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Networking Fun(damentals)
Part 1 - Getting Wired

by Bruce Preston

 

A large number of the questions that we have been getting during our Random Access sessions has been related to networking computers - either in a local network or connecting to the internet. This article (which is the first of several installments) will address some of the details as to how it works. Note to any EEs out there: some details will be sacrificed for simplicity.

Since ethernet is by far the most popular way for creating a wired network, I will start with the cabling that is used for creating a wired home network. The wiring must be Cat-5 (for Category 5) twisted pair cable. This cable had its origins in the telephone industry, but is most definitely not plain telephone wire. Let’s start with a bit of basic physics: When electricity flows through a wire there is a discernable magnetic field (disturbance) around the wire. If the wire is spun into a coil (think of a spool of thread) the magnetic field is greatly enhanced - and you have an electromagnet. Now the interesting part—if you have a magnetic field and a wire moves through it, the field can induce an electric current. This is the principle behind a generator. But physical movement isn’t the only way to induce a current - it can be induced if the field strength varies rapidly. Have you ever been on a telephone call and faintly heard someone else’s conversation? What is happening is that in some cable somewhere pulses from that conversation’s pair of wires are being picked up by your pair of wires - a phenomenon known as “cross talk”.

Data being sent through a pair of wires is done via pulses of ON and OFF voltages, millions of times (megabits) per second. That most definitely qualifies for rapid variation of field strength. The way to reduce the cross talk to an acceptable level is to make use of the fact that the signal is traveling up one wire and back down the other in the pair. By wrapping (or twisting) the wires of the pair around each other, the magnetic fields cancel each other out - voila - no cross talk.

Cat-5 cable has 4 pairs of wire. They are color coded: orange and orange/white stripe, green and green/white stripe, blue and blue/white stripe, and brown and brown/white stripe. If you get a magnifying glass and look at the plug end of a regular Cat-5 cable, you will see the individual wires in this sequence shown below as “End A”

Only the orange and green pairs are used for ethernet. In a regular connection a ‘patch’ cable is used to connect a PC to a hub or switch or modem device. If the PC’s network card is set to “send” on the orange pair and “receive” on the green pair, then the other device must be set to “receive” from the orange pair and “send” on the green pair. Fortunately, you usually connect a PC (a so-called “terminal” device) to a hub, switch, router, or modem (a so-called “communications” device) so the manufactures set them up as to which pair in the jack are send and which are receive.

At this point it might be well to explain the difference between a hub, switch, router and modem.

A hub is a device with accepts data from a device such as a PC and sends it out to all other connections. It has no ‘intelligence’ - it just broadcasts everything that it hears onto all connections. Fortunately within the ethernet and TCP/IP protocols (more on those later) there is information within the data transmission that identifies where the data came from and where it is going. The network software within the other devices determines whether it should listen to and accept the data, or just ignore it. This does make for a lot of data traveling over the network cables.

A switch has some intelligence within it. It keeps track of which device is attached to which of its ports (sockets) and directs the data transmission to the intended device without putting it on the cables for the other devices. This greatly reduces the amount of data traveling over the network cables.

A router is a specialized switch. It knows which devices are local and which destinations are ‘somewhere else’ - such as the internet. It determines whether to make a local connection or steer (route) the transmission to another router (say at your ISP’s site) and hand it off. Typical broadband Cable Modem/DSL Routers (such as by LinkSys, D-Link, Belkin, SMC, Netgear, etc.) also perform other functions which I will describe in a future article. Since the price of a router is marginally (if at all) more than a hub, you won’t find many hubs being made or sold today.

A modem is a device which converts the local ethernet signal to something that is suitable for transmission over your broadband connection - such as DSL or cable. It does bidirectional conversion, i.e. ethernet to broadband, or broadband to ethernet.

For an absolutely bare-bones network, you may connect two ‘terminal’ devices, such as two PCs, ‘back to back’ using a specialized Cat-5 cable, called a “cross over” cable. (Sometimes the cable will have the abbreviation “X-over” on it.) In this special-case cable, the wires at one end of the cable are in the plug in a sequence different than the other. In the diagrams below I only show two of the four pairs.

You can now see that a signal put on (say) pair 1-2 at one end will come out on pair 3-6 at the other, and visa versa. This effectively corrects the transmitter of one device to the receiver of the other, and visa versa.

Just as two similar “terminal” devices may be connected via a cross-over cable, two similar “communications” devices may be connected via a cross-over cable, effectively making a ‘larger’ switch or hub. Another reason to have multiple switches or hubs would be to place them in the centers of ‘work groups’, thus reducing the amount of cable necessary to connect the clusters of work groups to each other. For example, in an office that occupies more than one floor of a building, there might be one switch per floor, with an ‘uplink’ connection connecting the switches on each floor. More often, however, one of the ports will be designated an ‘up-link’ port which has the cross-over function implemented via an internal mechanism.

In the next installment I will discuss network cards and how to configure them and test your configuration using some very basic tools included in Windows - ping, winipcfg, ipconfig, and tracert.


Bruce Preston is president of West Mountain Systems, a consultancy in Ridgefield, CT specializing in database applications. A DACS director, Bruce also leads the Access SIG. Members may send tech queries to Bruce at askdacs@dacs.org.

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