expanded by Jozef Goetz, 2014 The McGraw-Hill Companies, Inc., 2007 Lesson 7 - 8 Wireless LANs Part I September 15, 2018 expanded by Jozef Goetz, 2014 The McGraw-Hill Companies, Inc., 2007

Topics discussed in this section: 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. Topics discussed in this section: Architecture MAC Sublayer Physical Layer

LANs Wireless LANs are found on college campuses, office buildings, and public areas. At home, a wireless LAN can connect roaming devices to the Internet. In this chapter, we concentrate on two promising wireless technologies for LANs: IEEE 802.11 wireless LANs, sometimes called wireless Ethernet, and Bluetooth, a complex technology for small wireless LANs.

LANs IEEE 802.11, which covers the physical and data link layers. The Industrial, Scientific and Medical (ISM) radio bands were originally reserved internationally for the use of RF electromagnetic fields for Industrial, Scientific and Medical purposes other than communications.

WIRELESS LANs A set of wireless LAN standards has been developed by the IEEE 802.11 committee. Three transmission schemes are defined in the current 802.11 standard: [1] Infrared: at 1 Mbps & 2 Mbps. [2] Direct-sequence spread spectrum: 2.4GHz ISM band [3] Frequency-hopping spread spectrum: 2.4GHz ISM band 802.11b - 11 Mbps 802.11g - 54 Mbps

Basic Service Set BSS (Cell) IEEE 802.11 defines the Basic Service Set (BSS) or Cell as the building block of a wireless LAN. The Basic Service Set (BSS) is made of stationary or mobile wireless stations and a possible central Base Station (BS), known as the Access Point (AP). BSS without an AP is a stand-alone network and cannot send data to other BSSs.

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.

Extended Service Set (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 IEEE LAN such as an Ethernet. Note that the extended service set 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, we have what is called an infrastructure network.

Extended Service Set (ESS) In this infrastructure network, 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. The idea is similar to communication in a cellular network if we consider each BSS to be a cell and each AP to be a base station. Note that a mobile station can belong to more than one BSS at the same time.

WIRELESS LANs ESS Wired Backbone LAN functions as a bridge BSS BSS

Wireless LAN A multicell 802.11 network (WiFi). The connection between the 802.11 system and the outside world is called a portal All the base stations are wired together using copper or fiber Unlike cellular tel. systems, each cell has only one channel, covering the entire available bandwidth ( 11 – 54 Mbps) and covering all the stations in its cell

Station Types IEEE 802.11 defines 3 types of stations based on their mobility in a wireless LAN: no-transition, BSS-transition, and ESS-transition. No-Transition Mobility A station with no-transition mobility is either stationary (not moving) or moving only inside a BSS. BSS-Transition Mobility A station with BSS-transition mobility can move from one BSS to another, but the movement is confined inside one ESS. ESS-Transition Mobility A station with ESS-transition mobility can move from one ESS to another. However, IEEE 802.11 does not guarantee that communication is continuous during the move.

The 802.11 Protocol Stack ISM: Abbreviation for Industrial, Scientific, and Medical applications (of radio frequency energy). range is 7 times greater

MAC layers in IEEE 802.11 standard

Reminder: IEEE standard for wired LANs Note: there is one LLC sublayer for all IEEE LANs Reminder: IEEE standard for wired LANs

Modulation In analog transmission, the sending device produces a high-frequency signal that acts as a base for the information signal. This base signal is called the carrier signal or carrier frequency. The receiving device is tuned to the frequency of the carrier signal that it expects from the sender. This kind of modification is called modulation (shift keying).

Modulation Modulation - is the process of encoding source data onto a carrier signal with frequency carrier fc i.e. process of modulating the carrier signal by modifying one or more of parameters: amplitude, frequency, & phase of a sine wave.

