Types of Wireless Links

littleparsimoniousMobile - Wireless

Nov 21, 2013 (3 years and 6 months ago)

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Computer Networks11/18/2009
CSC 257/457 -Fall 20091
Wireless
Wireless Communication LinksCommunication Links
Kai Shen
11/18/2009CSC 257/457 -Fall 20091
Types of Wireless Links

IEEE 802.11 wireless LAN (WiFi)
R t 100 t d t 54Mb

R
ange

up
t
o
100
me
t
ers;

spee
d
up
t
o
54Mb
ps

Intend to function as Ethernet without wires

Longer range (tens of kilometers) in point-to-point mode

IEEE 802.15 Bluetooth

Shorter range (~10 meters); slower speed (up to 3Mbps)

But less power; easier setup

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11/18/2009CSC 257/457 -Fall 20092

Appropriate for simple appliances: phone
,
game console

Cellular wireless access, IEEE 802.16 WiMAX

Longer distance, but typically lower rate than WiFi
Wireless Media Access Control

Collision if 2 or more nodes
transmit at same time
transmit at same time

Can we use CSMA/CD?

hidden terminal problem 
carrier sense and collision
detection don’t work

Repair collision detection:

add acknowled
g
ement
11/18/2009CSC 257/457 -Fall 20093
g

Repair carrier sense:

carrier sense with random
backoff

carrier probing
802.11 Media Access Control
802.11 sender

if sense channel idle for
if sense channel idle for
DIFSsec; then transmit
entire frame (no collision
detection)

if sense channel busy; then
wait for it become idle and
then backoff with random
delay (collision avoidance)
11/18/2009CSC 257/457 -Fall 20094
802.11 receiver

if received OK; return ACK
after SIFS
Computer Networks11/18/2009
CSC 257/457 -Fall 20092
Inter-Frame Spacing
Short inter
frame spacing:
Short inter
-
frame spacing:

give transmitter time to
switch to receive mode and
get ready to decode
Distributed inter-frame
spacing:

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⁷桹
11/18/2009CSC 257/457 -Fall 20095

DIFS
>
SIFS
,
why?
Better Collision Avoidance through
Carrier Probing

sender transmits short RTS
(request to send) packet
:

(request to send) packet

indicates transmission and its
duration

receiver replies with short CTS
(clear to send) packet

notifying (possibly hidden)
nodes

nodes hearing either RTS or
CTS will defer access
11/18/2009CSC 257/457 -Fall 20096
CTS will defer access

Collision of RTS/CTS?

less likely with short RTS/CTS
802.11 Media Access Control

CSMA/CA (Collision Avoidance)

Repair collision detection:

add acknowledgement

Repair insufficient carrier sense:

carrier sense with random backoff

carrier probing
I
f i
11/18/2009CSC 257/457 -Fall 20097

I
nter-
f
rame

spac
i
ng

prioritize different types of frames
802.11 Sub-standards

802.11b

Up to 11Mbps; frequency range 2.4-2.85 GHz

802.11a

Up to 54Mbps; frequency range 5.1-5.8 GHz –shorter
range or higher power consumption

802.11g

Up to 54Mbps; frequency range 2.4-2.85 GHz
11/18/2009CSC 257/457 -Fall 20098

802.11n

Even higher rate; still in the midst of standardization
Computer Networks11/18/2009
CSC 257/457 -Fall 20093
Power and Range

Power is of paramount importance in wireless devices

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Power usage is directly related to range

Friis transmission equation:

P
r
and
P
t
are received power and transmission power

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慮a
G
t
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2
4









R
GG
P
P
rt
t
r


11/18/2009CSC 257/457 -Fall 20099

G
r
and
G
t
are antenna gains at receiving and transmission
sides

R
is the range

is the signal wavelength
Quadratic relation between transmission power and range

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Friis transmission e
q
uation:
q

P
r
and
P
t
are received power and transmission power

is the signal wavelength

Wavelength relates directly to signal frequency
What relation?
2
4









R
GG
P
P
rt
t
r



11/18/2009CSC 257/457 -Fall 200910

What relation?

assume electro-magnetic signal at speed of light
2.4GHz 12.5cm; 5GHz 6cm

5GHz signals require more than four times transmission
power, or reaches less than half of the range
Wireless Channel

802.11 operates in specific frequency band

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2.412GHz (channel 1)

2.417GHz (channel 2)

… …

2.462GHz (channel 11)

Each channel operates at a width of 22MHz (11MHz on
each side of the center)
11/18/2009CSC 257/457 -Fall 200911
each side of the center)

How many interference-free channels we have?

