The capacity of the transmission medium is called the bandwidth, and it is measured in bits per second.
A transmission medium with a high bandwidth can transfer more data.
Transmission media are categorised as either wire transmission or wireless transmission.
Wire transmission transfers the data through wires and cables.
These cables must be protected from damage, they take up space, and they can be difficult to install.
However, wire transmission can carry large amounts of data with little interference from other signals.
Wire transmission media include twisted-pair, coaxial or fibre-optic cable .
Wired Transmission Media
Wired or bounded transmission media restricts the signal so that it is contained within a cable and therefore follows the path of the cable.
In addition wired media can be shielded to prevent (or at least limit) external electromagnetic forces from affecting the signal.
No cable is perfect in this regard which means signals do degrade as distances increase.
Different standards are in place, which specify various technical attributes of cables.
These attributes determine the maximum recommended distance between nodes.
Twisted-paircable
consists of two thin insulated copper wires, twisted to form a spiral.
Each copper wire is contained within plastic insulation and then the twisted pairs of wire are enclosed within an outer sheath.
Twisting reduces the amount of interference from other cabling.
The two main types of twisted-pair cables are unshielded twisted-pair (UTP) and shielded twisted-pair (STP).
UTP is the most common and economical for LAN and Telephone connections
UTP cable does not include any physical shield against outside electromagnetic interference (apart from somewhat limited shielding provided by the twists in each pair).
Most UTP cables contain four pairs – a total of eight copper wires.
STP is used in ‘noisy’ environments where its shield protects against excessive electromagnetic interference.
Sometimes, Screened Twisted Pair (ScTp)
Includes a metal shield or screen and a drain wire (see Fig 3.69).
STP and ScTP cable is significantly more expensive therefore its use is limited to applications where a high level of electromagnetic interference is present – primarily industrial applications
Twisted pair is the slowest medium, with a bandwidth up to 60 Kbps.
UTP is classified into categories where higher category cable supports higher frequencies and hence high data transfer speeds.
Cat 6 cable supports frequencies up to 250MHz whilst the more common Cat-5e cable supports frequencies up to 125MHz.
Lower specification Cat 3 cable supports frequencies up to 16MHz and was once popular for 10Mbps networks – today Cat 3 cable is used almost exclusively for telephone lines.
In general individual UTP cable runs should not exceed 100 metres from the central node (usually a switch) to the end node (usually a computer).
Coaxial cable
(or coax, pronounced ‘co-axe’) consists of a single copper wire surrounded by an insulator, grounded shielding and an outer insulator.
The shielding allows data to be transmitted with little distortion.
It is commonly used over distances of less than a few kilometres.
The bandwidth for a coaxial cable is 10 Mbps.
Coaxial cable was originally designed to transmit analog broadcast TV from antennas to television sets. As analog TV stations transmit on frequencies ranging from 30MHz to 3GHz (VHF and UHF bands) the cable also needed to support these high frequencies.
Furthermore coaxial cable is relatively immune to outside electromagnetic interference compared to twisted pair.
When computer networks emerged coaxial cable was the natural choice.
Early Ethernet standards and also IBM’s token ring standards used coaxial cable borrowed from the TV and radio industries. For example 10base5 (Thicknet) and 10base2 (Thinnet) Ethernet both used coaxial cable over a logical bus topology.
Compared to UTP, coaxial cable is expensive and furthermore it takes more space and is less flexible.
As a consequence coaxial cable is seldom used when cabling new baseband LANs.
Optic Fibre Cable
Fibre-optic cable uses a laser of light to carry data in small glass fibres about the diameter of a human hair. It is free from electromagnetic and radio interference, is very secure and can transmit data at high speeds without errors. Fibre-optic cables are replacing conventional copper-wire cables.
The bandwidth for fibre-optic cables is in excess of 400 Mbps.
A single strand of optical fibre can carry over 50 billion telephone conversations.
Most LANs use either twisted-pair cable or coaxial cable. Fibre-optic cable is usually too expensive and difficult to install.
Useful Terms: Baseband vs Broadband
There are two types of transmission used: baseband and broadband.
Baseband networks use the entire capacity of the cable to transmit only one signal at a time. Most LANs are baseband.
Broadband networks divide the cable so that several signals can be transmitted at the same time.
Less susceptible to electrical interference caused by nearby equipment or wires.
In turn are less likely to cause interference themselves.
Because it is electrically "cleaner", STP wire can carry data at a faster speed.
Disadvantages :
STP wire is that it is physically larger and more expensive than twisted pair wire.
STP is more difficult to connect to a terminating block.
Coaxial Pair
Advantages :
Coaxial cable can support greater cable lengths between network devices than twisted pair cable.
Thick coaxial cable has an extra protective plastic cover that help keep moisture away.
Disadvantages :
Thick coaxial is that it does not bend easily and is difficult to install.
Bulky size
Fiber Optics :
Advantages:
One single mode fiber can replace a metal of time larger and heavier.
Multi-mode optical cable has a larger diameter and can be used to carry signal over short distance.
Disadvantages :
Fiber optic versus metal cable is that it is difficult to make connections to fiber optic cable.
The optical fiber must be highly polished to allow light to pass with little loss.
Comms Media 1.mp4
Wireless Transmission
Wireless transmission moves the data through air and space.
Wireless or unbounded transmission uses the atmosphere as the medium to carry electromagnetic waves between nodes.
