Ch 6. Wireless and Mobile Networks

Myungchul Kim

mckim@icu.ac.kr

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– Wirless hosts– Wireless links– Base station: (e.g., cell towers, access points) handoff

Infrastructure mode Ad hoc mode

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Wireless network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects to base station (WiFi,WiMAX, cellular)

which connects to larger Internet

no base station, noconnection to larger Internet (Bluetooth,

ad hoc nets)

host may have torelay through several

wireless nodes to connect to larger Internet: mesh net

no base station, noconnection to larger Internet. May have torelay to reach other a given wireless node

MANET, VANET

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Wireless links and network characteristics Differences between a wired link and a wireless link

– Decreasing signal strength– Interference from other source– Multipath propagation

Signal-to-noise ration (SNR)– A relative measure of the strength of the received signal and the

noise.– dB

Bit error rate (BER)

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Physical-layer characteristics of higher-layer wireless communication protocols

– For a given modulation scheme, the higher the SNR, the lower the BER.

– For a given SNR, a modulation technique with a higher bit transmission rate will have a higher BER.

– Dynamic selection of the physical-layer modulation technique can be used to adapt the modulation technique to channel condition.

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CDMA– A partitioning protocol in that it partitions the codespace and assi

gns each node a dedicated piece of the codespace.

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WiFi: 802.11 Wireless LANs

– 802.11n: multiple-input multiple-output (MIMO) antennas, over 100Mbps, compatible with 802.11a/b/g devices

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The 802.11 architecture– Basic service set (BSS): service set identifer (SSID)

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Channels and association– 11 partically overlapping channels– Separated by four channels (e.g., 1, 6, and 11)– AP sends beacon frames (AP’s SSID and MAC address)

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– Association with authentication A station’s MAC address Usernames and passwds by RADIUS or DIAMETER

The 802.11 MAC protocol– CSMA/Collision Avoidance– Differences from Ethernet

Collision avoidance Link-layer acknowledgement/retransmission

– No collision detection Costly to build hardware that can detect a collision No detect all collisions due to the hidden terminal problem and fadin

g

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Dealing with hidden terminals: RTS and CTS– RTS (including the total time required to transmit the DATA and t

he ACK) and CTS to reserve access to the channel– CTS: gives the sender explicit permission to send and also instru

cts the other stations not to send for the reserved duration.– RTS/CTS is used only when the frame is longer than the thresho

ld.

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Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

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The IEEE 802.11 frame– The fourth address: APs forward frames to each other in ad hoc

mode.

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– Address 3 contains the MAC address of the router interface.

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Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr

dest. address source address

802.3 frame

802.11 frame: addressing

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Mobility in the same IP subnet

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Advanced features in 802.11– 802.11 rate adaptation: select the underlying physical-layer mod

ulation technique to use based on current channel characteristics.

– Power management: alternate between sleep and wake states, 250 microseconds for wakeup.

Bluetooth– IEEE 802.15.1– Wireless personal area network– 4Mbps– Ad hoc networks– A piconet of up to eight active devices

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WiMAX– IEEE 802.16– Wireless ADSL– Speeds of 70-80 miles per hour

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Cellular Internet Access

– Air interface access technologies A combination of FDM and TDM CDMA

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1G– Analog FDMA– Voice only

2G– Digital for voice– Global system for mobile communication (GSM)

Combined FDM/TDM

– IS-95 CDMA

2.5G– General packet radio service (GPRS): up to 9.6kbps packet-bas

ed data– Enhanced data rates for global evolution (EDGE): 384 kbps– CDMA-2000 phase: packet-data up to 144.4 kbps

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3G– 144 kbps at driving speeds– 384 kbps for outside stationary use or walking speeds– 2 Mbps for indoors– CDMA-2000

Evolved from IS-95 1xEVDO for 3 Mbps data

– Universal Mobile Telecommunication Service (UMTS) WCDMA for data HSDPA/HSUPA up to 14 Mbps

4G– Ubiquitout Internet access– The highest bit rate at that time and place– Vertical handoff– Seamless moblity– Voice and real-time video over IP

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Mobility management: principles

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Routing to a mobile node

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– A mobile-node-to-foreign-agent protocol– A foreign-agent-to-home-agent registration protocol– A home-agent datagram encapsulation protocol– A foreign-agent decampsulation protocol

Direct routing to a mobile node– Triangle routing problem– A mobile-user location protocol– Anchor foreign agent

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Mobile IP

– Agent discovery– Registration with the home agent– Indirect routing of datagrams

Agent discovery– Via agent advertisement or via agent solicitation– An extension to the router discovery protocol– Home agent bit (H), foreign agent bit (F), registration required bit

(R), M and G encapsulation bits, care-of-address (COA) fields.

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Managing mobility in cellular networks

GSM– Indirect routing – Home location register (HLR), Visitor location register (VLR)– Mobile station roaming number

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Handoffs in GSM

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Mobile Switching

Center

VLR

old BSS

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3

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GSM: handoff with common MSC

new BSS

1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs

2. MSC sets up path (allocates resources) to new BSS

3. new BSS allocates radio channel for use by mobile

4. new BSS signals MSC, old BSS: ready

5. old BSS tells mobile: perform handoff to new BSS

6. mobile, new BSS signal to activate new channel

7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call

8 MSC-old-BSS resources released

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Wireless and mobility: impact on higher layer protocols Packet loss in TCP

– Network congestion– Handoff– Wireless communication

Approaches– Local recovery– TCP sender awareness of wireless links– Split-connection approaches: two tranport-layer connections