Mobile ad hoc networks (MANET)

Mobile ad hoc networks (MANET)
Circle Lecture Communication Systems
Winter Term 2004/2005
1 Introduction
2 Media access
3 Routing
4 Current research areas
Prof. Dr. M. Zitterbart
Institute of Telematics
Dr.-Ing. Kilian Weniger
Outline
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1 Introduction
What are mobile ad hoc networks?
Wireless networks without infrastructure
no base station/access points, no backbone
Each terminal is host and router at the same time!
Terminals may be mobile
Packet-based data forwarding
Routes between two devices can be several hops long
– also called multi-hop ad-hoc networks
Ad-hoc networks are self-organizing
No central components
Example
r
transmission
range r
Node
Link
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Example of data packet routing in MANETs
transmission
range
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Advantages and applications
Mobile ad-hoc networks: Advantages
Simple, fast and cheap setup of networks
e.g. with 802.11b in the 2.4 GHz (ISM) band: no license required
transmit power can be reduced compared to wireless infrastructure networks
distance to nearest neighbor can be lower than to the base station
More robust concerning failure of single components due to decentralized structure
Military applications
formations of soldiers, tanks, planes,...
Civil applications
conferences, exhibitions, meetings, lectures
telematics applications in traffic
Extension of cell-based systems (WLAN, UMTS)
Entertainment on travels (file sharing, gaming,... in trains, cars or planes)
sport events
networks for cabs, police,…
sensor networks
In case of disasters
In case of infrastructure failure (telephone network,..): e.g. earthquakes
rescue operations, e.g. after avalanches
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Application 1: Sensor networks
Protection against or reaction to
disasters
Situation: Flood disaster
Sandbags protect a dyke
Support by sensor networks
Sandbags are equipped with
sensors
Sensors cooperate to detect
leaks in the dyke
Collection and preprocessing of
data
Source: Timmermann, Uni Rostock
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Application 2: Vehicle networks
Vehicles send immediate reports to other (following) vehicles, e.g. about
Traffic jams
Obstacles
Dangerous spots
Speed controls
...
Demonstrator Fleetnet-Project
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Application 3: Integration with cellular systems
Project IPonAir: „Integration of wireless networks with the Internet“
Internet access via several
intermediate terminals
UMTS base station
Internet
WLAN access point
e.g. no access network
in direct range
load balancing
Extension of
access network
Ad-hoc communication,
e.g. for P2P or
multi-player games
Ad-hoc
communication
Extension of access
network
Ad-hoc
Communication
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Properties of mobile ad-hoc networks
Highly dynamic network topology
Mobility of devices
Changing properties of the wireless channel (Fading, multipath propagation)
Partitioning and merging of ad-hoc networks possible
No infrastructure
No central „always on“ components
Limited battery power of mobile devices
Worsened by signaling traffic, e.g. of the routing protocol
Limited bandwidth
Worsened by signaling traffic, e.g. of the routing protocol and the MAC protocol
Asymmetric/Unidirectional links
Quality of connections can differ between both directions
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Motivation
2. Media access
Media access control is necessary to enable controlled access to a
medium shared by several devices
MAC algorithms for fixed networks (e.g. CSMA/CD) usually cannot be
applied in a wireless environment:
Construction of transceivers, being able to broadcast and receive on the
same frequency at the same time, is difficult
No collision detection at sender
Signal strength degrades quadratically with the distance
Problem of hidden or exposed terminals
Possible algorithm
IEEE 802.11 in Ad-hoc mode with
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
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802.11 MAC-layer: Distributed Coordination Function
Sender
DIFS
Data frame
Receiver
SIFS
Ack
Other
stations
Waiting time
DIFS
Contention period
(backoff)
Data frame
t
Station senses medium
Case 1: Medium is free for „Distributred InterFrame Space (DIFS)“
Station may send the data frame
Case 2: Medium ist busy
Station waits till medium is free for IFS
Transmission is delay by random backoff time
Backoff timer is freezed when other stations are sending
Ack for received data frame (after Short IFS)
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Problem of hidden terminals
Hidden terminal
A sends to B
C does not receive A anymore
