Chapter 14: Wireless Networks

Chapter 14:Wireless Networks

Principles of Computer Principles of Computer Networks and CommunicationsNetworks and Communications

M. Barry Dumas and Morris SchwartzM. Barry Dumas and Morris Schwartz

Principles of Computer Networks and Communications

2Chapter 14

Objectives

Describe the role of wireless transmission in computer communications and the physical phenomena that enable wireless communications

Identify characteristics that are common in all wireless networks Differentiate between contemporary wireless network configurations and

provide practical examples of their application Provide examples of alternative LAN protocol sets including their

capabilities and drawbacks Identify the essential elements of a wireless personal area network including

various configurations, protocols, advantages, and disadvantages associated with Bluetooth

Describe the essential elements of the IEEE802.15.1 WPAN and IEEE 802.16 wireless standards

Explain various aspects of cellular telephony including current configurations Provide characteristics of satellite communications including differentiating

between orbital differences


3Chapter 14

Overview

For transmission The electromagnetic carrier is modulated to represent the data signal Multiplexing allows many transmissions to take place simultaneously

without interfering with one another

Upon receipt At the receiver, the signal is demodulated to extract the data

“Wireless networks employ electromagnetic waves, primarily radio waves and microwaves,

to carry transmissions over the air or through the vacuum of space

using antennas to transmit and receive signals.”


4Chapter 14

Wireless Local Area Networks

Wireless local area networks (WLANs) Use radio wave carriers to transmit signals

among nodes Typically share the networking burden with

wired counterparts Provide flexibility and mobility over wired

LANs

Most WLANs operate in 2.4 GHz and 5 GHz bands


5Chapter 14


National information infrastructure (NII) Collection of network types that includes

Radio and television Public switched telecommunications network Private communications networks

Includes the U-NII that defines the industrial, scientific, medical (ISM) bands that are unlicensed in the United States and most countries

Band Definition Range900 MHz 915 ± 13 MHz 9.02 to 9.28 MHz

2.4 GHz 2.45 ± 0.05 GHz 2.40 to 2.50 GHz

5 GHz 5.8 ± 0.075 GHz 5.725 to 5.875 GHz


6Chapter 14


Wireless local area networks (WLANs)

Advantages Easy creation; no cables; can connect to wired LANs Provide access in places where wiring is not feasible/too costly Simple connection (usually automatic) for spontaneous participation Mobility and unconstrained physical configuration (within range)

Disadvantages Possible interference from electromagnetic radiation in ISM bands Potential for eavesdropping/security breaches Limited data rates compared to wired networks Incompatibilities due to proprietary schemes in the market


7Chapter 14


WLANs—topology

Fundamental structure of a WLAN is a Basic service set (BSS)

Computers in a WLAN are called stations Minimum BSS has two stations Stations can be mobile or fixed computers Can include an access point that connects

Wirelessly to the BSS By wire (through LAN/backbone) to the organization’s wired network

Independent basic service set (IBSS) (aka “ad hoc network”) An independent standalone LAN LAN stations can communicate only within the LAN

Mobile station vs. portable station

Mobile stationoperates while moving

Portable stationable to be moved within the LAN


8Chapter 14

WLANs—Independent basic service set (IBSS)

Fig 14.1A WLAN IBSS

with server

Devices within the IBSScan communicate

with the serveror with one another


9Chapter 14

WLANs—Basic service set (BSS)

Fig. 14.2A WLAN BSS with access point

wiredaccess


10Chapter 14


WLANs—Basic service set (BSS) (cont.)

Basic building blocks of extended WLANs

When two or more BSSs are connected by their access points to the same wired LAN Wired portion is called a distribution system (DS)

DS + BSSs extended service set (ESS)


11Chapter 14

WLANs BSS and ESS

Fig. 14.3

Wired access


12Chapter 14


The distribution system (DS) (wired portion of an ESS) provides the following participation services Association

Required for station participation Stations can associate with only one access point at a time

Disassociation When a station leaves a BSS or shuts down

Re-association Within an ESS, a station can move to another BSS (different access point)

Distribution Within an ESS, a station in one BSS needs to communicate with a station in a

different BSS Integration

The DS communicates between ESS stations and the other wired LANs in the corporate network

Inter-ESS movement Stations can move from one ESS to another ESS


13Chapter 14


The distribution system (DS) (wired portion of an ESS) provides the following station specific services

