1. Chapter 8: Wireless Services Part 2: Wireless LAN
Computer Data Communications

2. Introduction Overview of Wireless LANs Key Application Areas
Wireless LAN requirements
IEEE Architecture
IEEE Terminology
IEEE Services
IEEE Medium Access Control (MAC)
IEEE Protocol Architecture
IEEE Physical Layers
Wireless LAN Access Point
Wireless LAN Access Point/ Client Features
Wireless LAN Applications

3. Overview of Wireless LANs
A wireless local area network (LAN) is a flexible data communications system implemented as an extension to, or an alternative for, a wired LAN
Using radio frequency (RF) technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections.
Issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed
Key application areas:
LAN extension
cross-building interconnect
nomadic access
ad hoc networking

4. LAN extension
Originally targeted to reduce cost of wiring, but new buildings now have sufficient wiring in place
Still useful in buildings where wiring is problematic
buildings with large open areas,
historical buildings with insufficient twisted pair
small offices wired LANs are not economical
Typically, a wireless LAN will be linked into a wired LAN on the same premises

5. Single-Cell LAN extension
In addition, there is a control module (CM) that acts as an interface to a wireless LAN. The control module (CM) includes either bridge or router functionality to link the wireless LAN to the backbone. It includes some sort of access control logic, such as a polling or token-passing scheme, to regulate the access from the end systems. Figure 1 illustrates the Single-Cell LAN extension

6. Single-Cell LAN extension
Hubs or other user modules (UMs) that control a number of stations off a wired LAN may also be part of the wireless LAN configuration. This configuration can be referred to as a single-cell wireless LAN; all of the wireless end systems are within range of a single control module.

7. Single-Cell LAN Extension
Figure 1: Single-Cell LAN Extension

8. Multi-Cell LAN extension
A multiple-cell wireless LAN, there are multiple control modules interconnected by a wired LAN.
Each control module supports a number of wireless end systems within its transmission range.
For example, with an infrared LAN, transmission is limited to a single room; therefore, one cell is needed for each room in an office building that requires wireless support.
Figure 2 illustrates the Multi-Cell LAN extension

9. Multi-Cell LAN Extension
Figure 2: Multi-Cell LAN Extension

10. Cross-building interconnect
Connect LANs in nearby buildings, be they wired or wireless LANs
Point-to-point wireless link is used between two buildings (e.g. two microwave or infrared transmitter/receiver units can be placed on the rooftops of two buildings within the line of sight of each other)
Devices are typically bridges or routers.

11. Nomadic Access
Provides a wireless link between a LAN hub and a mobile data terminal (e.g. laptop computer) Figure 3 illustrates the Infrastructure Wireless LAN
Examples
Enable an employee returning from a trip to transfer data from a personal portable computer to a server in the office.
Access in an extended environment such as a campus or a business operating out of a cluster of buildings.
In both of these cases, users may wish access to the servers on a wired LAN from various locations.

12. Infrastructure Wireless LAN
Nomadic Access :-
Infrastructure Wireless LAN
Figure 3: Infrastructure Wireless LAN

13. Ad hoc networks
A peer-to-peer network (no centralized server) set up temporarily to meet some immediate need
For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting.
Figure 4 illustrates the Adhoc Networks

14. Ad hoc networks
Figure 4: Adhoc Networks

15. Wireless LAN Requirements
Efficient throughput
Support for multiple nodes
Connection to backbone LAN
Broad service area (~ m)
Allows for reduced power consumption while not using the network (e.g. sleep mode)
Transmission robustness and security
Co-located network operation
License-free operation
Handoff/roaming
Dynamic and automated addition, deletion, and relocation

16. Wireless LAN Technology
Infrared (IR) LANs
Individual cells are limited to a single room, because infrared light does not penetrate opaque walls
Spread spectrum LANs
In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands so that no FCC licensing is required for their use in the U.S.
Narrowband microwave
Do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands

17. IEEE Architecture
Figure 5: IEEE Architecture

18. IEEE Architecture
IEEE has defined the specifications for a wireless LAN, called IEEE , which covers the physical and data link layers.
The standard defines two kinds of services; the basic set (BSS) and the extended service set (ESS)
Figure 5 illustrates the IEEE Architecture.

19. IEEE 802.11 Architecture - BSS
IEEE defines the basic service set (BSS) as the building block of a wireless LAN.
A basic service set is made of stationary or mobile wireless stations and optional central base station, known as the access point (AP).
The BSS without an AP is a stand-alone network and cannot send data to other BSSs. It is called an ad hoc architecture.
A BSS with an AP is sometimes referred to an infrastructure network.
Figure 6 illustrates the BSS.

20. IEEE 802.11 Architecture - BSS
Figure 6: Basic service sets (BSSs)

21. IEEE 802.11 Architecture - ESS
An extended service set (ESS) is made up of two or more BSSs with APs.
In this case, the BSSs are connected through a distribution system, which is usually a wired LAN. The distribution system connects the APs in the BSSs.
IEEE does not restrict the distribution system; it can be any IEE LAN such as an Ethernet.

