April 23, 2011, 10:35 a.m.
posted by mv
To make a network operate under any standard requires actual hardware—pieces of equipment that plug into each other to make the physical network. Most of today's networking systems require three different kinds of hardware. Network interface cards link your computer to the network wiring system (or wireless signaling system). A hub or access point brings the signals from all the computers and other devices in the network together. And a medium (such as wires or radio waves) links these two together. You build a network using these three components from one network standard.
Network Interface Cards
The network interface card (NIC) and its associated driver software have the most challenging job in the network. The NIC takes the raw data from your computer, converts it into the proper format for the network you're using, and then converts the electrical signals to a format compatible with the rest of the network. If it doesn't work properly, your data would be forever trapped in your computer.
Because it converts the stream of data between two different worlds (inside your computer and the outside world of the network), it needs to match two standards: a connection system used by your computer and the connection system used by the network.
Traditionally, a network adapter has plugged into the expansion bus of the host computer. No other connection could provide the speed required for network use. Desktop computers used internal NICs that slid into expansion slots. Portable computers used NICs in the form of PC Cards.
With the advent of USB, however, that situation has changed. Although expansion bus–based NICs remain important, many networks are shifting to NICs that plug into USB ports. The primary reason for this is that USB-based NICs are easier to install. Plug in one connector, and you're finished. Although early USB-based NICs gave up speed for this convenience, a USB 2.0 port can handle the data rates required even by 100Base-T networking.
The simplified piggyback networking systems HomePlug and HomePNA rely on USB-based NICs for easy installation. In wireless networking, USB-based NICs often have greater range than their PC Card–based peers because they can operate at higher power and with more sophisticated antennae. Compared to installing a PC Card adapter in your desktop computer and then sliding a wireless NIC into the adapter, the USB-based NIC makes more sense, both for the added range as well as the greater convenience and lower cost.
The NIC you choose must match the standard and speed used by the rest of your network. The exception is, of course, that many NICs operate at more than one speed. For example, nearly all of today's 100Base-T NICs slow down to accommodate 10Base-T networks. Dual-speed 802.11b+ wireless NICs will accommodate either 11 or 22MHz operation.
Most dual-speed NICs are autosensing. That is, they detect the speed on the network wire to which they are connected and adjust their own operating speed to match. Autosensing makes a NIC easier to set up, particularly if you don't know (or care) the speed at which your network operates. You can just plug in the network wire and let your hardware worry about the details.
Some network adapters allow for optional boot ROMs, which allow computers to boot up using a remote disk drive. However, this feature is more applicable to larger businesses with dedicated network servers rather than a home or small business network.
Hubs and Access Points
A network hub passes signals from one computer to the next. A network access point is the radio base station for a wireless network. Piggyback networks such as HomePlug and HomePNA do not use dedicated hubs, although one of the computers connected to the network acts like a hub.
The design of Ethernet requires all the signals in the network loop to be shared. Every computer in the loop—that is, every computer connected to a single hub—sees exactly the same signals. The easiest way to do this would be to short all the wires together. Electrically, such a connection would be an anathema.
The circuitry of the hub prevents such disasters. It mixes all the signals it receives together and then sends out the final mixture in its output.
To make the cabling for the system easy, the jacks on a hub are wired the opposite of the jacks on NICs. That is, the send and receive connections are reserved—the connections the NIC uses for sending, the hub uses for receiving.
Inexpensive hubs are distinguished primarily by the number and nature of the ports they offer. You need one port on your hub for each computer or other device (such as a network printer or DSL router) in your network. You may want to have a few extra ports to allow for growth, but more ports cost more, too. Some hubs include a crossover jack or coaxial connection that serves as an uplink to tie additional hubs into your network.
Expensive hubs differ from the economic models chiefly by their management capabilities—things such as remote monitoring and reconfiguration, which are mostly irrelevant to a small network.
As with NICs, hubs can operate at multiple speeds. Although some require each network connection to be individually configured for a single speed (some are even physically set only at one speed), most modern dual-speed hubs are autosensing, much like NICs. Coupling a low-cost autosensing NIC and low-cost autosensing hub can sometimes be problematic with marginal wiring. The hub's sense may negotiate the highest-speed signals on the network, even though many packets fail to negotiate the wiring at higher speeds. In such instances, performance can fall below that of a low-speed-only network.
Some hubs are called switches; others have internal switches. A switch divides a network into segments and shifts data between them. Most dual-speed hubs actually act as switches when passing packets between 10Base-T and 100Base-T devices.
When used on a single-speed network, a switch can provide faster connections because the full bandwidth of the network gets switched to service each NIC. When several NICs contend for time on the network, however, the switch must arbitrate between them, and in the end, they share time as they would on a hub-based network.
Switches usually have internal buffers to make it easier to convert between speeds and arbitrate between NICs. In general, the larger the buffer the better, because a large buffer helps the switch operate at a higher speed more of the time.
