Transcript Ad Hoc Networks: Overview

Characteristics of
Wireless Environment
1
Radio Propagation Mechanism
2
Characteristics of Wireless Channel
Path loss
Pr/Pt = O(d-γ), where d: distance
• γ: 2 (free space), 5 (strong attenuation)
Fading: fluctuation of signal strength
Fast fading: due to multipath propagation
Slow fading: occurs when objects absorb the
transmission
May reduced by diversity or adaptive modulation
Interference
Adjacent channel interference guard band
Co-channel interference  cellular, directional
antenna, dynamic channel allocation
Inter-symbol interference  adaptive equalization
Doppler shift
3
Multiple Access Techniques
 FDMA
 OFDM
 TDMA
 Hard to compute good schedules in a distributed fashion.
 Schedule needs to be traffic dependent.
 Need synchronized clocks in hardware to implement slots
 CDMA
 FHSS
 DSSS
 SDMA
 Duplexing
 FDD
 TDD
4
CDMA DSSS
 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”)
5
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
6
d0 = 1
d1 = -1
slot 1
channel
output
slot 0
channel
output
CDMA: two-sender interference
7
Wireless Networks
8
Wireless MAC
 MAC (Medium Access Control)
 Sharing a Single Broadcast Medium among Multiple Users
 Contention : Most Widely Used, Suffer from Collision
 Non-Contention : Reservation/Round-Robin, Collision Free
 Wireless MAC vs. Ad Hoc MAC ?
 Ad Hoc Network: Multi-Hop Wireless Network
9
Ranges and Zones
Transmission range
Frame can be
successfully received
Carrier-sensing
zone (C-Zone)
Signal can be detected,
but not decoded.
Interfering range
Receiving node can be
interfered by another
transmission
 collision
10
ALOHA and CSMA
 ALOHA - University of Hawaii (1970)
 Transmit whenever it has data to send.
 Listen to the acknowledgement feedback from the receiver.
 If a collision occurs (no ACK), retransmits after a random delay.
 Utilization: pure Aloha = 18.5%, slotted Aloha = 37%
 CSMA - Kleinrock (1975)
 Listen (Carrier Sense) before transmission
 1. If channel is idle, transmit
 2. Otherwise, do one of the followings:
• Wait until channel become idle and transmit  1 Persistent-CSMA
• Wait until idle and transmit with probability p  p Persistent-CSMA
• Defer transmission and try again after a random delay  NP-CSMA
 Carrier sense not foolproof
• Propagation delay (also a problem in wireline).
• Can sense only at transmitter; but collision happens at receiver (a
wireless problem).
11
CSMA/CA (Collision Avoidance)
 Contention Window
Frame
Contention
Window
A
Remaining Backoff
Frame B
Data
Arrival
C
Frame
D
Frame
DIFS
DIFS
Backoff = Uniform[0, CW]
DIFS
 How about CSMA/CD (Collision Detection) ?
 Need the ability to Listen while transmitting to detect collision
 The strength of its own transmission would mask all other
signals on the air
12
Hidden and Exposed Terminal Problem
A
B
A transmits to B
Time
Packet
Transmission
From A to B
Collision
C
A
B
B transmits to A
C wants to transmit to B.
It does not hear A’s
transmission, accesses
the channel and collides
C
D
C wants to transmit to D. It
hears B’s transmission, and
unnecessarily defers, although it
could transmit in parallel as A
can’t hear C’s transmission
Time
Packet
Packet
Transmission Transmission
From B to A From B to A
Packet
Transmission
From C to B
Waste
Resource
C and A are Hidden Terminals relative
to each other – one can’t sense the other’s
transmission
Packet can be
transmitted
from C to D,
But don’t it.
C is an Exposed Terminal relative to B.
B’s transmission inhibits C, although there
would be no collision at the receiver (D). If C
were to transmit.
13
MACA: Multiple Access Collision Avoidance
 Proposed by Phil Karn (1990) as an alternative to
the CSMA
 Inspired by the CSMA/CA method
 Extend and Enhance the CA part of the CSMA/CA – Every one
overhearing CTS knows just how long to wait to avoid collision.
 Get rid of the CS in CSMA/CA and become MACA.
