mirror of https://github.com/tLDP/LDP
160 lines
11 KiB
XML
160 lines
11 KiB
XML
<sect1 id="Media-Types">
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<title>Media-Types</title>
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<para>
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Media Types
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Common network media include twisted pair (UTP and STP), coaxial cable, fiber-optic cable, and wireless transmissions via infrared and radio waves. Network media are usually chosen based on several criteria: their cost, their vulnerability to EMI and eavesdropping, their bandwidth, and the maximum distance at which they can be used.
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Twisted Pair Ethernet Cable
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Unshielded Twisted Pair (UTP)
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UTP is the most common type of network cable in use today. UTP consits of one or more pairs of insulated copper wires. The wires are twisted together to reduce crosstalk, and enclosed in a plastic insulator. UTP is the type of cable used in telephone systems.
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UTP cables are generally wires using RJ-45 jacks and plugs. These are 8-conductor connectors similar to the RJ-11 connectors used in telephone cables. UTP is inexpensive and easy to install compared with other types of cable, although the differenet types of UTP vary in cost.
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Copper wire has a high level of attenuation. YTP cable is limited to transmission distances of 100 meters or less. UTP is more susceptible to interference (EMI) than mos types of cable, and is vaulnerable to eavesdropping since its own emissions are not shielded.
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Types of UTP cable are defined by the EIA standards, which specify the three categories of cable. Category 3 is the minimum requiredment for networking, and Category 5 is the highest-quality network cable. The UTP categories and the bandwidth they support are summarized below.
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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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STP cable is similar to UTP, but inclues a foil or wire mesh shield between the wire pairs and the outer insulation. The shield is electrically grounded, and reduces emissions and susceptibility to EMI. STP cable is used 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 also uses grounded connection connectors, adding to the expense. STP uses the same copper wires as UTP as the same level of attenuation, and therefore the same maximum distance of about 100 meters. However, it is much less susceptible to EMI and eavesdropping. The reduced inteference allows for higher bandwidth, potentially as high as 500 Mbps.
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Coaxial
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Coaxial Cable consists of a single thick copper wire surrounded by an insulator. A shield surrounding the insulator is used as the second conductor, and is encased in an outer insulation. One type of coaxial cable is that used for cable television. The shielding cable makes coaxial cable less susceptible to EMI and emissions than UTP. The cable used in most nteworks is either Thick Ethernet (RG-8) or Thin Ethernet (RG-58). Thin coaxial cable is less expensive than the highest quality (Category 5) UTP, but is more difficult to install due to its thickness, its lack of flexibility, the connectors (you should keep the length of cable between the `T piece' and the actual ethernet card in the PC as short as possible, ideally the `T piece' will be plugged directly into the ethernet card), and the neccesity for terminators (a terminator is a 52 ohm resistor that helps to ensure that the signal is absorbed and not reflected when it reaches the end of the cable. Without a terminator at each end of the cabling you may find that the ethernet is unreliable or doesn't work at all).
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The generally available types of coaxial cable are described in the table below.
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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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Fiber Optic
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A fiber optic cable consists of a thin glass or clear plastic fiber encased in a protective jacket. Signals are sent through the cable in form of light. There are two types of fiber optic cable: single-mode, which uses a single wavelength, and multimode, which uses multiple multiple wavelengths in the same cable. Fiber optic cable is completely invulnerable to EMI, and has no detectable emissions. However, it and its associated equipment are expensive compared to other types of cable, and the most difficult to install. Single-mode cable is much more expensive than multimode cable. The advantages of fiber are high bandwidth (up to 2 Gbps (gigabits per second) and extremely low attenuation. Fiber cable can reach distances ranging from severak miles for multimode cable to hundreds of miles for single-mode cable.
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Infrared
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Wireless infrared networking systems are modulated beams of infrared light to transmit data. These types of networks require a line of sight, and are generally used for short distances such as networks within buildings or between nearby buildings. Infrared communications are not subject to EMI, but are vulnerable to obstructions (such as weather conditions) and bright light, and suseceptible to eavesdropping. There are two types of infrared networks:
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- Point-to-point networks use a focused beam, usually generated by a laser. They are less vulnerable to dispersion and can theorectically be used for long distance networking, although the need for precise alignment between receiever and transmitter and the vulneraibility to obstructions often makes it impractical. Bandwidth can be as high as 16 Mbps.
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- Broadcast networks use a less focused beam that disperses rapidly. These systems can transmit to multiple workstations ar once, but are much more vulnerable to dispersion, limiting their useful distance and bandwidth. Bandwidth is usually no more than 1 Mbps.
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Radio
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The most common type of wireless networks use radio waves. Tdaio-based networks have a reasonably high bandwidth, but are very sensitve to EMI and eavesdropping. Also, many radio frequencies are regulated by the FCC and are unavailable for use without a license. There are three types of radio links:
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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 uses a lower-power transmitter on a single radio frequency. The available range is approximately 30 meters. This is the lowest-cost method of radio networking. 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 for a much greater range, often covering an entire metropolitan area. Bandwidth is typically 10 Mbps. The greater range makes this type of network the most vulnerable to eavesdropping.
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- Spread-spectrum
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These systems use multiple frequencies, primarily to avoid eavesdropping. This is done in two ways, direct sequence modulation sends packets sequentially over several different frequencies, while frequency hopping transmission change frequencies at scheduled intervals known to both ends. Both of these are significantly less vulnerable less vulnerable to EMI and snooping than other radio networks.
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Microwave
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Another type of wireless network communications uses microwaves, which are similar to radio waves but at a higher frequency. Higher frequencies are less vulnerable to interference and snooping, and can provide greater bandwidth. Two common types of microwave networks are in use:
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- Terrestial
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This method provides for line-of-sight communication, usually across a short distance.
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Bandwidth can be as high as 10 Mbps. Microwaves are still vulnerable to interference and
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eavesropping, although not as much as conventional radio waves.
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- Satellite
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This method relays microwave transmissions via a satellite, allowing for a nearly global
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range. The bandwidth can be as high as 10 Mbps, but the satellite relays cause delays that
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may impair real-time communication. These systems are more expensive than wireless
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communication.
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Serial NULL Modem cable
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Not all NULL modem cables are alike. Many null modem cables do little more than trick your computer into thinking all the appropriate signals are present and swap transmit and receive data. This is ok but means that you must use software flow control (XON/XOFF) which is less efficient than hardware flow control. The following cable provides the best possible signalling between machines and allows you to use hardware (RTS/CTS) flow control.
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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 cable will work for you irrespective of what sort of parallel ports you 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 careful and double check every connection to ensure you don't cause yourself 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 it if you can. The specifications for the cable allow for a cable length of about 1 metre or so. Please be very careful when running long plip cables as sources of strong electromagnetic fields such as lightning, power lines and radio transmitters can interfere with and sometimes even damage your controller. If you really want to connect two of your computers over a large distance you really should be looking at alternatives such as obtaining a pair of thin-net ethernet cards and running some coaxial cable.
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</para>
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</sect1>
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