ECE 4450:427/527 - Computer NetworksSpring 2015

Dr. Nghi TranDepartment of Electrical & Computer Engineering

Lecture 5.6: Wireless Networks - MAC

Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 1

Wireless Mobile Networks

Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 2

• Brief Introduction

• Wireless Channel Characteristics

• WiFi: CSMA/CA - Collision Avoidance

Elements of Wireless Networks

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network infrastructure

wireless hosts laptop, PDA, IP phone run applications may be stationary (non-

mobile) or mobile wireless does not always

mean mobility

Elements of Wireless Mobile Networks

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network infrastructure

base station typically connected to

wired network relay - responsible for

sending packets between wired network and wireless host(s) in its “area” e.g., cell towers, 802.11

Access Points (AP) Handoff: Mobile changes

station

Characteristics of selected wireless link standards

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Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m – 4 Km

Long-rangeoutdoor

5Km – 20 Km

.056

.384

1

4

5-11

54

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 3G

802.15

802.11b

802.11a,g

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellularenhanced

802.16 (WiMAX)

802.11a,g point-to-point

200 802.11n

Dat

a ra

te (

Mbp

s)

data

Wireless Mobile Networks

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ad hoc mode no base stations nodes can only transmit

to other nodes within link coverage

nodes organize themselves into a network: route among themselves

Wireless Network Taxonomy

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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

Wireless Channel Characteristics

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Differences from wired link ….

– decreased signal strength: radio signal attenuates as it propagates through matter (path loss)

– interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well

– multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

Multipath propagation

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• Constructive and destructive interference: channel strengths change randomly with time → Fading

• When channel is weak, i.e., bad quality→Low reliability

Hidden Terminal Problem

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Multiple wireless senders and receivers create additional problems (beyond multiple access):

AB

C

Hidden terminal problem: Signals blocked B, A hear each other B, C hear each other A, C can not hear each othermeans A, C unaware of their interference at B

Hidden Terminal Problem

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A B C

A’s signalstrength

space

C’s signalstrength

Hidden terminal problem due to signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other interfering at B

Exposed Node Problem

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• Suppose B is sending to A. Node C is aware of this communication because it hears B’s transmission.

• It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission.

• Suppose C wants to transmit to node D. This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B.

IEEE 802.11 Wireless LAN

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• 802.11b– 2.4-5 GHz unlicensed spectrum– up to 11 Mbps– direct sequence spread spectrum

(DSSS) in physical layer

• all hosts use same chipping code

• 802.11a – 5-6 GHz range– up to 54 Mbps

• 802.11g – 2.4-5 GHz range– up to 54 Mbps

• 802.11n: multiple antenna– 2.4-5 GHz range– up to 600 Mbps

all use CSMA/CA for multiple access all have base-station and ad-hoc network versions

80211. Wireless LAN Architecture

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wireless host communicates with base station base station = access point

(AP) Basic Service Set (BSS) (aka

“cell”) in infrastructure mode contains: wireless hosts access point (AP): base

station ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP

IEEE 802.11 Multiple Access

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• avoid collisions: 2+ nodes transmitting at same time

• 802.11: CSMA - sense before transmitting– don’t collide with ongoing transmission by other node

• 802.11: no collision detection as in Ethernet!– difficult to receive (sense collisions) and transmitting at the same time

due to weak received signals: swamps the receiving circuitry– can’t sense all collisions in any case: hidden terminal, fading– goal: avoid collisions: CSMA/C(ollision)A(voidance)

IEEE 802.11 MAC Protocol: CSMA/CA

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sender receiver

DIFS

data

SIFS

ACK

802.11 sender1 if sense channel idle for DIFS then transmit entire

frame (no CD) – cannot sense when transmitting2 if sense channel busy then

- start random backoff time- timer counts down while channel idle. If

busy, timer frozen- transmit when timer expires. Wait for ACK- ACK: New frame, go to step 1; if no ACK,

increase random backoff interval, repeat 2

802.11 receiver- if frame received OK return ACK after SIFS

To deal with transmitting and sensing problems

IEEE 802.11 MAC Protocol: CSMA/CA

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idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of data frames

• sender first transmits small request-to-send (RTS) packets to BS using CSMA– RTSs may still collide with each other (but they’re short)

• BS broadcasts clear-to-send (CTS) in response to RTS• CTS heard by all nodes

– sender transmits data frame– other stations defer transmissions

avoid data frame collisions completely using small reservation packets!

To deal with hidden terminal problem: Using RTS and CTS

IEEE 802.11 MAC Protocol: CSMA/CA

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APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

IEEE 802.11 MAC Protocol: CSMA/CA

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Applets to check out:

• http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withhidden.html

• http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withouthidden.html