Post on 07-Oct-2020
Challenges and specifics of ad hoc networks
Lecturer: Dmitri A. Moltchanov
E-mail: moltchan@cs.tut.fi
http://www.cs.tut.fi/kurssit/TLT-2756/
Ad hoc networks D.Moltchanov, TUT, 2009
OUTLINE:
• Some history;
• Cellular vs. ad-hoc;
• Applications of ad-hoc networks;
• Technical challenges;
• Examples of special protocols:
– Data-link layer: MAC protocol for ad hoc networks (MACA);
– Network layer: location aided routing (LAR);
– Transport layer: split TCP.
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Ad hoc networks D.Moltchanov, TUT, 2009
1. HistoryThe idea of ad-hoc networks is the multi-hop wireless relaying of messages:
• 500 B.C. Darius I, King of Persia:
– the principle of multi-hop relaying was introduced;
– messages from the capital to remote provinces have been relayed using line of shooting men;
– more than 25 time faster than normal messengers available at that time;
– a lot of other ancient ad hoc systems: string of repeaters of drums, trumpets, or horns.
• 1970, Norman Abrahamson, ALOHAnet:
– aim: a network for University of Hawaii, to connect remote sites on islands;
– idea: fixed single-hop wireless packet switching with multiple access;
– extension: idea is applicable for multi-hop relaying also;
– outcome: stimulated the research in multi-hop relaying leading PRNET project.
Packet radio network (PRNET) sponsored by Defence Advanced Research Project Agency.
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• PRNET project:
– Aim: developing packet wireless network for military applications;
– Idea: evolved from centralized control to distributed wireless multi-hop system;
– How: ALOHA and CSMA for access to a shared media, DSSS over the channel;
– Features: self-organization, self-configuration, detection of radio connectivity.
The main issues that the RPNET faced were:
– obtaining, maintaining, and utilizing the topology information;
– error and flow control over the wireless links;
– reconfiguration of path to handle path breaks;
– processing and storage capability of nodes;
– distributed channel sharing.
The successful demonstration of PRNET proved:
– feasibility and efficiency of infrastructure-less networks;
– applicability to civilian and military purposes.
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• Just after PRNET: Survivable Radio Network (SURAN)
– What: extension of the DARPA PRNET project;
– Aim: providing efficiency in all aspects: size of devices, cost, scalability;
• In 1980th:
– What: military applications were extensively funded across the globe;
– Point: IETF created the WG called mobile ad-hoc network (MANET) group;
– Aim of MANET: provide standardized routing functionality for ad hoc networks.
• 1994: Bluetooth
– What: short-range, low-power, low-complexity radio;
– Aim: connectivity between heterogenous devices;
– How: Special Interest Group (SIG): 3Com, Ericsson, IBM, Lucent, Motorola, Nokia etc.
– Result: one of this first commercial realization.
Note: Bluetooth is not ad hoc itself! Just a possible platform for ad hoc networks.
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• Bluetooth standardized two major nodes’ formations:
– Piconet:
∗ single point-to-point wireless links formatted in group of nodes;
∗ in piconet every node can reach every other node in a group within a single hop.
– Scatternet:
∗ formation created by several piconets;
∗ multi-hop routing protocol should be used (not standardized).
D
S
Figure 1: Piconets and scatternets in Bluetooth.
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2. Cellular and ad-hoc wireless networks
Cellular networks Ad-hoc wireless networks
Fixed infrastructure No infrastructure
Single-hop wireless links Multi-hop wireless links
Guaranteed CBR bandwidth (voice traffic) Shared radio channel (data traffic)
Initially, circuit-switched Initially, packet-switched
High cost and time of deployment Very quick and cost-effective
Reuse of frequency via channel reuse Dynamic frequency sharing
Bandwidth reservation is achieved easily Complex MAC layer
Nowadays applications: civilian, commercial Nowadays applications: military, rescue
High cost of network maintenance Maintenance operations are built-in
Low complexity of mobile devices Intelligent mobile devices are required
Widely deployed, evolves Still under development in commercial sector
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3. Applications of ad-hoc wireless networksThere are a number of applications:
• military applications;
• collaborative and distributed computing;
• emergency and rescue operations;
• mesh networks;
• wireless sensor networks;
• hybrid cellular/ad-hoc wireless networks.
What makes ad hoc so attractive:
• quick deployment;
• inexpensive deployment and operation.
