Those of you who regularly grace this space with your presence know that we've spent the last year or so exploring WAN (Wide Area Network) technologies, with a focus on wireline, rather than wireless. If you missed all of that, why not spend a little time on those previous lessons? I guarantee that you'll find the experience worthwhile.

In any event, it occurs to me that we should spend a little time on LANs. We'll start off with this lesson on LAN Basics. We'll then explore Ethernet and then 10/100BaseT. The next steps will be WLANs in the forms of 802.11b and 802.11a, and we'll conclude with discussion of GbE (Gigabit Ethernet) and 10GbE.

We could also explore Token Ring, FDDI (Fiber Distributed Data Interface), ATM-based LANs, and so on, but that's up to you. If there's interest expressed in the Discussion Thread List, we'll do it. So, it's entirely up to you. (How often do you hear that?)

LAN Basics

A LAN (Local Area Network) is a form of local (i.e., limited-distance) shared packet network for computer communications, serving to interconnect computers and peripherals over a common medium. Thereby, users can share access to attached resources in the form of host computers and their resident databases, files and applications.

LANs also let users share access to networked peripherals, such as printers and storage devices. Finally, LANs let users share access to various WANs, including the Internet, generally through routers.

LANs are client/server networks, with client workstations such as PCs running against servers, which are multiport computers generally containing relatively large amounts of processing power and memory, and enabling multiple clients to share their resources.

Database servers are database engines capable of processing and managing client requests for data residing on them. Applications servers house application software, which they provide to clients on demand, assuming that the clients are authorized to access them.

Communications servers support communications through shared links to various WANs, such as the Internet. There are also print servers, and the list goes on. In small networks, such as mine, one server may provide all of these functions. In larger networks, there may be a great many specialized servers, with each providing a single function.

LANs offer raw bandwidth up to 100 Mbps and more, although actual throughput often is much less. LANs generally are limited to a maximum distance of only a few miles or kilometers, although their reach can be extended over considerable distances thanks to fiber optic transmission technologies.

LANs support the transmission of data in frame format, with the frames varying in size within minimum and maximum limits as determined by the software drivers in the NICs (Network Interface Cards. (Note: Frame is a term used to describe a unit (usually a subset) of information traveling over a link or Link Layer (Layer 2 of the OSI Reference Model) network, such as a LAN or Frame Relay. Packet is a term used to describe a unit of information traveling over a higher layer network, such as a Layer 3 (Network Layer) IP internetwork.) To be more exact in our definition of LANs, they run at Layer 1 (Physical Layer) and Layer 2.

Physical Media

LANs can run over virtually any physical medium, whether wired or wireless. Wired media include coaxial cable, twisted pair and optical fiber. Wireless media include RF (radio frequency) and Ir (Infrared).

Coaxial cable was the medium originally used in Ethernet and other LANs due to its ability to carry a high frequency signal over relatively long distances without significant signal attenuation. As a shielded medium, coax also offers the advantage of immunity to EMI (ElectroMagnetic Interference), thereby yielding fewer errors in transmission.

Over time, UTP (Unshielded Twisted Pair) largely replaced coax, due to its much lower costs of acquisition and configuration (and reconfiguration). However, UTP is a much thinner gauge copper medium than coax, which in fact increases electrical resistance and, therefore, results in greater signal attenuation.

Therefore, UTP cable runs are much shorter, generally in the neighborhood of 100 meters, in order to maintain signal strength at acceptable levels. Further, UTP is an unshielded medium, which translates into greater susceptibility to EMI.

Note: STP (Shielded Twisted Pair) and ScTP (Screened Twisted Pair) are used in some applications in order to reduce EMI. Optical fiber now is the medium of choice, at least in high speed LANs, particularly in applications involving longer distances.

WLANs (Wireless LANs) generally make use of RF. The highly popular 802.11b Ethernet standard, for example, runs at 2.4 GHz. The newer, upbanded 802.11a runs in the 5 GHz band. WLANs offer the advantage of portability, as the requirement for cabling is avoided.

Therefore, you can move your client workstations around with a reasonable degree of impunity, as long as you remain within the distance limitations imposed by the low power of the RF signal, and as long as you realize that any physical obstructions (i.e., walls, doors and windows) have considerable impact on signal attenuation.

Infrared is another option, although its heavy reliance on line-of-sight renders it much less acceptable in most applications. The previous lesson on Transmission Media provides a good deal more insight to the various media options available.

Topology

LAN topology most generally refers to the physical layout, or configuration, of the network, which may be in the form of a bus, ring or star. There are also distinctions between physical and logical topologies. For example, an Ethernet 10/100BaseT network has the physical appearance of a star, but logically operates as a bus. Confused? Well, lets explore the details.

Bus topologies (See Figure 1) are multipoint electrical circuits, originally of coaxial cable, although contemporary systems also make use of UTP or STP for connection to end points. Data transmission is bi-directional across the medium, with the attached devices transmitting and receiving in both directions.

Generally operating at a theoretical raw data rate of 10/100 Mbps, a bus typically offers much less in terms of actual throughput. (See the previous lesson Bandwidth, Throughput and Goodput.)

Devices attached to bus networks make independent decisions relative to media access and initiation of transmission. As you might expect, this approach results in data collisions, which are increasingly common as the geographical scope of the network increases and as larger numbers of users exercise the network more intensively.