C wants so send to B, Medium is free for C (CS fails)
Collision at B, A does not recognize this (CD fails)
A is „hidden“ for C
A
B
C
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MAC-layer: DCF with RTS/CTS
Request-to-Send (RTS)/Cleat-to-Send (CTS) option
Sender tansmits RTS first
contains Network Allocation Vector (NAV)
Receiver replies with CTS (after SIFS)
Sender sends data frame and receivers sends ack (after SIFS)
Other stations store NAV
Virtual reservation
Sender
DIFS
RTS
data
Receiver
SIFS
CTS
SIFS
SIFS
ACK
Other
Stations
NAV (RTS)
NAV (CTS)
Waiting time
Backoff
DIFS
data
t
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Exposed terminals
Exposed terminal
B sends to A
C wants to send to D
C must wait: CS signals an that medium is busy
This is unnecessary: A is outside C‘s range
C is “exposed“ to B
A
B
C
D
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3. Unicast routing protocols for mobile ad-hoc networks
Routing protocols developed for fixed networks (e.g. RIP, OSPF, ...) cannot be
applied to mobile ad-hoc networks without modifications
slow convergence
too much overhead
Routing protocols for ad-hoc networks must, in contrast, converge quickly and
use as little bandwidth as possible for control traffic
Different metrics are possible for mobile ad-hoc networks
Minimal number of hops
Minimal delay
Minimal packet loss
Maximal signal stability and route stability over time
Maximal battery runtime of a mobile device
Maximal lifetime of the whole network
e.g. until partitioning due to empty batteries of a network node
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Classification
Flooding for data transfer
Most simple “protocol“: Each node forwards each packet exactly once
very high overhead
Proactive Routing (Table-driven)
Routes to all nodes are maintained
Routes are permanently available
Constantly high control overhead
Reactive Routing (On-demand)
Routes are determined only when needed (Route Discovery)
Time delay at the beginning, as the route must be determined first
Control overhead depends on number of connections
Hybrid Routing
Mixture of proactive and reactive routing
There is not (yet) a one-fits-all routing protocol for mobile ad-hoc networks
Adequacy of a protocol depends on scenario
Standardization of ad-hoc routing protocols: IETF MANET WG [3]
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Principle
Proactive routing protocols
Nodes constantly determine routes to all other nodes in the network
Approaches
Distance Vector Routing
based on Bellman-Ford Algorithm
Each node determines for each other node
– its neighbor (next hop), via which the shortest route to the specific node leads
– the length of this route
This information is periodically broadcast to all neighbors
Example fixed network: Routing Information Protocol (RIP)
Link State Routing
Each node periodically broadcasts the status of its links, together with link state
information received from its neighbors
Therefore, each node knows the whole network topology
Shortest route to each node can be determined with Dijkstra‘s Shortest Path First
Algorithm
Example fixed network: Open Shortest Path First (OSPF)
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Example: Optimized Link State Routing (OLSR)
Goal: Efficient flooding of Link State Information!
1. Determination of neighborhood
Periodic broadcast of HELLO messages. These contain:
– Addresses of the nodes from which HELLO messages are currently received
– State of the links to these node: symmetric (bidirectional) or asymmetric
(unidirectional)
Neighborhood relationships computable from this information
– Y is a “1-hop neighbor” of X X receives HELLO message from Y
– Y is a “2-hop neighbor“ of X Y X and X sees Y in a HELLO message
received from Z
– Y is a „strict 2-hop neighbor“ of X Y is a 2-hop, but not a 1-hop neighbor of X
– Relationships are in each case defined as symmetric and as asymmetric
– Example: X sees itself in a HELLO message by Y Y is a symmetric
1-hop neighbor of X
Delete neighborhood information after timeout as a reaction to link breaks
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Multipoint Relays
2. Determination of „Multipoint Relays“ (MPR)
The MPRs of X are a subset of the symmetric 1-hop neighbors of X
This subset is determined such that each symmetric 2-hop neighbor
of X can be reached via at least one MPR
Determination of the subset according to heuristics on every change
detected within the 2-hop neigborhood
Selected MPRs are announced in HELLO messages
3. Determination of the „MPR Selectors“ (MS)
The MS of X are the nodes that have chosen X as their MPR
X gets to know about these nodes in the HELLO messages
received
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Example OLSR I
Determination of neighbors, MPR and MS (HELLO messages)
Only symmetric links in this
example!
