Study Unit 3 - Chapter Three Conducted and Wireless Media

Question 1

The world of computer networks would not exist if there were
no medium by which to transfer data
* The two major categories of media include:

Answer 1

– Conducted media
– Wireless media

Question 2

Twisted Pair Wire

Answer 2

Twisted Pair Wire
* One or more pairs of single conductor wires that have been
twisted around each other
* Twisted pair wire is classified by category. Twisted pair is
currently Category 1 through Category 7, although
Categories 1, 2 and 4 are nearly obsolete
* Twisting the wires helps to eliminate electromagnetic
interference between the two wires
* Shielding can further help to eliminate interference

Question 3

Twisted Pair Summary

Answer 3

Twisted Pair Summary
* Most common form of wire
* Relatively inexpensive
* Easy to install
* Carries high data rates (but not the highest)
* Can suffer from electromagnetic noise
* Can be easily wire-tapped
* Comes in shielded and unshielded forms

Question 4

Coaxial Cable

Answer 4

Coaxial Cable
* A single wire wrapped in a foam insulation surrounded by a
braided metal shield, then covered in a plastic jacket. Cable
comes in various thicknesses
* Baseband coaxial technology uses digital signaling in which
the cable carries only one channel of digital data
* Broadband coaxial technology transmits analog signals and
is capable of supporting multiple channels

Question 5

Coaxial Cable Summary

Answer 5

Coaxial Cable Summary
* A single wire surrounded by a braided shield
* Because of shielding, can carry a wide bandwidth of
frequencies
* Thus is good with applications such as cable television
* Not as easy to install as twisted pair
* More expensive than twisted pair

Question 6

Fiber-Optic Cable

Answer 6

• A thin glass cable approximately a little thicker than a human
  hair surrounded by a plastic coating and packaged into an
  insulated cable
• A photo diode or laser generates pulses of light which travel
  down the fiber optic cable and are received by a photo
  receptor

Question 7

Fibre optic cable summary

Answer 7

Fiber optic cable can carry the highest data rate for the
longest distances
* Initial cost-wise, more expensive than twisted pair, but less
than coaxial cable
* But when you consider the superiority of fiber, initial costs
outweighed by capacities
* Need to fibers for a round-trip connection
* Not affected by electromagnetic noise and cannot be easily
wiretapped, but still noise

Question 8

Wireless Media

Answer 8

Wireless Media
* Radio, satellite transmissions, and infrared light are all
different forms of electromagnetic waves that are used to
transmit data
* Technically speaking – in wireless transmissions, space is
the medium
* Note in the following figure how each source occupies a
different set of frequencies

Question 9

Terrestrial Microwave Transmission

Answer 9

Terrestrial Microwave Transmission
* Land-based, line-of-sight transmission
* Approximately 20-30 miles between towers
* Transmits data at hundreds of millions of bits per second
* Signals will not pass through solid objects
* Popular with telephone companies and business to business
transmissions

Question 10

Satellite Microwave Transmission

Answer 10

Satellite Microwave Transmission
* Similar to terrestrial microwave except the signal travels from
a ground station on earth to a satellite and back to another
ground station
* Can also transmit signals from one satellite to another
* Satellites can be classified by how far out into orbit each one
is (LEO, MEO, GEO, and HEO)

Question 11

LEO MEO GEO HEO

Answer 11

LEO (Low-Earth-Orbit) – 100 to 1000 miles out
– Used for wireless e-mail, special mobile telephones, pagers, spying,
videoconferencing
* MEO (Middle-Earth-Orbit) – 1000 to 22,300 miles
– Used for GPS (global positioning systems) and government
* GEO (Geosynchronous-Earth-Orbit) – 22,300 miles
– Always over the same position on earth (and always over the equator)
– Used for weather, television, government operations
32
Satellite Microwave Transmission
(continued)
* HEO (Highly Elliptical Earth orbit) – satellite follows an
elliptical orbit
– Used by the military for spying and by scientific organizations for
photographing celestial bodies

Question 12

Satellite microwave can also be classified by its configuration
(see next figure):

