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Wireless Networks CSE 3461: Introduction to Computer Networking Reading: 6.1–6.3, Kurose and Ross (6th ed.); 7.1–7.3, Kurose and Ross (7th ed.) 1

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Wireless Networks

CSE 3461: Introduction to Computer NetworkingReading: ��6.1–6.3, Kurose and Ross (≤ 6th ed.);

��7.1–7.3, Kurose and Ross (7th ed.)

1

Questions

• How do you use wireless network technology in daily life?

• How many of you have smartphones? Laptops?– Smartphones: iOS or Android?– Laptops: MS Windows, macOS, or Linux?

• Have you have set up a wireless LAN? What was the experience like?

2

Wireless Networks

Background:• Number of wireless (mobile) phone subscribers now exceeds

number of wired phone subscribers– 4.3 billion mobile broadband subscriptions worldwide (ITU, 2017) [1]– 334.6 million smartphones sold in Q1 2016 alone [2]

• Number of wireless Internet-connected devices equals number of wireline Internet-connected devices– Laptops, smartphones promise anytime untethered Internet access

• Key idea: Wireless: communication over wireless link

3

[1] ITU, https://www.itu.int/en/mediacentre/Pages/2017-PR37.aspx[2] D. Van Boom, “It’s not just Apple: Global smartphone market shrinks forthe first time ever,” 27 Apr. 2016, CNET / CBS Interactive, https://cnet.co/2mldOa3

Outline

• Introduction • Wireless links, characteristics– CDMA

• IEEE 802.11 wireless LANs (“Wi-Fi”)• Tools of the Trade

4

Elements of a Wireless Network (1)

5

Network Infrastructure

Elements of a Wireless Network (2)

6

Wireless Hosts• Laptop, smartphone• Run applications• May be stationary (non-

mobile) or mobile– Wireless does not always

mean mobility

Network Infrastructure

Elements of a Wireless Network (3)

7

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

Network Infrastructure

Elements of a Wireless Network (4)

8

Wireless link• Typically used to connect

mobile(s) to base station• Also used as backbone

link • Multiple access protocol

coordinates link access • Various data rates,

transmission distance

Network Infrastructure

Elements of a Wireless Network (5)

9

Infrastructure mode• Base station connects

mobiles into wired network

• Handoff: mobile changes base station providing connection into wired network

Network Infrastructure

Elements of a Wireless Network (6)

10

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

Characteristics of Selected Wireless Links

11

Indoor10–30 m

Outdoor50–200 m

Mid-rangeoutdoor

200 m – 4 km

Long-rangeoutdoor

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, CDMA2000-1xEV-DO

4G: LTE /WiMAX

802.11a,g point-to-point

450 802.11n

Dat

a ra

te (M

bps)

1300 802.11ac

Wireless Network Taxonomy

12

Single Hop Multiple Hops

Infrastructure (e.g., APs)

Host connects to base station (WiFi, WiMAX, cellular), which connects to larger Internet

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

No Infrastructure No base station, no connection to larger Internet (Bluetooth, ad hoc nets)

No base station, no connection to larger Internet. May have to relay to reach another given wireless node (MANET, VANET)

Outline

• Introduction • Wireless links, characteristics– CDMA

• IEEE 802.11 wireless LANs (“Wi-Fi”)• Tools of the Trade

13

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., phones); devices (motors, microwaves, etc.) 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”

14

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

– Larger SNR ⟹ easier to extract signal from noise (“good thing”)

• BER: bit error rate• SNR versus BER tradeoffs:

– Given physical layer: increase power ⟹ increase SNR ⟹decrease BER

– Given SNR: choose physical layer that meets BER requirement, giving highest throughput• SNR may change with

mobility: dynamically adapt physical layer (modulation technique, data rate)

• Details: – QAM: quadrature amplitude

modulation– BPSK: binary phase shift keying 15

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create additional

problems (beyond multiple access):

16

AB

C

Hidden terminal problem• B, A hear each other• B, C hear each other• A, C cannot hear each other

means A, C unaware of their interference at B

A B C

A’s signalstrength

Space

C’s signalstrength

Signal attenuation:• B, A hear each other• B, C hear each other• A, C cannot hear each other

interfering at B

Sign

al S

treng

th

Code Division Multiple Access (CDMA)

• Unique “pseudo-noise code” (PN code) assigned to each user; i.e., code set partitioning– All users share same frequency, but each user has

own “chip” sequence (i.e., PN code) to encode data– Allows multiple users to “coexist” and transmit

simultaneously with minimal interference (if codes are “orthogonal”)

• Encoded signal = (original data) � (chip sequence)

• Decoding: inner product of encoded signal and chip sequence

17

CDMA Encoding/ Decoding

18

Slot 1 Slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di . cm

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

Slot 0channeloutput

Slot 1channeloutput

Channel output Zi,m

SenderCode

Databits

Slot 1 Slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

Slot 0channeloutput

Slot 1channeloutputReceiver

Code

Receivedinput

CDMA: Two-Sender Interference

19

Using same code as sender 1, receiver recovers sender 1’s original data from summed channel data!

