Chapter 6
Wireless and Mobile
Networks
Modified by John Copeland,
Georgia Tech,
for use in ECE3600
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Computer Networking:
A Top Down Approach
Featuring the Internet,
6th edition.
Jim Kurose, Keith Ross
Addison-Wesley
Thanks and enjoy! JFK/KWR
All material copyright 1996-2009
J.F Kurose and K.W. Ross, All Rights Reserved
11/8/2014
6: Wireless and Mobile Networks
6-1
Chapter 6: Wireless and Mobile Networks
Background:
r # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers! (2008)
r computer nets: laptops, palmtops, PDAs,
Internet-enabled phone promise anytime
untethered Internet access
r two important (but different) challenges
m
m
communication over wireless link
handling mobile user who changes point of
attachment to network
6: Wireless and Mobile Networks
6-2
Chapter 6 outline
6.1 Introduction
Wireless
r 6.2 Wireless links,
characteristics
m
CDMA (Carrier Detect)
r 6.3 IEEE 802.11
wireless LANs (“wifi”)
r 6.4 Cellular Internet
Access
m
m
architecture
standards (e.g., GSM)
Mobility
r 6.5 Principles:
addressing and routing
to mobile users
r 6.6 Mobile IP
r 6.7 Handling mobility in
cellular networks
r 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-3
Elements of a wireless network
network
infrastructure
wireless hosts
r laptop, PDA, IP phone
r run applications
r may be stationary
(non-mobile) or mobile
m
wireless does not
always mean mobility
6: Wireless and Mobile Networks
6-4
Elements of a wireless network
network
infrastructure
base station
r typically connected to
wired network
r relay - responsible
for sending packets
between wired
network and wireless
host(s) in its “area”
m e.g., cell towers
802.11 access
points
6: Wireless and Mobile Networks
6-5
Elements of a wireless network
network
infrastructure
wireless link
r typically used to
connect mobile(s) to
base station
r also used as backbone
link
r multiple access
protocol coordinates
link access
r various data rates,
transmission distance
6: Wireless and Mobile Networks
6-6
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
802.11n
54
802.11a,g
5-11
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX (4G?))
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
Bluetooth
.384
UMTS/WCDMA, CDMA2000
.056
3G
2G
IS-95, CDMA, GSM
NFR
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
6: Wireless and Mobile Networks
6-7
Wireless Access Point (WAP) and Directional Antenna
("Cantenna") to extend range in one direction (4x).
6: Wireless and Mobile Networks
6-8
Elements of a cellular wireless network
infrastructure mode
r base station (or Wireless
Access Point, WAP)
r
network
infrastructure
connects mobiles into
wired network
handoff: mobile changes
base station providing
connection into wired
network
6: Wireless and Mobile Networks
6-9
Cellular Network
Frequency Reuse
1
15
8
Cell Tower
Freq. B
Freq. A
1
15
8
Freq. C
7 Types of Cells,
21 Sets of Frequencies
(3 for each cell type)
http://upload.wikimedia.org/wikipedia/en/5/57/CellTowersAtCorners.gif
Elements of a wireless network
Ad hoc mode
r no base stations
r nodes can only
transmit to other
nodes within link
coverage
r nodes organize
themselves into a
network: route among
themselves
6: Wireless and Mobile Networks
6-11
Wireless Link Characteristics
Differences from wired link ….
m
m
m
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”
6: Wireless and Mobile Networks
6-12
Wireless Link Characteristics (2)
r
SNR: signal-to-noise ratio
m larger SNR – easier to extract
signal from noise (a “good thing”)
SNR versus BER tradeoffs
m given physical layer: increase
power -> increase SNR->decrease
BER
m given SNR: choose physical layer
that meets BER requirement,
giving highest thruput
• SNR may change with mobility:
dynamically adapt physical layer
(modulation technique, rate)
10-1
10-2
Unusable
10-3
BER
r
Weak <- SIGNAL -> Strong
10-4
10-5
10-6
10-7
10
20
30
40
SNR(dB)
BER = bit error rate
QAM256 (8 Mbps) (1M symbols/s)
10-4 means 1 error per 10,000 bits
QAM16 (4 Mbps)
BPSK (1 Mbps)
6: Wireless and Mobile Networks
6-13
Wireless network characteristics
Multiple wireless senders and receivers create
additional problems (beyond multiple access):
C
A
B
Hidden terminal problem
B, A hear each other
r B, C hear each other
r A, C can not hear each other
means A, C unaware of their
interference at B
r
B
A
C
C’s signal
strength
A’s signal
strength
space
Signal fading:
r
r
r
B, A hear each other
B, C hear each other
A, C can not hear each other
interfering at B
6: Wireless and Mobile Networks
6-14
Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into
“pieces”
r pieces allocated to
calls
r resource piece idle if
not used by owning
call (no sharing)
 dividing link bandwidth into
“pieces”
 frequency division (FDM)
 time division (TDM)
 code division (CDM)
FDM
4 users
frequency
TDM
time
frequency
time
6: Wireless and Mobile Networks
6-15
Code Division Multiple Access (CDMA)
r used in several wireless broadcast channels
r
r
r
r
r
(cellular, satellite, etc) standards
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
encoded signal = (original data) X (chipping
sequence)
decoding: inner-product of encoded signal and
chipping sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes
are “orthogonal”)
6: Wireless and Mobile Networks
6-16
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
6: Wireless and Mobile Networks
6-17
CDMA: two-sender interference
Orthogonal Codes
A
B
1
1
1
-1
1
-1
-1
-1
---0
AxB
1
-1
1
1
1
-1
1
1
---0
1
-1
1
-1
1
1
-1
-1
---0
Sum over i
Ai = 0
Bi = 0
A i x Bi = 0
Ai x(-Bi) = 0
6: Wireless and Mobile Networks
6-18
Chapter 6 outline
6.1 Introduction
Wireless
r 6.2 Wireless links,
characteristics
m
CDMA
r 6.3 IEEE 802.11
wireless LANs (“WiFi”)
r 6.4 Cellular Internet
Access
m
m
architecture
standards (e.g., GSM)
Mobility
r 6.5 Principles:
addressing and routing
to mobile users
r 6.6 Mobile IP
r 6.7 Handling mobility in
cellular networks
r 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-19
IEEE 802.11 Wireless LAN (WiFi)
r 802.11a
r 802.11b
m 5-6 GHz range
m 2.4-5 GHz unlicensed spectrum
m up to 54 Mbps
m up to 11 Mbps
r 802.11g
m direct sequence spread
spectrum (DSSS) in physical
m 2.4-5 GHz range
layer
m up to 54 Mbps
• all hosts use same chipping
r 802.11n: multiple antennae
code
m 2.4-5 GHz range
m up to 200 Mbps
r
r
all use CSMA/CA for multiple access
all have base-station and ad-hoc network versions
6: Wireless and Mobile Networks
6-20
802.11 LAN architecture
r
Internet
r
AP
hub, switch
or router
BSS 1
AP
BSS 2
wireless host communicates
with base station
m base station = access
point (AP)
Basic Service Set (BSS)
(aka “cell”) in
infrastructure mode
contains:
m wireless hosts
m access point (AP or
WAP): base station
m ad hoc mode: hosts only
6: Wireless and Mobile Networks
6-21
802.11: Channels, association
r
802.11b: 2.4GHz - 2.485GHz spectrum divided into 11
channels at different frequencies (only 1, 6, 11 don't
overlap)
m AP admin chooses frequency for AP
m interference possible: channel can be same as that
chosen by neighboring AP!
r host: must associate with an AP
m scans channels, listening for beacon frames containing
AP’s name (SSID) and MAC address
m selects AP to associate with
m may perform authentication [Chapter 8]
m will typically run DHCP to get IP address in AP’s subnet
6: Wireless and Mobile Networks
6-22
IEEE 802.11: multiple access
r avoid collisions: 2+ nodes transmitting at same time
r 802.11: CSMA - sense before transmitting
m don’t collide with ongoing transmission by other node
r 802.11: no collision detection!
m difficult to receive (sense collisions) when transmitting due
to weak received signals
m can’t sense all collisions in any case: hidden terminal, fading
m goal: avoid collisions: CSMA/C(ollision)A(voidance)
C
A
B
A
B
C
C’s signal
strength
A’s signal
strength
space
6: Wireless and Mobile Networks
6-23
IEEE 802.11 MAC Protocol: CSMA/CA
(without CA - no collision avoidance)
802.11 sender
1 if sense channel idle for DIFS then
sender
transmit entire frame (no CD)
2 if sense channel busy then
start random back-off time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff
interval, repeat 2
receiver
DIFS
data
SIFS
ACK
802.11 receiver
- if frame received OK
return ACK after SIFS (ACK needed due
to hidden terminal problem)
6: Wireless and Mobile Networks
6-24
Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random
r
r
r
access of data frames: avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets
to BS (WAP) using CSMA
m 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 within range of the receiver.
m sender transmits data frame
m other stations defer transmissions
BS-base station, WAP-wireless access point,
CSMA-carrier sense multiple access
Avoid data frame collisions completely*
using small reservation packets!
(*for static wireless hosts)
6: Wireless and Mobile Networks
6-25
Collision Avoidance: RTS-CTS exchange
A
B
AP
reservation collision
DATA (A)
defer
time
6: Wireless and Mobile Networks
6-26
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
2
Sequence No.: needed for
ARQ (ACK required) mode.
