Transcript Document 7333748

ADHOC NETWORKS
Evolution :
Computer Networks
Wireless Mobile Networks
Adhoc Networks
Sensor Networks
Networks provide users access to information &
communication
Computer Networks: Interconnected collection of
autonomous Computers
1. Wired Network : Physically connected through
cables
2. Wireless Mobile Networks : Supported by a fixed
wired infrastructure:
A
single hop wireless radio communication to access a base
station that connects it to the wired infrastructure
3. Adhoc Networks : Does not use any fixed
infrastructure:
High mobility. Therefore Mobile Adhoc
Networks
(MANET)
Network Classification by Size
Classification of interconnected processors by scale.
Client-Server Model
The client-server model involves requests and replies.
Peer-to-Peer Applications
In a peer-to-peer system there are no fixed clients and servers.
Wireless LANs
(a) Wireless networking with a base station.
(b) Ad hoc networking.
What is an ad hoc network
• A collection of nodes that can communicate with each other
without the use of existing infrastructure
• Each node is a sender, a receiver, and a relay
• There are no “special nodes”
• No specialized routers
• Nodes can be static or mobile
• Can be thought of as: peer-to-peer communication
• Ad hoc networks are wireless networks
• Decentralized networks, in which each node acts as both an
endpoint and a router for other nodes
• increase the redundancy of the network and open up the
possibilities for network scaling as well
• self-organizing networks which automatically reconfigure
without human intervention in the event of degraded or
broken communication links between transceivers
• Also called Self Healing Networks
A node in an ad hoc network
Two main components of a node
Local Task
From neighbors
To neighbors
Routing
Routing methods
•Each node maintains a table of routing information.
•Table entry: destination X preferred neighbor.
•Data packet contains a destination ID in header.
• Packet received: forward packet to the preferred
neighbor. Use table entry for the destination.
Some features
•These networks may have bridges or gateways to other networks
such as wired Ethernet or 802.11
•the strength of their architecture is that they do not require a base
station or central point of control.
•Automated network analysis through link and route discovery and
evaluation are the distinguishing features of self-healing
network algorithms
•Through discovery, networks establish one or more routes between
the originator and the recipient of a message.
•Through evaluation, networks detect route failures, trigger renewed
discovery, and in some cases select the best route
available for a message.
Mode of Operation:
• Peer to peer multihop mobile wireless networks
• Information packets transmitted in a store and forward
manner from a source to an arbitrary destination via
intermediate nodes
• Topology information noted in each node (Mobile Host)
• All MHs need not be in the range of each other;
MHs move topology changes.
as
Symmetric & Asymmetric links:
If MH1 is within radio range of MH3, then MH3 is also within
the Radio range of MH1 : Communication links are
symmetric or bidirectional.
Asymmetric links are unidirectionaL
Mobile Adhoc Network (MANET)
An autonomous system of Mobile Hosts (MH: also serving
as routers) connected by wireless links, forming a
communication network.
Contrast: In Cellular networks, communication between
two mobile nodes completely rely on the wired backbone
and the fixed Base Stations
In a MANET no such infrastructure exists.
Network topology may dynamically change in an
unpredictable manner since the nodes are free to move
Example: Ad hoc network
Nodes have power range
Communication happens between nodes within range
What Is Different Here?
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Broadcasts of nodes can “overlap” -> collision
How do we handle this?
A MAC layer protocol could be the answer
If one node broadcasts, neighbors keep quiet
Thus, nearby nodes compete for air time
This is called contention
The Hidden Terminal Effect
• hidden terminals: A, C cannot hear each other
– obstacles, signal attenuation
– collisions at B
• goal: avoid collisions at B
• CSMA/CA: CSMA with Collision Avoidance
(a)The hidden station problem.
(b) The exposed station problem.
The Hidden Terminal Problem
• Wireless stations have transmission ranges and not
all stations are within radio range of each other.
• Simple CSMA will not work!
• C transmits to B.
• If A “senses” the channel, it will not hear C’s
transmission and falsely conclude that A can begin a
transmission to B.
The Exposed Station Problem
• This is the inverse problem.
• B wants to send to C and listens to the channel.
• When B hears A’s transmission, B falsely assumes
that it cannot send to C.
