The Physical Layer


The first and most obvious aspect of any network is the physical components and how they interact. This is called the physical layer. The physical layer is the hardware. It is the wires and all the devices that they connect. Parts of this layer include the network port on your computer or printer, the network cables, network hubs or switches, AirPort Base Stations, an AirPort Card installed in a computer, broadband modems, and much more. Together they make up the physical connection between every device.

These components of the physical layer require a lot of configuration. How fast does your network port send information to the network switch? Which cable leads from the network switch to the printer? What happens when your network card has to send and receive at exactly the same time? It is an incredibly daunting task! Fortunately, all these components can actually configure themselves. They do this by talking to each other using a common language, or protocol. There are several different hardware protocols. However, the Ethernet protocol is the only one that you are likely to see.

Ethernet protocol

For most people, the term Ethernet is synonymous with network, and they aren’t far off. Ethernet is by far the most common wired and wireless network in use in homes and offices. An Ethernet network can include computers using Mac OS X, earlier Mac OS versions, Windows, Unix, and many other operating systems. If it can be networked and you’ve heard of it, you can bet it uses Ethernet.

Token Ring networks are another type of networking. Like Ethernet, the components of a Token Ring network speak to each other. But instead of using the Ethernet protocol, the components use the Token Ring protocol. This makes Ethernet components and Token Ring components incompatible.

Every Ethernet port — whether it is on a computer, printer, cable modem or any other end device — has a unique address called an Ethernet address, or MAC address. (MAC is an acronym for Media Access Control, by the way, not an abbreviation for Macintosh.) This address is a long series of numbers and letters in the form 00:30:65:00:C2:4B. This Ethernet address is how network devices identify each other. Table 15-1 shows OS X 10.3–compatible Macintoshes and their built-in Ethernet capabilities.

Table 15-1: Macs with Built-in Ethernet Capabilities

Mac Model

Gigabit

100Base-T

10Base-T

iMac - all models

eMac - all models

iBook - all models

PowerBook G4 Titanium – (550Mhz-1Ghz)

PowerBook G4 Titanium (400-500Mhz)

PowerBook G4 12” (867Mhz-1Ghz)

PowerBook G4 15” (Aluminum)

PowerBook G4 17” (1.25-1.33 GHz)

PowerBook G3 Series - Bronze keyboard (Lombard)

PowerBook G3 - FireWire (Pismo)

PowerMac G4 - QuickSilver

Power Mac G4 - Digital Audio

Power Mac G4 - Gigabit Ethernet

Power Mac G4 - AGP Graphics

Power Mac G4 - PCI Graphics

Power Mac G4 Cube

Power Macintosh G3 - Blue and White

Ethernet devices communicate with each other using very small bundles of data called frames. Ethernet frames are the Post Office envelopes of your network. Each frame has both the sender’s address and the destination’s address. Data sent over the network is broken up into pieces that can fit into a frame. Those frames, then, are delivered to the destination. Finally, the destination’s network port reassembles these pieces back into the whole.

Making the Ethernet Connection

All Macs that are qualified for Mac OS X are ready to be connected to an Ethernet network. Older Macs commonly used another type of wiring called LocalTalk, but this type of network can’t be used with Mac OS X. A LocalTalk to Ethernet adaptor can enable these older Macs for use on an Ethernet network, though.

Ethernet cabling

When you think of an Ethernet network, you probably think of wires. But technically, AirPort wireless networks also use the Ethernet protocol as their physical layer language. AirPort networks are actually wireless Ethernet networks. However, they are rarely, if ever, called that. The common practice is to refer to wired networks as Ethernet networks and wireless networks as AirPort networks or wireless networks.

Looking at Ethernet cables

Ethernet networks may be wired with several types of cable, the most popular being unshielded twisted-pair (UTP). This type of cable looks like telephone cable and uses RJ-45 connectors that look like modular phone connectors, only a little larger. Other kinds of cable include Thinnet, thick coax, and fiber-optic Ethernet cables.

Twisted-pair cable is used in three different capacity networks: 10baseT, 100baseT (also called Fast Ethernet), and 1000baseT (also called Gigabit). All twisted-pair cable is graded according to how well it protects against electrical interference. Category 3 (Cat 3) cable is adequate for 10baseT networks, which have a maximum data transfer rate of 10 Mbps (megabits per second). While adequate for many small networks, this type of network is considered slow by today’s standards.

