This section is on mostly hardware media. See also Communicating for mostly software media.

Intro

In computers, media refers to whatever medium is used to communicate data. Media is usually the copper or fiber optic glass cables but data can also be sent through the air via electromagnetic frequencies such as infrared, microwave, or radio waves. Media is important because it is often half the cost of the network!

Important factors in determining media include:

  • required speed
  • distance
  • ease of installation and maintenance access
  • technical expertise required to install and utilize
  • resistance to internal EMI (ElectroMagnetic Interference) inside the cable, esp. the cross talk of parallel wires
  • resistance to external EMI outside the cable
  • resistance to other environmental hazards such as workers carelessly drilling into walls, fire and the weather.

Ethernet Media

Ethernet is the most popular protocol for connecting LANs (Local Area Networks) and usually transmits data at 10 Mb/s. Ethernet is also known as CSMA/CD (Carrier-Sense Multiple Access with Collision Detection).

Ethernet can be applied over a number of different kinds of media. Please note that these media can also be used by non-Ethernet protocols and advanced versions of Ethernet including 100 Mb/s and 1 Gb/s Ethernet. In order of popularity, the top varieties include:

  • UTP (Unshielded Twisted Pair), aka 10BASE-T.
  • thin coaxial cable, aka 10BASE-2.
  • thick coaxial cable, aka 10BASE-5.
  • fiber optic cable, aka 10BASE-F.
  • STP (Shielded Twisted Pair).

IEEE (Institute of Electrical and Electronic Engineers) defines Ethernet in both the Data Link and Physical Layers of the OSI (Open Systems Interconnection) Reference Model.

Ethernet is usually referred to as IEEE Standard 802.3, eg 802.3z refers to Gigabit Ethernet. IEEE also uses shorthand for the baseband media segments that connect LANs (as opposed to broadband). The shorthand format is:

<n Mb/s>BASE-<Code for type of media>

Here are the different media, listed by in historical order:

10BASE-5 (aka thick coaxial cable or thicknet or yellow cable or thick coax or RG-8 or RG-11) is the original media for Ethernet networks. It is a copper cable coaxially surrounded by insulation, braided copper shielding, and an outer jacket, approximately 1/2 inch thick. It is not used as often as thinnet and UTP since it is expensive, heavy, and inflexible. However it is well shielded and a segment can be up to 500 m (1,640 feet) (Thus the -5 in 10BASE-5.).

Each end of 10BASE-5 has an AUI external transciever (Attached Unit Interface) tapped into the cable with a vampire tap. The AUI is connects to NICs (Network Interface Card) with an AUI connector, usually a DB15.

10BASE-2 (aka thin coaxial cable or thinnet or coax or RG-58) is more common than 10BASE-5. It is a copper cable coaxially surrounded by insulation, braided copper shielding, and an outer jacket, approximately 1/4 inch thick. Thinnet is very fast, physically medium in size, fairly inexpensive, flexible, and can go up to 185 m (607 feet) per segment (This is nearly 200 m, thus the -2 in 10BASE-2).

The 5-4-3 rule applies for 10BASE-2 Ethernet: A max of 5 trunk segments (185m x 5 = 925 m (3,035 feet)) can be connected with 4 repeaters, and no more than three of the segments can be populated with stations. Each trunk segment can have up to 30 stations. The entire network can have no more than 1,024 stations.

Unlike 10BASE-5 thinnet usually has its transceiver within the NIC. Thinnet also utilizes convenient BNC connectors which can connect segments with repeaters (to reduce signal degradation), barrel connectors, T connectors, 50 ohm terminators, etc.

10BASE-T (aka UTP or Unshielded Twisted-Pair or twisted pair or Category 5 or Cat 5) is even more popular and cheaper than 10BASE-2. It is 2-4 pairs of insulated, color coded wire inside one outer jacket. Its connector, RJ-45, looks similar to the typical phone connector, RJ-11, which only has one pair. Each pair is twisted (to cancel out electrical noise) and each pair is also twisted a different number of twists per inch. UTP is fast, physically small, the least expensive, but has a fairly short maximum segment length of 100 m (328 feet) (This would have made it 10BASE-1, but instead the -T in 10BASE-T stands for "Twisted").

In general, try to use straight-thru UTP instead of crossed UTP. In crossed UTP, wire 1 and wire 3 switch, and wire 2 and wire 6 switch. The color code of the 8 wires are as follows:

  • Wire 1: White with Orange stripe
  • Wire 2: Orange
  • Wire 3: White with Green stripe
  • Wire 4: Blue
  • Wire 5: White with Blue stripe
  • Wire 6: Green
  • Wire 7: White with Brown stripe
  • Wire 8: Brown

A maximum of 1,024 stations are allowed on a segment without bridging. The big difference between coaxial cabling and UTP is that UTP networks need a hub at the center where each computer connects to. The advantage of the hub is that a downed computer does not affect the network as much as it would in a linearly connected coaxial cable network.