Sender 1

Sender 2

Channel sums together transmissions by sender 1 and 2

Outline

• Introduction • Wireless links, characteristics– CDMA

• IEEE 802.11 wireless LANs (“Wi-Fi”)• Tools of the Trade

20

IEEE 802.11 Wireless LAN (WLAN)802.11a

• Freq.: 5–6 GHz• Data rate: up to 54 Mbps

802.11b• 2.4, 5 GHz unlicensed spectrum• Data rate: up to 11 Mbps• Direct sequence spread spectrum

(DSSS) in physical layer– All hosts use same chip code

802.11g• Freq.: 2.4, 5 GHz• Data rate: up to 54 Mbps

802.11n: multiple antennas• 2.4, 5 GHz range• Data rate: up to 450 Mbps

802.11ac: higher data rate (> 400 Mbps), 5-GHz only802.11ad: ≥ 1 Gbps data rate, 60-GHz band, range: 10s of cm

21

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

802.11 WLAN Architecture

• Wireless hosts communicate 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

22

802.11: Channels and Association• 802.11: 2.4–2.485 GHz 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!– Numerous channels in 5-GHz frequency band

• 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 – Typically runs DHCP to get IP address in AP’s subnet

23

802.11: Passive/Active Scanning

24

AP 2AP 1

H1

BSS 2BSS 1

12 3

1

Passive scanning:1. Beacon frames sent from APs2. Association Request frame sent: H1 to

selected AP 3. Association Response frame sent from

selected AP to H1

AP 2AP 1

H1

BBS 2BSS 1

122

3 4

Active scanning: 1. Probe Request frame broadcast from

H12. Probe Response frames sent from

APs3. Association Request frame sent: H1

to selected AP 4. Association Response frame sent

from selected AP to H1

IEEE 802.11: Multiple Access• Avoid collisions: 2 or more 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!– 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/CA (Collision Avoidance)

25

Space

AB

CA B C

A’s signalstrength

C’s signalstrength

Sign

al S

treng

th

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender1. if sense channel idle for DIFS* then

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

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, then increase random backoff

interval; repeat 2

802.11 receiver1. if frame received OK:returnACK after SIFS* (ACK needed due to hidden terminal problem)

26

sender receiver

DIFS

data

SIFS

ACK

* DIFS: DCF Inter-Frame Space; SIFS: Short Inter-Frame space; DCF: Distributed Coordination Function

802.11: Collision AvoidanceIdea: allow sender to �reserve� channel rather than random

access of data frames: avoid collisions of long 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– Sther stations defer transmissions

27

Avoid data frame collisions completely using small reservation packets!

802.11: Collision Avoidance: RTS-CTS Exchange

28

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

802.11 Frame: Addressing (1)

29

802.11 Frame: Addressing (2)

30

802.11 Frame: More Details

31

• H1 remains in same IP subnet: IP address can remain same

• Switch: which AP is associated with H1?– Self-learning (Ch. 5):

switch will see frame from H1 and “remember” which switch port can be used to reach H1

32

802.11: Mobility in the Same Subnet

H1 BSS 2BSS 1

Rate Adaptation• Base station, mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

33

802.11: Advanced Capabilities (1)

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1. SNR decreases, BER increase as node moves away from base station

2. When BER becomes too high, switch to lower transmission rate but with lower BER

34

Power Management• Node-to-AP: �I am going to sleep until next beacon

frame�– AP knows not to transmit frames to this node– Node wakes up before next beacon frame

• Beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent– Node will stay awake if AP-to-mobile frames to be sent;

otherwise sleep again until next beacon frame

802.11: Advanced Capabilities (2)

• 10-meter diameter• Replacement for cables (mouse,

keyboard, headphones)• Ad hoc: no infrastructure• Master/slaves:

– Slaves request permission to send (to master)

– Master grants requests

• 802.15: evolved from Bluetooth specification– 2.4–2.5 GHz radio band– Data rate: up to 721 kbps

35

M radius ofcoverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)P

802.15: Personal Area Network

Outline

• Introduction • Wireless links, characteristics– CDMA

• IEEE 802.11 wireless LANs (“Wi-Fi”)• Tools of the Trade

36

Tools of the Trade (Linux) (1)

• iw: Scan for WLANs and configure them (https://wireless.wiki.kernel.org/en/users/documentation/iw )

37

Examples run onDebian Linux 9(amd64 machinearchitecture).

Tools of the Trade (Linux) (2)

• iw (cont’d): Wi-Fi scan at OSU (Au2017):– 13 osuwireless SSIDs (plus many more, e.g., WiFi@OSU)– Frequencies: 2.412, 2.462 GHz; 5.220, 5.3 GHz;

5.52, 5.54, 5.56, 5.6, 5.62, 5.66, 5.7, 5.785, 5.8 GHz– Signal strengths: –84 dBm to –53 dBm

(0 dBm corresponds to 1 mW; –3 dBm corresponds to 0.5 mW,and so on)

38

Tools of the Trade (Linux) (3)

• hcitool: Scan for nearby Bluetooth devices – Both “Classic” Bluetooth and Bluetooth Low Energy– Part of Linux BlueZ suite (http://www.bluez.org)

39

Tools of the Trade: Hardware• RTL-SDR (https://www.rtl-sdr.com/): $25 receiver

(24 MHz – 1.7 GHz) that supports software-defined radio (SDR)

40

GQRX: Free SDR program(http://www.gqrx.dk)

Summary

• Introduction • Wireless links, characteristics– CDMA

• IEEE 802.11 wireless LANs (�Wi-Fi�)• Tools of the Trade

41