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CHAPTER 6:WIRELESS & MOBILE NETWORKSCode Division Multiple AccessSpread SpectrumCellular Telephones4G TechnologyIEEE 802.11Mobile IPWireless SecuritySatellite Networks

CODE DIVISION MULTIPLE ACCESSPage 159Chapter 6CS 447FDMA: Everyone gets to talk at the same time, but only across their narrow channels.TDMA: Everyone gets to talk using the entire bandwidth, but they have to take turns talking.CDMA: Everyone gets to talk simultaneously, using the entire bandwidth! They do this by coding their transmissions in a unique fashion (as if every pair were speaking a different language, and each other language merely sounds like background noise).

COMPARING CELLULAR APPROACHESPage 160Chapter 6CS 447

FDMAsubject to impairment due to selective channel fadinglimited to fixed number of concurrent usersnot amenable to privacy needswasteful guard bands are needed between channelsTDMAsubject to impairment due to noise burstslimited to fixed number of concurrent usersnot amenable to privacy needswasteful guard times are needed between time slots to ensure synchronizationCDMAfrequency diversity avoids transmission impairmentsexperiences graceful degradation with more usersinherent privacy due to noise-like characteristics of other messagesefficiently utilizes the entire available bandwidth spectrum

SPREAD-SPECTRUM SIGNALINGPage 161Chapter 6CS 447To eliminate the noise interference and privacy violations that have plagued wireless communication, spread-spectrum signaling was developed in two principal forms:

Frequency Hopping Spread Spectrum (FHSS)Each transmitter changes frequencies at a regular time interval, following a pseudorandom pattern known only to the transmitter and the receiver.The receiver demodulates the received signal, filtering out any received signals that are not at the appropriate frequency.FHSS is commonly used in Bluetooth systems.

Page 162Chapter 6CS 447Direct-Sequence Spread Spectrum (DSSS)Each data bit is multiplied by a long bit sequence and modulated for transmission (the sequence used for 0 is the twos complement of the signal used for 1).Other transmitters must use orthogonal sequences so theyll be filtered out at the receiver.

Page 163Chapter 6CS 447ORTHOGONAL DIGITAL SIGNALSA chip set of orthogonal vectors is used to spread the signals.

Chip Code A: (1,1,1,1)Chip Code B: (1,1,-1,-1)Chip Code C: (1,-1,-1,1)Chip Code D: (1,-1, 1,-1)Note that the chip codes are mutually orthogonal (i.e., the dot product of any pair of them is zero).

Page 164Chapter 6CS 447CODING OUTGOING MESSAGESAssume that there are four senders...

A: (1,1,1,1)B: (1,1,-1,-1)C: (1,-1,-1,1)D: (1,-1, 1,-1)SenderMessage1 VersionChip CodeEncoded MessageW010010-1 1 -1 -1 1 -1A(-1,-1,-1,-1), (1,1,1,1),(-1,-1,-1,-1), (-1,-1,-1,-1),(1,1,1,1), (-1,-1,-1,-1)X1110111 1 1 -1 1 1B(1,1,-1,-1), (1,1,-1,-1),(1,1,-1,-1), (-1,-1,1,1),(1,1,-1,-1), (1,1,-1,-1)Y000111-1 -1 -1 1 1 1C(-1,1,1,-1), (-1,1,1,-1),(-1,1,1,-1), (1,-1,-1,1),(1,-1,-1,1), (1,-1,-1,1)Z1010011 -1 1 -1 -1 1D(1,-1,1,-1), (-1,1,-1,1),(1,-1,1,-1), (-1,1,-1,1),(-1,1,-1,1), (1,-1,1,-1)Summing the four encoded messages yields:(0,0,0,-4), (0,4,0,0), (0,0,0,-4), (-2,-2,-2,2), (2,2,-2,2), (2,-2,-2,-2)

