Chapter 7 v7 0

©1996-2016, J.F Kurose and K.W. Ross

Chapter 7
Wireless and
Mobile Networks

Computer
Networking: A Top
Down Approach
7th Edition, Global Edition
Jim Kurose, Keith Ross
Pearson
April 2016
Wireless and Mobile Networks 7-1


Ch. 7: Wireless and Mobile Networks
Background:
§ # wireless (mobile) phone subscribers now exceeds #
wired phone subscribers (5-to-1)!
§ # wireless Internet-connected devices equals #
wireline Internet-connected devices

â1996-2016, J.F Kurose and K.W. Ross

ã laptops, Internet-enabled phones promise anytime untethered
Internet access

§ two important (but different) challenges
• wireless: communication over wireless link
• mobility: handling the mobile user who changes point of

attachment to network

Wireless and Mobile Networks 7-2


Chapter 7 outline
7.1 Introduction

Mobility

Wireless

7.5 Principles: addressing and
routing to mobile users
7.6 Mobile IP
7.7 Handling mobility in
cellular networks
7.8 Mobility and higher-layer
protocols

7.2 Wireless links,
characteristics

â1996-2016, J.F Kurose and K.W. Ross

ã CDMA

7.3 IEEE 802.11 wireless
LANs (“Wi-Fi”)
7.4 Cellular Internet Access

• architecture
• standards (e.g., 3G, LTE)

Wireless and Mobile Networks 7-3


Elements of a wireless network

©1996-2016, J.F Kurose and K.W. Ross

network
infrastructure

Wireless and Mobile Networks 7-4


Elements of a wireless network
wireless hosts

©1996-2016, J.F Kurose and K.W. Ross

network
infrastructure

§ laptop, smartphone
§ run applications
§ may be stationary (nonmobile) or mobile
• wireless does not always
mean mobility

Wireless and Mobile Networks 7-5


Elements of a wireless network
base station

©1996-2016, J.F Kurose and K.W. Ross

network
infrastructure

§ 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

Wireless and Mobile Networks 7-6


Elements of a wireless network
wireless link

©1996-2016, J.F Kurose and K.W. Ross

network

infrastructure

§ 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

Wireless and Mobile Networks 7-7


Characteristics of selected wireless links
1300

Data rate (Mbps)

©1996-2016, J.F Kurose and K.W. Ross

450
54
5-11

802.11ac
802.11n
802.11a,g
802.11b

4

1

802.11a,g point-to-point
4G: LTWE, WIMAX
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO

802.15

.384

2.5G: UMTS/WCDMA, CDMA2000

.056

2G: IS-95, CDMA, GSM

Indoor

Outdoor

10-30m

50-200m

Mid-range
outdoor

Long-range
outdoor

200m – 4 Km

5Km – 20 Km
Wireless and Mobile Networks 7-8


Elements of a wireless network
infrastructure mode

©1996-2016, J.F Kurose and K.W. Ross

network
infrastructure

§ base station connects
mobiles into wired
network
§ handoff: mobile changes
base station providing
connection into wired
network

Wireless and Mobile Networks 7-9


©1996-2016, J.F Kurose and K.W. Ross

Elements of a wireless network
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 and Mobile Networks 7-10


Wireless network taxonomy
single hop

©1996-2016, J.F Kurose and K.W. Ross

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)

multiple hops
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

Wireless and Mobile Networks 7-11


Chapter 7 outline
7.1 Introduction

Mobility

Wireless

7.5 Principles: addressing and
routing to mobile users
7.6 Mobile IP

7.7 Handling mobility in
cellular networks
7.8 Mobility and higher-layer
protocols

7.2 Wireless links,
characteristics

â1996-2016, J.F Kurose and K.W. Ross

ã CDMA

7.3 IEEE 802.11 wireless
LANs (“Wi-Fi”)
7.4 Cellular Internet Access
• architecture
• standards (e.g., 3G, LTE)

Wireless and Mobile Networks 7-12


Wireless Link Characteristics (1)

©1996-2016, J.F Kurose and K.W. Ross

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 ad destination at slightly different
times
…. make communication across (even a point to point)
wireless link much more “difficult”
Wireless and Mobile Networks 7-13


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

10-1

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

©1996-2016, J.F Kurose and K.W. Ross

• 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)

10-3

BER

§ BER: bit error rate
§ SNR versus BER tradeoffs

10-2

10-4
10-5
10-6
10-7

10

20

30

40

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

Wireless and Mobile Networks 7-14

Wireless network characteristics
Multiple wireless senders and receivers create additional
problems (beyond multiple access):
B

A

C

C

â1996-2016, J.F Kurose and K.W. Ross

A

B

Hidden terminal problem
Đ B, A hear each other
§ B, C hear each other
§ A, C can not hear each other
means A, C unaware of their
interference at B

C’s signal
strength

A’s signal

strength

space

Signal attenuation:
§ B, A hear each other
§ B, C hear each other
§ A, C can not hear each other
interfering at B
Wireless and Mobile Networks 7-15