Modulation of Digital Data Basic Digital-to-Analog conversion or modulation or encoding Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM)

The digital data must be modulated on an analog signal Digital-to-analog modulation The digital data must be modulated on an analog signal Modulation of binary data or (digital-to-analog modulation) is the process of changing one of the characteristics of an analog signal a sine wave is defined by 3 characteristics: amplitude, frequency, and phase based on the information in a digital signal (0s and 1 s).

Digital-to-analog conversion

Types of digital-to-analog modulation Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM)

Note: Bit rate is the number of bits per second [bps] Baud rate is the number of signal elements (or impulses or symbols or signal units) per second [baud] Baud rate <= Bit rate.

Bit rate N is the # of bits (passengers by analogy) per second. Note Bit rate N is the # of bits (passengers by analogy) per second. Baud rate S is the # of signal elements (vehicles by analogy) per second. In the analog and digital transmission of digital data, S <= N

In data transmission: a signal element as the smallest unit of a signal that is constant The baud rate determines the bandwidth required to send the signal.

In analog transmission: We can define the data rate (bit rate) and the S signal rate (baud rate) baud rate: S = N / r [baud] = N / r [signal/sec] = N / r [paulse/sec] where N is the data rate (bps) and r is the # of data elements (bits) carried in one signal element. The value of r in analog transmission is r = log2 L where L is the # of signal elements (pulses), not the level e.g. for FSK is # of different frequencies S = N / r => N = S r

Example An analog signal carries 4 bits in each signal element (unit). If 1000 signal elements (units or pulses) are sent per second, find the baud rate and the bit rate Solution Given: r = 4 [bit/pulse] , S = 1000 [pulse/sec] Find N = ? S = N / r => N = S r S = Signal rate =Baud rate = 1000 pulse/sec N = Data rate = Bit rate = S r = 1000 [pulse/sec] 4 [bit/pulse] = 4000 bps

Example The bit rate of a signal is 3000. If each signal element carries 6 bits, what is the baud rate? Solution Baud rate S = N / r = 3000 [bit/sec] / 6 [bit/pulse] = 500 [pulse/sec] = 500 baud

DIGITAL DATA, ANALOG SIGNALS AMPLITUDE-SHIFT KEYING (ASK) In ASK, the two binary values are represented by two different amplitudes A1 and A0 of the carrier frequency f sub c; t is a time .

DIGITAL DATA, ANALOG SIGNALS FREQUENCY-SHIFT KEYING (FSK) In FSK, the two binary values are represented by two different frequencies. The frequencies of the modulated signal is constant for the duration of one signal element.

DIGITAL DATA, ANALOG SIGNALS PHASE-SHIFT KEYING (PSK) In PSK, the phase of the carrier signal is shifted to represent data.

Analog-to-analog modulation needed when the medium has a band-pass nature or if only band-pass bandwidth is available and e.g. a radio produced by each station is a low-pass signal. So a low-pass signal need to be shifted.

ANALOG DATA, ANALOG SIGNALS There are two principal reasons for analog modulation of analog signals: A higher frequency may be needed for effective transmission. Modulation permits frequency-division multiplexing. Three techniques: Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Amplitude modulation AM works by varying the strength (amplitude) of the carrier in proportion to the waveform being sent while the frequency remains constant

ANALOG DATA, ANALOG SIGNALS AM - MODULATION Amplitude modulation (AM) is the simplest form of modulation. Mathematically, the process can be expressed as Carrier Input signal Modulation index

Figure Amplitude modulation

ANALOG DATA, ANALOG SIGNALS FM - MODULATION In frequency modulation (FM) we modulate the instantaneous frequency fi(t), with the signal s(t) - he frequency of the carrier signal is varied. Mathematically, the process can be expressed as Carrier Input signal Modulation index

ANALOG DATA, ANALOG SIGNALS PM - MODULATION For phase modulation (PM), the phase is proportional to the modulating signal s(t). Mathematically, the process can be expressed as Carrier Input signal

Industrial, Scientific, and Medical (ISM) band 3 unlicensed bands in the 3 ranges