10BASE-F (fiber optic) is more often used between LANs than within a LAN. Fiber optic cabling consists of threads of glass surrounded by plastic shielding, Kevlar, and an outer jacket. It will typically use an ST fiber optic connector. Instead of sending electrical signals, it sends pulses of light and which are totally unaffected by external electrical fields. Fiber optics is the fastest, physically small, the most expensive, and can go up to 100 km.

Please note that 100BASE-T and 1000BASE-T are higher quality UTP used in 100MB/s and 1 Gb/s Ethernet LANs.

UTP and STP are both varieties of TP (Twisted Pair) cabling. The key to TP cabling is that each pair of insulated wires is twisted around each other and then enclosed within a sheath. The twisting helps to resist the crosstalk between wires.

UTP usually comes in different categories of quality, from CAT1 to CAT7. CAT1 is the lowest quality but is frequently used in phone systems. CAT5 is the most commonly used in Ethernet systems.

Note that the coaxial cables look similar to RG-59, (cable TV cabling), and RG-62 (ARCNet cabling (Attached Resource Computer NETwork)). But whereas thicknet and thinnet are both 50 ohms, RG-59 is 50-75 ohms and RG-62 is 93 ohms.

Wireless Media

Wireless media is usually used either out of necessity (the distance may be so long or impassable by physical media) or convenience (why fiddle with wires when you can point a laptop to another laptop?).

The three main kinds are radio waves, microwaves, and infrared.

  • Radio waves have virtually no distance limitations. However the radio waves are government regulated, expensive, and can be tapped into. This can be used across continents.
  • Microwaves have a medium distance limitation and require line of sight. This is good between buildings or between satellites and satellite dishes. Weather and solar conditions may affect transmission.
  • Infrared has a very short distance limitation and definitely requires line of sight. A common usage would be to pass info between two laptops.

Connectors

A connector is the part of the media that connects to other pieces of media or ports/interfaces on various devices. Most connectors are either male (with prongs or pins) or female (with receiving holes or slots). Take a look at an excellent image on the most common connectors from www.BlackBox.com.

Common connector types include:

  • O-type. Shaped like an "O".
    • DIN6 (Deutsche Industry Norm). 6 pins. Used for PC mice (PS/2 Port).
    • DIN8. Used for modems and certain printers.
    • ADB (Apple Desktop Bus). Used on older Macs to daisy chain mice, keyboards, etc. (RS-422).
    • BNC (British Naval Connector or Bayonet Nut Connector or Bayonet Neill Concelman). Sliding-locking thingies. Used for Ethernet LANs on 10BASE-2 (aka thin coaxial cable or thinnet or coax or RG-58 A/U).
    • TNC. Like BNC but no sliding-locking thingy.
    • N-Type or RCA or phono plugs. Small ring-and-pin red, white, and yellow plugs. Used for sound systems and composite video systems. See TV.
    • F-Type or video plugs. Large ring-and-pin plug. Used for cable TV and Ethernet LANs on 10BASE-5 (aka thick coaxial cable or yellow cable or thick coax).
    • Twinax. Instead of one pin it has two.
  • D-type. Shaped like a "D".
    • DB9. 9 pins. Used in serial ports (RS-232). The most common serial port.
    • DB15. 15 pins. Used in monitors.
    • DB25. 25 pins. Used for parallel ports (printers), serial ports (RS-232), and SCSI cable. The most common parallel port.
    • EPP (Enhanced Parallel Port) or ECP (Extended Capabilities Port). BEWARE!
      These looks like DB25 but support bi-directional printers.
    • MiniD50 or Micro DB50. Large but dense 50 pins. Used for SCSI cable.
    • MiniD68 or Micro DB68. Large but dense 68 pins. Used for SCSI-3 cable.
    • HD15. Tiny 15 pins. Used in mobile computers.
    • HD30. Tiny 30 pins. Used in mobile SCSI jacks.
  • RJ. Looks like clicking phone jack.
    • RJ-11. 2, 4, or 6 wires. Is also used for phones and faxes.
    • RJ-45. Eight-wire jack for Ethernet LANs on 10BASE-T (aka UTP or Unshielded Twisted-Pair or twisted pair). Quality often described as CAT5 or Level 7.
    • MT-RJ. Long and black. Used for fiber optic cabling.
  • Other
    • Ceramic. Has a white lollipop stick poking out. Used for fiber optic cabling.
    • C36 (Centronics). Like a D but it has a slot with metal strips. Serves like DB25.
    • C50. 50 pins. May serve same functions.
    • USB Type A (Universal Serial Bus). Connects at a computer, host, or device.
    • USB Type B. Connects at a hub
    • FireWire. (IEEE 1394). 4-6 pins on a slot.
    • S-Plug. L-shaped slotted thingy. Used in component video systems. See TV.


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