Chapter 11: Wireless LANs

1. Chapter 11: Wireless LANs Business Data Communications, 6e

2. Wireless LAN Applications • LAN extension • Cross-building interconnect • Nomadic access • Ad hoc networks

3. 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

4. Single-Cell Wireless LAN

5. Multiple-Cell Wireless LAN

6. 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.

7. Nomadic Access • Provides a wireless link between a LAN hub and a mobile data terminal (e.g. laptop computer) • 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

8. 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

9. 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

10. Wireless LAN Technology • 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. • OFDM LANs • For higher speeds; this is known as orthogonal frequency division multiplexing and is superior to spread spectrum. • Infrared (IR) LANs • Individual cells are limited to a single room, because infrared light does not penetrate opaque walls

11. IEEE 802.11 Architecture

12. IEEE 802.11 Services

13. IEEE 802.11Medium 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 • DFWMAC (distributed foundation wireless MAC)

14. IEEE 802.11Protocol Architecture

15. IEEE 802.11 Physical Layer • 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) - operates in the 2.4-Ghz band and 54 Mbps • IEEE 802.n • operates in the 2.4-Ghz band and hundreds of Mbps

16. Original 802.11Physical 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

17. 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

18. 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

19. 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

20. Higher-Speed 802.11 Options • 802.11n • Uses both the 2.4 GHz and 5-GHz band. • Improves data transmission and effective throughput • Uses Multiple Input Multiple Output (MIMO) antenna architecture • Uses channel bonding , allowing for 2X as many subchannels, doubling the transmission rate • Uses orthogonal frequency division multiplexing (OFDM) rather than spread spectrum • Aggregates multiple MAC frames into a single block for transmission

21. IEEE 802.11 Security Considerations • Workstations are not physically connected to the network • Privacy concerns since any station in range can receive data • Wi_FiProtected Access (WPA) used to address these concerns