Computer Networks
Wireless Communication and Wi-Fi
Networks
Lin Gu
lingu@acm.org
Computer Networking: A Top Down Approach 6th edition. Jim Kurose and
Keith Ross, Addison-Wesley.
Part of the slides are adapted from course companion materials.
6-1
Ch. 6: Wireless and Mobile Networks
Background:

# wireless (mobile) phone subscribers now exceeds #
wired phone subscribers (5-to-1)!
 As of Sept., 2009, there are 720 million out of the 1 billion
phone service subscriptions are mobile phone subscriptions
 726 million mobile phone end-of-year 2009
(http://labs.chinamobile.com/news/29571 )

# wireless Internet-connected devices equals #
wireline Internet-connected devices
 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 Mobile Networks
Wireless,
6-2
Future Applications
6: Wireless and Mobile Networks
6-3
6: Wireless and Mobile Networks
6-4
Chapter 6 outline
6.1 Introduction
Mobility
Wireless
6.5 Principles: addressing and
routing to mobile users
6.6 Mobile IP
6.7 Handling mobility in
cellular networks
6.8 Mobility and higher-layer
protocols
6.2 Wireless links,
characteristics
 CDMA
6.3 IEEE 802.11 wireless
LANs (“Wi-Fi”)
6.4 Cellular Internet Access
 architecture
 standards (e.g., GSM)
6.9 Summary
Wireless, Mobile Networks
6-5
Elements of a wireless network
network
infrastructure
Wireless, Mobile Networks
6-6
Elements of a wireless network
wireless hosts



network
infrastructure
laptop, smartphone
run applications
may be stationary (nonmobile) or mobile
 wireless does not always
mean mobility
Wireless, Mobile Networks
6-7
Elements of a wireless network
base station


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, Mobile Networks
6-8
Elements of a wireless network
wireless link


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, Mobile Networks
6-9
Elements of a wireless network
infrastructure mode


network
infrastructure
base station connects
mobiles into wired
network
handoff: mobile changes
base station providing
connection into wired
network
Wireless, Mobile Networks 6-10
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, Mobile Networks 6-11
Characteristics of selected wireless links
Data rate (Mbps)
200
54
5-11
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, Mobile Networks 6-12
Wireless network taxonomy
single hop
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, Mobile Networks 6-13
Chapter 6 outline
6.1 Introduction
Mobility
Wireless
6.5 Principles: addressing and
routing to mobile users
6.6 Mobile IP
6.7 Handling mobility in
cellular networks
6.8 Mobility and higher-layer
protocols
6.2 Wireless links,
characteristics
 CDMA
6.3 IEEE 802.11 wireless
LANs (“Wi-Fi”)
6.4 Cellular Internet Access
 architecture
 standards (e.g., GSM)
6.9 Summary
Wireless, Mobile Networks 6-14
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 ad destination at slightly different
times
…. make communication across (even a point to point)
wireless link much more “difficult”
Wireless, Mobile Networks 6-15
Wireless Link Characteristics (2)

SNR: signal-to-noise ratio
10-1
 larger SNR – easier to
extract signal from noise (a
“good thing”)
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)
10-3
BER

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, Mobile Networks 6-16
Wireless network characteristics
Multiple wireless senders and receivers create additional
problems (beyond multiple access):
B
A
C
C
B
C’s signal
strength
A’s signal
strength
A
Hidden terminal problem



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
space
Signal attenuation:



B, A hear each other
B, C hear each other
A, C can not hear each other
interfering at B
Wireless, Mobile Networks 6-17
Code Division Multiple Access (CDMA)