Answer 12

Bulk carrier configuration
– Multiplexed configuration
– Single-user earth station configuration (e.g. VSAT)

Question 13

Cellular Telephones

Answer 13

Cellular Telephones
* Wireless telephone service, also called mobile telephone,
cell phone, and PCS
* To support multiple users in a metropolitan area (market),
the market is broken into cells
* Each cell has its own transmission tower and set of
assignable channels

Question 14

Placing a call on a cell phone

Answer 14

You enter a phone number on your cell phone and press Send. Your
cell phone contacts the nearest cell tower and grabs a set-up
channel. Your mobile identification information is exchanged to make
sure you are a current subscriber.
– If you are current, you are dynamically assigned two channels: one
for talking, and one for listening. The telephone call is placed. You
talk.

Question 15

Receiving a call on a cell phone

Answer 15

Receiving a call on a cell phone
– Whenever a cell phone is on, it “pings” the nearest cell tower every
several seconds, exchanging mobile ID information. This way, the
cell phone system knows where each cell phone is.
– When someone calls your cell phone number, since the cell phone
system knows what cell you are in, the tower “calls” your cell phone.

Question 16

Cellular Telephones (continued)
* 1st Generation

Answer 16

Cellular Telephones (continued)
* 1st Generation
– AMPS (Advanced Mobile Phone Service) – first popular cell phone
service; used analog signals and dynamically assigned channels
– D-AMPS (Digital AMPS) – applied digital multiplexing techniques on
top of AMPS analog channels

Question 17

2nd Generation

Answer 17

2nd Generation
– PCS (Personal Communication Systems) – essentially all-digital cell
phone service
– PCS phones came in three technologies:
* TDMA – Time Division Multiple Access
* CDMA – Code Division Multiple Access
* GSM – Global System for Mobile Communications

Question 18

2.5 Generation

Answer 18

2.5 Generation
– Using GPRS (General Packet Radio Service) in a GSM networks (can
transmit data at 30 kbps to 40 kbps)
– Using CDMA2000 1xRTT (one carrier radio- transmission technology)
(50 kbps to 75 kbps)

Question 19

3rd Generation

Answer 19

3rd Generation
– UMTS (Universal Mobile Telecommunications System) – also called
Wideband CDMA
* The 3G version of GPRS
* UMTS not backward compatible with GSM (thus requires phones with
multiple decoders)
– 1XEV (1 x Enhanced Version) –3G replacement for 1xRTT
* two forms:
– 1xEV-DO for data only
– 1xEV-DV for data and voice

Question 20

4th Generation

Answer 20

4th Generation
– LTE (Long Term Evolution) – theoretical speeds of 100 Mbps or
more, actual download speeds 10-15 Mbps
– WiMax – introduced in a couple slides – theoretical speeds of 128
Mbps; actual download speeds 4 Mbps
– HSPA (High Speed Packet Access) – 14 Mbps downlink, 5.8 Mbps
uplink; is this 3.5G or 4G?
– HSPA+ – theoretical downlink of 84 Mbps, 22 Mbps uplink 3.5G or
4G?

Question 21

WiMax - Broadband Wireless Systems

Answer 21

WiMax - Broadband Wireless Systems
* Delivers Internet services into homes, businesses and
mobile devices
* Designed to bypass the local loop telephone line
* Transmits voice, data, and video over high frequency radio
signals
* Maximum range of 20-30 miles and transmission speeds in
Mbps
* IEEE 802.16 set of standards

Question 22

Bluetooth

Answer 22

Bluetooth
* Bluetooth is a specification for short-range, point-to-point or
point-to-multipoint voice and data transfer
* Bluetooth can transmit through solid, non-metal objects
* Its typical link range is from 10 cm to 10 m, but can be
extended to 100 m by increasing the power

Question 23

Wireless Local Area Networks
(IEEE 802.11)

Answer 23

Wireless Local Area Networks
(IEEE 802.11)
* This technology transmits data between workstations and
local area networks using high-speed radio frequencies
* Current technologies allow up to 100 Mbps (theoretical) data
transfer at distances up to hundreds of feet
* Three popular standards: IEEE 802.11b, a, g, n
* More on this in Chapter Seven (LANs)