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4. Technical challengesThere are many challenges in design, deployment, and performance of ad hoc:
• Medium access scheme;
• Routing and multicasting;
• Transport layer protocol;
• Pricing scheme;
• Quality of service provisioning;
• Security;
• Energy management;
• Addressing and service discovery;
• Scalability;
• Deployment considerations.
NO SINGLE SOLUTION TO ANY OF THESE PROBLEMS!
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4.1. Medium access scheme
Why it is so important:
• MAC is responsible for shared use of the transmission medium;
• performance depends on MAC protocol (e.g. Token Ring vs. Ethernet).
Challenges of MAC protocol in ad-hoc wireless network are:
• Distributed operation;
• Maximum throughput;
• Minimum access delay;
• Fairness;
• Real-time traffic support;
• Power control capabilities;
• Use of directional antennas.
• Hidden terminal problem;
• Exposed terminal problem.
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receiver senderhidden terminal
collisionpackets packets
Figure 2: Hidden terminal problem (transmission ranges are circular!).
sender receiverexposed terminal
packets
receiver
packets
Figure 3: Exposed terminal problem (transmission ranges are circular!).
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4.2. Routing
The responsibility of any routing protocol:
• determining a feasible path to a destination based on a certain criterion;
• discovering, storing, and exchanging routing information;
• gathering information about a path breaks and updating route information accordingly.
Challenges for routing protocol in ad-hoc networks:
• Mobility;
• Bandwidth constraints;
• Resource constraints;
• Erroneous transmission medium;
• Location-dependent contention:
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Requirements on a routing protocol in ad-hoc networks:
• Minimum route acquisition delay:
• Quick route reconfiguration:
• Loop-free routing:
• Distributed routing:
• Low overhead;
• Scalability;
• Privacy;
• Support of time-sensitive traffic.
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4.3. Multicasting
Multicasting is an important feature in wireless ad-hoc networks:
• search and rescue operations: distribution of commands;
• military applications: distribution of commands.
Why not to adapt something from fixed networks (CBT, PIM, DVMRP):
• core base trees (CBT), distance vector multicast routing protocol (DVMRP), etc.;
• mobility of nodes changes the topology of the network! Trees are unstable!
There are following challenges in ad-hoc environment for multicasting:
• Fast recovery;
• Control overhead;
• Efficient group management;
• Scalability;
• Security.
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4.4. Transport layer protocol
There are following major function of connection-based transport layer protocol:
• setting up and maintaining end-to-end connection;
• reliable end-to-end delivery of data packets;
• flow control;
• congestion control.
Why not to go with UDP:
• does not perform flow and congestion control and reliable end-to-end transfer;
• result: increase on contention → losses.
Performance degradation stems from:
• high error rate;
• frequent path breaks;
• presence of ’old’ routing information;
• network partitioning.
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4.5. Quality of service provisioning
Effect of service performance determining the degree of satisfaction of a user of the service.
QoS includes a number of concepts including:
• traffic performance in the network;
• service support performance;
• service operability performance;
• service security performance.
To satisfy QoS:
• use values of traffic engineering variables that constitute the so-called Grade of Service (GoS).
Provision of QoS requires:
• negotiation between the host and a network;
• resource reservation schemes;
• priority scheduling;
• call admission control.
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4.6. Self-organization
Self-organization is the main attractive property of ad-hoc networks.
To perform self-organization the following things are required:
• neighbor discovery:
– first phase when a node switches on;
– a node should gather network information (transmission of reception of discovery packets).
• topology organization:
– every nodes gathers information about the entire network (a part of);
– construct and maintain the network topology.
• topology reorganization:
– when links break, nodes switch off etc.
– requires periodic or aperiodic exchange of topology information.
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4.7. Security
What makes ad hoc more vulnerable to attacks:
• lack of central coordination;
• shared wireless medium.
The attacks against ad-hoc networks are generally classified into:
• passive attacks:
– how: malicious nodes attempt to obtain information relayed in the network;
– damage: no damage to operation of the network, just capture if information.
• active attacks:
Active attacks disrupt the operation of the network and classified into the following types:
– external attacks: attacks executed by nodes outside the network;
– internal attacks: attacks executed by nodes belonging to the same network.
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There are following security threads in ad-hoc networks:
• Denial of service:
– make the network resource unavailable for service to other nodes (i.e. flood packets).
• Resource consumption:
– consume the scarce resource in ad hoc networks including:
– Energy depletion:
to deplete the power of the node relaying the traffic through them.