Authentication A station must identify itself before it can associate with a BSS Two versions of authentication

Open system authentication Station access is never denied Station simply identifies itself during association

Shared key Controls station access Station must possess a secret key to be authenticated A secret key is distributed by Wired Equivalent Privacy (WEP)

De-authentication A station’s authentication is terminated Occurs when a station leaves a BSS or is disassociated


14Chapter 14


WLAN protocols

Contained in the 802.11 specifications Exist in lowest two architecture model layers

Physical—defines electrical/spectrum and bit transmission/receipt Data link—responsible for

Frame assembly Node-to-node error control Physical addressing Inter-node synchronization Medium (channel) access

Two protocol sets Client/server (LAN paradigm)

Employs 802.x protocols used by wired LANs Ad hoc (wireless personal area networks paradigm)

Small coverage areas Used in Bluetooth networks


15Chapter 14


WLAN protocols—physical layer of 802.11

Physical—defines electrical/spectrum and bit transmission/receipt Defines four transmission methods (infrared or radio frequency)

1. (IR) Infrared (IR)2. (RF) Frequency hopping spread spectrum (FHSS)3. (RF) Direct sequence spread spectrum (including high rate)

(HR/DSSS)4. (RF) Orthogonal frequency division multiplexing (OFDM)

For nodes to communicate, each must use the same transmission method.


16Chapter 14



1. Infrared (IR) Signals are carried by infrared light Very short useful range [5 to 6 meters; 15 to 20 feet] Commonly found in TV remote controls and wireless computer peripherals

(keyboard, mouse, etc.) Standards developed by the infrared data association (IRDA)

Advantages Works in electrically noisy environments without interference Signals can reflect (off walls, ceilings, etc.) to reach target Inexpensive

Disadvantages Very limited span Line-of-sight required Unable to penetrate solid objects

Could be an advantage if security is an issue(i.e., difficult to intercept)

Except for Bluetoothrarely used in WLANs


17Chapter 14



1. Infrared (IR) irDA-defined physical layer protocols

Protocol Descriptor Data RateIrDA-FIR fast infrared up to 4 Mbps

IrDA-MIR medium infrared up to 1.15 Mbps

IrDA-SIRserial infrared“slow infrared”

up to 115 Kbps


18Chapter 14



2. (RF) Frequency hopping spread spectrum (FHSS) Narrow bandwidth, only a small portion of 2.4 GHz

spectrum Entire spectrum is used by constantly shifting the signal

(hopping) across the spectrum A master station establishes the hopping sequence that is

followed by participating stations Transmissions appear to take place over a single (virtual)

communications channel

Particularly popular in Bluetooth and HomeRF networks


19Chapter 14



3. (RF) Direct sequence spread spectrum (high rate) (HR/DSSS)

Spreads the signal over the entire 2.4 GHz spectrum Entire spectrum is used by substituting a redundant sequence of

bits (chipping code) for each bit of the signal to be transmitted Because the chipping code data rate is higher than the

original signal rate, there is no delay in signal transmission Most often used in WiFi 802.11b (11 Mbps) or 802.11g

(below 20 Mbps)

DSSS and FHSS will interfere with each other!These are not usually found in business environments.


20Chapter 14



4. (RF) Orthogonal frequency division multiplexing (OFDM) Similar to frequency division multiplexing (FDM), except

FDM transmits signals from multiple sources at the same time, with each source assigned a separate sub-band frequency

OFDM assigns all of the sub-bands to a single source for a specified time

Carrier frequencies are produced so that peak amplitudes of each frequency coincide with minimum amplitudes of adjacent frequencies

Modulators see frequencies in only a particular carrier sub-band


21Chapter 14


WLAN protocols—802.11 variations

(2001) 802.11a (54 Mbps, 5 GHz) (1999) 802.11b (11 Mbps, 2.4 GHz)

Original WiFi standard (2003) 802.11g (54 Mbps, 2.4 GHz)

Backward compatible with 802.11b Essentially eliminated need for 802.11a

(2006) 802.11n (100 to 600 Mbps, 5 GHz) Uses multiple input/multiple output (MIMO) signaling with

many data streams traveling over the same frequencies, and each data stream carrying different information


22Chapter 14


WLAN protocols—802.11 variations summarized

Standard Speed Range Frequency Method

802.11a 54 Mbps 60 ft 5 GHz OFDM

802.11b 11 Mbps 300 ft 2.4 GHz DSSS

802.11g 54 Mbps 300 ft 2.4 GHz OFDM

802.11n100 Mbps to

600 Mbps60 ft 5 GHz MIMO

Susceptible to microwave/portable phone interference!


