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The LAN Connection
Introduction
Load that program! Send me that file! The efficiency of a modern university hinges on moving information quickly and conveniently. If personal computers are the houses where information lives, Local Area Networks (LANs) are the avenues over which it moves.
Back in the days of prehistory, some 20 years ago, we didn't need networks, we had The Mainframe. We could share our files because they all resided in the same place; we only had to give other people permission to use them. We could share data, because the database was on the mainframe and everyone could log in and use it. We could share programs, because each program was stored in one place and everyone got a copy when needed. And we could share printers, because all printers were attached to the one computer that we all used.
Mainframes have worked well for us, but they have disadvantages. The two most significant are their lack of flexibility and their high cost. As personal computers came of age, many people started using them for their flexibility and economy. Everyone could choose his or her own word processor, own operating system, own printer, own database, and his or her own headaches. The only problem was that some important things that mainframes did well were missing -- convenient communication and coordination among its users. (Carrying a floppy disk from one machine to the next, aka sneakernet, does not count as convenient communication.) LANs were invented to replace these missing functions, without sacrificing the economy and individuality of the small machines.
Types of LANs
A Local Area Network, as the name suggests, connects machines in close geographical proximity, although exactly what "proximity" means can be stretched. The term "Wide Area Network" (WAN) is used for networks that expand beyond the campus or office; the Internet is the best known example of a WAN. (The Internet is also an "internet", a collection of networks acting as one.) Usually a WAN will have one or more slow links, perhaps over telephone lines between cities, whereas all the links in a LAN will be fast. This difference in speed is important for optimizing the overall network performance.
The usual use of the of term "LAN," however, implies more services than simply making connections between local machines. On a LAN, we expect to share files, programs, or printers, all without being particularly aware of where the physical resources we're using are actually located. LANs providing these types of services are typically set up either as "peer-to-peer" or "client-server" LANs, or perhaps as a combination of the two.
Peer-to-peer LANs
All the machines on a peer-to-peer LAN are equal. Provided that the file's owners give permission, a file on machine A can be accessed from machine B, and vice versa. Peer-to-peer LANs do not require any one machine to be a dedicated, high-performance server; service by a peer-to- peer LAN is often cheaper for this reason. Peer-to-peer LANs work well when only a small number of machines are connected to it. But as the size of the LAN grows, peer-to-peer services can become quite disorganized, and because each machine on the LAN must be powerful enough to serve all of its peers, the cost increases. For larger LANs, the dedicated client-server LAN architecture becomes more cost effective.
Client-server LANs
A client-server LAN consists of one or more server machines on which shared files and programs reside and many client machines where people do their work. The LAN server machines are usually big and fast because they must serve many users, while the client machines need only be fast enough for one person to use at a time. Shared printers are either attached directly to a server, or to a print server (a specialized computer attached to the network), or to a personal computer on network that acts as a print server.
Connecting a LAN: The Physical Connection
Regardless of type, all LANs require special hardware. The usual parallel and serial ports that come with personal computers are not fast enough for most uses on a LAN. At UIC, and probably most universities, each desktop computer that will be networked on a LAN must have an Ethernet card, which gives the desktop computer a third, very fast, type of communications port.
In addition to LANs based on Ethernet hardware, in which data is broken up into small "packets" for transmission, including Twisted Pair Ethernet, Fast Ethernet (100 Mbps vs only 10 Mbps for regular Ethernets), and Gigabit Ethernet, there are "token passing" networks such as Token Ring and FDDI (Fiber Distributed Data Interface), and "cell relay" networks such as ATM (Asynchronous Transfer Model). Ethernet, Token Ring, and FDDI are all "baseband" networks -- the wires carry only one signal (aka channel) at a time. ATM, which is part of B-ISDN (Broadband Integrated Services Digital Network), on the other hand, is "broadband", in which a single wire can carry multiple channels simultaneously.
All types of networks differ, of course, in cost and speed, and in necessary software and hardware. And all of these types of network are, or have been, used at UIC. (For more information about ATM, see Building the Data Highway and the other articles in the November/December 1996 ADN Connection newsletter.)
Once a desktop computer has an appropriate network card installed, it can be connected to other computers through cables, hubs, and routers, so that the information can flow from one to another as quickly as possible. This is precisely how computers at UIC are connected to the UIC campus network and ultimately to the Internet. Commonly, "LAN hardware" also includes dedicated machines like file or print servers, which provide LAN services to really make the network a LAN.
Connecting a LAN: The Logical Connection
Desktop machines on a LAN also require special software, which breaks a file or other information into packet-sized chunks, adds information about where the packet should be sent (this is called an envelope), and tells the network card to send the packet. This software allows the machines on a LAN to make use of the various services available on the LAN. There are several logical protocols used for network traffic. At UIC, most network traffic uses IP (Internet Protocol); Novell Netware's IPX (Internetwork Packet Exchange) and ATM are also quite important. Apple LocalTalk networks use DDP (Datagram Delivery Protocol).
An important point is that the same network card can handle many different software protocols and can do so simultaneously. In particular, your computer can be connected to the UIC campus network and to a departmental LAN through the same Ethernet card and cable. This situation already exists in the ACCC public PC labs. The lab PCs are part of a Novell LAN and also are attached to the UIC campus network and the Internet; you use IP on these machines when you logon to a workstation account or use a Web browser to explore the Internet, but you use IPX when you do a dir your H: home directory provided for you by the LAN server.
Behavior of a LAN Client
What changes can you expect when your personal computer is added to a LAN? There will naturally be a few differences in the way it acts. First, you will need to logon to the LAN.
After you logon to the LAN, remote files will appear to be directly on your machine. You'll have new drives on your PC, with new drive letters. For example, in the ACCC PC labs and with Server Services, you'll have a new H: drive that is a directory on the server that provides you with private disk space. (This is usually called your home directory.) There might be other drives holding software and data that the machines on the LAN can use (provided that suitable accesses permissions are set). The file permissions associated with your PC's LAN account will specify which of the LAN services you can use, which disks you can access, and whether you can write on or otherwise modify the disks or just read their contents.
Often, special licensing and installation procedures are needed when installing software on LAN servers, but once it's installed, you can use software on LAN drives exactly as you'd use it if it were on your own drive.
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