The document discusses some limitations and difficulties of wireless technologies. It notes that while wireless is convenient, there are political and technical challenges that inhibit the technologies. Specifically, it points out a lack of industry standards and limitations of devices, such as small screens that can only display a few lines of text and use of WML instead of HTML on most mobile browsers.

1. Limitations and Difficulties of Wireless
Technologies
• Wireless is convenient and less expensive
• Limitations includes: political and technical
difficulties which inhibit wireless technologies
• Lack of an industry-wide standard
• Device limitations
– E.g., small LCD on a mobile telephone can only displaying
a few lines of text
– E.g., browsers of most mobile wireless devices use wireless
markup language (WML) instead of HTML
Engr. Sana Mukhtar

2. Wireless Technologies
 Cellular wireless.
 Wireless local loop.
 Wireless local area networks.
 Mesh networks.
 Satellites.
 Multi-hop wireless.
Engr. Sana Mukhtar

3. Cellular Concept: Motivation
 Early mobile radio systems:
 Large coverage with single, high-powered
transmitter.
 But, no frequency re-use due to interference.
 With limited spectrum allocation, capacity (in
terms of number of users) is limited.
Engr. Sana Mukhtar

4. Some Cellular Terminology
 Mobile.
 Base station.
 Mobile Switching Center (MSC).
 Handoff.
 Cell.
Engr. Sana Mukhtar

5. Cellular Fundamentals
 System-level idea, no major technological changes.
 Many low-power transmitters instead of single, high
power one (large cell).
 Service area divided into small cells covered by each
low power transmitter.
 Each transmitter (base station) allocated a portion of
the spectrum.
 Nearby BSs assigned different channel groups to
minimize interference.
 Scalability: as more users subscribe, more BSs can be
added using lower transmission power): mini-cells.
Engr. Sana Mukhtar

6. Frequency Reuse
E
B
G
C
A G
F D
E F
Engr. Sana Mukhtar

7. Handoff/Handover
 Mobile hosts can change cells while
communicating.
 Hand-off occurs when a mobile host starts
communicating via a new base station.
 Handoff decision made based on signal
strength.
Engr. Sana Mukhtar

8. Handoff Strategies: Network-initiated
 Used in 1G.
 Based solely on measurements of received
signals from MH.
 Each BS monitors signal strengths of
mobiles with calls in progress.
 MSC decides if handoff necessary.
Engr. Sana Mukhtar

9. Cellular Networks: Evolution
 Evidence of the wireless success!
 Since 1996, number of new mobile phone
subscribers exceeded number of new fixed
phone subscribers!
 1st Generation (1G): analog technology
 FDMA.
 Analog FM.
Engr. Sana Mukhtar

10. Second Generation (2G)
• Second generation 2G cellular telecom networks were
commercially launched on the GSM standard in Finland in 1991
• radio signals on 2G networks are digital
• But…both systems use digital signaling to connect the radio
towers
• Three primary benefits of 2G networks over their predecessors
were:
– phone conversations were digitally encrypted
– significantly more efficient on the spectrum allowing for far
greater mobile phone penetration levels
– 2G introduced data services for mobile, starting with SMS text
• 2G has been superseded by newer technologies such as 2.5G,
2.75G, 3G, and 4G
Engr. Sana Mukhtar

11. Second Generation (2G) ….
 Most of today’s cellular networks use 2G
standards.
 Early 90s.
 Digital technology.
 Digital modulation.
 TDMA and CDMA based on the type of multiplexing techniques.
 Lighter, smaller devices with longer battery life.
 Better reception and channel utilization.
Engr. Sana Mukhtar

12. Example 2G Standards
 TDMA standards:
 Global System Mobile (GSM).

Europe, Asia, Australia, South America.
 Interim Standard 13 (IS-136 or NDSC).

North and South America and Australia.
 Pacific Digital Cellular (PDC).
 Similar to IS-136.
 Japan.
 CDMA standard
 Interim Standard 95 (IS-95)
 North and South America, Korea, Japan, China, Australia.
Engr. Sana Mukhtar

13. 2G Evolution
 Shift from voice to data.
 New wireless devices: pagers, PDAs.
 New services: Web access, e-mail, instant
messaging, etc.
 New “data-centric” standards.
 “Retrofit” 2G to support higher data throughput.
 2.5G standards supports GPRS.MMS, WWW.
 Moved from 56 Kbs to 115Kbs
 Support higher data rates for Web browsing (e.g., WAP), e-mail, m-
commerce, etc.
Engr. Sana Mukhtar

14. 3G Wireless Networks
 Multi-megabit Internet access, VoIP,
ubiquitous “always-on” access.
 Single mobile device for everything
(integrated service approach).
 New, world-wide standard.
 International Mobile Telephone 2000 (IMT
2000)
Engr. Sana Mukhtar

15. Wireless Local Loop (WLL)
Home
Base
station
Office
Switching
Center
Engr. Sana Mukhtar

16. WLL
 Wireless “last mile”.
 Between central office and homes and businesses close-by.
 Fixed wireless service.
 Developing countries, remote areas.
 Broadband access.
 Microwave or millimeter radio frequencies.
 Directional antennas.
 Allow for very high data rate signals (tens or hundreds Mbs).
 But need LOS: no obstacles!
Engr. Sana Mukhtar

17. Wireless Local Area Networks
 Local area connectivity using wireless
communication.
 IEEE 802.11 WLAN standard.
 Multitude of commercially available devices:
WaveLan, Aironet, etc.
 Wireless LAN may be used for
 Last hop to a wireless host.
 Wireless connectivity between hosts on the LAN.
Engr. Sana Mukhtar

18. Satellite Communications
 Satellite-based antenna(e) in stable orbit above
earth.
 Two or more (earth) stations communicate via
one or more satellites serving as relay(s) in space.
 Uplink: earth->satellite.
 Downlink: satellite->earth.
 Transponder: satellite electronics converting
uplink signal to downlink.
Engr. Sana Mukhtar

19. Satellite Communications
SAT
ground stations
Engr. Sana Mukhtar

20. Orbits
 Shape: circular, elliptical.
 Plane: equatorial, polar.
 Altitude: geostationary (GEO), medium
earth (MEO), low earth (LEO).
Engr. Sana Mukhtar

21. GEO Satellites
 Most common type.
 Orbit at 35,863 Km above earth and rotates
in equatorial plane.
 Many GEO satellites up there!
Engr. Sana Mukhtar

22. Time-Domain Concepts
• Analog signal - signal intensity varies in a smooth fashion
over time
– No breaks or discontinuities in the signal
• Digital signal - signal intensity maintains a constant level for
some period of time and then changes to another constant
level
• Peak amplitude (A) - maximum value or strength of the signal
over time; typically measured in volts
• Frequency (f )
– Rate, in cycles per second, or Hertz (Hz) at which the signal repeats
• Periodic signal - analog or digital signal pattern that repeats
over time
– s(t +T ) = s(t ) -¥< t < +¥
• where T is the period of the signal
Engr. Sana Mukhtar

23. Time-Domain Concepts
• Period (T ) - amount of time it takes for one repetition
of the signal
– T = 1/f
• Phase (φ) - measure of the relative position in time
within a single period of a signal
• Wavelength (λ) - distance occupied by a single cycle
of the signal
– Or, the distance between two points of corresponding phase
of two consecutive cycles
Engr. Sana Mukhtar