When you talk about network technologies, you're talking about a specific type of hardware and associated driver software that, when added to a PC, can produce a working network connection. The working part, however, is contingent on the right infrastructure — cables, connections, and ancillary equipment (such as bridges or routers) — also being available.
For the purposes of this book, there are two primary network technologies (but that's a tremendous oversimplification, as you'll find out later):
Ethernet Token ring Other
Okay, you caught us sneaking in a third, catch-all entry (we almost called it miscellaneous) to give us the opportunity to say something about several of the multitude of other available (but less common) network technologies that we choose not to cover in detail in this book. In fact, recent industry analyses indicate that it's about 75 percent likely that your network uses (or will use) one or both of the first two network technologies mentioned in the preceding list. Thus, even though we cover only a small number of technologies in depth, we cover most networks to some degree.
Meet Ethernet, the most popular network technology
Ethernet is the best known, most widely used, versatile, and readily available network technology around. As such things go, Ethernet has been around longer than most, since the mid-to-late 1970s. Ethernet was the brainchild of Xerox's Palo Alto Research Center (PARC) and later adopted by Digital, Intel, and Xerox (which is why older 15-pin connectors for ThickWire Ethernet are sometimes called DIX connectors). Ethernet has long been a networking commodity, which means that plenty of vendors play in this market and that lots of options and choices are available for this technology.
Ethernet uses the CSMA/CD access method. The sidebar titled "Ethernet: Network bumper cars," shown later in this chapter, explains what this stuff means in everyday English, insofar as the subject allows — which isn't so very far, alas.
KEY CONCEPT The easiest way to describe CSMA/CD is like this: "Listen before sending. Listen while sending. If garbage happens, quit sending and try again later."
Ethernet's strengths and weaknesses
Ethernet's strengths are as follows: It's robust and reliable, and it comes in a broad, affordable range of flavors. Ethernet's weaknesses include the inevitability of collisions and the more difficult troubleshooting techniques that a bus network requires. Ethernet's base speed of 10 Mbps (short for megabits per second) is on the slow end for modern networks, but plenty of higher-speed Ethernet versions are now available. (We give you the goods on these in Chapter 7.)
Ethernet does not perform well for high-traffic applications or when real-time delivery is needed (for video and multimedia), nor does it degrade gracefully when high traffic volumes occur. In fact, with Ethernet's CSMA/CD access method, the effective ceiling on its bandwidth is between 56 and 60 percent of total bandwidth (or between 5.6 and 6.0 Mbps on a 10-Mbps Ethernet). That's the level of use beyond which the increasing probability of collisions often results in network slowdowns or failures.
Ethernet: Network bumper cars
The acronym that describes Ethernet's media access method is CSMA/CD, for Carrier Sense Multiple Access with Collision Detection. (The auditory equivalent of a collision is an echo.) When a collision occurs, you must repeat the transmission. The following list provides a definition for each term in this access method acronym:
Carrier Sense: Everyone attached to the network is always listening to the wire, and no one can send while someone else is sending. When a message moves across the wire, an electrical signal called a carrier is used. By listening to the wire, a device knows when it's busy, because it senses the presence of the carrier.
Multiple Access: Any device attached to the network can send a message whenever it wants, as long as no carrier is sensed at the time. This means that multiple senders can (and sometimes do) begin sending at roughly the same time — when they think things are quiet — and that's why it's called multiple access.
Collision Detection: If two or more senders begin transmitting at roughly the same time, sooner or later their messages run into each other, causing a collision. Collisions are easy to recognize because they produce a garbage signal that is completely unlike a valid transmission. Ethernet hardware includes collision-detection circuitry that immediately halts transmission when a collision is observed. When a collision occurs, each sender waits a random time interval before listening to the wire to retry its transmission.
Warning When planning bandwidth consumption for an Ethernet network, use 55 percent of the total bandwidth (5.5 Mbps on a 10-Mbps network, 55 Mbps on a 100-Mbps network, and so on) as the ceiling for usable bandwidth on any network segment. If you plan to consume Ethernet's entire bandwidth when designing a network, you'll be designing a network that's headed for trouble!
There's no shortage of bandwidth available to Ethernet customers today. Most newer Ethernet network interface cards (NICs) are 10/100 designs, which means they can sense whether they're used on a 10-Mbps or 100-Mbps Ethernet network and set their speeds accordingly. Today, Gigabit Ethernet, with an amazing 1,000 Mbps of total theoretical bandwidth, lifts the ceiling on network capacity to new heights but retains compatibility with other Ethernet versions.
All the flavors of Ethernet
Ethernet comes in all the basic flavors. That is, Ethernet runs on any of the major media types — twisted-pair, coaxial cable (multiple versions, in fact), and fiber optic — and works with both bus and star topologies. One unusual variant — 100BaseVG-AnyLAN — uses a different access method called demand priority that gives this implementation
interesting capabilities. (100BaseVG-AnyLAN is covered in detail in Chapter 7.)
Also, you can easily find Ethernet devices that allow you to mix and match media; so you can use Ethernet to build networks of just about any size and for even the most hostile environments (such as factory floors or engine rooms,
where lots of heavy-duty equipment can create major interference).
In addition, Ethernet technologies support some innovative uses of bandwidth, so you'll occasionally hear about varieties such as switched Ethernet and full-duplex Ethernet. The former variety depends on a special kind of device (called a switch, naturally) that allows any two nodes to establish a private end-to-end connection. Therefore, switched Ethernet allows pairs of machines to use the entire bandwidth of the network medium. (This is a great way to squeeze extra life out of 10-Mbps Ethernet systems.) Full-duplex Ethernet is limited to 100BaseVG-AnyLAN and uses two pairs of wires so that machines can send and receive data at the same time, thereby doubling overall bandwidth.
The business end of Ethernet
Despite its age, Ethernet remains the most widespread and popular network technology. Of the major media types available, twisted-pair leads the pack for new Ethernet installations, but a lot of coaxial cable is still in use. Fiberbased Ethernet is usually limited to networks in campus environments, where long distances and electrical interference issues are greatest. However, it's also used in hostile environments or for high-bandwidth applications, including both
100-Mbps and gigabit implementations.
The primary reasons for Ethernet's unshaken popularity are as follows:
Affordability: Cabling is cheap and interfaces range from $20 for bottom-end NICs to less than $200 for powerful server NICs. Ethernet is not the cheapest of all the network technologies, but it's darn close!
Freedom of choice: Ethernet supports all types of media, numerous bandwidths, and lots of gear to build hybrid networks. Vendors galore offer Ethernet hardware. For specialized network hardware or media needs, chances are good that some Ethernet variety meets them. If some option isn't available, it's probably on someone's drawing board.
Experience: Ethernet's longevity means that Ethernet-savvy individuals are easy to find. Also, lots of technical and training material on Ethernet makes expertise relatively easy to build.
Continuing innovation: At 10 Mbps, basic Ethernet is no speed demon. However, vendors make high-speed network switches for Ethernet that can deliver the entire 10 Mbps to individual
connections, and higher-speed Ethernet varieties are readily available and widely used. As bandwidth needs grow, engineers have found ways to increase Ethernet's capabilities to match those needs, as the formalization of Gigabit Ethernet as an IEEE standard (802.3z, in fact) attests.
Tip If you're asked to build a new network and no compelling reasons exist to choose another network technology, choose Ethernet because of all the reasons previously mentioned!