more
users, multiplies the value of your capital investment.  Network
system users can share peripherals easily.  MSDOS batch files allow
one word to execute a short list of Network commands.

   You can store printouts in memory until the current user is
finished with the printer.  Because the network accumulates
printing tasks in memory, the pc quickly sends out the print job
and resumes the program.  A pc is available while waiting for the
printer to finish the job.  This feature is called SPOOLING.

   Hard disks offer vast storage and very fast access to files.
Every user in the system can now use any disk drive, floppy or
hard, connected to the system.

- SYSTEM ARCHITECTURE -

   The 'VOLUME' system used by most networks offers added
flexibility.  A volume is a designated disk storage area of 64 K
or more.  Any user can create a volume, depending on the unused
space in the mass storage device.  Think of a volume as a 'user
created' disk drive.  Each volume, or logical drive, is represented by
a letter.

   Volumes have different types of access.  Volumes can allow other
users to only read them.  Files in these read/only public volumes
cannot be changed by users.  Volumes can also be totally private to
one user only.  This flexibility allows tightly controlled access to
programs and data files and prevents unauthorized access.

- MANAGEMENT -

   Peripheral sharing is the greatest benefit of the network, when
the users have little guidance.  Simple batch files can easily allow
untrained users to share peripherals and other resources.  However,
valuable facets of networking become available with management
by one person, the system administrator.

   The system administrator designs and maintains the system.  For
more complex functions than peripheral sharing, one person needs
to control file access.  Anybody can be the administrator.  The
perfect candidate is the employee whose hobby is computers.

   The need for an administrator also lies in the interaction of the
network and the MSDOS operating system.  The multi-user network
allows more than one person to use a file in a shared volume.
MSDOS has no understanding of a shared file.  The administrator
must prevent shared data files from changing in physical size.

   In unadministrated systems, access to shared files should only be
allowed through batch files using the LLOCK command.  This
command loops until the file is available.

- THE 'DEADLY EMBRACE' -

   The infamous 'Deadly Embrace' happens when two or more users
try to change the same data at exactly the same time.  This data-
threatening standoff will usually result in neither user gaining
access to the file, and possibly, corruption of the data file.

- THE POTION -

   Network commands, LOCK, UNLOCK, and LLOCK, or the like, provide
the solution to the 'Deadly Embrace'.  The file LOCK and UNLOCK
functions act like a flag.  Because the file lock is voluntary, a
user, disregarding the lock command, can access a file already in use
and corrupt the data through collision.

- THE KEY : THE LOCK -

   Because of variations in data file structure, a network cannot be
expected to provide record LOCKING.  Almost every word processor,
programming language, database system, and application program
has its own file structure.  When record locking is necessary,
most applicaions have some form of their own record locking
available.

- BACK UP -

   Another concern for the administrator is the value of the data in
the system.  Conscientious use of a back-up system once a day will
minimize your damages in case of fatal error.  No precautions
substitute for periodic backup.  Both Networks and MSDOS provide
commands for backin up files larger than a floppy.  Avoid
finger-pointing by assigning an administrator.

- REMOTE CONTROL -

   Each network generally consists of one or more master compouters,
called the shared pcs, and one or more user pcs.  A user pc can send
commands to the shared pc.  A shared pc will execute those remote
commands when it is running no local program.  Remote commands
are executed in the order they are received.

The author own his own consulting firm and is a specialist in
the design and implementation of local area networks.  He may
be reached at [phone].