mirror of https://github.com/tLDP/LDP
290 lines
11 KiB
XML
290 lines
11 KiB
XML
<sect1 id="Media-Types">
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<title>Media-Types</title>
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Media Types
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<para>
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Common network media include twisted pair (UTP and STP), coaxial cable,
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fiber-optic cable, and wireless transmissions via infrared and radio waves.
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Network media are usually chosen based on several criteria: their cost,
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their vulnerability to EMI and eavesdropping, their bandwidth, and the
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maximum distance at which they can be used.
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</para>
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Twisted Pair Ethernet Cable
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Unshielded Twisted Pair (UTP)
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<para>
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UTP is the most common type of network cable in use today. UTP consits of
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one or more pairs of insulated copper wires. The wires are twisted together
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to reduce crosstalk, and enclosed in a plastic insulator. UTP is the type of
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cable used in telephone systems.
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</para>
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<para>
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UTP cables are generally wires using RJ-45 jacks and plugs. These are
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8-conductor connectors similar to the RJ-11 connectors used in telephone
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cables. UTP is inexpensive and easy to install compared with other types of
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cable, although the differenet types of UTP vary in cost.
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</para>
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<para>
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Copper wire has a high level of attenuation. YTP cable is limited to
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transmission distances of 100 meters or less. UTP is more susceptible
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to interference (EMI) than mos types of cable, and is vaulnerable to
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eavesdropping since its own emissions are not shielded.
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</para>
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<para>
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Types of UTP cable are defined by the EIA standards, which specify the three
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categories of cable. Category 3 is the minimum requiredment for networking,
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and Category 5 is the highest-quality network cable. The UTP categories and
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the bandwidth they support are summarized below.
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</para>
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Category Maximum Data and Transfer Rate Description
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3 10 Mbps Least expensive network cable, commonly used
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4 16 Mbps Medium quality; rarely used
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5 100 Mbps Highest quality
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> Start Binh
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5 1000Mbps Highest Quality
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> End Binh
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Shielded Twisted Pair (STP)
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<para>
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STP cable is similar to UTP, but inclues a foil or wire mesh shield
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between the wire pairs and the outer insulation. The shield is electrically
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grounded, and reduces emissions and susceptibility to EMI. STP cable is used
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in some Token Ting and AppleTalk networks. STP is more expensive than
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UTP, and its thickness and rigidity make it more difficult to install. It
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also uses grounded connection connectors, adding to the expense. STP uses
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the same copper wires as UTP as the same level of attenuation, and therefore
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the same maximum distance of about 100 meters. However, it is much less
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susceptible to EMI and eavesdropping. The reduced inteference allows for higher
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bandwidth, potentially as high as 500 Mbps.
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</para>
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Coaxial
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<para>
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Coaxial Cable consists of a single thick copper wire surrounded by an
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insulator. A shield surrounding the insulator is used as the second conductor,
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and is encased in an outer insulation. One type of coaxial cable is that used
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for cable television. The shielding cable makes coaxial cable less susceptible
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to EMI and emissions than UTP. The cable used in most nteworks is either
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Thick Ethernet (RG-8) or Thin Ethernet (RG-58). Thin coaxial cable is less
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expensive than the highest quality (Category 5) UTP, but is more difficult to
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install due to its thickness, its lack of flexibility, the connectors (you
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should keep the length of cable between the `T piece' and the actual ethernet
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card in the PC as short as possible, ideally the `T piece' will be plugged
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directly into the ethernet card), and the neccesity for terminators (a
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terminator is a 52 ohm resistor that helps to ensure that the signal is
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absorbed and not reflected when it reaches the end of the cable. Without a
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terminator at each end of the cabling you may find that the ethernet is
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unreliable or doesn't work at all).
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</para>
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<para>
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The most commonly available types of coaxial cable are described in the table
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below.
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</para>
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<para>
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<screen>
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Type Impendance Common Cable
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RG-8 50 ohms Thick Ethernet (thicknet)
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RG-11 50 ohms Thick Ethernet (thicknet)
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RG-58 50 ohms Thin Ethernet (thinnet)
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RG-59 75 ohms Cable TV
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RG-62 93 ohms ARCnet
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</screen>
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</para>
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Fiber Optic
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<para>
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A fiber optic cable consists of a thin glass or clear plastic fiber encased
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in a protective jacket. Signals are sent through the cable in form of light.
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There are two types of fiber optic cable: single-mode, which uses a single
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wavelength, and multimode, which uses multiple multiple wavelengths in the
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same cable. Fiber optic cable is completely invulnerable to EMI, and has no
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detectable emissions. However, it and its associated equipment are expensive
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compared to other types of cable, and the most difficult to install. Single-mode
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cable is much more expensive than multimode cable. The advantages of fiber are
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high bandwidth (up to 2 Gbps (gigabits per second) and extremely low attenuation.
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Fiber cable can reach distances ranging from severak miles for multimode cable
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to hundreds of miles for single-mode cable.
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Infrared
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<para>
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Wireless infrared networking systems are modulated beams of infrared light to
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transmit data. These types of networks require a line of sight, and are
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generally used for short distances such as networks within buildings or between
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nearby buildings. Infrared communications are not subject to EMI, but are
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vulnerable to obstructions (such as weather conditions) and bright light, and
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suseceptible to eavesdropping. There are two types of infrared networks:
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</para>
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- Point-to-point networks use a focused beam, usually generated by a laser.
