Lecture #2 Chapter 14 Wireless LANs

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.

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

Figure 14.1 Basic service sets (BSSs)

Figure 14.2 Extended service sets (ESSs)

Figure 14.3 MAC layers in IEEE 802.11 standard

Why Wireless LANs cannot Implement CSMA/CD A station must be able to send data and receive collision signals at the same time. This can be very costly. Collision may not be detected because of the hidden station problem. The distance between stations can be great. Signal fading could prevent a station a station at one end from hearing a collision at the other end.

Figure 14.4 CSMA/CA flowchart Distributed Interframe Space Short Interframe Space

Figure 14.5 CSMA/CA and NAV Network Allocation Vector

Network Allocation Vector How do other stations defer sending their data if one station acquires access? When a station sends an RTS frame, it includes the duration of time that it needs to occupy the channel. The stations that are affected by this transmission create a timer called a NAV that shows how much time must pass before these stations are allowed to check the channel for idleness. Each station, before sensing the physical medium to see if it is idle, first checks its NAV to see if it is expired.

Collision During Handshaking Two or more station may try to send RTS frames at the same time….these may collide. There is no mechanism for collision detection. The sender assumes there has been a collision if it has not received a CTS frame from the receiver. The back-off strategy is employed and the sender tries again.

Point Coordination Function PCF Is an optional access method that can be implemented in an infrastructure network (not an AD HOC). It is implemented on top of the DCF and is used for time-sensitive transmission. PCF has a centralized, contention-free polling access method. The AP performs polling for stations that are capable of being polled. The stations are polled one after the other, sending any data they have to the AP. To give priority to PCF over DCF, another set of interframe spaces has been defined: PIFS and SIFS. The SIFS is the same as that in DCF, but the PIFS (PCF IFS) is shorter than in DIFS. This means that if, at the same time, a station wants to use only DCF and an AP wants to use PCF, the AP has priority. Due to the priority of PCF over DCF, stations that only use DCF may not gain access to the medium. To prevent this, a repetition interval has been designed to cover both contention-free(PCF) and contention-based(DCF) traffic. The repetition interval which is repeated continuously, starts with a special control frame B (Beacon Frame). When the stations hear the beacon frame, they start their NAV for the duration of the contention-free period of the .repetition interval

Figure 14.6 Example of repetition interval

Figure 14.7 Frame format Frame Control Duration of transmission (in one only: frame ID)

Table 14.1 Subfields in FC field

Figure 14.8 Control frames

Table 14.2 Values of subfields in control frames

Table 14.3 Addresses

Figure 14.9 Addressing mechanisms

Figure 14.10 Hidden station problem

Hidden station problem Assume that station B is sending data to station A. In the middle of this transmission, station C also has data to send to A. However, station C is out of B’s range and transmission from B to A cannot reach C. C thinks the medium is free, and sends its dta to A which results in a collision at A because A is receiving from B and C. In this case we say that B and C are hidden from each other with respect to A. Hidden stations can reduce the capacity of the network because of the possibility of collision. The solution is the use of the handshake frames. C receives CTS and hence knows that some hidden station is using the channel and refrains from transmitting until the duration is over.

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

Figure 14.11 Use of handshaking to prevent hidden station problem

Figure 14.12 Exposed station problem A is sending to B C has data to send to D C is exposed to transmission from A. C refrains!! A station refrains from using a channel when it is ,in fact, available.

Figure 14.13 Use of handshaking in exposed station problem

Table 14.4 Physical layers

Figure 14.14 Industrial, scientific, and medical (ISM) band

Figure 14.15 Physical layer of IEEE 802.11 FHSS

Figure 14.16 Physical layer of IEEE 802.11 DSSS

Figure 14.17 Physical layer of IEEE 802.11 infrared

IEEE 802.11a OFDM IEEE 802.11a OFDM describes the orthogonal frequency division multiplexing . For signal generation in a 5-Ghz ISM band. OFDM is similar to FDM with one major difference: All the subbands are used by one source at a given time. Sources contend with one another at the data link layer for access. The band is divided into 52 subbands, with 48 subbands for sending 48 groups of bits at a time and 4 subbands for control information. The scheme is similar to ADSL. Dividing the ban into subbands diminishes the effects of interference. If the subbands are used randomly, security can also be increased. OFDM uses PSK and QAM for modulation. The common rates are 18Mbps (PSK) and 54 Mbps(QAM).

Figure 14.18 Physical layer of IEEE 802.11b HR-DSSS Complementary code keying

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. A Bluetooth LAN can be connected to the Internet if one of the devices has this capability. Bluetooth technology is the implementation of a protocol defined by IEEE802.15 standard. The protocol defines a wireless PAN operable in an area the size of a room or hall. Bluetooth defines two types of networks: piconet and scatternet.

Figure 14.19 Piconet All the secondaries synchronize their clocks and hopping sequence with the primary. Communication between the P and S can be 1: or 1:N. Max. 7 secondaries, but 8 secs. Can be in parked state.

Figure 14.20 Scatternet

Bluetooth Devices A Bluetooth device has a built-in short-range radio transmitter. The current data rate is 1 Mbps with a 2.4-Ghz bandwidth. This means that there is a possibility of interference between the IEEE 802.11b wireless and Bluetooth LANs.

Figure 14.21 Bluetooth layers

Bluetooth Band , FHSS and Modulation Bluetooth uses a 2.4GHz ISM band divided into 79 channels of 1MHz each. Bluetooth uses the FHSS method in the physical layer to avoid interference from other devices or other networks. Bluetooth hops 1600 times per second. A device uses a frequency for only 625 µs before it hops to another frequency; the dwell time is 625 µs. Bluetooth uses a sophisticated version of FSK called GFSK. (Guassian Bandwidth Filtering). The Baseband layer is equivalent to the MAC sublayer in LANs. The access method is TDMA. The primary and secondary communicate using time slots. Communication is only between a primary and a secondary. Secondaries cannot communicate directly.

Figure 14.22 Single-secondary communication

Figure 14.23 Multiple-secondary communication

Figure 14.24 Frame format types