unique “code” assigned to each user; i.e., code set
partitioning
 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, Mobile Networks 6-18
CDMA encode/decode
sender
d0 = 1
data
bits
code
Zi,m= di.cm
-1 -1 -1
1
-1
1 1 1
-1 -1 -1
slot 1
-1
slot 1
channel
output
1
-1
1 1 1 1 1 1
1
d1 = -1
1 1 1
channel output Zi,m
-1 -1 -1
slot 0
1
-1
-1 -1 -1
slot 0
channel
output
M
Di = S Zi,m.cm
m=1
received
input
code
receiver
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
slot 1
M
1
1
-1
-1 -1 -1
slot 0
d0 = 1
d1 = -1
slot 1
channel
output
slot 0
channel
output
Wireless, Mobile Networks 6-19
CDMA: two-sender interference
Sender 1
channel sums together
transmissions by sender 1
and 2
Sender 2
using same code as
sender 1, receiver recovers
sender 1’s original data
from summed channel
data!
Wireless, Mobile Networks 6-20
CDMA’s Evolution
1xEV Phase 1
(1xEV-DO)
cdma2000
1X
IS-95-A
• Voice
• 14.4 kbps
• High Capacity Voice
• 153 kbps Packet
• RF Backward Comp.
• 2.4 Mbps Packet
• RF Backward Comp.
？
1xEV Phase 2
(1xEV-DV)
• Higher Cap Voice/ Data
• RF Backward Comp.
cdma2000
3X MC
• High Capacity Voice
• 384+ kbps Packet
• RF Backward Comp.
1995
1999
2000
2001
2002
2003+
6: Wireless and Mobile Networks
6-21
Chapter 6 outline
6.1 Introduction
Mobility
Wireless
6.5 Principles: addressing and
routing to mobile users
6.6 Mobile IP
6.7 Handling mobility in
cellular networks
6.8 Mobility and higher-layer
protocols
6.2 Wireless links,
characteristics
 CDMA
6.3 IEEE 802.11 wireless
LANs (“Wi-Fi”)
6.4 Cellular Internet Access
 architecture
 standards (e.g., GSM)
6.9 Summary
Wireless, Mobile Networks 6-22
IEEE 802.11 Wireless LAN
802.11a
802.11b
 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
 up to 54 Mbps
 all hosts use same chipping
802.11n: multiple antennae
code
 2.4-5 GHz range
 up to 200 Mbps


all use CSMA/CA for multiple access
all have base-station and ad-hoc network versions
Wireless, Mobile Networks 6-23
802.11 LAN architecture

Internet
wireless host
communicates with base
station
 base station = access point
(AP)
hub, switch
or router

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
Wireless, Mobile Networks 6-24
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!

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
Wireless, Mobile Networks 6-25
802.11: passive/active scanning
BBS 1
BBS 1
BBS 2
BBS 2
1
AP 1
1
1
2
AP 2
AP 1
2
3
2
3
AP 2
4
H1
H1
passive scanning:
active 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
(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, Mobile Networks 6-26
Association and Authentication
 The association process is a good opportunity
for exercising authentication rules
 802.11 provide a number of alternatives for
authentication
MAC based authentication
 Username/password

 Authentication may involves a specialized
authentication server
One authentication server may work for a number of
APs
 Separating authentication from APs keeps the AP
cost and complexity low

6: Wireless and Mobile Networks
6-27
IEEE 802.11: multiple access


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!
 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)
B
A
C
C
A
B
C’s signal
strength
A’s signal
strength
space
Wireless, Mobile Networks 6-28
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for DIFS then
sender
transmit entire frame (no CD)
2 if sense channel busy then
DIFS
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 (ACK needed due to
hidden terminal problem)
Wireless, Mobile Networks 6-29
Why not CSMA/CD?
802.11 and Ethernet
CD is impossible: Low receive signal
sender
receiver
strength, Collision occurs at the receiver’s
location
DIFS
Transmission is usually in its
entirety: No collision detection means no
“stop sign” for transmission
Nuances in the 802.11 and Ethernet
design
Transmit right after busy period ends?
- SIFS and DIFS useful for Ethernet?
- When to backoff in 802.11?
-
data
SIFS
ACK
6: Wireless and Mobile Networks
6-30
Avoiding collisions (more)
idea: 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
 other stations defer transmissions
avoid data frame collisions completely
using small reservation packets!
Wireless, Mobile Networks 6-31
Collision Avoidance: RTS-CTS exchange
A
B
AP
reservation collision
DATA (A)
defer
time
Wireless, Mobile Networks 6-32
Realistic Radio Model
Question: Would RTS/CTS be effective in realistic settings?
 Assumptions in RTS/CTS
 Realities in radio communication



