Transcript Chapter 6 slides, Computer Networking, 3rd edition

Elements of a wireless network
network
infrastructure
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary
(non-mobile) or mobile

wireless does not
always mean mobility
6-1
Elements of a wireless network
network
infrastructure
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
6-2
Elements of a wireless network
network
infrastructure
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
6-3
Elements of a wireless network
network
infrastructure
infrastructure mode
 base station connects
mobiles into wired
network
 handoff: mobile
changes base station
providing connection
into wired network
6-4
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
6-5
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
6-6
Wireless Link Characteristics (1)
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”
6-7
Wireless Link Characteristics (2)
 SNR: signal-to-noise ratio
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
10-1

• SNR may change with
mobility: dynamically adapt
physical layer (modulation
technique, rate)
10-2
BER
10-3
10-4
10-5
10-6
10-7
10
20
30
40
SNR(dB)
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
6-8
Cellular networks: the first hop
Two techniques for sharing
mobile-to-BS radio
spectrum
 combined FDMA/TDMA:
divide spectrum in
frequency channels, divide
each channel into time
slots
frequency
bands
 CDMA: code division
multiple access
time slots
6-9
6-10
Cellular standards: brief survey
2G systems: voice channels
 IS-136 TDMA: combined FDMA/TDMA (north
america)
 GSM (global system for mobile communications):
combined FDMA/TDMA

most widely deployed
 IS-95 CDMA: code division multiple access
GSM
Don’t drown in a bowl
of alphabet soup: use this
for reference only
6-11
6-12
Cellular standards: brief survey
2.5 G systems: voice and data channels
 for those who can’t wait for 3G service: 2G extensions
 general packet radio service (GPRS)
 evolved from GSM
 data sent on multiple channels (if available)
 enhanced data rates for global evolution (EDGE)
 also evolved from GSM, using enhanced modulation
 data rates up to 384K
 CDMA-2000 (phase 1)
 data rates up to 144K
 evolved from IS-95
6-13
Cellular standards: brief survey
3G systems: voice/data
 Universal Mobile Telecommunications Service (UMTS)
 data service: High Speed Uplink/Downlink packet
Access (HSDPA/HSUPA): 3 Mbps
 CDMA-2000: CDMA in TDMA slots
 data service: 1xEvlution Data Optimized (1xEVDO)
up to 14 Mbps
….. more (and more interesting) cellular topics due to mobility (stay
tuned for details)
6-14
6-15
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX)
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
.384
UMTS/WCDMA, CDMA2000
.056
3G
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
6-16
Cellular Concept
 Given a propagation environment, increasing
transmitted power will increase the service
coverage area.

The coverage area can be controlled by using a proper
transmitted power level.
 In cellular systems, the total service area is
divided into a number of smaller areas, each of
which is a radio cell.

Advantages:
• Low transmitted power
• Frequency reuse possible.
 Regular polygons may be used to represent the cell
coverage.
6-17
Cellular Concept
 Hexagonal cells are popular because
 closest to a circle
 tight cellular packing
 perfect partitioning of the service area.
 Frequency reuse is limited by co-channel
interference. Cells which use the same frequency
channels are called co-channel cells.
 Frequency is reused from cell cluster to cell
cluster. No frequency channel is reused among
cells in the same cell cluster.

Cells in each cell cluster use unique frequency channels.
6-18
Multiple Access Techniques
 Radio cell: a geographical coverage area in
which the services of mobile stations
(MSs) are supported by a single base
station (BS)
 Forward link (downlink): BS → multiples
MSs (one to many broadcasting)
 Reverse link (uplink): MSs →BS (many to
one multiple access)
6-19
Multiple Access Techniques
 Multiple MSs want to access the common BS
simultaneously
 If two or more user signals arrive at the BS at
the same time, there will be interferences,
unless the signals are orthogonal
 How can we achieve the orthogonality?
6-20
FDMA
 The total bandwidth is divided into
nonoverlapping frequency bands (channels)
 Each user occupies a channel for the
duration of the connection
 waste
of resources
 Narrowband transmission
 Forward and reverse links use FDD
6-21
TDMA
 Time is partitioned into frames
 Each frame consists of Nslot data slots plus a header
and a trailer
 Each slot is for transmission of one information unit
 A user continues to use the same slot in every frame
during call connection

waste of resources
 TDMA systems require strict time synchronization.
6-22
TDMA
6-23
TDMA
 W-TDMA: Each user occupies the total
frequency bandwidth during its slots
 N-TDMA: The total frequency spectrum is
divided into frequency subbands (channels);
within each frequency channel, TDMA is
used. −→Both time and frequency are
partitioned.
6-24
Code Division Multiple Access (CDMA)
 used in several wireless broadcast channels





(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”)
6-25
CDMA
6-26
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
6-27
CDMA: two-sender interference
6-28