- 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 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 at the DOS prompt. The real name that you substitute for 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 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 (404) 434-8478.