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

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

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

14.4 Figure 14.1 Basic service sets (BSSs)

14.5 Figure 14.2 Extended service sets (ESSs)

14.6 WLAN Standards ReleaseFreqTyp ThroughputMax Net BitrateMod GHz 0.9 Mbps 2IR/FHSS/DSSS a OFDM b DSSS g OFDM n / OFDM

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

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 b and g. IEEE a 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

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

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

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

14.12 Figure 14.6 Example of repetition interval

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 Table 14.1 Subfields in FC field

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

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

14.17 Table 14.2 Values of subfields in control frames

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

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

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

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

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

14.23 Figure Hidden station problem

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

14.25 Figure 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.

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

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

14.28 Table 14.4 Physical layers

14.29 Figure Industrial, scientific, and medical (ISM) band

14.30 Figure Physical layer of IEEE FHSS

14.31 Figure Physical layer of IEEE DSSS

14.32 Figure Physical layer of IEEE infrared

14.33 Figure Physical layer of IEEE b

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:

14.35 Figure Piconet

14.36 Figure Scatternet

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

14.38 Figure 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.

14.39 Figure Single-secondary communication

14.40 Figure Multiple-secondary communication

14.41 Figure 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

14.42 Figure 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