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The LAN Connection |
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Write this memo! 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 15 to 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. |
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| Types of LANs | ||||
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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 LANsAll 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 LANsA 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. |
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| Connecting a LAN | ||||
The Physical ConnectionRegardless of type, all LANs require special hardware. The usual parallel and serial ports that come with IBM-type PCs are not fast enough for most uses on a LAN. At UIC, and probably most universities, each PC that will be networked on a LAN must have an Ethernet card, which gives the PC a third, very fast, type of communications port. There are also Ethernet cards for Apple Macintoshes, and you'll probably need one if you want your Mac to be connected to the ADN-ii.In addition to LANs based on Ethernet hardware, there are "token passing" networks like Token Ring and FDDI (Fiber Distributed Data Interface), and ATM (Asynchronous Transfer Mode), which uses cell relay. Some of these are used at UIC. All types of networks differ, of course, in cost and speed, and in necessary software and hardware. 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 ADN-ii 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. The Logical ConnectionDesktop 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) also is 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 ADN-ii and to a departmental LAN through the same Ethernet card and cable. This situation already exists in the ADN public PC labs. The lab PCs are part of a Novell LAN and also are attached to the ADN-ii; you use IP on these machines when you logon to a mainframe or use Mosaic to explore the Internet, but you use IPX when you do a dir of the temporary C: drive provided for the PC by the LAN server. Behavior of a PC LAN ClientWhat changes can you expect when your IBM-compatible PC 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, although this can be done behind the scenes as we do in the public PC labs.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, your new H: drive might be a directory on the server that provides you with additional 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. When attached to a LAN, a certain amount of a PC's memory is used by the network software; you might find that you don't have enough memory left on your machine to do your normal work. For this reason, we don't recommend networking a PC less capable than a 386 with a couple of extra megabytes of RAM. Behavior of a Mac LAN ClientThe behavior of an Apple Macintosh on a LAN is a bit different than a PC. Macs have built-in networking capabilities; the Mac System 7 operating system comes with software supporting Apple's AppleTalk network protocols (the equivalent of TCP/IP software for an Ethernet network), and Macs come with the hardware needed to connect to an Apple LocalTalk network (the equivalent of an Ethernet card in a PC connected to an Ethernet network). So all you need to connect a group of Macs into a Mac LocalTalk peer-to-peer LAN is the cable to connect them. The Apple User Guide explains LocalTalk networks in detail. You also can use AppleTalk over the ADN-ii to exchange files with other AppleTalk users on campus.Macs also can participate in an Ethernet Internet network using AppleTalk protocol software with the addition of an Ethernet card and cable. Using AppleTalk software on an Ethernet is called EtherTalk. When using a Mac, you access an AppleTalk network via the Chooser desk accessory. Chooser presents you with a list of network clients, called rdevs. If your Mac is also on an AppleTalk Ethernet network, you will additionally see a list of AppleTalk zones. When you select a service (and a zone, if appropriate), Chooser will identify the names of any known entities of the specified type (and within the specified zone). By selecting the name of the particular service you are interested in, you are given direct access to that service. AppleTalk services often include file servers and printers. The Computer Center's own Novell file servers can be accessed via the Chooser in this way, as well as its distributed printers. One caveat: the Mac's AppleTalk software can be used with a wide variety of physical networks, but it can only be used to communicate with one physical network at a time. For example, you cannot use AppleTalk to communicate with others on a LocalTalk network at the same time you are using it to communicate over an Ethernet network. Before you can use either type of network (and whenever you want to change from LocalTalk to EtherTalk, or vice versa), you must use the Network control panel to select the physical hardware AppleTalk will use. |
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| LAN Servers | ||||
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All of the ADN's LAN servers run the Novell NetWare network operating system,
which enables them to share their resources with the machines in the ADN
public personal computer labs (and in some departments; see "What
LAN Services Are Available?"). To serve the machines in the labs in
the fastest manner possible, each ADN LAN server is connected to the ADN
backbone via a fiber-optic connection, and each has a large amount of disk
and memory available.
Of course, because we have so many machines depending on our servers, we cannot afford to have them be unavailable! So we maintain multiple servers, each a mirror image of the other. Currently, ADN_CC_S1 through ADN_CC_S6 serve the public labs, and LOKI services departments and faculty. The big advantage the ADN LAN servers give us is that we maintain a single copy of each program on the server, for example WordPerfect, and all the client computers use that single copy. (We do, however, pay for as many licenses as we have client machines!). But it's much easier to upgrade a single copy when newer versions are released than it is to upgrade 100 different machines. This is the same philosophy behind the Server Services mentioned in "What LAN Services Are Available?". In addition to providing public access to software that the Computer Center has purchased for the campus, such as WordPerfect or Paradox or Maple or SAS, we also install software on our servers, at a faculty member's request (licensing permitting), to be used by their students. Typically, this software remains on the servers for one semester. We can install software in our labs for shorter periods, perhaps to support a conference, tutorial, or other university sponsored activity. And we also provide long-term storage to support research projects. Please let us know what kind of services you would like to see in out public micro labs and on our servers, and how we can improve our existing setups. We will strive to bring the latest technologies to our labs, be it new hardware such the Power Macintoshes or advanced operating systems such as IBM's OS/2, in a timely manner. Your suggestions, compliments, and complaints are welcomed by suggest@uic.edu, or call the Client Services office, (312) 413-0003. Comments are appreciated; send them to |
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| The ADN Connection, January/February 1995 | Previous: January/February Contents | Next: What LAN Services Are Available? |
| 1999-9-1 connect@uic.edu |
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