23Chapter 14


WLAN protocols—data link layer of 802.11

As with all 802 LANs, the data link layer is subdivided: Logical link control (LLC) Media access control (MAC)

When an ESS (collection of BSSs) is created, component BSSs appear to the LLC as a single IBSS Stations can communicate with other stations on the ESS Stations can move to any BSS on the ESS

A station’s physical address is one of the 48-bit MAC addresses of the (wireless) NIC


24Chapter 14

Wireless Personal Area Networks

Wireless personal area network (WPAN) Accommodates data sharing and connectivity Small, often impromptu groups Limited span (e.g., same room) Originally designed to replace desktop cable

connections

Predominantly Bluetooth!


25Chapter 14


WPAN—Bluetooth

Based on the 802.11 standard However, does not use 802.x LAN protocols Not designed for LAN communications, large-scale data Operates in the 2.4 GHz band Operates in a piconet (supporting 2 to 8 devices) Uses FHSS to hop from channel to channel within the 79

(1 MHz) sub-bands (channels) of the 2.4 GHz band

FHSS avoids interference from other 2.4 GHz devices(e.g., portable phones, baby monitors, microwaves, etc.)

802.15.1 establishes Bluetoothas a de jure standard


26Chapter 14


WPAN—piconet

Supports 2 to 8 devices (needs at least two active members) Is established automatically (on the fly) Devices entering a piconet

[with less than 8 devices] are assigned an address [with 8 or more devices] can be on standby

First member assumes the role of master; others act as slaves Members can be mobile or stationary Mobile members can move within a piconet

as long as they stay within range of the master

A collection of piconets is a scatternet


27Chapter 14

WPAN—piconet

Fig 14.4A and B

M1

2

3

45

6

7

M 1


28Chapter 14

WPAN—piconet

Fig 14.4C

M 1

M1

M2

M3

1

21 2

1

2

3


29Chapter 14

Wireless Metropolitan Area Networks Wireless metropolitan area network (WMAN)

[aka WiMAX] 802.16 Operates in 2 to 11 GHz band [as of 802.16a] High data-data-rate broadband system (to 70 Mbps) Can operate over substantial distances (> 30 miles) Uses same logical link control (LLC) as other 802

networks, which means: WiMAX and WiFi networkscan interconnect!


30Chapter 14

Wireless Metropolitan Area Networks WMAN (WiMAX)

Provides four key wireless functionalities High-speed connectivity

Alternative to contracting for wired services Last-mile broadband

High speed without need for telco last-mile local loops Hot spot (hot zone) coverage

Connects mobile devices to access points Backhaul alternative

Provides wireless access from remote sites to the core network


31Chapter 14

Wireless Metropolitan Area Networks WMAN (WiMAX)—standards in other countries

European Telecommunications Standard Institute (ETSI) wireless standards 802.11—WiFi HiperLAN—high performance radio LAN 802.15—PAN HiperPAN—high performance radio PAN 802.16—WiMAX HiperMAN—high performance radio MAN

Korean Telecommunications Technology Association (KTTA) 802.16—WiMAX WiBro—wireless broadband

Compatible!


32Chapter 14

Cellular Telephony

Terms Base stations

Stationary, ground-based sites linked to neighboring sites Are connected to and controlled by MSCs

Cell Logical way of thinking about a coverage region (usually hexagonal) Base station coverage areas

Cell phone Low-power transmitter/receiver for voice and data Communicating wirelessly through a collection of base stations

Mobile switching centers (MSCs)(aka mobile telephone switching offices—MTSOs)

Establish call connections Coordinate all base stations Provide links to the wired telephone network and the Internet Keep calling and billing records


33Chapter 14

Cellular Telephony

Basic functionality (simple)

When a call is initiated, a connection is established between the caller’s cell phone and the base station of the cell the caller is in

As the caller begins to move out of range for that cell,

the base station senses the drop in signal power and relays that information to the MSC

