CHAPTER 6:WIRELESS & MOBILE NETWORKS

• Code Division Multiple Access• Spread Spectrum• Cellular Telephones• 4G Technology• IEEE 802.11• Mobile IP• Wireless Security• Satellite Networks

CODE DIVISION MULTIPLE ACCESS

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FDMA: 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 APPROACHES

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FDMA…• subject to impairment due

to selective channel fading• limited to fixed number of

concurrent users• not amenable to privacy

needs• wasteful guard bands are

needed between channels

TDMA…• subject to impairment due to

noise bursts• limited to fixed number of

concurrent users• not amenable to privacy

needs• wasteful guard times are

needed between time slots to ensure synchronization

CDMA…• frequency diversity avoids

transmission impairments• experiences graceful

degradation with more users• inherent privacy due to noise-like

characteristics of other messages

• efficiently utilizes the entire available bandwidth spectrum

SPREAD-SPECTRUM SIGNALING

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To 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.

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Direct-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 two’s complement of the signal used for 1).

• Other transmitters must use orthogonal sequences so they’ll be filtered out at the receiver.

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ORTHOGONAL 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).

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CODING 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)

Sender

Message

±1 Version

Chip Code

Encoded Message

W 010010 -1 1 -1 -1 1 -1 A(-1,-1,-1,-1), (1,1,1,1),(-1,-1,-1,-1), (-1,-1,-1,-

1),(1,1,1,1), (-1,-1,-1,-1)

X 111011 1 1 1 -1 1 1 B(1,1,-1,-1), (1,1,-1,-1),(1,1,-1,-1), (-1,-1,1,1),(1,1,-1,-1), (1,1,-1,-1)

Y 000111 -1 -1 -1 1 1 1 C(-1,1,1,-1), (-1,1,1,-1),(-1,1,1,-1), (1,-1,-1,1),(1,-1,-1,1), (1,-1,-1,1)

Z 101001 1 -1 1 -1 -1 1 D(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)

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DECODING 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?

Receiver

OriginalMessag

eChip Code

Decoded Message

Decoded Message After

ScalingW′ 010010 A -4, 4, -4, -4, 4, -4 010010

X′ 111011 B 4,4,4,-4,4,4 111011

Y′ 000111 C -4,-4,-4,4,4,4 000111

Z′ 101001 D 4,-4,4,-4,-4,4 101001

(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.

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CELLULAR 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.

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CELLULAR BASE STATIONSCellular 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.

1) The caller dials and the phone sends the phone number, programmed system ID, and phone serial number to the nearest base station.

2) After verifying everything, the base station relays the info to a mobile switching center, which then uses optical fiber or wireless to forward the caller’s signals.

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CELLULAR HANDOFF

The two base stations coordinate with each other through the Mobile Telephone Switching Office, and at some point, the user’s 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 user’s signal strength increasing.

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

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MULTIPATH PROPAGATIONCellular 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 signal’s 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.

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1G 2G 3G 4GFirst-generation cellular systems

were analog, using FDMA and

emphasizing voice

applications.

CDMA’s 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.

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4G’S 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.

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WIRELESS 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 messages• Distributed 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 (it’s too hard to tell the difference between external noise and one’s own transmission on such a wide-open medium).Corrupted signals must be handled at a higher protocol layer.

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IEEE 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 BSS’s, where the access points communicate with each other to forward traffic from one BSS to another in order to facilitate movement of stations between BSS’s.

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IEEE 802.11 FRAME FORMATPreamble: 10 bytes of alternating 0’s and 1’s (for synch) & a 2-byte start delimiter

PLCP (Physical Layer Convergence Protocol) Header: Length & rate fields, and a HEC.

MAC Data: The MAC frame, described below.

Preamble PLCP Header MAC Data CRC

CRC: 4 bytes to error-check the entire frame.

Preamble PLCP Header MAC Data CRC

Frame 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 flag

Duration/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.

FC DI Address1 DataAddress2 Address3 SC Address4

Sequence Control: 4-bit fragment number and 12-bit sequence number.Data: 0-2312 bytes of LLC data or control info.

FC DI DataSCAddress1 Address2 Address3 Address4IEEE 802.11 MAC FRAME FORMAT

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MOBILE 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 doesn’t 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 user’s home network, and (if used) Foreign Agent software on a router in the remote network.

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WIRELESS SECURITYExtra 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)

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SATELLITE NETWORKSEach method of medium access has problems when applied to satellites:

FDMA•Wide guard bands are needed to separate channels. •Power must be carefully controlled to preserve the signal’s integrity.

•Digital communication isn’t supported; only analog is.

TDMA•Ground stations have varying propagation times, so synchronization is tough.

•Ground stations must be capable of high burst speeds.

CDMA•Channels have low capacity due to noise and lack of synchronization.

•Transmitters must have extremely fast rates to accommodate spread.