Interference range
Asymmetric wireless links
Capture effect
No model is correct
but some are useful!
6: Wireless and Mobile Networks
6-33
802.11 Point-to-Point Link
Question: How far can Wi-Fi signals reach?
 Depends on transmission power, antennae, and environmental
conditions
 In omnidirectional radio propagation, the electromagnetic
energy disseminates in all directions
 Directional antennae focus the energy toward one direction
 Extends communication range to tens of kilometers
 Similar use of directional antennae is seen in microwave
radio relay, satellite communication, and wireless energy
transmission
6: Wireless and Mobile Networks
6-34
802.11 frame: addressing
2
2
6
6
6
frame
address address address
duration
control
1
2
3
Address 1: MAC address
of wireless host or AP
to receive this frame
Address 2: MAC address
of wireless host or AP
transmitting this frame
2
6
seq address
4
control
0 - 2312
4
payload
CRC
Address 4: used only in
ad hoc mode
Address 3: MAC address
of router interface to
which AP is attached
Wireless, Mobile Networks 6-35
802.11 frame: addressing
Internet
R1 router
H1
R1 MAC addr H1 MAC addr
dest. address
source address
802.3 frame
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.11 frame
Wireless, Mobile Networks 6-36
802.11 frame: more
frame seq #
(for RDT)
duration of reserved
transmission time (RTS/CTS)
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
Protocol
version
2
4
1
Type
Subtype
To
AP
6
2
1
seq address
4
control
1
From More
AP
frag
1
Retry
1
0 - 2312
4
payload
CRC
1
Power More
mgt
data
1
1
WEP
Rsvd
frame type
(RTS, CTS, ACK, data)
Wireless, Mobile Networks 6-37
802.11: mobility within same subnet
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
BBS 1
H1
BBS 2
Wireless, Mobile Networks 6-38
802.11: advanced capabilities
Rate adaptation
base station, mobile
dynamically change
transmission rate
(physical layer modulation
technique) as mobile
moves, SNR varies
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
operating point
10-2
10-3
BER

10-1
10-4
10-5
10-6
10-7
10
20
30
SNR(dB)
40
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
Wireless, Mobile Networks 6-39
802.11: advanced capabilities
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 APto-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
Wireless, Mobile Networks 6-40
802.15: personal area network




less than 10 m 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
 up to 721 kbps
P
S
P
radius of
coverage
M
S
P
S
P
M Master device
S Slave device
P Parked device (inactive)
Wireless, Mobile Networks 6-41
Appendix
Wireless, Mobile Networks 6-42
802.16: WiMAX
 like 802.11 & cellular:
point-to-point
base station model
transmissions to/from
base station by hosts
with omnidirectional
antenna
 base station-to-base
station backhaul with
point-to-point antenna

point-to-multipoint
 unlike 802.11:
 range ~ 6 miles (“city
rather than coffee
shop”)
 ~14 Mbps
6: Wireless and Mobile Networks
6-43
802.16 Applications
Businesses
RESIDENTIAL & SoHo DSL
Transportation
BACKHAUL
ALWAYS BEST CONNECTED
802.16
Multi-Point
BACKHAUL
802.11
802.11
802.11
Copyright Worldwide Interoperability for
Microwave Access Forum
802.16: WiMAX: downlink, uplink scheduling
 transmission frame
down-link subframe: base station to node
 uplink subframe: node to base station

pream.
…
DL- ULMAP MAP
DL
burst 1
DL
burst 2
…
…
DL
burst n
Initial request
SS #1 SS #2
maint. conn.
downlink subframe
SS #k
…
uplink subframe
base station tells nodes who will get to receive (DL map)
and who will get to send (UL map), and when
 Initial maintenance
 Ranging (SS requests power and timing measurement and adjustment)
 Collisions may occur in this interval
 WiMAX standard provide mechanism for scheduling, but not scheduling
algorithm
6-45