6
4
0 - 2312
seq address
4
control
payload
CRC
Address 4: MAC address
of wireless relay host (ad hoc
networks only)
Address 3: MAC address of router
interface to which AP is attached [Serves
as E'net destination address]
Address 2: MAC address
of wireless host or AP
transmitting this frame
[Serves as E'net source address]
6: Wireless and Mobile Networks
6-27
802.11 frame: addressing
Ethernet
R1 router
H1
Internet
AP
R1 MAC addr H1 MAC addr
dest. address
source address
802.3 (Eth) frame
AP MAC addr H1 MAC addr R1 MAC addr
address 1
1st dest (AP)
address 2
source
address 3
2nd dest (eth)
802.11 (WiFi) frame
6: Wireless and Mobile Networks
6-28
802.11 frame: more
frame seq #
(for reliable ARQ)
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)
6: Wireless and Mobile Networks
6-29
802.11: mobility within same subnet
r H1 remains in same IP
subnet: IP address
can remain same
r 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
m [What about a Hub?]
m
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
6: Wireless and Mobile Networks
6-30
802.11: advanced capabilities
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
operating point
10-1
10-2
10-3
BER
Rate Adaptation
r base station, mobile
dynamically change
transmission rate
(physical layer
modulation technique)
as mobile moves, SNR
varies
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
6: Wireless and Mobile Networks
6-31
802.15: personal area network
(like BlueTooth)
r less than 10 m diameter
r replacement for cables
(mouse, keyboard,
headphones)
r ad hoc: no infrastructure
r master/slaves:
m
m
slaves request permission to
send (to master)
master grants requests
r 802.15: evolved from
Bluetooth specification
m
m
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)
6: Wireless and Mobile Networks
6-32
802.16: WiMAX
r like 802.11 & cellular:
point-to-point
base station model
m
m
transmissions to/from base
station by hosts with omnidirectional antenna
base station-to-base
station backhaul with pointto-point antenna
point-to-multipoint
r unlike 802.11:
m range ~ 6 miles (“city
rather than coffee shop”)
m ~14 Mbps
6: Wireless and Mobile Networks
6-33
6: Wireless and Mobile Networks
6-34
ISS
iStumbler (Mac)
6: Wireless and Mobile Networks
6-35
Chapter 6 outline
6.1 Introduction
Wireless
r 6.2 Wireless links,
characteristics
m
CDMA
r 6.3 IEEE 802.11
wireless LANs (“wifi”)
r 6.4 Cellular Internet
Access
m
m
architecture
standards (e.g., GSM)
Mobility
r 6.5 Principles:
addressing and routing
to mobile users
r 6.6 Mobile IP
r 6.7 Handling mobility in
cellular networks
r 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-36
Components of cellular network architecture
MSC
cell
 connects cells to wide area net
 manages call setup (more later!)
 handles mobility (more later!)
 covers geographical
region
 base station (BS)
analogous to 802.11 AP
 mobile users attach
to network through BS
 air-interface:
physical and link layer
protocol between
mobile and BS
Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network
6: Wireless and Mobile Networks
6-37
Cellular networks: the first hop
Two techniques for sharing
mobile-to-BS radio
spectrum
r combined FDMA/TDMA:
divide spectrum in
frequency channels, divide
each channel into time
slots
frequency
bands
r CDMA: code division
multiple access
time slots
6: Wireless and Mobile Networks
6-38
Cellular standards: Generations
1G systems: Cellular, FM, voice, 1 mobile per frequency
2G systems: Digital, voice, many mobiles per freq.
3G systems: Digital, voice & data
Universal Mobile Telecommunications Service (UMTS)
m GSM next step, but using CDMA
CDMA-2000: CDMA in TDMA slots
(HSPA) 28 Mbps (AT&T)
m
1xEvolution Data Optimized
(1xEVDO) 14 Mbps (Verizon)
4G systems: Higher speed, like 802.17 WiMAX
As of August 2011, U.S. carriers currently use the following (most
will go to LTE):
m
AT&T = HSPA+
(63 Mbps) (2011 -> LTE)
m
Sprint = WiMax
(47 Mbps) (2013 -> LTE)
m
Verizon = LTE
(93 Mbps)
m
T-Mobile = HSPA+ (75 Mbps) (2013 -> LTE)
m
LTE
– 300 Mbps max. (700 MHz band, OFDMA)
LTE Advanced – 1000 Mbps max. (8x8 MIMO and 128 QAM)
6: Wireless and Mobile Networks
6-39
Cell Tower Coverage
30 mi
16 mi
9 mi
8 mi
The 450 MHz band was made available by the switch to digital
(HiDef) TV and the release of higher frequency TV channels for other
services (first responders, universal Internet, ...).
Frequency bands in the 700 MHz range were auctioned by the FCC in
2012, and there are plans to buy back TV channels in the 600 MHz
band.
The gain of a half-wave antenna varies as 1/f2. Lower frequencies
6: Wireless and Mobile Networks
also penetrate walls better.
6-40
Wireless, mobility: impact on higher layer protocols
r logically, impact should be minimal …
best effort service model remains unchanged
m TCP and UDP can (and do) run over wireless, mobile
r … but performance-wise:
m packet loss/delay due to bit-errors (discarded
packets, delays for link-layer retransmissions), and
handoff
m TCP interprets loss as congestion, will decrease
congestion window un-necessarily
m delay impairments for real-time traffic
m limited bandwidth (bits/second) of wireless links
m
6: Wireless and Mobile Networks
6-41