The 802.11 MAC protocol with CA
RTS
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RTS
A
B
CTS D
C
CTS
Introduced to reduce collisions
Sender sends Request To Send (RTS): ask permission
Case A: Receiver gives permission Clear To Send (CTS)
Sender sends Data
Receiver sends ACK, if received correctly
Case B: Receiver does not respond
– Sender waits, times out, exponential back-off, and tries
again
Why is this necessary?
RTS
A
C
B
CTS
D
• A sends RTS, and B replies with a CTS
• C hears RTS and avoids sending anything
– C could have been near B (not shown here)
• D hears CTS so it does not send anything to B
ROUTING ISSUES IN ADHOC NETWORKS::
• Nodes liable to move
• Highly dynamic network
• Rapid topological changes causing route failures
• Wireless channel acting as shared medium
• Available bandwidth per node is lower
• Nodes run on batteries which have limited energy supply
Hence routing to be bandwidth efficient, having low
overheads, energy efficient
Proactive Routing : Maintains routes between all pairs of nodes
regardless of whether all routes are actually used.
Two optimised variations of these protocols are
(1) Distance Vector and (2) Link State
In (1) a node exchanges with its neighbours a vector containing the
current distance information to all known destinations; the distance
information propagates across the network and routes are computed
in a distributed manner at each node.
In (2) each node disseminates the status of each of its outgoing
links throughout the network in the form of link state updates; each
node locally computes routes using the complete topology
information
On Demand (Reactive) Routing : Find and maintain only
needed routes
Attractive when traffic is sporadic, bursty and directed mostly
towards a small subset of nodes.
Queuing delays occur at the source as routes are created at
session initiation when need arises
(1)Dynamic Source routing: Sender knows the complete hopby-hop route to the destination. These routes are stored in
a route cache The data packet carries the source route in
the packet header.
Route Discovery: By flooding the network with Route Request (Query)
packets. Each node retransmits it unless it is the destination or it has a
route to the destination in its route cache. Such a node replies to the
request with a route reply packet routed back to the original source.
The route of the reply packet is cached at the source for future use.
Hybrid Approaches: Combination of Proactive and Reactive :
• Augments a reactive protocol with some proactive functionality
• Each active destination periodically refresh routes
• Zone Routing Protocol is the example
• Defines zones for each node X which includes all nodes that
are within a certain distance in hops, around the node X
• A proactive link state protocol is used to keep every node it
reactively initiates a zone aware of the complete topology
within its zone
• When X needs a route to Y not in its zone, it reactively
initiates a route discovery
Common feature for the 3 routing schemes :
Nodes exchange routing messages and use this information to guide
future routing decisions.
An entirely different routing scheme is Location based routing:
• Assumes each node knows its own location
• Nodes use GPS – GLOBAL POSITIONING SYSTEM
• Every node learns location of its immediate neighbours by
exchanging hello messages
• Location of potential destination nodes is assumed to be available
via this location service
• Source sending a packet to destination uses d’s location to find a
neighbour closest in geographic distance to d & forwards packet to
that neighbour. That neighbour repeats the process until reaching d
Flooding
Source Node simply broadcasts data to neighboring nodes
Each Node hearing the broadcast for the first time
rebroadcasts it
Broadcast propagates until every node has heard the packet
and transmitted it once
Delivers data to every node in the connected component of
the network
Suitable when node mobility is high; otherwise inefficient
A node may receive the same packet from several neighbours
Other flooding techniques also available
Wireless Routing Protocol (WRP)
An important proactive routing approach
Table driven protocol with the goal of maintaining routing
information among all nodes in the network
Each node responsible for maintaining four tables :
• Distance table
• Routing table
• Link-cost table
• Message Retransmission List (MRL) table
Mode of Operation:
• Peer to peer multihop mobile wireless networks
• Information packets transmitted in a store and forward
manner from a source to an arbitrary destination via
intermediate nodes
• Topology information noted in each node (Mobile Host)
• All MHs need not be in the range of each other;
MHs move topology changes.
as
Symmetric & Asymmetric links:
If MH1 is within radio range of MH3, then MH3 is also within
the Radio range of MH1 : Communication links are
symmetric or bidirectional.
Asymmetric links are unidirectionaL
SOME ROUTING ATTACKS
• Wormhole attacks : Two collaborating malicious nodes create a
tunnel to falsify the hop count metric.