Question 24

Free Space Optics and
Ultra-Wideband

Answer 24

Free Space Optics and
Ultra-Wideband
* Free space optics
– Uses lasers, or more economically, infrared transmitting devices
– Line of sight between buildings
– Typically short distances, such as across the street
– Newer auto-tracking systems keep lasers aligned when buildings
shake from wind and traffic

Question 25

Ultra-wideband (continued)

Answer 25

Ultra-wideband (continued) – Proponents for UWB say it gets something for nothing, since it shares frequencies with other sources. Opponents disagree – Cell phone industry against UWB because CDMA most susceptible to interference of UWB – GPS may also be affected – One solution may be to have two types of systems – one for indoors (stronger) and one for outdoors (1/10 the power) Ultra-wideband – UWB not limited to a fixed bandwidth but broadcasts over a wide range of frequencies simultaneously – Many of these frequencies are used by other sources, but UWB uses such low power that it “should not” interfere with these other sources – Can achieve speeds up to 100 Mbps but for small distances such as wireless LANs

Question 26

Infrared Transmissions

Answer 26

Infrared Transmissions * Transmissions that use a focused ray of light in the infrared frequency range * Very common with remote control devices, but can also be used for device-to-device transfers, such as PDA to computer

Question 27

ZigBee

Answer 27

ZigBee * Based upon IEEE 802.15.4 standard * Used for low data transfer rates (20-250 Kbps) * Also uses low power consumption * Ideal for heating, cooling, security, lighting, and smoke and CO detector systems * ZigBee can use a mesh design – a ZigBee-enabled device can both accept and then pass on ZigBee signals

Question 28

Media Selection Criteria

Answer 28

Media Selection Criteria * Cost * Speed * Distance and expandability * Environment * Security

Question 29

Cost * Different types of costs

Answer 29

Cost * Different types of costs – Initial cost – what does a particular type of medium cost to purchase? To install? – Maintenance / support cost * ROI (return on investment) – if one medium is cheaper to purchase and install but is not cost effective, where are the savings?

Question 30

Speed * Two different forms of speed:

Answer 30

Speed * Two different forms of speed: – Propagation speed – the time to send the first bit across the medium * This speed depends upon the medium * Airwaves and fiber are speed of light * Copper wire is two thirds the speed of light – Data transfer speed – the time to transmit the rest of the bits in the message * This speed is measured in bits per second

Question 31

Expandability and Distance

Answer 31

Certain media lend themselves more easily to expansion * Don’t forget right-of-way issue for conducted media and lineof-sight for certain wireless media

Question 32

Environment * Many types of environments are hazardous to certain media

Answer 32

Environment * Many types of environments are hazardous to certain media – Electromagnetic noise – Scintillation and movement – Extreme environmental conditions

Question 33

Security

Answer 33

Security * If data must be secure during transmission, it is important that the medium not be easy to tap – Make the wire impervious to electromagnetic wiretapping – Encrypt the signal going over the medium

Question 34

Conducted Media in Action: Two Examples

Answer 34

Conducted Media in Action: Two Examples * First example – simple local area network – Hub typically used – To select proper medium, consider: * Cable distance * Data rate Second example – company wishes to transmit data between buildings that are one mile apart – Is property between buildings owned by company? * If not consider using wireless * When making decision, need to consider: – Cost – Speed – Expandability and distance – Environment – Security

Question 35

Wireless Media In Action: Three Examples

Answer 35

Wireless Media In Action: Three Examples * First example – you wish to connect two computers in your home to Internet, and want both computers to share a printer – Can purchase wireless network interface cards – May consider using Bluetooth devices * Second example – company wants to transmit data between two locations, Chicago and Los Angeles – Company considering two-way data communications service offered through VSAT satellite system 72 Wireless Media In Action: Three Examples (continued) 73 Wireless Media In Action: Three Examples (continued) * Third example – second company wishes to transmit data between offices two miles apart – Considering terrestrial microwave system