UNIX System Administration Handbook - Evi Nemeth [225]
A single logical segment may consist of physical segments operating at different speeds (10 Mb/s, 100 Mb/s, or 1 Gb/s); hence switches must have buffering and timing capabilities in order to eliminate potential conflicts.
Unshielded twisted pair
Unshielded twisted pair (UTP) is the preferred cable medium for Ethernet. It is based on a star topology and has several advantages over other media:
• It uses inexpensive, readily available copper wire. (Sometimes, existing phone wiring can be used.)
• UTP wire is much easier to install and debug than coax or fiber. Custom lengths are easily made.
• UTP uses RJ-45 connectors, which are cheap, reliable, and easy to install.
• The link to each machine is independent (and private!), so a cabling problem on one link is unlikely to affect other hosts on the network.
The general “shape” of a UTP network is illustrated in Exhibit B on the next page.
Exhibit B A UTP installation
UTP wire suitable for use in modern LANs is commonly broken down into eight classifications. The performance rating system was first introduced by Anixter, a large cable supplier. Today, these classifications are known as Category 1 through Category 7, with a special Category 5E in the middle.
Category 1 and Category 2 cables are suitable only for voice applications (if that). Category 3 cable is as low as you can go for a LAN; it is the standard for 10 Mb/s 10BaseT. Category 4 cable is something of a orphan, not exactly suited for any particular application. It is occasionally used for 16 Mb/s UTP token ring or for fancy 10BaseT installations. Category 5 cable can support 100 Mb/s and is the most common standard currently in use for data cabling. Category 5E and Category 6 cabling support 1 Gb/s.
See page 386 for more information about wiring.
10BaseT connections require two pairs of Category 3 wire, and each link is limited to a length of 100 meters; 100BaseTX has the same length limitation but requires two pairs of Category 5 wire. Both PVC-coated and Teflon-coated wire are available. Your choice of jacketing should be based on the environment in which the cable will be installed. Enclosed areas that feed into the building’s ventilation system (“return air plenums”) typically require Teflon. PVC is less expensive and easier to work with.
RJ-45 connectors wired with pins 1, 2, 3, and 6 are used to make the connections. Although only two pairs of wire are needed for a working 10 Mb/s or 100 Mb/s connection, we recommend that when installing a new network you use four-pair Category 5E wire and connect all eight pins of the RJ-45 jack.
See page 93 for more information about the RS-232 standard.
For terminating the four-pair UTP cable at patch panels and RJ-45 wall jacks, we suggest that you use the TIA/EIA-568A RJ-45 wiring standard. This standard, which is compatible with other uses of RJ-45 (e.g., RS-232), is a convenient way to keep the wiring at both ends of the connection consistent, regardless of whether you can easily access the cable pairs themselves. The 568A standard is detailed in Table 15.2.
Table 15.2 TIA/EIA-568A standard for wiring four-pair UTP to an RJ-45 jack
Existing building wiring may or may not be suitable for network use, depending on how and when it was installed. Many old buildings were retrofitted with new cable in the 1950s and 1960s. Unfortunately, this cable usually won’t support even 10 Mb/s.
Connecting and expanding Ethernets
Ethernets can be logically connected at several points in the seven-layer ISO network model. At layer 1, the physical layer, you can use either hardware connectors or repeaters (commonly called hubs in modern times). They transfer the signal directly, much like two tin cans connected by string.
At layer 2, the data link layer, switches are used. Switches transfer frames on the basis of the hardware source and