Broadly defined as anything that can carry information from a source to a destination.

In data communication, the transmission medium is usually free space, metallic cable, or fiber-optic cable. The information is usually a signal that is the result of a conversion of data from another form.

19th century – invention of the telegraph by Morse: slow and dependent on a metallic medium.

1869 – invention of telephone by Alexander Graham Bell: use metallic medium. Unreliable due to poor quality of wires. The lines were often noisy and the technology was unsophisticated.

1895 – Wireless communication started when Hertz was able to send high-frequency signals.

Current – Better metallic medium such as twisted-pair and coaxial cable. Better technology offer more efficient communication via Optical fibers and Free space (air, vacuum and water).

Divided into two broad categories: Guided and Unguided

Are those that provide a conduit from one device to another, includeI.Twisted-pair cableII.Coaxial cableIII.Fiber-optic cable

A signal travelling along any of these media is directed and controlled by the physical limits of the medium.

Twisted-pair cable and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current.

Optical fiber is a cable that accepts and transports signals in the form of light.

There are two type of twisted-pair cables:

Shielded Twisted Pair (STP) Unshielded Twisted Pair (UTP)

A typical twisted-pair cable consists of four pairs of thin copper wires. Each pair consists of two conductors (normally copper), each with its own insulation coated with PVC or Teflon, spiraled around one another (twisted).

Why are the pairs of wire twisted?To minimized the interference (crosstalk/noise) between the wires. As when two copper wires running parallel to each other they interfere with each other quite a bit.

There are two type of twisted-pair cables:

Shielded Twisted Pair (STP) Unshielded Twisted Pair (UTP)

Crosstalk - When wires pick up electromagnetic signals from nearby wires also carrying signals

Noise - Noise is an electrical disturbance that can degrade communication.

An extra layer of metal foil is present between the twisted pairs of copper wires and the outer sheath.

The purpose of this layer is to provide additional protection from Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI).

However, this shielding reflects back the normal radiation emitted by the wires. This radiation may corrupt the signal.

To prevent this reflection, a coating of dielectric insulator, which absorbs the radiation, is provided on the internal surface of the metal foil.

Shielding - Used to minimize interference.

STP is more expensive than UTP Generally used in networks where

cables pass closer to devices that cause high EMI.

The presence of shielding increases the resistance to the signal and therefore, restricts the cable length and the throughput offered by STP

Foil Shields:Offers excellent protection against electromagnetic interference at audio and radio frequencies.

They weigh and cost less than braid shields. Due to their small size, foil shields are commonly used to shield individual pairs of multi-pair cables to reduce crosstalk.

Drain wires are generally used with foil shields to ease termination to the shell connection on the connector.

Braid Shields:Provides effective shielding at audio, as well as RF ranges.

Also provides excellent structural integrity while maintaining good flexibility.

Braid shields are better than foil for minimizing low frequency interference and have lower DC resistance.

UTP is the most commonly used network wiring. It is inexpensive, flexible, and light, thus making it very easy to work with.

UTP is terminated with an RJ-45 connector, which is kind of the big brother to the RJ-11 male connector that you find on a telephone cord.

Of the four pairs in a UTP cable, only two pairs are actually used for communication in LANs and provide speeds up to 100 Mbps. All the four pairs are used only for higher transmission rates such as 1,000 Mbps.

Copper Wire Twisted-Pair Cable

Connectors

UTP cables are inexpensive and are resistant to physical stress.

They also offer the flexibility to select a particular category of cable depending on the network requirements and resist attenuation over distances that are adequate for most LANs (185 meters).

UTP cables should not be used in an environment with high EMI.

Category

Speed Notes

Cat-1 None Supports analog voice data and is commonly used for telephone communication. E.g. old telephone systems

Cat-2 Up to 4Mbps

Supports digital voice communication. Commonly used in IBM Token Ring networks.

Cat-3 10Mbps The minimum cable requirement for data networks running Ethernet. Cat-3 is also known as 10baseT.Maximum data transfer rate supported is 16 Mbps but generally used to transmit data at 10 Mbps.

Cat-4 16Mbps Supports a maximum data transfer rate of 20Mbps, which is only slightly higher than Cat-3. Therefore, Cat-4 is rarely used.

Depending on the bandwidth offered, the following categories of UTP are defined:

Category

Speed Notes

Cat-5 100Mbps

Used by most networks today and has become the standard for new LAN installations. Used with Fast Ethernet.

Cat-6 1,000 Mbps

Two pair with foil and braided shield, Data patchSupports data transfer rates up to 1,600 and is used to implement Gigabit Ethernet.

Cat-7 Gbps Supports data transfer rates up to 10Gbps over copperUsed as a cabling infrastructure for 1000BASE-T (Gigabit Ethernet, or GbE) and 10GBASE-T (10-Gigabit Ethernet, or 10 GbE) networks. 

Cat-8 Flat cable for under carpets with two twisted pair

Cat-9 Plenum cable with two twisted pair. It is safe if you're having a fire

Wires (or RF, IR) connecting together the devices that make up a LAN

Twisted pairMost common:

STP: shielded twisted pairUTP: unshielded twisted pair

(e.g. Telephone cable, Ethernet 10BaseT)

Coaxial cableThin (similar to TV cable)Thick (e.g., 10Base5, ThickNet)

FiberWireless

Attenuation - Signal loss due to impedance. Bandwidth - Indicates the amount of data that

can be sent in a time period. Measured in Mbps which is one million bits per second.

Impedance - The amount of resistance to the transmission device.

Interference - Electromagnetic Interference (EMI). Crosstalk - When wires pick up electromagnetic signals from nearby wires also carrying signals.

Plenum - Space above a false ceiling in an office area where heat ducts and cables may be run. Plenum cabling is special fire resistant cabling required for use in these areas due to fire hazards.

Shielding - Used to minimize interference.

http://www.youtube.com/watch?v=0S6cjJS5y1I

http://networktechinc.com/article-noise.html