Transcript Media - DePaul University

Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 3
The Media : Conducted and Wireless
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Data Communications and Computer Networks
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Introduction
The world of computer networks and data
communications would not exist if there were no medium
by which to transfer data.
The two major categories of media include:
• Conducted media
• Wireless media
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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
wire is currently Category 1 through Category 6,
although Categories 2 and 4 are obsolete.
Twisting the wires helps to eliminate electromagnetic
interference between the two wires.
Shielding can further help to eliminate interference.
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Coaxial Cable
A single wire wrapped in a foam insulation surrounded
by a braided metal shield, then covered in a plastic jacket.
Cable can be thick or thin.
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 of data.
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Fiber Optic Cable
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.
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Fiber Optic Cable
Fiber optic cable is capable of supporting millions of bits
per second for 1000s of meters
Thick cable (62.5/125 micron) causes more ray
collisions, so you have to transmit slower. This is step
index multimode fiber. Typically use LED for light
source, shorter distance transmissions
Thin cable – very thin (8.3/125 microns) – very little
reflection, fast transmission, typically uses a laser, longer
distance transmissions. AKA single mode fiber
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Fiber Optic Cable
Fiber optic cable is susceptible to reflection (where the
light source bounces around inside the cable) and
refraction (where the light source passes out of the core
and into the surrounding cladding).
Thus, fiber optic cable is not perfect either. Noise is still
a potential problem.
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It is very common to mix fiber with twisted pair in LANs.
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Wireless Media
Radio, satellite transmissions, and infrared light are all
different forms of electromagnetic waves that are used to
transmit data.
Note in the following figure how each source occupies a
different set of frequencies.
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Terrestrial Microwave
Land-based, line-of-sight transmission
Approximately 20-30 miles maximum between towers
Transmits data at hundreds of millions of bits per second
Popular with telephone companies and business to
business transmissions
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Often the microwave antennas are on towers and buildings.
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Satellite Microwave
Similar to terrestrial microwave except the signal travels
from a ground station on earth to a satellite and back to
another ground station.
Satellites can be classified by how far out into orbit each
one is (LEO, MEO, GEO, and HEO).
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Satellite Microwave
LEO - Low Earth Orbit - 100 miles to 1000 miles. Used
for pagers, wireless e-mail, special mobile telephones,
spying, videoconferencing.
MEO - Middle Earth Orbit - 1000 to 22,300 miles. Used
for GPS (global positioning systems) and government.
GEO - Geosynchronous Orbit - 22,300 miles. Always
over the same position on earth (and usually over the
equator). Used for weather, television, and government
operations.
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Satellite Microwave
HEO – Highly Elliptical Orbit
A fourth type of orbit used by the military for spying and
by scientific organizations for photographing celestial
bodies.
When satellite is far out into space, it takes photos.
When satellite is close to earth, it transmits data.
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Satellite Microwave
Satellite microwave can also be classified by its
configuration:
Bulk carrier configuration
Multiplexed configuration
Single-user earth station configuration (e.g. VSAT)
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Cellular Telephone
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.
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Cellular Telephone – 1st Generation
AMPS - Advanced Mobile Phone Service - First popular
mobile phone service, uses analog signals and
dynamically assigned frequency division multiplexing.
D-AMPS - Digital Advanced Mobile Phone Service Applies digital time division multiplexing on top of
AMPS.
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Cellular Telephone – 2nd Generation
PCS - Personal Communication Systems - All-digital
mobile phone service
2nd generation PCS phones came in three technologies:
TDMA - Time division multiple access
CDMA - Code division multiple access
GSM - Global system for mobile communications
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Cellular Telephone – 2.5 Generation
AT&T Wireless, Cingular Wireless, and T-Mobile
(formerly VoiceStream) now using GPRS (General
Packet Radio Service) in their GSM networks (can
transmit data at 30 kbps to 40 kbps)
Verizon Wireless, Alltel, U.S. Cellular, and Sprint PCS
are using CDMA2000 1xRTT (one carrier radio transmission technology) (50 kbps to 75 kbps)
Nextel uses IDEN technology
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Cellular Telephone – 3rd Generation
UMTS (Universal Mobile Telecommunications System) also called Wideband CDMA; the 3G version of GPRS;
UMTS is not backward compatible with GSM, so first
UMTS phones will have to be dual-mode; based on
TDMA (same as D-AMPS and GSM)
1XEV (1 X Enhanced Version) - The 3G replacement for
1xRTT; will come in two flavors
1xEV-DO for data only
1xEV-DV for data and voice
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Cellular Digital Packet Data
Technology that supports a wireless connection for the
transfer of computer data from a mobile location to the
public telephone network and the Internet.
