CS计算机代考程序代写 DHCP 11.Wireless

11.Wireless

COMP 3331/9331:
Computer Networks and

Applications
Week 9

Wireless Networks

Reading Guide: Chapter 7, Sections 7.1 – 7.3

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Wireless Networks
Background:
v # wireless (mobile) phone subscribers now exceeds #

wired phone subscribers (5-to-1)!
v # wireless Internet-connected devices equals #

wireline Internet-connected devices
§ laptops, Internet-enabled phones promise anytime untethered

Internet access
v two important (but different) challenges

§ wireless: communication over wireless link
§ mobility: handling the mobile user who changes point of

attachment to network

We will only focus on wireless challenges

Outline

7.1 Introduction

Wireless
7.2 Wireless links,

characteristics
7.3 IEEE 802.11 wireless

LANs (“Wi-Fi”)

5

Wireless 101

6

Elements of a wireless network

network
infrastructure

7

wireless hosts
v laptop, smartphone
v run applications
v may be stationary (non-

mobile) or mobile
§ wireless does not always

mean mobility

Elements of a wireless network

network
infrastructure

8

base station
v typically connected to wired

network
v relay – responsible for

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

802.11 access points

Elements of a wireless network

network
infrastructure

9

wireless link
v typically used to connect

mobile(s) to base station
v also used as backbone link
v multiple access protocol

coordinates link access
v various data rates,

transmission distance

Elements of a wireless network

network
infrastructure

10

Characteristics of selected wireless links

Indoor
10-30m

Outdoor
50-200m

Mid-range
outdoor

200m – 4 Km

Long-range
outdoor

5Km – 20 Km

.056

.384

1

4

5-11

54

2G: IS-95, CDMA, GSM

2.5G: UMTS/WCDMA, CDMA2000

802.15

802.11b

802.11a,g

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

4G: LTWE WIMAX

802.11a,g point-to-point

450 802.11n

D
at

a
ra

te
(

M
bp

s)

7-11

1300 802.11 ac

Wireless Networks 12

13

infrastructure mode
v base station connects

mobiles into wired network
v handoff: mobile changes

base station providing
connection into wired
network

Elements of a wireless network

network
infrastructure

14

ad hoc mode
v no base stations
v nodes can only

transmit to other
nodes within link
coverage

v nodes organize
themselves into a
network: route among
themselves

Elements of a wireless network

15

Wireless network taxonomy

single hop multiple hops

infrastructure
(e.g., APs)

no
infrastructure

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

larger Internet

no base station, no
connection to larger
Internet (Bluetooth,

ad hoc nets)

host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net

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

MANET, VANET

16

Outline

7.1 Introduction

Wireless
7.2 Wireless links,

characteristics
7.3 IEEE 802.11 wireless

LANs (“Wi-Fi”)

17

Wireless Link Characteristics (1)

important 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 at destination at slightly different
times

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

18

Path Loss/Path Attenuation

v Free Space Path Loss
d: distance

: wavelength
f: frequency
c: speed of light

v Reflection, Diffraction, Absorption
v Terrain contours (urban, rural, vegetation)
v Humidity

19

Multipath Effects

v Signals bounce off surface and interfere
(constructive or destructive) with one another

v Self-interference

20

Ideal Radios

21

Real Radios

22

Wireless Link Characteristics (2)
v SNR: signal-to-noise ratio

§ larger SNR – easier to
extract signal from noise (a
“good thing”)

v SNR versus BER tradeoffs
§ given physical layer: increase

power -> increase SNR-
>decrease BER

§ given SNR: choose physical layer
that meets BER requirement,
giving highest thruput

• SNR may change with
mobility: dynamically adapt
physical layer (modulation
technique, rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE
R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

23

SNR = signal to noise ratio

BER = Bit Error Rate

Wireless network characteristics
Multiple wireless senders and receivers create additional

problems (beyond multiple access):

A
B

C

Hidden terminal problem
v B, A hear each other
v B, C hear each other
v A, C can not hear each other

means A, C unaware of their
interference at B

v Carrier sense will be ineffective

A B C

A’s signal
strength

space

C’s signal
strength

Signal attenuation:
v B, A hear each other
v B, C hear each other
v A, C can not hear each other

interfering at B

24

v Exposed Terminals

v Node B sends a packet to A; C hears this and
decides not to send a packet to D (despite the fact
that this will not cause interference) !!

v Carrier sense would prevent a successful
transmission

25

Wireless network characteristics

Outline

7.1 Introduction

Wireless
7.2 Wireless links,

characteristics
7.3 IEEE 802.11 wireless

LANs (“Wi-Fi”)

