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Chapter 8: Wireless Services Part 2: Wireless LAN

Chapter 8: Wireless Services Part 2: Wireless LAN. Computer Data Communications. Introduction. Overview of Wireless LANs Key Application Areas Wireless LAN requirements IEEE 802.11 Architecture IEEE 802.11 Terminology IEEE 802.11 Services

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Chapter 8: Wireless Services Part 2: Wireless LAN

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  1. Chapter 8:Wireless ServicesPart 2: Wireless LAN Computer Data Communications

  2. Introduction • Overview of Wireless LANs • Key Application Areas • Wireless LAN requirements • IEEE 802.11 Architecture • IEEE 802.11 Terminology • IEEE 802.11 Services • IEEE 802.11 Medium Access Control (MAC) • IEEE 802.11 Protocol Architecture • IEEE 802.11 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. 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 (~ 100-300m) • 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 802.11 Architecture Figure 5: IEEE 802.11 Architecture

  18. IEEE 802.11 Architecture • IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, 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 802.11 Architecture.

  19. IEEE 802.11 Architecture - BSS • IEEE 802.11 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 802.11 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 802.11 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: • 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: • 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: • 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 802.11 Protocol Architecture. • Security • The prevention of unauthorised access using wireless network.

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

  32. IEEE 802.11 Physical Layer Figure 9: IEEE 802.11 Physical Layer

  33. IEEE 802.11 Physical Layer • The physical layer for IEEE 802.11 has been issued in four stages: - • 802.11 (1997) • MAC layer and three physical layer specifications; two 2.4-GHz band, one infrared, all operating at 1 and 2 Mbps • IEEE 802.11a (1999) • operates in the 5-GHz band at up to 54 Mbps • IEEE 802.11b (1999) • operates in the 2.4-Ghz band at 5.5 and 11 Mbps. • IEEE 802.g (2002) • extends IEEE 802.11b 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 802.11 products were of limited utility because of the low data rates

  35. IEEE 802.11b • Extension of the IEEE 802.11 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 802.11b products

  36. Problems with 802.11 and 802.11b • Original 802.11 and 802.11b 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 • 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 802.11b. • Combines physical layer encoding techniques used in 802.11a and 802.11b 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 15-50 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.

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