100baseT networks require at least Category 5 (Cat 5) cable, properly installed. These networks have a maximum data transfer rate of 100 Mbps. The rules for installing cable on a 100baseT network, however, are more stringent than for 10baseT. For example, sharp bends in the cable are not allowed.

Gigabit networking requires at least Category 5e (Cat 5e) cable and can sometimes require Category 6 (Cat 6) cable. Gigabit networks have a maximum data transfer rate of 1000 Mbps, or 1 Gigabit per second (Gbps). The rules for installing cable on a Gigabit network are stringent as well.

Some have successfully used Cat 5 cable to run a Gigabit network. While this may work for them, don’t count on it. The costs of any cable are insignificant when compared to the costs of installing it properly. If you are going through the trouble of installing the cabling properly, do not waste your time and money on the hope that Cat 5 will suffice. Get the right stuff: Cat 5e or Cat 6.

Using crossover cables

Twisted-pair cables actually contain eight separate wires, twisted into four pairs (hence the name). 10baseT and 100baseT networks use only two of the four pairs, while the other two pairs lie unused. Gigabit uses all four pairs. In each case, however, half of the wires in use are for inbound traffic while the other half are for outbound traffic. Your computer sends data on the outbound wires only and it receives data on the inbound wires only.

Connecting two computers using a standard Ethernet cable does not usually work. All the outbound connectors on one computer end up going right into the outbound connectors on the other. This won’t work! You need some way to cross over the wire somewhere along the line so that outbound goes into inbound.

Hubs, switches, and sometimes devices such as broadband modems and routers do crossing over for you inside the port. To reflect this fact, for example, the ports on a 100baseT switch are designated 100baseTX — the X indicates that the port performs this crossover. Sometimes using a hub or switch between two computers isn’t an option, though. For example, what if your friend brought her laptop over and wanted to connect it to your computer for a while? In that case, you need a special cable called a crossover cable. These are specially made cables that crossover on one end, turning outbound wires into inbound wires.

Most devices that are designed to plug into your computer do so by using a standard Ethernet cable. Put more technically, connecting a 100baseT network port to a 100baseTX network port requires a standard Ethernet cable. The same is true for both 10baseT and Gigabit. Connecting a computer (100baseT) to a cable modem or a network hub (100baseTX) is a good example of this very common kind of configuration.

But when you want to connect two similar devices, such as two computers or two hubs, you need a crossover cable. Put more technically, connecting two 100baseT devices to each other or two 100baseTX devices to each other requires a crossover cable. Connecting a computer (100baseT) to a computer (100baseT) or a hub (100baseTX) to a hub (100baseTX) are good examples of this kind of configuration.

Tip

If you try to connect a computer to a hub or switch using a crossover cable, you’ve just double-crossed yourself! Keep crossover cables clearly labeled so that you don’t make this mistake.

Smart network ports

Wouldn’t it be nice if each network port could just switch automatically? Then, every cable could be a standard cable. Well, someone figured it out. Some devices now have network ports that automatically change from 100baseT to 100baseTX or 1000baseT to 1000baseTX whenever they need to. Any Macintosh model with built-in 1000baseT (Gigabit) Ethernet has this feature, as do certain versions of Apple’s AirPort Base Station. Some of the more expensive network switches will also do this. This feature typically requires at least a standard, four-pair Category 5 cable.

Hubs and switches

At the center of a network is the hub or switch. Both devices serve the same purpose: to connect the various devices on your network at a central point. For many purposes, the subtle differences between a hub and a switch are not important. You could take a hub or a switch and plug in a few computers, a printer or two, a router, and a broadband connection without needing to know the difference between the two devices. The subtle differences, however, quickly become important — even critical — as the demand on your network grows. The wrong device in an otherwise fast network will bring your network to its knees in short order.

Remember that when you send information on an Ethernet network, it is broken down into frames for the journey. Each frame has both the sender’s address and the destination’s address. So, when your computer sends your document to the printer, that information is broken down into frames and sent down your Ethernet cable to the hub or switch. The hub or switch, then magically sends your frames down the correct cable to the printer. The difference between hubs and switches lies in how they get the right information down the right cable to the right place.

When a hub needs to get the right frame down the right cable, it takes a very simple, brute force approach: just send that frame down every cable. When you send an Ethernet frame through a hub to your printer, the hub duplicates that frame and sends it to every single device on the network. Only the destination device, however, recognizes the destination address of the frame and lets it in. Everything else on your network just ignores it.