B
H G F A D E
C
Node
A
B
C
D
E
F
G
H
1-hop neighbors
2-hop neighbors
MPR
MS
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Example OLSR II
Determination of neighbors, MPR and MS (HELLO messages)
B
H G F A D E
C
Node
A
B
C
D
E
F
G
H
1-hop neighbors
A
A
A, E
A, G
G
2-hop neighbors
MPR
MS
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Example OLSR III
Determination of neighbors, MPR and MS (HELLO messages)
B
H G F A D E
C
Node
A
B
C
D
E
F
G
H
1-hop neighbors
F, D
A, D, F
A, D, F
A, E
D
A, G
F, H
G
2-hop neighbors
E, G
E, G
E, G
A
A
MPR
D, F
D, F
D, F
D
F
MS
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Example OLSR IV
Determination of neighbors, MPR and MS (HELLO messages)
B
H G F A D E
C
Node
A
B
C
D
E
F
G
H
1-hop neighbors
B, C, D, F
A, D, F
A, D, F
A, B, C, E
D
A, B, C, G
F, H
G
2-hop neighbors
E, G
E, G
E, G
F
A
D
A
MPR
D, F
D, F
D, F
B
D
B
F
MS
B, C
B, C
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Example OLSR V
Determination of neighbors, MPR and MS (HELLO messages)
B
H G F A D E
C
Node
A
B
C
D
E
F
G
H
1-hop neighbors
B, C, D, F
A, D, F
A, D, F
A, B, C, E
D
A, B, C, G
F, H
G
2-hop neighbors
E, G
C, E, G
B, E, G
F
A, B, C
D
A, B, C
MPR
D, F
D, F
D, F
B
D
B
F
MS
D, F
B, C
B, C
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Example OLSR VI
Determination of neighbors, MPR and MS (HELLO messages)
B
H G F A D E
C
Node
A
B
C
D
E
F
G
H
1-hop neighbors
B, C, D, F
A, D, F
A, D, F
A, B, C, E
D
A, B, C, G
F, H
G
2-hop neighbors
E, G
C, E, G
B, E, G
F
A, B, C
D, H
A, B, C
F
MPR
D, F
D, F
D, F
B
D
B, G
F
G
MS
D, F
A, B, C, E
A, B, C, G
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Example OLSR VII
Determination of neighbors, MPR and MS (HELLO messages)
B
H G F A D E
C
S
T
A
B
L
E
!
Node
A
B
C
D
E
F
G
H
1-hop neighbors
B, C, D, F
A, D, F
A, D, F
A, B, C, E
D
A, B, C, G
F, H
G
2-hop neighbors
E, G
C, E, G
B, E, G
F
A, B, C
D, H
A, B, C
F
MPR
D, F
D, F
D, F
B
D
B, G
F
G
MS
D, F
A, B, C, E
A, B, C, G
F, H
S
T
A
B
L
E
!
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Optimized Link State Routing (OLSR)
Efficient flooding possible with this algorithm
Source node broadcasts message
Optimization compared to the standard flooding algorithm:
Not all nodes forward the message, but only the MPRs
– Y forwards a message received from X, if X is an MS of Y
Forwarding also done as a broadcast
Forwarding of duplicates is avoided by the use of sequence numbers
Number of selected MPRs has a direct effect on network load
Number of MPRs should be optimal (minimal) to reduce traffic
Standard
flooding
Efficient
flooding with MPR
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Example OLSR VIII
Efficient flooding of messages, example: source node B
B
H G F A D E
C
MPRs of B
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Example OLSR IX
Efficient flooding of messages, example: source node B
MPR of F
B
MPR of D
H G F A D E
C
• Nodes F and D are MPRs of B and forward the packet received
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Example OLSR X
Efficient flooding of messages, example: source node B
MPR of G
B
H G F A D E
C
• Node B does not forward the packet received by F and D again (sequence numbers)
• Node G is an MPR of F and forwards the packet received from F
• Node F does not forward the packet received by G again (sequence numbers)
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Optimized Link State Routing (OLSR)
Efficient flooding of link state information
Each node X with MS Ø periodically sends a TC (Topology Control) message.