Page 165Chapter 6CS 447DECODING INCOMING MESSAGES

A: (1,1,1,1)B: (1,1,-1,-1)C: (1,-1,-1,1)D: (1,-1, 1,-1)What happens when the incoming message arrives?ReceiverOriginalMessageChip CodeDecoded MessageDecoded Message After ScalingW010010A-4, 4, -4, -4, 4, -4010010X111011B4,4,4,-4,4,4111011Y000111C-4,-4,-4,4,4,4000111Z101001D4,-4,4,-4,-4,4101001(0,0,0,-4), (0,4,0,0), (0,0,0,-4), (-2,-2,-2,2), (2,2,-2,2), (2,-2,-2,-2)The decoded message is scaled by converting all positive values to 1, and all non-positive values to 0.

Page 166Chapter 6CS 447CELLULAR TELEPHONESCellular phones transmit and receive on two kinds of channels (FDM for analog, TDM for digital):Control Channels Used for overhead messages (e.g., network system ID), pages (incoming call signals), access info (connection requests), and channel assignments (when connection is established).Communication Channels Used for voice/data, handoff control, and maintenance monitoring.

Page 167Chapter 6CS 447CELLULAR BASE STATIONS

Cellular providers typically have 832 (analog) channels to use.42 are reserved for control info.The remaining 790 are split into 395 duplex pairs for voice/data.The service region is split into a hexagonal grid, with approximately 56 channel pairs allocated to each base station.

The caller dials and the phone sends the phone number, programmed system ID, and phone serial number to the nearest base station.After verifying everything, the base station relays the info to a mobile switching center, which then uses optical fiber or wireless to forward the callers signals.

Page 168Chapter 6CS 447CELLULAR HANDOFF

The two base stations coordinate with each other through the Mobile Telephone Switching Office, and at some point, the users phone gets a signal on a control channel telling it to change frequencies.Meanwhile, the base station in the cell towards which the user is moving (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) detects the users signal strength increasing.

As a user moves toward the edge of a cell, the local base station notes that the users signal strength is diminishing.

Page 169Chapter 6CS 447MULTIPATH PROPAGATION

Cellular signals may be altered via four principal types of interference.Reflection occurs when a radio wave collides with an object which has very large dimensions compared to the wavelength of the propagating signal. This is often caused by the surface of the earth, buildings, and wallsDiffraction occurs when the path between the transmitter and the receiver is obstructed by an object with sharp edges, causing secondary waves to bend around the object and provide an artificial line-of-sight between the communicating pair.Scattering occurs when the signal travels through a medium containing objects with dimensions smaller than the signals wavelength, such as when the transmission encounters a rough surface or small objects.Shadowing occurs when the signal power fluctuates due to objects obstructing the propagation path between the transmitter and the receiver.

Page 170Chapter 6CS 4471G 2G 3G 4GFirst-generation cellular systems were analog, using FDMA and emphasizing voice applications.CDMAs big advantages Frequency diversity limits transmission impairments. Noise-like signal system improves privacy. Graceful service degradation with increased usage.Second-generation cellular systems are digital, using TDMA (or CDMA) and emphasizing e-mail & Internet access.Third-generation cellular systems are also digital, using CDMA and stressing video telephony and high-speed Internet access.Fourth-generation cellular systems will extend the bandwidth and throughput capabilities of 3G.

Page 171Chapter 6CS 4474GS MULTITECHNOLOGY APPROACHNumerous technologies are competing for a share of the 4G market, including:Orthogonal Frequency-Division Multiple Access (OFDMA) supports multi-user transmissions by separating them in both the time domain and in the frequency domain.

Ultra-Wideband (UWB) uses the radio spectrum at low energy levels to produce short-range, high-bandwidth wireless communications.

Multiple-Input/Multiple-Output (MIMO) uses multiple antennas at both the transmitter and the receiver to improve communication performance.

