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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Selecting the Proper Cable

As the demands of network users for ever increasing amounts of bandwidth continue, and n networking systems are developed to accommodate them, it soon becomes necessary to examine the capabilities of the most fundamental part of the network infrastructure: the cable itself. Ethernet over UTP cable, or 10BaseT, is the medium of choice in the majority of LAN installations today.

The cable used for such networks has traditionally been the same as that used for business telephone wiring. This is known as Category 3, or voice grade UTP cable, measured according to a scale that quantifies the cable’s data transmission capabilities. The cable itself is 24 AWG (American Wire Gauge, a standard for measuring the diameter of a wire), copper tinned, with solid conductors, 100[nd]105 ohm characteristic impedance, and a minimum of two twists per foot. Category 3 cable is adequate for networks running at up to 16Mbps.

Newer, faster network types require greater performance levels, however. Fast Ethernet technologies that run at 100Mbps using the same number of wires as standard Ethernet need a greater resistance to signal crosstalk and attenuation, and so the use of Category 5 cabling is essential. If, when you are building a LAN, you can use Category 3 wiring that is already in place, by all means do so. If, however, you are pulling new cable for your network, the use of Category 5 cable is recommended. Even if you are not running a high-speed network today, you will probably want to consider it in the future.

In a token-passing network, the cables from the workstations (or from the wall faceplates) connect centrally to a MSAU. The MSAU keeps track of which workstations on the LAN are neighbors and which neighbor is upstream or downstream. It is an easy job; the MSAU usually does not even need to be plugged into an electrical power outlet. The exceptions to this need for external power are MSAUs that support longer cable distances, or the use of UTP (Type 3) cable in high-speed LANs. The externally powered MSAU helps the signal along by regenerating it.

An IBM MSAU has eight ports for connecting one to eight Token-Ring devices. Each connection is made with a genderless data connector (as specified in the IBM cabling system). The MSAU has two additional ports, labeled RI (Ring-In) and RO (Ring-Out), that daisy-chain several MSAUs together when you have more than eight workstations on the LAN.

It takes several seconds to open the adapter connection on a Token-Ring LAN (something you may have noticed). During this time, the MSAU and your Token-Ring adapter card perform a small diagnostic check, after which the MSAU establishes you as a new neighbor on the ring. After being established as an active workstation, your computer is linked on both sides to your upstream and downstream neighbors (as defined by your position on the MSAU). In its turn, your Token-Ring adapter card accepts the token or frame, regenerates its electrical signals, and gives the token or frame a swift kick to send it through the MSAU in the direction of your downstream neighbor.

In an Ethernet network, the number of connections (taps) and their intervening distances are network’s limiting factors. Repeaters regenerate the signal every 500 meters or so. If repeaters were not used, standing waves (additive signal reflections) would distort the signal and cause errors. Because collision detection is highly dependent on timing, only five 500-meter segments and four repeaters can be placed in series before the propagation delay becomes longer than the maximum allowed period for the detection of a collision. Otherwise, the workstations farthest from the sender would be unable to determine whether a collision had occurred.

The people who design computer systems love to find ways to circumvent limitations. Manufacturers of Ethernet products have made it possible to create Ethernet networks in star, branch, and tree designs that overcome the basic limitations already mentioned. You can have thousands of workstations on a complex Ethernet network.

LANs are local because the network adapters and other hardware components cannot send LAN messages more than about a few hundred feet. Table 1.6 reveals the distance limitations of different kinds of LAN cable. In addition to the limitations shown in the table, keep in mind that you cannot connect more than 30 computers on single Thinnet Ethernet segment, more than 100 computers on a Thicknet Ethernet segment, more than 72 computers on UTP Token-Ring cable, or more than 260 computers on STP Token Ring cable.

Table 1.6 Network Distance Limitations.

Network Adapter Cable Type Maximum Minimum

Ethernet Thin 607 ft. 20 in.
Thick (drop cable) 164 ft. 8 ft.
Thick (backbone) 1,640 ft. 8 ft.
UTP 328 ft. 8 ft.
Token Ring STP 328 ft. 8 ft.
UTP 148 ft. 8 ft.
ARCnet (passive hub) 393 ft. Depends on cable
ARCnet (active hub) 1,988 ft. Depends on cable

Examining Protocols, Frames, and Communications

The network adapter sends and receives messages among the LAN computers, and the network cable carries the messages. It is the less tangible elements, however—the layers of networking protocols in each computer—that turn the individual machines into a local area network.

At the lowest level, networked computers communicate with one another by using message packets, often called frames. These frames, so-called because they surround and encapsulate that actual information to be transmitted, are the foundation on which all LAN activity is based. The network adapter, along with its support software, sends and receives these frames. Each computer on the LAN is identified by a unique address to which frames can be sent.

Frames are sent over the network for many different purposes, including the following:

  Opening a communications session with another adapter
  Sending data (perhaps a record from a file) to a PC
  Acknowledging the receipt of a data frame
  Broadcasting a message to all other adapters
  Closing a communications session


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