It contains:
own address
complete list of own MS
Each node in the network collects information about MS from TC messages
Network topology can be reconstructed to a sufficient extent
Shortest routes can be determined according to Dijkstra
Optimization: Combination (Piggyback) of HELLO and TC messages
Reduction of media accesses
Particular effective in networks with a high node density
Only few nodes act as MPR
OLSR is a „standard protocol“ (experimental) in the IETF (RFC 3626)
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Principle
Reactive routing protocols
A node only knows the routes it actually requires
No periodic updates
Tasks of a reactive routing protocol
Finding a route (Route Discovery)
Only when a node wants so send data to a node, but does not yet have a route to that
node
Maintaining a Route (Route Maintenance)
Only active routes are maintained
Comparison to proactive routing protocols
Advantage
Only routes that are actually needed are determined and maintained
No periodic transmission of messages necessary less use of resources
Disadvantage
Time delay at the beginning of a communication: route has to be determined first
Control overhead depends on number of connections and mobility
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Route Discovery
Example: Ad Hoc On Demand Distance Vector Routing (AODV)
Sender S floods the ad-hoc network with a Route Request (RREQ)
RREQ contains source address S and destination address D
Nodes that are involved in forwarding the RREQs store the address of
the node from which they received the RREQ
The so-called „Reverse Path“ into the direction of the sender S is built
up
Only works with bidirectional links!
When the destination node D receives the RREQ, it answers with a
Route Reply (RREP)
RREP contains source address S
RREP is forwarded via unicast to node S (use of Reverse Path)
With the forwarding of the RREPs, the so-called “Forward Path” into the
direction of the destination node D is built up
The Forward Path is used for data traffic
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AODV: Route Request I
Source: N. Vaidya, Tutorial Mobicom 01
Y
Z
A
B
H
S
C
I
E
G
F
K
J
M
D
N
L
Node that has received a Route Request (RREQ)
Node in transmission range
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AODV: Route Request II
Source: N. Vaidya, Tutorial Mobicom 01
Y
Broadcast
Z
A
B
H
S
C
I
E
G
F
K
J
M
D
N
L
RREQ
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AODV: Route Request III
Source: N. Vaidya, Tutorial Mobicom 01
Y
Z
S
E
A
B
H
C
I
G
F
K
J
M
D
N
L
Reverse Path
Each node stores the address of the neighbor from which a RREQ packet
has been received
Requires state maintenance in all nodes
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AODV: Route Request IV
Source: N. Vaidya, Tutorial Mobicom 01
Y
Z
S
E
A
B
H
C
I
G
F
K
J
M
D
N
L
The Reverse Path from each intermediate node to the sender is built up
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AODV: Route Request V
Quelle: N. Vaidya, Tutorial Mobicom 01
Y
Z
A
B
H
S
C
I
E
G
F
K
J
M
D
N
L
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AODV: Route Request VI
Source: N. Vaidya, Tutorial Mobicom 01
Y
Z
A
B
H
S
C
I
E
G
F
K
J
M
D
N
L
When the RREQ has arrived at D, there is a Reverse Path from D to S
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AODV: Route Reply
Source: N. Vaidya, Tutorial Mobicom 01
Y
Forward Path for data
Z
S
E
A
B
H
C
I
RREP
G
F
K
J
M
D
N
L
Thereupon, D sends a RREP to S via the Reverse Path
By the RREP, the Forward Path for data traffic from S to D is built up
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AODV: Sequence numbers
Each node has a Destination Sequence Number
Determines the “up-to-dateness” of routing information
Is only incremented, never decremented
Exception: Overflows
RREQ contains last known Destination Sequence Number of the
destination node
Optimization: Destination nodes or intermediate nodes knowing a newer route
(higher Destination Sequence Number) reply with RREP
RREP also contains Destination Sequence Number
If more than one RREP has been received, the RREP with the highest
Destination Sequence Number will be used
In case of equal Destination Sequence Numbers, the one with the lowest Hop Count
is selected
May accelerate the establishment of a route and save protocol overhead in
combination with Expanding-Ring-Search
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AODV: Optimizations
Expanding Ring Search
RREQs are not flooded in the whole network
First, RREQs with low time-to-live (TTL) are sent
If the search fails, a RREQ with a higher TTL is sent
Reduces network load when determining routes to nearby nodes
In case of distant nodes
Slower route establishment
Nearby nodes have to forward many RREQs
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AODV: Maintenance of a route
Reverse/Forwarding Path entries are deleted after timeout
Soft-state approach
The transport of data along a route refreshes this route
Timers are reseted
Detection of link breaks
Missed HELLO messages
Missed link-layer acknowledgements (Receipts on MAC layer)
Reaction to Link breaks: Sending of a RERR
contains increased Destination Sequence Number
Is forwarded to the source
Source tries to find a new route by issueing a RREQ
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AODV: Maintenance of a route
Destination Sequence Numbers prevent loops
A sends to D, Link from C to D breaks
RERR from C to A gets lost
Later, C wants to send to D
RREQ contains higher Destination Sequence Number
A receives RREQ via C-E-A
A sees higher Destination Sequence Number and does not sent RREP
for D
A B C D
E
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Local Route Repair
AODV: Optimizations
In case of a link break, RERR is not sent to the source immediately
Node detecting a link break searches a new route to the destination itself
(RREQ)
Can reduce network load in the average
Can lead to worse (longer) routes, as only sections of the route are
newly determined
AODV is a „standard protocol“ (experimental) in the IETF
(RFC 3561)
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4. Current research areas
There are intensive research activities in many areas of mobile ad-hoc
networks (also at our institute)
Multicast-Routing
Lots of group applications, like E-Learning, are based on IP-Multicast routing
protocols
Autoconfiguration
dynamic assignment of unique IP addresses with a distributed mechanism
Service Awareness
In a mobile environment, finding mobile services is of crucial importance
Integration with fixed networks
How can the integration of mobility protocols (Mobile IP) and routing in mobile ad-hoc
networks be realized?