Code Division Multiple Access (CDMA)
§ unique “code” assigned to each user; i.e., code set
partitioning

â1996-2016, J.F Kurose and K.W. Ross

ã all users share same frequency, but each user has own
“chipping” sequence (i.e., code) to encode data
• allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes are
“orthogonal”)

§ encoded signal = (original data) X (chipping
sequence)
§ decoding: inner-product of encoded signal and
chipping sequence

Wireless and Mobile Networks 7-16

CDMA encode/decode
sender

data
bits
code

Zi,m= di.cm

d0 = 1

-1 -1 -1

1
-1

11 1
-1 -1 -1

slot 1

-1

slot 1
channel
output

1

-1

11 1 11 1

1

d1 = -1
11 1

channel output Zi,m

-1 -1 -1

slot 0

1
-1

-1 -1 -1

slot 0
channel
output

©1996-2016, J.F Kurose and K.W. Ross

M

Di = S Zi,m.cm
m=1

received
input
code

receiver

11 1 11 1

1
-1 -1 -1

-1

11 1

1
-1

-1 -1 -1

-1
11 1
-1 -1 -1

slot 1

M

1

1
-1

-1 -1 -1

slot 0

d0 = 1
d1 = -1

slot 1
channel
output

slot 0
channel
output

Wireless and Mobile Networks 7-17


CDMA: two-sender interference
Sender 1

channel sums together
transmissions by sender 1
and 2

©1996-2016, J.F Kurose and K.W. Ross

Sender 2

using same code as
sender 1, receiver recovers
sender 1’s original data
from summed channel
data!
Wireless and Mobile Networks 7-18


Chapter 7 outline
7.1 Introduction

Mobility

Wireless

7.5 Principles: addressing and
routing to mobile users
7.6 Mobile IP
7.7 Handling mobility in
cellular networks
7.8 Mobility and higher-layer
protocols

7.2 Wireless links,
characteristics

â1996-2016, J.F Kurose and K.W. Ross

ã CDMA

7.3 IEEE 802.11 wireless
LANs (“Wi-Fi”)
7.4 Cellular Internet Access
• architecture
• standards (e.g., 3G, LTE)

Wireless and Mobile Networks 7-19


IEEE 802.11 Wireless LAN

â1996-2016, J.F Kurose and K.W. Ross

802.11a
802.11b
Đ 5-6 GHz range
§ 2.4-5 GHz unlicensed spectrum
§ up to 54 Mbps
§ up to 11 Mbps
802.11g
§ direct sequence spread spectrum
§ 2.4-5 GHz range
(DSSS) in physical layer
Đ up to 54 Mbps
ã all hosts use same chipping code
802.11n: multiple antennae
802.11ac

Đ 2.4-5 GHz range
ã 5GHz spectrum
Đ up to 200 Mbps
ã Beamforming
Đ all use CSMA/CA for multiple access
§ all have base-station and ad-hoc network versions
Wireless and Mobile Networks 7-20


802.11 LAN architecture
Internet

©1996-2016, J.F Kurose and K.W. Ross

hub, switch
or router

BSS 1

Đ wireless host
communicates with base
station
ã base station = access
point (AP)
Đ Basic Service Set (BSS) (aka
“cell”) in infrastructure
mode contains:
• wireless hosts
• access point (AP): base
station

• ad hoc mode: hosts only

BSS 2
Wireless and Mobile Networks 7-21


802.11: Channels, association
§ 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!
©1996-2016, J.F Kurose and K.W. Ross

Đ host: must associate with an AP
ã 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
Wireless and Mobile Networks 7-22


802.11: passive/active scanning
BBS 1

AP 1

BBS 1

BBS 2

1

1
2

BBS 2

1

AP 2

AP 1

2

3

©1996-2016, J.F Kurose and K.W. Ross

(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
4

H1

H1

passive scanning:

2
3

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
Wireless and Mobile Networks 7-23


IEEE 802.11: multiple access
§ avoid collisions: 2+ nodes transmitting at same time
Đ 802.11: CSMA - sense before transmitting
ã dont collide with ongoing transmission by other node

Đ 802.11: no collision detection!

â1996-2016, J.F Kurose and K.W. Ross

• 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)

C
A

B

A

B

C
C’s signal
strength

A’s signal
strength

space
Wireless and Mobile Networks 7-24


IEEE 802.11 MAC Protocol: CSMA/CA

©1996-2016, J.F Kurose and K.W. Ross

802.11 sender
1) if sense channel idle for DIFS (Distributed

sender
coordination function - DCF - Interframe
Space, e.g., 36µs/802.11ac) then
transmit entire frame (no CD)
DIFS
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

receiver

data

SIFS

ACK

return ACK after SIFS (Short Interframe Space,
e.g., 16µs/802.11ac)
(ACK needed due to hidden terminal problem)
Wireless and Mobile Networks 7-25