Page 172Chapter 6CS 447WIRELESS LANSMedium access control on wireless LANs has certain similarities to CSMA/CD.Three interframe space time values are defined:Short IFS: time interval for high-priority traffic (e.g., ACKs and Clear-To-Sends)Point Coordination Function IFS: time interval for polling messagesDistributed Coordination Function IFS: time interval for regular trafficA transmitting wireless station waits the appropriate IFS time, listening for traffic.If it hears a message, it waits until the message has passed, and then waits another IFS, plus an added exponential backoff time.

Note that there is no collision detection (its too hard to tell the difference between external noise and ones own transmission on such a wide-open medium).Corrupted signals must be handled at a higher protocol layer.

Page 173Chapter 6CS 447IEEE 802.11 TOPOLOGIESThe 802.11 architecture hierarchically defines its topologies.An Independent Basic Service Set has wireless stations communicating directly with each other in a peer-to-peer fashion.An Infrastructure Basic Service Set has a component called an Access Point that serves as a relay through which the stations communicate with each other and which provides a connection to an external Distribution System.An Extended Service Set is a set of infrastructure BSSs, where the access points communicate with each other to forward traffic from one BSS to another in order to facilitate movement of stations between BSSs.

Page 174Chapter 6CS 447IEEE 802.11 FRAME FORMATPreamble: 10 bytes of alternating 0s and 1s (for synch) & a 2-byte start delimiterPLCP (Physical Layer Convergence Protocol) Header: Length & rate fields, and a HEC.MAC Data: The MAC frame, described below.PreamblePLCP HeaderMAC DataCRCCRC: 4 bytes to error-check the entire frame.PreamblePLCP HeaderMAC DataCRCFrame Control: 2-bit protocol version; 6-bit message type; 1-bit flag indicating transmission to the Distribution System; 1-bit flag indicating transmission from the DS; 1-bit fragmentation flag; 1-bit retransmission flag; 1-bit power management flag (Power Save vs. Active); 1-bit Access Point polling flag; 1-bit encryption flag; 1-bit Strictly-Ordered flagDuration/ID: Either the time that the channel is being allocated, or, in control frames, the station ID number for Power-Save polling responses.Address1-4: The 6-byte addresses for the source, destination, source Access Point, and destination Access Point.FCDIAddress1DataAddress2Address3SCAddress4Sequence Control: 4-bit fragment number and 12-bit sequence number.Data: 0-2312 bytes of LLC data or control info.FCDIDataSCAddress1Address2Address3Address4IEEE 802.11 MAC FRAME FORMAT

Page 175Chapter 6CS 447MOBILE IPMobile IP was developed to allow users to seamlessly roam among wireless networks, with no interruptions to IP applications like media streaming or VoIP as network boundaries are crossed.

Mobile IP datagrams may flow in a network without a Foreign Agent, as long as the Mobile Node has a public IP address in the visited network.If the visited network has a Foreign Agent, the Mobile Node doesnt require any IP address, and the Foreign Agent only requires one public IP address for all Mobile Nodes.Mobile IP service requires every mobile device to contain Mobile Node software, Home Agent software on a router in the users home network, and (if used) Foreign Agent software on a router in the remote network.

Page 176Chapter 6CS 447WIRELESS SECURITY

Extra security mechanisms are required in wireless environments, due to the lack of physical connections:Authentication - Establishing identity (e.g., passwords)Deauthentication - Terminating authentication (e.g., if reauthentication is needed)Encryption - Ensuring privacy (e.g., encoding to inhibit unauthorized reading)

Page 177Chapter 6CS 447SATELLITE NETWORKSEach method of medium access has problems when applied to satellites:FDMAWide guard bands are needed to separate channels. Power must be carefully controlled to preserve the signals integrity.Digital communication isnt supported; only analog is.TDMAGround stations have varying propagation times, so synchronization is tough.Ground stations must be capable of high burst speeds.CDMAChannels have low capacity due to noise and lack of synchronization.Transmitters must have extremely fast rates to accommodate spread.