This works remarkably well for small networks. The problem with this approach is that it generates a lot of extra network traffic because each frame needs to be duplicated for every device on your network. As your network grows, this problem gets worse — and fast. As the network becomes more and more crowded, the Ethernet frames start colliding. Your network is quite capable of dealing with these collisions. The problem is, these network collisions dramatically reduce the efficiency and speed of your network. Even on networks with only a handful of computers, something as ordinary as backing up a computer over the network could cause a very large number of collisions, noticeably reducing the speed of your network. Hubs, therefore, are only appropriate on small networks.

Switches are smart. Instead of broadcasting every single frame to every single device, switches send each Ethernet frame only where it needs to go, creating private paths between devices on your network. Without all of those duplicate frames that a hub generates, your switch can accomplish the same task with significantly less network traffic. More importantly, however, a switch dramatically reduces the number of collisions on your network, even under heavy traffic. This makes switches far more efficient on busy networks or any medium or large network.

It also makes them more secure. The astute or paranoid might have noticed something disconcerting about the nature of a hub. That is that everything you send over the network ends up going to every single device — including every single computer and printer on the network. How can you be sure they aren’t reading the information that you send? In short, you can’t. This type of network assumes that every other device is going to dutifully ignore your information. And they do, usually. It is possible, albeit difficult, to set up a computer to listen to all the data it receives. If this computer is connected to a hub, receiving every frame on the network, the computer is able to listen to all the traffic on the network. This is called sniffing a network. A network switch, on the other hand, sends the information only where it needs to go. From a security standpoint, this makes it significantly harder to listen to all the traffic on a network because your computer is receiving exactly what you are being sent only. But chances are if you are using a hub, your network is very small. So, if your network uses a hub and you have a hacker on your network sniffing around, the problem is the hacker in your bedroom, and not your insecure hub.

Hubs and switches come in many different sizes, measured by the number of network ports they have. The most common sizes are 4-port, 8-port, and 16-port, although you can also find 24-port and 48-port sizes readily. Even larger configurations are available, but these tend to be expensive and very sophisticated. Prices start at about $20 for a generic 4-port 10baseT hub and rise with the number of ports. 100baseT hubs cost more than 10baseT hubs, and switches cost more than hubs. Generally, if a hub or switch supports 100baseT it also supports 10baseT. A hub or switch that supports both speeds will say that it is a dual speed or 10/100 hub or switch. Generally, only switches support Gigabit. Switches that support Gigabit will say so. These switches may or may not support both Gigabit and the slower speeds. Some Gigabit switches offer full support for all three speeds. This is usually indicated by the phrase “10/100/1000.” Some Gigabit switches offer only one or two Gigabit-only ports while the rest offer the more standard 10/100 ports. You will often have to look very carefully at the documentation for a switch to determine which ports support Gigabit.

Connecting hubs and switches

If you use up all the ports on one hub or switch, you can connect another hub or switch to it. You can even use hubs and switches together. Connecting hubs is called daisy chaining. Up to four hubs can be daisy-chained with twisted-pair cable. Switches are not subject to this restriction.

If you want to connect two hubs or switches, you can do so easily using a crossover cable between any two network ports. Most hubs and switches, however, have a specially designated port for this kind of connection called an uplink port.

You don’t need a crossover cable if your hub or switch has an uplink port or a port that you can make into an uplink port by setting a switch. Plugging a regular cable into an uplink port has the same effect as using a crossover cable, because the uplink port’s wiring is reversed. On some hubs and switches, the uplink port and the port next to it cannot be used at the same time because they are actually the same port, only with two different jacks. Choose one and use it. On other hubs and switches, the uplink port has a button or switch that allows you to change the port from a regular port to an uplink port. In either case, simply use a regular cable to connect the uplink port on one hub or switch to any regular port on the other hub or switch. Note that connecting two uplink ports would require a crossover cable.

Understanding the lights on a hub or switch

All hubs and switches have lights on them indicating all sorts of information. The most important lights on any switch or hub — aside from the power light, of course, are the link lights. Each port has a link light associated with it. Link lights are usually either right next to each port or listed all together on one end of the hub or switch. When two network ports on either end of a cable are connected properly, they automatically establish a link. When this happens, the port’s link light turns on. Multiple speed hubs and switches show you what speed a port is using. Hubs and switches also indicate traffic and collisions by using lights to indicate such occurrences. Look at the front panel of your device or check the documentation.




Mac OS X Bible, Panther Edition
Mac OS X Bible, Panther Edition
ISBN: 0764543997
EAN: 2147483647
Year: 2003
Pages: 290

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