1. Computer Networks - part IV (Wireless: Introduction & topologies)

2. A Wireless Network
Definition: A Group of interconnected nodes that exchange information and share resources through a wireless transmission medium

3. TYPES OF WIRELESS NETWORKS
Wireless PAN
Wireless LAN
Wireless Broadband
Wireless WAN (satellite , Microwave ,..etc)
Cellular Networks

4. Broadband Wireless Technology
Higher data rates obtainable with broadband wireless technology
Graphics, video, audio
Shares same advantages of all wireless services: convenience and reduced cost
Service can be deployed faster than fixed service
No cost of cable plant
Service is mobile, deployed almost anywhere

5. Characteristics of Wireless Transmission
Similarities with wired
Layer 3 and higher protocols
Signal origination
From electrical current, travel along conductor
Differences from wired
Signal transmission
No fixed path, guidance
Antenna
Signal transmission and reception
Same frequency required on each antenna
Share same channel

6. Characteristics of Wireless Transmission (cont’d.)

7. The Wireless Spectrum Continuum of electromagnetic waves
Data, voice communication
Arranged by frequencies
Lowest to highest
Spans 9 KHz and 300 GHz
Wireless services associated with one area
FCC oversees United States frequencies
ITU oversees international frequencies
Air signals propagate across borders

8. The Wireless Spectrum (cont’d.)

9. Antennas Radiation pattern Directional antenna Omnidirectional antenna
Relative strength over three-dimensional area
All electromagnetic energy antenna sends, receives
Directional antenna
Issues wireless signals along single direction
Omnidirectional antenna
Issues, receives wireless signals
Equal strength, clarity
All directions
Range
Reachable geographical area

10. Signal Propagation LOS (line-of-sight) Obstacles affect signal travel
Signal travels
In straight line, directly from transmitter to receiver
Obstacles affect signal travel
Pass through them
Absorb into them
Subject signal to three phenomena
Reflection: bounce back to source
Diffraction: splits into secondary waves
Scattering: diffusion in multiple different directions

11. Signal Propagation (cont’d.)
Multipath signals
Wireless signals follow different paths to destination
Caused by reflection, diffraction, scattering
Advantage
Better chance of reaching destination
Disadvantage
Signal delay

12. Multipath signal propagation

13. Signal Degradation Fading Attenuation Noise Change in signal strength
Electromagnetic energy scattered, reflected, diffracted
Attenuation
Signal weakens
Moving away from transmission antenna
Correcting signal attenuation
Amplify (analog), repeat (digital)
Noise
Significant problem
No wireless conduit, shielding

14. Narrowband, Broadband, and Spread Spectrum Signals
Defines wireless spectrum use:
Narrowband
Transmitter concentrates signal energy at single frequency, very small frequency range
Broadband
Relatively wide wireless spectrum band
Higher throughputs than narrowband
Spread-spectrum
Multiple frequencies used to transmit signal
Offers security

15. Narrowband, Broadband, and Spread Spectrum Signals (cont’d.)
FHSS (frequency hopping spread spectrum)
Signal jumps between several different frequencies within band
Synchronization pattern known only to channel’s receiver, transmitter
DSSS (direct-sequence spread spectrum)
Signal’s bits distributed over entire frequency band at once
Each bit coded
Receiver reassembles original signal upon receiving bits

16. Fixed versus Mobile Fixed communications wireless systems
Transmitter, receiver locations do not move
Transmitting antenna focuses energy directly toward receiving antenna
Point-to-point link results
Advantage
No wasted energy issuing signals
More energy used for signal itself
Mobile communications wireless systems
Receiver located anywhere within transmitter’s range
Receiver can roam

17. The Need for Wireless MANs
Traditional point-to-point and switched network techniques used in WANs are inadequate for growing needs of organizations
Need for high capacity and low costs over large area
MAN provides:
Service to customers in metropolitan areas
Required capacity
Lower cost and greater efficiency than equivalent service from telephone company
Standards for Wireless MANS is WI-MAX

18. Overview of WLAN Topologies
Three types of WLAN Topologies:
Independent Basic Service Sets (IBSS)
Basic Service Set (BSS)
Extended Service Set (ESS)
Service Set – A logical grouping of devices.
WLANs provide network access by broadcasting a signal across a wireless radio frequency (Beaconning)
Transmitter prefaces its transmissions with a Service Set Identifier (SSID)
A station may receive transmissions from transmitters with the same or different SSIDs.

19. Independent Basic Service Sets (IBSS)
IBSS consists of a group of stations directly communicating with each other.
No Access Point used
Also known as an ad-hoc network.
Usage: Few stations setup up for a specific purpose for a short period of time. (ex. file transfers.)
We will have a an IBSS lab, but our main focus will be BSSs and ESSs.

20. Basic Service Set (BSS)
BSS, also known as an Infrastructure BSS.
Requires an Access Point (AP)
Converts frames to Ethernet and visa versa
Known as a translation bridge
Stations do not communicate directly, but via the AP
APs typically have an uplink port that connects the BSS to a wired network (usually Ethernet), known as the Distribution System (DS).