Examples of unbounded media include point-to-point terrestrial (ground-based) microwave, satellites, wireless networks such as 802.11, Bluetooth networks, infrared and of course mobile phones.
It does not need a fixed physical connection between the source and the destination.
Radio and television are examples of wireless transmission.
Point-to-Point Terrestrial Microwave
A microwave is a high-frequency radio signal sent through space in a straight line from one antenna to another.
Microwaves have been used for several decades to transmit both voice and data.
Antennas are placed on tall buildings or mountain tops to continue transmission over long distances (see Figure 3.15). Microwave transmission is faster than telephone lines or coaxial cables.
Even though it is reasonably error free, weather conditions or such objects as trees and buildings can obstruct the signal and affect the transmission.
Point-to-point ground based (terrestrial) microwave is used to relay wireless signals across large distances.
A direct and uninterrupted line of sight between the transmitter and the receiver is required.
Generally sequences of transmitter/receivers, known as transponders, are arranged into a chain.
Each transponder receives the signal, amplifies it and transmits it precisely to the next transponder.
Distance between transponders varies considerably depending on the terrain, however generally transponders are around 40km apart.
Satellite
A satellite is a specialised receiver and transmitter that is launched by a rocket and placed in orbit around the earth.
A signal is sent from one ground station to the satellite, which receives and retransmits the signal to another ground station.
Each ground station uses a satellite dish to send and receive the signals
Satellites can transmit large amounts of data over long distances and at great speeds.
There are hundreds of satellites currently in operation.
They are used for weather forecasting, television broadcasts, radio communications and Internet communications.
Satellites use microwaves to carry digital signals from and to both ground based stations and also between satellites.
Satellites contain transponders that receive microwaves on one frequency, amplify and then transmit microwaves on a different frequency
Wireless LANS
• Wireless LANs use radio waves, not cables, as their transmission medium.
802.11g WLANs communicate using microwaves with frequencies in the vicinity of 2.4GHz.
Currently the range of frequencies around 2.4GHz is unlicensed, which means manufacturers are free to use such frequencies for any purpose they desire.
Common applications include cordless phones, Bluetooth devices, remote control toys and even microwave ovens.
Such devices can and do influence the performance of 802.11g WLANs.
802.11ac is the latest WiFi standard and uses the 5.8GHz frequency band, which is much less congested than 2.4GHz
Although it has less penetrating power, the 5GHz band is free from high noise and congestion.
Bluetooth
Bluetooth is a communication system for short-range transmission; it was designed to
replace the cables that connect portable devices.
Bluetooth operates within the unlicensed 2.4GHz part of the spectrum.
Many portable and other devices include support for Bluetooth,
for example, mobile phones, PDAs (portable Digital Assistants), car and home audio systems, MP3 and MP4 players, laptop computers gaming consoles and numerous other devices.
Specialised devices that use Bluetooth are beginning to emerge, for instance the electric motor in Fig 3.80 is controlled via a Bluetooth connection.
Bluetooth devices automatically recognise each other and form an “ad-hoc” network known as a piconet.
Up to seven devices can join each piconet, and each device can simultaneously connect to multiple piconets.
For instance, a Bluetooth headset can form a piconet with a mobile phone, whilst the mobile phone is transferring data to a laptop over another piconet.
Infrared
Infrared waves occur above microwaves and below visible light.
For communication systems, frequencies just above microwaves are used.
Infrared waves travel in straight lines hence a direct line of sight is required between source and destination.
Currently infrared is only used over short distances.
Common applications include remote controls used within many consumer products and for transferring data between a variety of portable devices and computers.
The IrDA (Infrared Data Association) maintains a set of IrDA standards.
In general, these standards provide a simple and relatively inexpensive means for transferring data between two devices.
Mobile Phones
Mobile phones transmit data to a grid of cellular stations that are linked to the wire transmission telephone network.
Mobile phones use radio waves to communicate with the cellular station.
They are portable devices, and monthly service fees and per call costs are higher than those for a normal telephone in most countries.
Mobile phone networks are split into areas known as cells.
Each cell contains its own central base station that transmits and receives data to and from individual mobile phones.
Each base station is connected to the PSTN (and Internet) using either a cabled link or via a microwave relay link.
As users roam from one cell to another the current base station passes the call onto the next base station.
Mobile phones automatically adjust the power output by their transmitters based on the signal level received from their current base station – this reduces electromagnetic radiation and also extends battery life.
GSM (Global System for Mobile communication) networks are currently the most popular mobile phone networks in Australia.
In GSM networks adjoining cells transmit and receive on different frequencies.
At least three different frequency bands are required to avoid overlap between adjoining cells.
Each GSM cell supports an equal number of users.
In areas of high usage the number of cells is increased and the effective coverage area of each cell is reduced.
In large cities and within shopping malls some cells cover areas of just a few hundred metres.
Wireless Communications
How Does Wi-Fi Work? - Brit Lab
How Does WiFi Work?
Wireless - Introduction and Preview
Tutorial Wireless Technology
Student Activity
1. Take notes (in OneNote) on the three wired and wireless transmission media, under the following headings:
TWP
COAXIAL CABLE
FIBRE OPTIC
MICROWAVE
Terrestrial
Satellite
RADIOWAVES
Broadcast Radio
Cellular Radio
INFRARED
2. Answer the following questions:
Give two examples of microwave communications media
Give two examples of DIFFERENT types of Twisted wire pairs and explain why one is preferred?