 Lack of carrier doesn’t always mean it’s OK to transmit
 Presence of carrier doesn’t always mean it’s bad to transmit
 It’s too hard to build a good DCD (Data Carrier Detect) circuit
 MACA uses signaling packets for CA
 RTS/CTS
 Contain: sender address, receiver address, packet size
 If a packet transmitted is lost, use BEB algorithm
 Variants of this method are used in IEEE 802.11
14
MACA example
 MACA avoids HTP
R
S1
RTS
S2
R1
RTS
CTS
Data
 MACA avoids ETP
S1
RTS
CTS
S2
RTS
Overhearing RTS
CTS
Data
Data
R2
Data
RTS
No CTS
RTS
CTS
Vulnerable period is
known to C by CTS
MACA could greatly relieve both problems,
but not completely solve them.
15
Data
Data
Wireless Network Technology
16
Wireless Networks
 Wireless Networks
 Infrastructured Network
• Cellular Network (3GPP or 3GPP2)
• Wireless LAN (IEEE 802.11)
 Infrastructureless Network
• Ad Hoc Network
Internet
WLAN
Cellular
[Mobile/Wireless] Ad Hoc Networks
17
Wireless Sensor Networks
18
Wireless Sensor Networks
 A sort of ad-hoc networks
Ad Hoc
 A network of low cost,
densely deployed,
Wireless
untethered sensor nodes
Sensor
 Application areas:
Network
heath, military, and home
 Placed in inaccessible terrains or disaster areas
Net
 It may be impossible to recharge batteries
 Different Node Characteristics from Traditional nodes
 # of nodes in a sensor network can be several orders of magnitude
higher than the nodes in an Ad Hoc network (100s to 1000s nodes)
 Densely deployed (20 nodes/m3)
 Prone to failures
 Topology changes very frequently
 Mainly use a broadcast communication, whereas most Ad Hoc
networks are based on point-to-point
 Limited in power, computing capacities, and memory
 May not have global ID because of the large amount of overhead and
large number of sensors
19
Existing Wireless Net vs. Sensor Net
Cellular system
Bluetooth, MANET
Sensor Network
Single Hop
Multi-hop
Multi-hop
High QOS
Bandwidth
efficiency
High QOS
Power conservation
Limited bandwidth
Large number of
node
Narrow radio range
Frequent topology
change
Station to Base
station
Peer to peer
Peer to multi node
20
Peer to multi node
Sensor Network Model
Sink
Stimulus
Sink
Source
21
Wireless Sensor Networks
 Sensor node
 Made up of four basic
components
• Sensing unit, Processing
unit, Transceiver unit, and
Power unit
 Additional applicationdependent components
• Location finding system,
power generator, and
mobilizer
 Scattered in a sensor field
 Collect data and route data
back to the sink
 Sink
 Communicate with the task
manager node (user) via
Internet or satellite
22
General Layered Architecture
 Sensing – detect events
 Processing – perform local data processing
 Communication – transmit/receive the data
23
Berkeley Mote (MICA II)
 2002 UC Berkeley
 2 AA batteries





4Mhz, 8bit MCU (ATMEL)
4KB SRAM, 128KB Flash
512KB EEPROM storage
916Mhz Radio, 100 ft, 10/40Kb/s
25 A inactive, 20 mA peak, 2xAA
Alk batt. (2850 mAh)
Many more sensors, faster
communication, but size still
limited by power
requirements
24
Wireless Sensor Network Protocols
Goal: building long-lived, massivelydistributed, physically-coupled systems
User queries, External database
In-network processing: Data
aggregation, query processing, …
Congestion avoidance
Data dissemination, storage, caching
Topology management, localization
MAC, time, power mgnt
modulation, sensing, MUXing
25
Energy Management
Tx power mgmt
MAC: sleep mode
Routing: consider battery life time: load balancing
Transport: reduce ReTx
App
Battery energy mgmt
Extend battery life by taking adv of chemical
properties, discharge patterns, and by the selection of
a battery from a set of batteries
Processor power mgmt
Device power mgmt
26
Energy Limitations
Sensor node has limited energy supply,
and may not be rechargeable
Sensor node expends maximum energy in
communication
20
Power
(mW)
15
10
5
0
Sensing
CPU
TX
RX
IDLE
SLEEP
Power consumption of a typical senor node
27
Military
28
Emergency Service
29