Channel 1 interferes with channels 2/3/4/5, but not 6
http://en.wikipedia.org/wiki/File:2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.svg
Utilization of Multiple Channels

Utilization of multiple channels reduce interference
c

In an infrastructured wireless network

Multiple access points (at different channels) can be put
near each other without interference

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c
hannel 6
11/18/2009CSC 257/457 -Fall 200912

good coverage and strong signal strength
Computer Networks11/18/2009
CSC 257/457 -Fall 20094
A Case Study

Channel usage at 6
th
floor CSB:
th dtt AP (hl 1/6/11) th

th
ree
d
epar
t
men
t AP
s
(
c
h
anne
l
s
1/6/11)
;
th
ree

university APs (channels 1/6/11)

Outside this classroom (measurement by Amal):

CS, channel 1, power 0.000020uw

CS, channel 6, power 0.000016uw

CS, channel 11, power 1.3uw

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11/18/2009CSC 257/457 -Fall 200913

UR
,
channel 1
,
power 0
.
0040uw

UR, channel 6, power 0.000020uw

UR, channel 11, power 0.00032uw

Outside 703, strong power from two channel-6 APs
Channel Hopping in Ad Hoc Networks

Use multiple channels in a wireless ad hoc networks

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Nodes dynamically hop between multiple channels

Challenge: coordinate sender/receiver at same channel

[Bahl et al. 2004]
channel 6
11/18/2009CSC 257/457 -Fall 200914
[Bahl et al. 2004]

Each node maintains a channel hopping schedule; nodes
learn each other’s schedule

A sender adjust its channel to match receiver

Issues: What if the receiver wants to send to somebody
else and change its channel? How about broadcast?
Partially Overlapped Channels
[Mishra et al. 2006]

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Channels 1 and 6 are interference-free

Channels 1 and 4 have partial interference

I-factor: amount of overlap between two channels

Measurement: P
i/Pj
Given a signal sent at channel j, P
j
is the received power at
channel j, P
i
is the received at channel i.

I
-
晡捴cr

01 for 3
-
channel separation
11/18/2009CSC 257/457 -Fall 200915

I
factor 0
.
1 for 3
channel separation

Range:

range ratio = 0.32 for 3-channel separation [error in the paper?]
If same channel signal disappears into noise at 100M, then 32M
distance is enough for 3-channel separation
Power/Range-to-Antenna Gain

Friis transmission e
q
uation:
q

P
r
and
P
t
are received power and transmission power

G
r
and
G
t
are antenna gains at receiving and transmission
sides

Antenna gain [wikipedia] –the ratio between:
2
4









R
GG
P
P
rt
t
r


11/18/2009CSC 257/457 -Fall 200916

the radiation intensity of an antenna in a given direction

the intensity that would be produced by a hypothetical
ideal antenna that radiates equally in all directions

Point-to-point WiFi can go very long distance
Computer Networks11/18/2009
CSC 257/457 -Fall 20095
Long-Distance Point-to-Point WiFi

Particularly suited for under-developed, rural areas
Ll
t WiFi it

L
oca
l
-cos
t WiFi
equ
i
pmen
t

Use unlicensed frequency spectrum and require no base
stations (unlike cellular networks)

[Raman and Chebrolu 2005/2007]

Wireless mesh networks formed over long-distance WiFi
in rural India

Less inter-node inter
f
erence
11/18/2009CSC 257/457 -Fall 200917
f

Mixedsend/receive over multiple links
at a node causes unacceptable interfere

New wireless MAC protocol (2P):
coordination to prevent mixed
send/receive
Long-Distance Point-to-Point WiFi
[Patra et al 2007]
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瑩t/kld

L
ong
l
a
t
ency

over
l
ong
di
s
t
ance


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湯n
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Solution: bulk transmissions with a single acknowledgement

High loss rate due to external WiFi interference (long-
distance WiFi interfere with regular short-distance WiFi
near receiver)
11/18/2009CSC 257/457 -Fall 200918
near receiver)

Solution: repeated retransmission or redundancy coding
Disclaimer

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11/18/2009CSC 257/457 -Fall 200919