Can be used in conjunction with mobile telephones and
laptop computers.
All digital transfer but relatively slow at 19,200 bps.
Emergency services make use of CDPD.
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Infrared Transmissions
Special 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.
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WAP (Wireless Application Protocol)
WAP allows wireless devices such as mobile telephones,
PDAs, pagers, and two-way radios to access the Internet.
WAP is designed to work with small screens and with
limited interactive controls.
WAP incorporates Wireless Markup Language (WML)
which is used to specify the format and presentation of
text on the screen.
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WAP (Wireless Application Protocol)
WAP may be used for applications such as:
- travel directions
- sports scores
- e-mail
- online address books
- traffic alerts
- banking
- news
Possible short-comings of WAP include low speeds,
security, and a very small user interface.
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Broadband Wireless Systems
Delivers Internet services into homes and businesses.
Designed to bypass the local loop telephone line.
Transmits voice, data and video over high frequency
radio signals.
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Broadband Wireless Systems
Multichannel multipoint distribution service (MMDS)
and local multipoint distribution service (LMDS) looked
promising a couple years ago but died off.
Now companies are eyeing Wi-Max – an IEEE 802.16
spec; initially 300k to 2M bps over a range of as much as
30 miles; forthcoming spec (802.16e) will allow for
mobile devices
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Bluetooth
Bluetooth is a Radio Frequency specification for shortrange, 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.
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Bluetooth
Bluetooth will enable users to connect to a wide range of
computing and telecommunication devices without the
need of connecting cables.
Typical uses include phones and pagers, modems, LAN
access devices, headsets, notebooks, desktop computers,
and PDAs.
Want to go to the movies?
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Wireless LAN (IEEE 802.11)
This technology transmits data between workstations and
local area networks using high speed radio frequencies.
Current technologies allow up to 54 Mbps data transfer at
distances up to hundreds of feet. (IEEE 802.11b, a, g)
More on this in Chapter Seven (LANs)
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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
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Free Space Optics
Current FSO speeds go from T-3 (45 Mbps) up to OC-48
(2.5 Gbps) with faster systems in the lab
Major weakness is fog
A typical FSO has a link margin of about 20 dB
Under perfect conditions air reduces a system’s power by
approx 1 dB/km
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Free Space Optics
Heavy fog can cause a loss of 400 db/km (rendering 20
dB systems to 50 meters)
Scintillation is also a problem (especially in hot weather).
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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 (unshared) but for
small distances such as wireless LANs.
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Ultra-wideband
Proponents say UWB gets something for nothing, since it
shares frequencies with other sources. Opponents say too
much interference.
Cell phone industry really against UWB because CDMA
most susceptible to interference.
GPS may also be affected
One solution may be have two types of systems – one for
indoors (stronger) and one for outdoors (1/10 the power)
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Media Selection Criteria
Cost
Speed
Distance and expandability
Environment
Security
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Media Selection Criteria - Cost
Different types of cost:
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 is the
savings?
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Media Selection Criteria - 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.
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Media Selection Criteria - Distance
and Expandability
Can this choice of medium be expanded easily?
What is needed to extend the distance? A repeater? An
amplifier?
How much noise is introduced with this expansion?
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Media Selection Criteria Environment
Is the intended environment electromagnetically noisy?
If so, should you use shielding? Or fiber?
If using wireless, are there other wireless signals that can
interfere?
Will the microwave or free space optics be affected by
bad weather?
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Media Selection Criteria - Security
Is the medium going to be carrying secure data? Should
you worry about wiretapping?
Encryption of the signal/data can help, but may not be the
perfect solution.
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Conducted Media In Action
How do we wire a local area network?
Remember : using Category 5e unshielded twisted pair,
the maximum segment length is 100 meters.
A wall jack is a passive device and does not regenerate a
signal.
Hub to hub connections are often fiber optic cable.
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Interconnecting Two Buildings
Two buildings are separated by 400 meters. How do we
interconnect them?
Twisted pair? (Do we even have access?)
Coax?
Fiber?
Wireless?
Other? (Chapter 12)
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Wireless Media In Action
DataMining Corporation has one office in Chicago and
one in Los Angeles.
There is a need to transmit large amounts of data between
the two sites.
DataMining is considering using a Very Small Aperture
Terminal satellite system.
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Wireless Media In Action
Cost is proportional to high amount of traffic with very
high reliability.
Speed is high enough to support company’s needs.
Distance can easily expand across the U.S.
Satellite systems are robust in most environments.
Security can be very good with encryption.
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