1. 14.1 Chapter 14 Wireless LANs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 14.2 14-1 IEEE 802.11 IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, which covers the physical and data link layers. Architecture MAC Sublayer Physical Layer Topics discussed in this section:

3. 14.3 A BSS without an AP is called an ad hoc network; a BSS with an AP is called an infrastructure network. Note

4. 14.4 Figure 14.1 Basic service sets (BSSs)

5. 14.5 Figure 14.2 Extended service sets (ESSs)

6. 14.6 WLAN Standards 802.11 ReleaseFreqTyp ThroughputMax Net BitrateMod ----19972.4 GHz 0.9 Mbps 2IR/FHSS/DSSS a20035 23 54OFDM b19992.4 4.3 11DSSS g20032.4 19 54OFDM n20092.4 / 5 74 600OFDM

7. An Access Point (AP) broadcasts is SSID (service set identifier) roughly every 100 ms and at 1 Mbps (to accommodate the slowest client) The Wi-Fi standard leaves connection criteria open to the client (?) The Wi-Fi spectrum is divided into a fixed number of channels 11 in North America 13 in most of Europe and China 14 in Japan 14.7 Creating WLAN Connections

8. But not all channels are used due to the concern of overlapping frequencies In North America, only channels 1, 6 and 11 are recommended for 802.11b and g. IEEE 802.11a has 42 channels, of which only 24 are used in North America, from which only about 12 are used to reduce overlapping frequencies 14.8 Creating WLAN Connections

9. 14.9 Figure 14.3 MAC layers in IEEE 802.11 standard FHSS - frequency hopping spread spectrum DSSS - direct sequence spread spectrum OFDM - orthogonal frequency division multiplexing

10. 14.10 Figure 14.4 CSMA/CA flowchart DIFS: distributed interframe space SIFS: short interframe space

11. 14.11 Figure 14.5 CSMA/CA and NAV (Network Allocation Vector) When a station sends its RTS, it includes a time of how long it needs the medium. Other stations then set their NAV timer to this time so they don’t transmit. DIFS: Distributed interframe space; SIFS: short interframe space

12. 14.12 Figure 14.6 Example of repetition interval

13. 14.13 Figure 14.7 Frame format FC: Frame Control D: duration of the transmission that is used to set the value of NAV SC: sequence control: defines the sequence number of the frame to be used in flow control

14. 14.14 Table 14.1 Subfields in FC field

15. 14.15 Figure 14.8 Control frames FC: Frame Control D: duration of the transmission that is used to set the value of NAV

16. 14.16 Frame Types Three types of frames: 1. Management - used for initial communication between stations and access points 2. Control - used for accessing the channel (RTS) and acknowledging frames (CTS or ACK) (See Figure 15-10). 3. Data - used for carrying data and control information

17. 14.17 Table 14.2 Values of subfields in control frames

18. 14.18 Table 14.3 Addresses Note: Address 1 is always address of next device Address 2 is always address of previous device Address 3 is address of final destination if not defined by Address 1 Address 4 is address of original source if not defined by Address 2

19. 14.19 Figure 14-9 Addressing mechanism: case 1 Frame is going directly from one client to another. No intervening distribution system. To DS = 0, From DS = 0

20. 14.20 Addressing mechanism: case 2 To DS = 0, From DS = 1 - frame is coming from a DS (Access Point)

21. 14.21 Addressing mechanism: case 3 To DS = 1, From DS = 0 - frame is going to a DS (or AP)

22. 14.22 Addressing mechanism: case 4 To DS = 1 and From DS = 1

23. 14.23 Figure 14.10 Hidden station problem

24. 14.24 The CTS frame in CSMA/CA handshake can prevent collision from a hidden station. Note

25. 14.25 Figure 14.11 Use of handshaking to prevent hidden station problem Station C doesn’t hear RTS from B, but it does hear CTS from A, so it knows something is up.

26. 14.26 Figure 14.12 Exposed station problem C wants to send to D, but hears A talking to B, so assumes the medium is (incorrectly) busy.

27. 14.27 Figure 14.13 Use of handshaking in exposed station problem Looking for a CTS handshake does not work in this case.

28. 14.28 Table 14.4 Physical layers

29. 14.29 Figure 14.14 Industrial, scientific, and medical (ISM) band

30. 14.30 Figure 14.15 Physical layer of IEEE 802.11 FHSS

31. 14.31 Figure 14.16 Physical layer of IEEE 802.11 DSSS

32. 14.32 Figure 14.17 Physical layer of IEEE 802.11 infrared

33. 14.33 Figure 14.18 Physical layer of IEEE 802.11b

34. 14.34 14-2 BLUETOOTH Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously. Architecture Bluetooth Layers Baseband Layer L2CAP Topics discussed in this section:

35. 14.35 Figure 14.19 Piconet

36. 14.36 Figure 14.20 Scatternet

37. 14.37 Figure 14.21 Bluetooth layers See next slide for description of some of these layers

38. 14.38 Figure 15.17 Bluetooth layers Radio layer - roughly equivalent to physical layer. Uses 2.4 GHz ISM divided into 79 channels of 1 MHz each. Uses FHSS: 1600 hops/sec, so each frequency lasts for only 625 microseconds (1/1600). This is the dwell time. Baseband layer - roughly equivalent to MAC sublayer and uses TDD-TDMA (time-division duplexing TDMA). Similar to walkie-talkies using different carrier frequencies.

39. 14.39 Figure 14.22 Single-secondary communication

40. 14.40 Figure 14.23 Multiple-secondary communication

41. 14.41 Figure 14.24 Frame format types Access code: 72-bit field normally contains sync bits and ID of the primary to distinguish the frame of one piconet from another Address: up to 7 secondaries; 0 means broadcast Type: defines the type of data coming from the upper layer F: flow control (1 indicates buffer full); A: ACK (bluetooth uses stop and wait) S: sequence number for stop and wait

42. 14.42 Figure 14.25 L2CAP data packet format L2CAP layer roughly equivalent to LLC layer in LANs Length: length of data coming from upper layers Channel ID: defines a unique ID for the virtual channel created at this level