Power Control
Control over topology by variation of transmit power of each device and thereby
minimization of interference between the nodes
Security
Integration of security mechanisms into mobile ad-hoc networks is very difficult
Scalability, ...
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IP-Autoconfiguration in MANETs
Internet Protocol (IP) requires unique IP addresses
Autoconfiguration of the network
Requirements
Efficiency concerning communication complexity
Robustness against packet loss
Support for network partitioning and
merging
Resolution of address conflicts
As few address changes as possible
Flexibility concerning routing protocol
Hierarchical addressing, if necessary
Rapidness of address assignment
1
A
Data path
B
2
4
C
F
Address
7
?
1
E
D
5
Example of network merging
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The PACMAN concept
Robustness by a hybrid approach as well as coupling with routing
Distributed allocation table and Duplicate Address Detection (DAD)
Implicit support for network merging
Efficiency by cross-layer/passive mechanisms
Passive DAD by anomaly detection in routing protocol traffic
Passive synchronization of the allocation table
Reduction of dependencies by modular architecture
Routing protocol
instance
Application
comm.
Transport.
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Routing protocol
packets
PACMAN
Network
Data Link
Physical
PACMAN:
Passive Autoconfiguration
for Mobile Ad hoc Networks
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Example: Neighborhood History (NH) Module
Model of a proactive link-state routing protocol
Nodes periodically send routing protocol messages
Source address (SA)
Sequence number (SN)
Set of bidirectional link states (LS)
Each node forwards them
Principle of the NH module
Utilization of bidirectional
neighborhood relationships
Maintenance of an NH table
with addresses which were
neighbors in the past time span t d
3
A
B
SA: 4
SN: 5
LS: 1,2,3
4 1
D E
2
NH table(C)
2,3
C
1
NH: Neighborhood History
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Prototype Implementation
Implementation for Pocket
PCs with Linux operating
system
Modification of routing
protocol implementations
not necessary
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Multicast-Routing
Multicast is a 1:n communication which enables group communication
Group of nodes is identified by an IP multicast address
Application only has to send a data packet once
Addressed to the IP multicast address of the corresponding group
Multicast data packet is forwarded to all group members
Application scenarios for multicast
Streaming
E.g. Internet radio
CSCW (Computer Supported Cooperative Work)
E.g. video conferences, whiteboards, distributed editors
Multiplayer games
E-Learning
Service Discovery
…
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ODOMP - On-demand Overlay Multicast Protocol
Creation of an overlay network between the group members
Group member are connected via IP-in-IP tunnels
Multicast data packets are encapsulated in IP unicast packets
Routing is done by an arbitrary underlying unicast routing protocol (e.g. AODV)
Overlay has the form of a tree
Source node is root of the tree
Data duplication by none-leaf nodes
Group members forward a copy of the multicast data packet to each of their
downstream members
sender
Group member
Non-group member
„Real“ link
Overlay link
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References
[1] C.-K. Toh, Ad Hoc Mobile Wireless Networks: Protocols and Systems,
Prentice Hall, 2002
[2] C. Perkins, Ad-hoc Networking, Addison Wesley, 2000
[3] IETF MANET Working Group, http://www.ietf.org/html.charters/manetcharter.html
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