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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www.zeetings.com/salil
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
39
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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