Access points earn their name because they provide access to the airwaves. Although in theory you could have a wireless-only hub to link together a small network of computers, such a network would shortchange you. You could not access the Internet or any devices that have built-in wired-networking adapters, such a network printers. In other words, most wireless networks also require a wired connection. The access point provides access to the airwaves for those wired connections.
The centerpiece of the access point is a combination of radio transmitter and receiver (which engineers often talk about simply as the radio). Any adjustment you might possibly make to the radio is hidden from you, so you don't have to be a radio engineer to use one. The access point's radio is both functionally and physically a sealed box, permanently set to the frequencies used by the standard used by the networking system supported by the access point. Even the power at which the access point operates is fixed. All access points use about the same power because the maximum limit is set by government regulations.
Access points differ in several ways, however. Because every access point has a wired connection, they differ just as wired network do—they may support different speed standards. In fact, most access points have multiple wired connections, so they are effectively wired hubs, too. The same considerations that apply to wired hubs therefore apply to the wired circuitry of access points.
Access points also differ in the interface between their radios and the airwaves—namely, their antennae. Lower-cost access points have a single, permanently attached antenna. More expensive hubs may have two or (rarely) more antennae. By using diversity techniques, multiple-antenna access points can select which of the antennae has the better signal and use it for individual transmissions. Consequently, dual-antennae access points often give better coverage.
Some access points make their antennae removable. You can replace a removable antenna with a different one. Some manufacturers offer directional antennae, which increase the range of an access point in one direction by sacrificing the range in all other directions. In other words, they concentrate the radio energy in one direction. Directional antennae are particularly useful in making an access point into a radio link between two nearby buildings.
When you want to link more than one network together—for example, you want to have both wired and wireless networks in your office—the device that provides the necessary bridge is called a router because it gives the network signals a route from one system to another. When you want to link a home network to the Internet through a DSL line, you need a router. Routers are more complex than hubs because they must match not only the physical medium but also the protocols used across the different media.
Sometimes what you buy as a hub actually functions as a router. Most wireless access points include 10Base-T or 100Base-T connections to link with a wired network. Many DSL adapters are sold as hubs and have multiple network connections (or even a wireless access point), even though they are, at heart, routers.
Network wires must carry what are essentially radio signals between NICs and hubs, and they have to do this without letting the signals actually become radio waves—broadcasting as interference—or finding and mixing with companion radio waves, thus creating error-causing noise on the network line. Two strategies are commonly used to combat noise and interference in network wiring: shielding the signals with coaxial cable and preventing the problem with balanced signaling on twisted-pair wiring.
Coaxial cables get their name because they all have a central conductor surrounded by one or more shields that may be a continuous braid or metalized plastic film. Each shield amounts to a long, thin tube, and each shares the same longitudinal axis—the central conductor. The surrounding shield typically operates at ground potential, which shields stray signals from leaking out of the central conductor or prevents noise from seeping in. Figure shows the construction of a simple coaxial cable.
Coaxial cables are tuned as transmission lines—signals propagate down the wire and are completely absorbed by circuitry at the other end—which allows them to carry signals for long distances without degradation. For this reason, they are often used in networks for connecting hubs, which may be widely separated.
The primary alternative is twisted-pair wiring, which earns its name from being made of two identical insulated conducting wires that are twisted around one another in a loose double-helix. The most common form of twisted-pair wiring lacks the shield of coaxial cable and is often denoted by the acronym UTP, which stands unshielded twisted pair. Figure shows a simplified twisted-pair cable.
Manufacturers rate their twisted-pair cables by category, which defines the speed of the network they are capable of carrying. Currently standards define six categories: 1 through 5 and 5e (which stands for 5 enhanced). Two higher levels, 6 and 7, exist on the periphery (proposed but not official standards). As yet, they are unnecessary because Gigabit Ethernet operates with Category 5 wiring. Note that, strictly speaking, until the standards for Levels 6 and 7 get approved, wires of these quality levels are not termed Category 6 and Category 7, because the categories are not yet defined. Instead, they are termed Level 6 and Level 7.
Those in the know don't bother with the full word Category. They abbreviate it Cat, so look for Cat 5 or Cat 5e wire when you shop. Figure lists the standards and applications of wiring categories.
Most UTP wiring is installed in the form of multipair cables with up to several hundred pairs inside a single plastic sheath. The most common varieties have 4 to 25 twisted pairs in a single cable. Standard Cat 5 wire for networking has four pairs, two of which are used by most 10Base-T and 100Base-T systems.
The pairs inside the cable are distinguished from one another by color codes. The body of the wiring is one color alternating with a thinner band of another color. In the two wires of a given pair, the background and banding color are opposites—that is, one wire will have a white background with a blue band, and its mate will have a blue background with a white band. Each pair has a different color code.
To minimize radiation and interference, most systems that are based on UTP use differential signals. For extra protection, some twisted-pair wiring is available with shielding. As with coaxial cable, the shielding prevents interference from getting to the signal conductors.
The specifications for most UTP networking systems limit the separation between any NIC and hub to no more than 100 meters (about 325 feet). Longer runs require repeaters or some other cabling system.