Upgrading and Repairing PCs

Network Protocols Overview

The protocol you choose to run is the single most important decision you make when setting up a local area network. This protocol defines the speed of the network, the medium access control mechanism it uses, the types of cables you can use, the network interface adapters you must buy, and the adapter drivers you install in the network client software.

The Institute of Electrical and Electronic Engineers (IEEE) has defined and documented a set of standards for the physical characteristics of both collision-detection and token-passing networks. These standards are known as IEEE 802.3 (Ethernet) and IEEE 802.5 (Token-Ring). IEEE 802.11 defines wireless versions of Ethernet.

Note Be aware, however, that the colloquial names Ethernet and Token-Ring actually refer to earlier versions of these protocols, on which the IEEE standards were based. Minor differences exist between the frame definitions for true Ethernet and true IEEE 802.3. In terms of the standards, IBM's 16Mbps Token-Ring products are an extension of the IEEE 802.5 standard. There is also an older data-link protocol called ARCnet that is now rarely used.

The two major choices today for wired networks are Ethernet and Token-Ring, although Ethernet and its variations are by far the most popular. Other network data-link protocols you might also encounter are summarized in Table 20.2. The abbreviations used for the cable types are explained in the following sections.

Table 20.2. Wired LAN Protocol Summary

Network Type	Speed	Maximum Number of Stations	Cable Types	Notes
ARCnet	2.5Mbps	255 stations	RG-62 coax UTP/Type 1 STP	Obsolete for new installations; was used to replace IBM 3270 terminals (which used the same coax cable).
Ethernet	10Mbps	Per segment: 10BASE-T-2 10BASE-2-30 10BASE-5-100 10BASE-FL-2	UTP Cat 3 (10BASE-T), Thicknet (coax; 10BASE-5), Thinnet (RG-58 coax; 10BASE-2), fiber-optic (10BASE-F)	Largely replaced by Fast Ethernet; can be interconnected with Fast Ethernet by use of dual-speed hubs and switches; use switches and routers to overcome "5-4-3" rule in building very large networks.
Fast Ethernet	100Mbps	Per segment: 2	Cat 5 UTP	Fast Ethernet can be interconnected with standard Ethernet through the use of dual-speed hubs, switches, and routers. The most common variety is 100BASE-TX; alternative 100BASE-T4 is not widely supported.
Gigabit Ethernet	1,000Mbps	Per segment: 2	Cat 5 UTP	Gigabit Ethernet can be interconnected with Fast or standard Ethernet through the use of multispeed hubs, switches, and routers.
Token-Ring	4Mbps or 16Mbps	72 on UTP, 250-260 on type 1 STP	UTP, Type 1 STP, and fiber optic	High price for NICs and MSAUs to interconnect clients; primarily used with IBM mid-size and mainframe systems.

A few years ago, the choice between Token-Ring or Ethernet wasn't easy. The original versions of standard Ethernet (10BASE-5 "Thick Ethernet" and 10BASE-2 "Thin Ethernet") used hard-to-install coaxial cable and were expensive to build beyond a certain point because of the technical limitations expressed by the "5-4-3" rule (see the following note).

Note The 5-4-3 rule gets its name from the fact that Ethernet signals can travel through no more than five segments, four repeaters or hubs, and three populated segments (cable segments with two or more stations) before they become unreliable. Because 10BASE-T hubs act as repeaters and a 10BASE-T segment has only one station per cable, creating large networks is easier with 10BASE-T Ethernet than with older forms of Ethernet.

Initially, Token-Ring's 16Mbps version was substantially faster than 10BASE versions of Ethernet and had larger limits on the numbers of workstations permitted per segment. Currently, however, the popularity and low cost of Fast Ethernet; the use of easy-to-install twisted-pair cabling for standard, 100Mbps Fast, and even 1,000Mbps Gigabit Ethernet; and the use of hubs and switches to overcome classic Ethernet station limitations have made Fast Ethernet the preferred choice for workgroup-size networks and a competitor to Token-Ring for larger networks. A properly designed Fast Ethernet network can be upgraded to Gigabit Ethernet in the future.

Ethernet

With tens of millions of computers connected by Ethernet cards and cables, Ethernet is the most widely used data link layer protocol in the world. Ethernet-based LANs enable you to interconnect a wide variety of equipment, including Unix and Linux workstations, Apple computers, printers, and PCs. You can buy Ethernet adapters from dozens of competing manufacturers. Older adapters supported one, two, or all three of the cable types defined in the standard: Thinnet, Thicknet, and Unshielded Twisted Pair (UTP). Current adapters, on the other hand, almost always support UTP only. Traditional Ethernet operates at a speed of 10Mbps, but the more recent (and most popular of the Ethernet flavors) Fast Ethernet standards push this speed to 100Mbps. The latest version of Ethernet, Gigabit Ethernet, reaches speeds of 1,000Mbps, or 100 times the speed of original Ethernet.

Fast Ethernet

Fast Ethernet requires adapters, hubs, and UTP or fiber-optic cables designed to support the higher speed. Some early Fast Ethernet products supported only 100Mbps, but almost all current Fast Ethernet products are combination devices that run at both 10Mbps and 100Mbps, enabling you to gradually upgrade an older 10Mbps Ethernet network by installing new NICs and hubs over an extended period of time.

Both the most popular form of Fast Ethernet (100BASE-TX) and 10BASE-T standard Ethernet use two of the four wire pairs found in UTP Category 5 cable. An alternative Fast Ethernet standard called 100BASE-T4 uses all four wire pairs in UTP Category 5 cable, but this Fast Ethernet standard was never popular and is seldom seen today.

Gigabit Ethernet

Gigabit Ethernet also requires special adapters, hubs, and cables. Most users of Gigabit Ethernet use fiber-optic cables, but you can run Gigabit Ethernet over the same Category 5 UTP cabling that Fast Ethernet and newer installations of standard Ethernet use. Gigabit Ethernet for UTP is also referred to as 1000BASE-T.

Unlike Fast Ethernet and standard Ethernet over UTP, Gigabit Ethernet uses all four wire pairs. Thus, Gigabit Ethernet requires dedicated Ethernet cabling; you can't "borrow" two wire pairs for telephone or other data signaling with Gigabit Ethernet as you can with the slower versions. Most Gigabit Ethernet adapters can also handle 10BASE-T and 100BASE-TX Fast Ethernet traffic, enabling you to interconnect all three UTP-based forms of Ethernet on a single network.

Neither Fast Ethernet nor Gigabit Ethernet support the use of thin or thick coaxial cable originally used with traditional Ethernet, although you can interconnect coaxial-cable-based and UTP-based Ethernet networks by using media converters or specially designed hubs and switches.

Note For more information about Ethernet, Fast Ethernet, Token-Ring, and other network data-link standards, see the "Data Link Layer Protocols" and "High-Speed Networking Technologies" sections found in Chapter 19 of Upgrading and Repairing PCs, 11th Edition, available in electronic form on the DVD provided with this book.