– Buffer overflow:
fill the routing table with ’bad’ entries to consume the buffer space of the target node.
• Host impersonalization:
– act as an another node responding with control packets and interrupting data traffic.
• Information disclosure:
– disclose information destined for a certain node.
• Interference:
– just create some noise on a shared media!
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4.8. Addressing and service discovery
No centralized control.
The following features are required for ad-hoc addressing scheme:
• Global unique address;
• Autoconfiguration of addresses;
• Duplicate address detection mechanism.
The following features are required for ad-hoc network to be meaningful:
• automatic service advertisement mechanism:
– should allow to identify the current location of the service;
– why current: it is not possible to assume static service locations in ad hoc networks.
• integration of service discovery protocols and routing protocols:
– may allow to easily find the necessary service in a network;
– why not so good: violate the traditional design objectives of the routing protocol.
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4.9. Energy management
This may be done via following:
• shaping the energy discharge pattern;
• use routes with minimal total energy consumption;
• use special task scheduling schemes;
• proper handling the processor and interface devices.
Energy savings can be achieved by:
• Transmission power management;
• Battery energy management;
• Processor power management;
• Interface power management.
There will be no breakthrough in battery technology in the following decade!
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4.10. Scalability
Testbeds and operational ad hoc networks made so far:
• contain only a limited number of nodes;
• may not be good examples of ad hoc performance.
What we may expect in real implementations:
• performance of ad-hoc network degrades drastically with the increase of the number of nodes;
• one may expect commercial realization of, at least, thousands of nodes.
4.11. Deployment
The deployment of ad-hoc network has the following benefits:
• Low cost: no cables, no configuration, no maintenance;
• Incremental: functioning starts immediately after minimum configuration is done;
• Short time: no cables, no configuration, no maintenance;
• Reconfigurability: no cables, no configuration, no maintenance.
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5. Example: data-link/network/transport5.1. Data-link layer: MACA
MAC protocols for ad-hoc
Contention-basedContention-based
with reservation
Contention-based
with schedulingOther protocols
- MMAC;
- MCSMA;
- PCM;
- RBAR.
- DPS;
- DWOP;
- DLPS.
synchronous asynchronous
- D-PRMA;
- CATA;
- HRMA;
- SRMA/PA;
- FPRP.
- MACA/PR;
- RTMAC.
sender initiated receiver initiated
- RI-BTMA;
- MACA-BI;
- MARCH.
multiple channelsingle channel
- MACAW;
- FAMA.
- BTMA;
- DBTMA;
- ICSMA.
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MACA: stands for MAC protocol for ad hoc networks.
What are major facts:
• contention-based without reservation and scheduling;
• MACA was proposed as an extension for CSMA/CA protocol;
• was further extended and adopted for IEEE 802.11.
CSMA operates as follows:
• the sender sense the channel for the carrier signal;
• if the carrier is present it retries to sense the channel after some time (exp. back-off);
• if not, the sender transmits a packet.
The following shortcomings are inherent to CSMA/CA:
• −: hidden terminal problem leading to frequent collisions;
• −: exposed terminal problem leading to worse bandwidth utilization.
MACA avoids hidden and exposed terminal problems using the RTS-CTS.
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Neighbor Sender Receiver Neighbor
RTS
CTS CTS
Data
RTS
Data
Figure 4: Packet transmission in MACA.
• RTS and CTS packets carry the expected duration of transmission;
– a node near the sender: that hearing RTS do not transmit for a time to receive CTS;
– a node near the receiver: after hearing CTS differs its transmission;
– if the neighbor hears the RTS only:, it is free to transmit.
• if the data packet is lost, a node backs off for a random period of time T ∈ {CW}, CW = 2CW .
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5.2. Network layer: Location Aided Routing (LAR)
Routing protocols for ad-hoc networks
Routing information update Temporal information for routing
HybridReactiveProactive Past history Predictions
- DSDV;
- WRP;
- CGSR;
- STAR;
- OLSR;
- FSR;
- HSR;
- GSR.
- DSR;
- AODV;
- ABR;
- SSA;
- FORP;
- PLBR.
- CEDAR;
- ZRP;
- ZHLS.
- DSDV;
- WRP;
- STAR;
- DSR;
- AODV;
- FSR;
- HSR;
- GSR.
- FORP;
- RABR;
- LBR.