22. IEEE 802.11 Architecture - ESS
The extended service set (ESS) uses two types of stations: mobile and stationary.
The mobile stations are normal stations inside a BSS.
The stationary stations are AP stations that are part of a wired LAN.
When BSSs are connected, the stations within reach of one another can communicate without the use of an AP.
However, communication between two stations in two different BSSs usually occurs via two APs. Figure 7 illustrates an ESS

23. IEEE 802.11 Architecture - ESS
Figure 7: Extended service sets (ESSs)

24. IEEE Terminology

25. IEEE 802.11 Services The IEEE 802.11 services are listed below:
Association
Reassociation
Disassociation
Authentication
Deauthentication
Privacy

26. IEEE 802.11 Services Association: Reassociation:
Establishes an initial association between a station and an AP. Before a station can transmit or receive frames on a wireless LAN, its identity and address must be known.
For this purpose, a station must establish an association within an AP within a particular BSS.
The AP can then communicate this information to other Aps within the ESS to facilitate routing and delivery of frames.
Reassociation:
Enables an established association to be transferred from one AP to another, allowing a mobile station to move from one BSS to another.

27. IEEE 802.11 Services Disassociation: Authentication:
A notification from either a station or an AP that an existing association is terminated.
A station should give this notification before leaving an ESS or shutting down.
Authentication:
Used to establish the identity of a stations to each other.
For a wireless LAN, in which connectivity is achieved simply by having an attached antenna that is properly tuned.
The authentication service is used by stations to establish their identity with stations they wish to communicate with.

28. IEEE 802.11 Services Deauthentication: Privacy:
This service is invoked whenever an existing authentication is to be terminated.
Privacy:
Used to prevent the contents of messages from being read by other than the intended recipient.
The standard provides for the optional use of encryption to assure privacy.

29. IEEE 802.11 Medium Access Control (MAC)
MAC (Medium Access Control) layer covers three functional areas:
Reliable data delivery
Access control
Security

30. IEEE 802.11 Medium Access Control
Reliable Data Delivery
Basic data transfer mechanism involves an exchange of two or four frames (data, ACK, and optional CTS/RTS)
Access Control
Used DFWMAC (distributed foundation wireless MAC) that provides a distributed access control mechanism with an optional centralised control built on top of that. Figure 8 illustrates the IEEE Protocol Architecture.
Security
The prevention of unauthorised access using wireless network.

31. IEEE 802.11 Protocol Architecture
Figure 8: IEEE Protocol Architecture

32. IEEE Physical Layer
Figure 9: IEEE Physical Layer

33. IEEE Physical Layer
The physical layer for IEEE has been issued in four stages: -
(1997)
MAC layer and three physical layer specifications; two 2.4-GHz band, one infrared, all operating at 1 and 2 Mbps
IEEE a (1999)
operates in the 5-GHz band at up to 54 Mbps
IEEE b (1999)
operates in the 2.4-Ghz band at 5.5 and 11 Mbps.
IEEE 802.g (2002)
extends IEEE b to higher data rates

34. Original 802.11 Physical Media Definitions
Direct-sequence spread spectrum (DSSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps
Frequency-hopping spread spectrum (FHSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps
Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm
All of the original products were of limited utility because of the low data rates

35. IEEE b
Extension of the IEEE DSSS scheme, providing data rates of 5.5 and 11 Mbps (higher data rate is achieved with more complex modulation)
Apple Computer was first, with AirPort wireless networking, followed by other vendors
Wireless Ethernet Compatibility Alliance created to certify interoperability for b products

36. Problems with and b
Original and b may interfere with other systems that operate in the 2.4-GHz band
Bluetooth
HomeRF
other devices--including baby monitors and garage door openers
Limited data rate results in limited appeal

37. Higher-Speed 802.11 Options 802.11a 802.11g Uses 5-GHz band.
Uses orthogonal frequency division multiplexing (OFDM) rather than spread spectrum
Possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
802.11g
Higher-speed extension to IEEE b.
Combines physical layer encoding techniques used in a and b to provide service at a variety of data rates

38. Wireless LAN Access Point
Access Point is a transceiver device which connects to the wired network from a fixed location.
It receives, buffers and transmit data between the WLAN and the wired network infrastructure.
It supports client devices and can function within a range of less than a hundred to several hundred feet
End users access the wireless LAN through PC cards.

39. Wireless LAN Access Point /Client Features
Roaming
Provides mobility within a subnet, across subnets or across SSID subnets
Protect against AP failure, client PCs will detect link loss and roam to alternate AP.
Best AP selection
PC Clients should scan all channels for the best AP and rescan periodically and move to a better AP if possible.

40. Wireless LAN Access Point /Client Features
Load balancing
Load balancing provides the capability for PC clients to improve network performance by switching to the least utilized Access Point
PC client driver should periodically look for a better AP during network idle
The AP needs to monitor utilization and signal clients to discourage association and encourage roaming during sustained periods of high utilization

41. Wireless LAN Access Point /Client Features
Network traffic filtering
Used in AP to eliminate unnecessary traffic to improve network performance.
Management
To provide capability to centrally monitor and manage an enterprise wide wireless network. (SNMP protocol support)

42. Wireless LAN – Applications
Home
Home networking of computers and internet applicances
Small Office Home Office (SOHO)
Ease of installations and re-location, eliminate wiring cost, scalability
Public Hot-spots
High speed wireless internet access at hot spots such as hotels, airports, conference centre and other areas while travelling

43. Wireless LAN - Applications
Enterprise
Extension to existing infrastructure network, provide constant connectivity to corporate facilities while supporting users mobility. E.g. University, wireless internet and intranet access across the campus.