The MSC automatically “hands off” the call to the base station of the cell the caller is moving into


34Chapter 14

Cellular Telephony

Where are cells located? Some viewpoints:

1

Fig 14.5

coverageBase station 1


35Chapter 14

Cellular Telephony

Generations and systems First Generation (1G—early 1980s)

Analog based, multiplexed by FDMA Advanced mobile phone system (AMPS) Used 850 MHz band (824–894 MHz)

824–849 MHz mobile unit to base station 869–894 MHz base station to mobile unit

Problems Noise and poor quality Coverage was limited Cells had limited capacity Easy to tap airborne signals (steal phone codes)


36Chapter 14

Cellular Telephony

Generations and systems

Second Generation (2G—late 1980s to 1990s) Introduced digital service Employ powerful authentication techniques Three schemes

Digital AMPS [D-AMPS] Digital version of AMPS, based on TDMA Uses 850 MHz band [824–894 MHz; same as AMPS] Phone voice coders (vocoders) converted analog voice

to digital

European and U.S. GSMsare not compatible


37Chapter 14

Cellular Telephony

Personal communication system (PCS) Uses code division multiple access (CDMA) Digital system combines DSSS with chipping codes Uses 1,900 MHz band (1,850–1,910 MHz)

Global system for mobile communications (GSM)—developed in Europe

Uses combination of FDMA to divide bands into channelsand TDMA to create time slots within the channels

Uses 850 MHz and 1,900 MHz band in United States Uses 900 MHz and 1,800 MHz band in Europe and Asia

Sprint Verizon

AT&T CingularNextelT-Mobile


38Chapter 14

Cellular Telephony

Generations and systems

Third Generation (3G) Addressed speed shortcomings of 2G cell phones (144 Kbps to 2+ Mbps) [With the speed] Enabled access to more services

Web browsing Web-based applications Multimedia E-mail (with or without attachments)

Works with smart phones (i.e., cell phones, PDAs with cell phone features)

Problems Memory Online costs

Although 3G mobile devices can access broadband services,connection cost (at cell phone rates) is still a limiting factor.


39Chapter 14

Cellular Telephony

Generations and systems Evolving Third Generation (3G+)

Three schemes Universal mobile telephone service (UMTS)

(GSM-type, wide-band code division) Designed to run over existing GSM networks Will probably replace GSM

CDMA20000 (enhanced 2G code division multiple access) TD-SCDMA (time division + synchronous code division)

Data rates as high as 14 Mbps

Fourth generation (4G) technology holds the prospect of data rates between 100 Mbps–1 Gbps


40Chapter 14

Satellites

There cannot be successful communication If the (transmit/receive) earth stations cannot “see”

the satellite If the satellites cannot “see” each other

“…line of sight still is required from the earth transmitter to the satellite,

from the satellite to the earth receiver, and indeed from one satellite to another.”

Echo I (994 mile altitude) orbited the earth every 90 minutes

A spot on earth could “see” Echo I for only 10 minutes each orbit!


41Chapter 14

Satellites

Transmission signals Uplink—from earth location to satellite Downlink—from satellite to an earthbound station

Orbits Geosynchronous earth orbits (GEOs) Medium earth orbits (MEOs) Low earth orbits (LEOs) Highly elliptical orbits (HEOs)

None of these are synchronous


42Chapter 14

Satellites

Geosynchronous earth orbits (GEOs) Appear stationary to an observer on earth Match the rotation of the earth 22,240 miles (35,786 km) above the earth Typically centered around the equator Can see 35 to 40% of the earth within latitude bands

If a GEO satellite is in line of sight,

it will always be in line of sight.


43Chapter 14

Satellites

For non-synchronous orbits Satellites do not appear to be stationary Constellations (parades of satellites) are used for coverage Transmissions from a “departing” satellite (moving out of line of sight)

are handed off to an incoming satellite Medium earth orbits (MEOs)

Range from 5,000–15,000 km (3,100–9,300 miles)

Low earth orbits (LEOs) Range from 100–2,000 km

(100–1,240 miles) Highly elliptical orbits (HEOs)

Ranges in altitude from 500–50,000 km (less than 311–more than 31,000 miles)

Only orbit used for polar regions


44Chapter 14

Satellites

Communications satellites use microwave signals between 1.5 and 30 GHz

Table 14.1

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Chapter 14: Wireless Networks

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