26

IEEE 802.11 Wireless LAN
802.11b
v 2.4-5 GHz unlicensed spectrum
v up to 11 Mbps
v 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 antennae
§ 2.4-5 GHz range
§ up to 200 Mbps

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

27

802.11 LAN architecture
v wireless host communicates

with base station
§ base station = access point

(AP)

v Basic Service Set (BSS) (aka
“cell”) in infrastructure mode
contains:
§ wireless hosts
§ access point (AP): base station
§ ad hoc mode: hosts onlyBSS 1

BSS 2

Internet

hub, switch
or router

28

802.11: Channels, association

v 802.11b: 2.4GHz-2.485GHz spectrum divided into 11
channels at different frequencies
§ AP admin chooses frequency for AP
§ interference possible: channel can be same as that

chosen by neighboring AP!

v host: must associate with an AP
§ scans channels, listening for beacon frames containing

AP’s name (SSID) and MAC address
§ selects AP to associate with
§ may perform authentication [Chapter 8]
§ will typically run DHCP to get IP address in AP’s

subnet

29

30

802.11b channels

802.11: passive/active scanning

AP 2AP 1

H1

BBS 2BBS 1

1
2 3

1

passive scanning:
(1) beacon frames sent from APs
(2) association Request frame sent: H1 to

selected AP
(3) association Response frame sent from

selected AP to H1

AP 2
AP 1

H1

BBS 2BBS 1

1
22

3 4

active scanning:
(1) Probe Request frame broadcast

from H1
(2) Probe Response frames sent

from APs
(3) Association Request frame sent:

H1 to selected AP
(4) Association Response frame sent

from selected AP to H1

31

IEEE 802.11: multiple access
v avoid collisions: 2+ nodes transmitting at same time
v 802.11: CSMA – sense before transmitting

§ don’t collide with ongoing transmission by other node
v 802.11: no collision detection!

§ difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)

§ can’t sense all collisions in any case: hidden terminal, fading
§ goal: avoid collisions: CSMA/C(ollision)A(voidance)

space

A
B

C
A B C

A’s signal
strength

C’s signal
strength

32

Multiple access: Key Points
v No concept of a global collision

§ Different receivers hear different signals
§ Different senders reach different receivers

v Collisions are at receiver, not sender
§ Only care if receiver can hear the sender clearly
§ It does not matter if sender can hear someone else
§ As long as that signal does not interfere with receiver

v Goal of protocol
§ Detect if receiver can hear sender
§ Tell senders who might interfere with receiver to shut up

33

IEEE 802.11 MAC Protocol: CSMA/CA
Distributed Coordination Function (DCF)
802.11 sender
1 if sense channel idle for DIFS then

transmit entire frame (no CD)
2 if sense channel busy then

start random backoff time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff interval,

repeat 2
802.11 receiver
– if frame received OK

return ACK after SIFS (ACK needed due to
hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

34

DIFS = DCF Inter Frame space
SIFS = Short Inter Frame Space

Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random

access of data frames: avoid collisions of long data frames
v sender first transmits small request-to-send (RTS) packets

to BS using CSMA
§ RTSs may still collide with each other (but they’re short)

v BS broadcasts clear-to-send CTS in response to RTS
v CTS heard by all nodes

§ sender transmits data frame
§ other stations defer transmissions

v RTS and CTS contain the duration for transmitting the
subsequent data frame

avoid data frame collisions completely
using small reservation packets!

35

Collision Avoidance: RTS-CTS exchange

AP
A B

time

RTS(A)
RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

36

Collision Avoidance: RTS-CTS exchange

AP
A B

time

CTS(A)

DATA (A)

37

RTS (A)

ACK(A)

RTS (A)

DATA (A)

Quiz

v The following is the correct sequence of message
exchanges as per the reservation process of
802.11 CSMA/CA

A. RTS->CTS->DATA->CTS

B. CTS->RTS->DATA->ACK

C. RTS->CTS->DATA->ACK

D. RTS->ACK->DATA->CTS
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Answer: C

Quiz

v Which multiple access technique is used by IEEE
802.11?

A. CSMA/CD

B. Slotted ALOHA

C. CSMA/CA

D. TDMA

E. FDMA

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www.zeetings.com/salil

Answer: C

Summary

Wireless
v wireless links:

§ capacity, distance
§ channel impairments

v IEEE 802.11 (“Wi-Fi”)
§ CSMA/CA reflects wireless

channel characteristics

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