21. Extended Service Set (ESS)
Multiple BSSs can be connected together with a layer 2 “backbone network” to form an Extended Service Set (ESS).
does not specify the backbone network
The backbone network is also known as the Distribution System (DS) and could be wired or wireless.
Stations are “associated” with only one AP at a time.
The SSID is the same for all BSS areas in the ESS (unless creating multiple BSSs, i.e. one for Marketing and another for Sales).

22. Extended Service Set (ESS)
What if you want to be able to move between access points without the latency of re-association and re-authentication (these will be explained)?
Roaming gives stations true mobility allowing them to move seamlessly between BSSs. (More later)
APs need to be able to communicate between themselves since stations can only associate with one AP at a time.
Currently, inter-access point communication can only be achieved with proprietary, non-standard technologies.
IEEE working group (Task Group F) is working on standardizing IAPP (Inter-Access Point Protocol)

23. Access Points Access Point (AP)
Translates (converts) frames to Ethernet and visa versa
Known as a translation bridge
Typically provides wireless-to-wired bridging function
All BSS communications must go through the AP, even between two wireless statsions

24. Quick Preview: Station/AP Connectivity
SSID (Service Set Identity)
At a minimum a client station and the access point must be configured to be using the same SSID.
An SSID is:
Between 2 and 32 alphanumeric characters
Spaces okay
Must match EXACTLY, including upper and lower case
Sometimes called the ESSID
Not the same as BSSID (MAC address of the AP)

25. WLAN Interconnection. Can connect two separate LANs
Fixed link, directional antennas between two access points
Allows access points 1000 feet apart
Support for same protocols, operating systems as wired LANs
Ensures compatibility

26. Association Packet exchanged between computer, access point Scanning
Gain Internet access
Scanning
Surveying surroundings for access point
Active scanning transmits special frame
Probe
Passive scanning listens for special signal
Beacon fame

27. Association (cont’d.) SSID (service set identifier)
Unique character string identifying access point
In beacon fame information
Configured in access point
Better security, easier network management

28. Association (cont’d.) ESS with several authorized access points
Must allow station association with any access point
While maintaining network connectivity
Reassociation
Mobile user moves from one access point’s range into another’s range
Occurs by simply moving, high error rate
Stations’ scanning feature
Used to automatically balance transmission loads
Between access points

29. Factors To Consider For Wireless LAN
Same as any LAN
High capacity, short distances, full connectivity, broadcast capability
Throughput: efficient use wireless medium
Number of nodes:Hundreds of nodes across multiple cells
Connection to backbone LAN: Use control modules to connect to both types of LANs
Service area: 100 to 300 m
Low power consumption:Need long battery life on mobile stations
Mustn't require nodes to monitor access points or frequent handshakes
Transmission robustness and security:Interference prone and easily eavesdropped

30. Factors To Consider For Wireless LAN
Collocated network operation:Two or more wireless LANs in same area
License-free operation
Handoff/roaming: Move from one cell to another
Dynamic configuration: Addition, deletion, and relocation of end systems without disruption to users

31. Wireless LAN Applications
LAN Extension
Cross-Building Interconnect
Nomadic Access
Ad Hoc Networking

32. Applications - LAN Extension
Saves installation of LAN cabling
Eases relocation and other modifications to network structure
Wireless LAN to replace wired LANs has not happened
In some environments, role for the wireless LAN
Buildings with large open areas
Manufacturing plants, stock exchange trading floors, warehouses
Historical buildings
Small offices where wired LANs not economical
May also have wired LAN
Servers and stationary workstations

33. Applications – Cross-Building Interconnect
Connect LANs in nearby buildings
Point-to-point wireless link
Connect bridges or routers
Not a LAN per se
Usual to include this application under heading of wireless LAN

34. Applications - Nomadic Access
Link between LAN hub and mobile data terminal
Laptop or notepad computer
Enable employee returning from trip to transfer data from portable computer to server
Also useful in extended environment such as campus or cluster of buildings
Users move around with portable computers
May wish access to servers on wired LAN

35. Applications – Ad Hoc Networking
Peer-to-peer network
Set up temporarily to meet some immediate need
E.g. group of employees, each with laptop or palmtop, in business or classroom meeting
Network for duration of meeting

36. Bluetooth Networks Version 1.1 Version 2.0 (2004)
Maximum theoretical throughput: 1 Mbps
Effective throughput: 723 Kbps
10 meter node difference
Designed for PANs (personal area networks)
Version 2.0 (2004)
Different encoding schemes
2.1-Mbps throughput
30 meters node difference
Usage: cellular telephones, phone headsets, computer peripherals, PDAs

37. Bluetooth Networks Ericson’s original goals
Wireless technology compatible with multiple devices
Require little power
Cover short ranges
Aim of Bluetooth Special Interest Group (SIG)
Refine and standardize technology
Result: Bluetooth
Mobile wireless networking standard using FHSS (frequency hopping spread spectrum) RF signaling in 2.4-GHz band