• Rushing attack : targets routing protocols that choose routes on
what message arrives first. Malicious route message will be
rushed to block legitimate messages
• Sybil attacks : one malicious node takes up multiple identities to
project a false topology
Sensor networks
Low-bit-rate, low-grade data that is aggregated and distilled
into characterized by a large quantity of nodes per network,
where each node produces useful information.
Some possible applications:
• Traffic monitoring & Control
Sensors placed at strategic locations in road junctions assess
traffic intensity/congestion and convey information to
controlling officers so as to select alternative route for
incoming traffic
• Goods management applications:
commercial goods equipped with inexpensive wireless nodes
that communicate information about their state and place of
origin.
WIRELESS SENSOR NETWORKS
WSN is a special case of Adhoc Networks with reduced or no
mobility
Combine wireless communication & minimal computation
facilities with sensing of physical phenomenon which can be
easily embedded in our physical environment
A sensor node consists of a radio front end, a microcontroller,
power supply and the actual sensor all in a single device
A sensor consists of a transducer, an embedded processor for
local processing, small memory unit for storage of data and a
wireless transceiver to transmit or receive data, all these run
on the power supplied by the attached battery
Eg The Mica Mote
Some advantages of WSN:
Ease of deployment – can be put anywhere, anytime.
Extended range – One large wired-sensor can be replaced
by many smaller wireless sensors for the same cost
Fault tolerant – if one macro-sensor fails, monitoring of its
area is gone. Failure of one node in WSN does not affect
operation
Mobility – ease of redeployment
Some Challenges:
Limited energy supply, limited computing power, limited
bandwidth of the wireless connecting links
Energy management technique – an important issue
Some numbers for 802.11
• Typical radius of power-range: 250m
• Interference range: 500m
– At 500m one can not hear, but they are
bothered!
• RTS packet 40 bytes
• CTS and ACK 39 bytes
• MAC header is 47 bytes
Some issues for investigative research:
•Scalability
•Quality of Service
•Security
•Interoperation with the Internet
•Energy conservation
•Node cooperation
•etc
Performance Metrics
(General Definitions)
• Utilization :: the percentage of time a device is busy
servicing a “customer”.
• Throughput :: the number of jobs processed by the
“system” per unit time.
• Response time :: the time required to receive a response
to a request (round-trip time).
• Delay :: the time to traverse from one end to the other in
a system.
Network Performance Measures
• Latency:: usually implies the minimum possible delay.
Latency assumes no queuing and no contention
encountered along the path.
• Goodput:: {measured at the receiver} rate in bits per
second of useful traffic received. Goodput excludes
duplicate packets and packets dropped along the
path.
• Fairness:: either Jain’s fairness or max-min fairness
are used to measure fair treatment among competing
flows.
• Quality of Service:: a QoS measure accounts for
importance of specific metric to one type of
application. [e.g. jitter for streaming media]
Network Performance Measures
• Channel utilization :: the average fraction of time a
channel is busy [e.g. Util = 0.8]
– when overhead is taken into account (i.e., excluded
from useful bits, channel utilization is often referred to
as channel efficiency
• Throughput :: bits/sec. successfully transmitted
[e.g. Tput = 10 Mbps]
End-to-end packet delay
End-to-end packet delay :: the time to deliver a packet
from source to destination.
{most often we are interested in the packet delay within
the communications subnet}
This delay is the sum of the delays on each subnet link
traversed by the packet.
Each link delay consists of four components
Packet Delay
1.
2.
3.
4.
The processing delay [PROC] between the time the packet is
correctly received at the head node of the incoming link and
the time the packet is assigned to an outgoing link queue for
transmission.
The queueing delay [QD] between the time the packet is
assigned to a queue for transmission and the time it starts
being transmitted. During this time, the packet waits while
other packets in the transmission queue are transmitted.
The transmission delay [TRANS] between the times that the
first and last bits of the packet are transmitted.
The propagation delay [PROP] between the time the last bit is
transmitted at the head node of the link queue and the time
the last bit is received at the next router. This is proportional to
the physical distance between transmitter and receiver.
End-to-End Packet Delay
Link packet delay = PROC + QD + TRANS+ PROP.
end-to-end packet delay = sum of ALL link packet delays.
Be Careful !! end-to-end can be defined
either from Host-to-Host or only within the sub-network.