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They are less vulnerable to dispersion and can theorectically be used for long
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distance networking, although the need for precise alignment between receiever
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and transmitter and the vulneraibility to obstructions often makes it
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impractical. Bandwidth can be as high as 16 Mbps.
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- Broadcast networks use a less focused beam that disperses rapidly. These
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systems can transmit to multiple workstations ar once, but are much more
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vulnerable to dispersion, limiting their useful distance and bandwidth.
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Bandwidth is usually no more than 1 Mbps.
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Radio
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<para>
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The most common type of wireless networks use radio waves. Tdaio-based networks
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have a reasonably high bandwidth, but are very sensitve to EMI and eavesdropping.
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Also, many radio frequencies are regulated by the FCC and are unavailable for use
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without a license. There are three types of radio links:
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</para>
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- Low power single frequency
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This type of system is best suited for small ares, such as within a building. It
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uses a lower-power transmitter on a single radio frequency. The available range is
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approximately 30 meters. This is the lowest-cost method of radio networking.
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Bandwidth may be as high as 10 Mbps.
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- High power single frequency
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This system is also uses a single frequency, but at a higher power. This allows
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for a much greater range, often covering an entire metropolitan area. Bandwidth is
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typically 10 Mbps. The greater range makes this type of network the most vulnerable
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to eavesdropping.
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- Spread-spectrum
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These systems use multiple frequencies, primarily to avoid eavesdropping. This
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is done in two ways, direct sequence modulation sends packets sequentially over
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several different frequencies, while frequency hopping transmission change
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frequencies at scheduled intervals known to both ends. Both of these are
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significantly less vulnerable less vulnerable to EMI and snooping than other
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radio networks.
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Microwave
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<para>
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Another type of wireless network communications uses microwaves, which are
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similar to radio waves but at a higher frequency. Higher frequencies are less
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vulnerable to interference and snooping, and can provide greater bandwidth.
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Two common types of microwave networks are in use:
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</para>
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- Terrestial
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This method provides for line-of-sight communication, usually across a short
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distance. Bandwidth can be as high as 10 Mbps. Microwaves are still vulnerable
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to interference and eavesropping, although not as much as conventional radio
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waves.
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- Satellite
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This method relays microwave transmissions via a satellite, allowing for a
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nearly global range. The bandwidth can be as high as 10 Mbps, but the satellite
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relays cause delays that may impair real-time communication. These systems are
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more expensive than wireless communication.
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<para>
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Please note that any information after this point in this section is largely
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redundant as these technologies have been superceded (from the perspective of
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being economically more viable and also technologically superior) by
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those that have been outlined above.
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</para>
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Serial NULL Modem cable
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<para>
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Not all NULL modem cables are alike. Many null modem cables do little more
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than trick your computer into thinking all the appropriate signals are present
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and swap transmit and receive data. This is ok but means that you must use
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software flow control (XON/XOFF) which is less efficient than hardware flow
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control. The following cable provides the best possible signalling between
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machines and allows you to use hardware (RTS/CTS) flow control.
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</para>
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<para>
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<screen>
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Pin Name Pin Pin
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Tx Data 2 ----------------------------- 3
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Rx Data 3 ----------------------------- 2
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RTS 4 ----------------------------- 5
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CTS 5 ----------------------------- 4
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Ground 7 ----------------------------- 7
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DTR 20 -\--------------------------- 8
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DSR 6 -/
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RLSD/DCD 8 ---------------------------/- 20
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\- 6
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</screen>
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</para>
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Parallel port cable (PLIP cable)
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<para>
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If you intend to use the PLIP protocol between two machines then this
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cable will work for you irrespective of what sort of parallel ports you
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have installed.
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</para>
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<para>
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<screen>
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Pin Name pin pin
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STROBE 1*
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D0->ERROR 2 ----------- 15
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D1->SLCT 3 ----------- 13
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D2->PAPOUT 4 ----------- 12
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D3->ACK 5 ----------- 10
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D4->BUSY 6 ----------- 11
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D5 7*
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D6 8*
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D7 9*
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ACK->D3 10 ----------- 5
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BUSY->D4 11 ----------- 6
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PAPOUT->D2 12 ----------- 4
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SLCT->D1 13 ----------- 3
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FEED 14*
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ERROR->D0 15 ----------- 2
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INIT 16*
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SLCTIN 17*
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GROUND 25 ----------- 25
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</screen>
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</para>
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Notes:
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<EFBFBD> Do not connect the pins marked with an asterisk `*'.
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<EFBFBD> Extra grounds are 18,19,20,21,22,23 and 24.
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<EFBFBD> If the cable you are using has a metallic shield, it should be
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connected to the metallic DB-25 shell at one end only.
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<para>
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Warning: A miswired PLIP cable can destroy your controller card. Be very
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careful and double check every connection to ensure you don't cause yourself
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any unnecessary work or heartache.
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</para>
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<para>
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While you may be able to run PLIP cables for long distances, you should avoid
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it if you can. The specifications for the cable allow for a cable length of
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about 1 metre or so. Please be very careful when running long plip cables as
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sources of strong electromagnetic fields such as lightning, power lines and
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radio transmitters can interfere with and sometimes even damage your controller.
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If you really want to connect two of your computers over a large distance you
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really should be looking at alternatives such as obtaining a pair of thin-net
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ethernet cards and running some coaxial cable.
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</para>
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</sect1>
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