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Routing protocols for ad-hoc networks
Utilization of specific resource Topology information
FloodingGeographicalPower-aware Flat Hierarchial
- PAR. - LAR. - DSR;
- AODV;
- ABR;
- SSA;
- FORP;
- PLBR.
- CGSR;
- FSR;
- HSR.
Proactive Reactive
- OLSR. - PLBR.
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What are basics of LAR:
• uses the location information (assumes the availability of GPS);
• reactive (on-demand) protocol.
LAR designates two zones for selective forwarding of control packets:
• ExpectedZone:
This is a geographical zone in which the location of the terminal is predicted based on:
– location of the terminal in the past;
– mobility information of the terminal.
There is no info about previous location of the terminal the whole network is the ExpectedZone.
• RequestZone:
This is a geographical zone within which control packets are allowed to propagate:
– this area is determined by the sender of the data packet;
– control packets are forwarded by node within a RequestZone only;
– if the node is not found using the first RequestZone, the size of RequestZone is increased.
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8
5
6
7
2
9
4
1
3
Source
Dest.
10
3
Figure 5: Example of ExpectedZone and RequestZone.
Nodes decide whether to forward or discard packets based on two algorithms:
• LAR type 1;
• LAR type 2.
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LAR type 1 algorithm works as follows:
• the sender explicitly specifies the RequestZone in the RouteRequest packet;
• the RequestZone is the smallest rectangle that includes the source and the ExpectedZone;
• when the node is in ExpectedZone, the RequestZone is reduced to the ExpectedZone;
• if the ReouteRequest packet is received by the node within RequestZone, it forwards it.
8
5
6
7
2
9
4
1
3
Source
Dest.
10
3
Figure 6: Routing procedure in LAR type 1.
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LAR type 2 algorithm operates as follows:
• the sender includes the distance to the source in the RouteRequest packet;
• intermediate nodes compute the distance to the destination:
– if this distance is less than the distance between source and destination packet is forwarded;
– otherwise the packet is discarded.
• distance in the packet is updated at every node with lower distance to destination.
8
5
6
7
2
9
4
1
3
Source
Dest.
10
11
12
13
14
15
16
18
17
Figure 7: Routing procedure in LAR type 2.
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5.3. Transport-layer protocols: Split TCP
Transport layer protocols for ad-hoc wireless
networks
TCP modifications Other protocols
- ACTP;
- ATP.Split approaches
End-to-end
approaches
- Split-TCP. - TCP-ELFN;
- TCP-F;
- TCP-BuS;
- ATCP.
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The following are two major problems with TCP:
• degradation of throughput with increase of the path length:
– short connections get more throughput than long connections.
Th
rou
gh
pu
t
Number of hops2 4 6 8
Figure 8: Throughput as a function of the number of hops.
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• unfairness among TCP flows:
– MAC layer contention (channel capture effect):
– lengthy TCP flows – more points to contend.
Figure 9: Unfairness in TCP flows in ad-hoc networks.
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Split-TCP provides the solution by splitting the TCP functionalities into two aims:
• congestion control;
• end-to-end reliability.
Why it is possible?
• congestion control: local phenomenon due to high contention for resources;
• end-to-end reliability: end-to-end phenomenon.
CongestionsEnd-to-end delivery
Figure 10: Congestion control and end-to-end reliability.
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Split TCP: splits the connection into a set of concatenated TCP connections.
S
R
proxy node
segment 1 (zone 1)
segment 2 (zone 2)
Figure 11: Splitting of the TCP connection into segments (zones).
Proxy node is responsible for:
• terminating the connection from the sender/precessor proxy node;
• setting up a connection with receiver/successor node.
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Proxy nodes are chosen using the distributed algorithm:
• simplest way: packet traversed n hops - behave as a proxy.
Transmission control at the TCP sender window is split into:
• end-to-end CW :
updated according to arrival of end-to-end ACKs.
• local CW : (local CW ≤ end-to-end CW )
updated according to arrival of local ACKs (LACKs) from the next node.
The proxy node behaves as follows:
• it maintains local CW governing transmission in a segment;
• when packet arrives from predecessor the LACK is sent back;
• arrived packet is buffered;
• the buffered packet is forwarded to the next node.
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S
R
proxy node
segment 1 (zone 1)
segment 2 (zone 2)
TCP data flow;
LACK;
end-to-end ACK;
network link.
Figure 12: Flows in split-TCP.
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