Ad Hoc Networks Advanced Mobile Communication Networks Integrated Communication Systems Group Ilmenau University of Technology
Outline Introduction Medium Access Control (MAC) in Multi-Channel Scenario Routing Conclusion References 2
Introduction 3
Mobile Ad Hoc Networks (MANETs) Spontaneous federation of wireless devices No infrastructure (base station / access point), no backbone Devices can be mobile Packet-based forwarding Each device must serve as a router Routes between devices can span multiple hops Ad hoc networks are self organizing No central components r Transmission range r 4
Advantages and Application Scenarios Advantages Easy and cheap deployment E.g. using 802.11 in license free ISM band (2.4 GHz) Reduced transmission power Robust to component failures Application: where there is no access to infrastructure Military applications Groups of soldiers, tanks, planes... Civil applications Conferences, exhibitions, meetings, lectures, gaming, Car-to-car-communication, network for taxis, police, Extension of cellular networks Disaster recovery ( Int. Graduate School on Mobile Communications) After crash of infrastructure (e.g. telephone network after earthquake) Rescue 5
Applications Automotive networks Military communications Interactive lectures 6
Scenarios Wireless Mesh Networks (WMNs) Wireless Sensor Networks (WSNs) Vehicular Ad Hoc NETworks (VANETs) 7
Scenarios Low bit rate data coverage order region (no coverage) High bit rate data coverage Infrastructure mode Multi-hop communication (Ad Hoc mode) Mobile in infrastructure mode Mobile gateway (works in two modes) 8
MANET Properties Highly dynamic topology Mobility of devices Changing of quality of wireless channel (fading) Partitioning and merging of ad hoc networks possible Asymmetric / unidirectional links Different quality in both directions Wireless medium is semi-broadcast medium Hidden and exposed terminals Limited battery capacity of mobile devices Additional battery drain due to (e.g.) routing functionality Limited bandwidth Additional bandwidth required for routing and MAC functionality Time synchronization difficult Problem for low power modes (e.g. sleeping periodically) Security mechanism hard to apply Every devices must be able to forward packets no encryption of routing headers 9
Medium Access Control (MAC) in Multi-Channel MANET 10
asics Multiple users compete for access to a common, shared medium. Thus, suitable MAC mechanisms are required see Medium Access Schemes Lecture for details Problems Hidden and exposed terminals Problems related to the use of multiple channels - Node has a single interface - Node has multiple interfaces Problems related to broadcast - Redundancy: all nodes forward broadcast packets same packet is received from many nodes - Contention: nodes compete to access the medium if the medium is free, broadcast packet will be sent - Collision: no RTS/CTS dialog hidden terminal problem 11
Problems Related to the Use of Multiple Channels Node has a single interface Fixed at a particular channel (traditional solution) - Problems: how to synchronize with others using the same channel, etc. May be switched among different channels - Problems: how to select the suitable channel to switch to, how long will this switch take, how long can you use this channel, etc. Node has multiple interfaces Each interface may be fixed at a particular channel or switched dynamically among different channels A combination between fixed and dynamically switching interfaces, i.e. some are fixed at certain channels, while others are switched dynamically Problems: how to select suitable channels for each interface (depends on neighbors information), when to switch, how to synchronize with other nodes in the network, broadcasting, etc. 12
Problems Related to the Use of Multiple Channels A C Interface1 Ch1 Ch6 Interface1 Ch6 Ch11 Interface1 Ch1 Interface2 Interface2 Ch6 Interface3 Ch11 Ch6 Interface2 13
Multi-Channel MAC Approaches Dedicated control channel One channel for control messages and others for data traffic Needs usually two or more interfaces Split phase Communication in two phases - Channel negotiation phase (a default channel is used by all nodes) - Data transfer phase (all channels are used to transmit data during this phase including the default one) Works with one interface Common hopping sequence All nodes follow the same channel hopping sequence Works with one interface Multiple rendezvous Each node follows its own channel hopping sequence Works with one interface 14
Dedicated Control Channel A C D Ch0 1 A C A Ch1 3 C D D Ch0 A Ch1 2 C D Ch1 A 4 C D Ch2 Ch3 Ch2 Ch1 Ch0 (control) RTS1 CTS1 2 RTS2 Data (A ) CTS2 4 Data (C D) 1 3 RTS CTS DATA 15
Dedicated Control Channel A wants to send data to (A ) 1. Selection of communication channel A exchanges RTS/CTS with to determine the channel to be used C and D receive RTS/CTS due to using a common single channel for signaling 2. After the channel has been selected, A sends data to After some time, C wants to send data to D (C D) 3. Selection of communication channel - C exchanges RTS/CTS with D to determine the channel to be used 4. After that, C sends data to D as well. 16
Split Phase Control phase Data phase A C D A and C D A Ch0 1 C D A 2 C D Ch0 Wait for data phase A Ch0 3 C D Ch2 Ch3 3 Ch2 Ch1 1 2 Data (C D) Ch0 RTS1 CTS1 RTS2 CTS2 Data (A ) RTS CTS DATA 17
Split Phase Control phase 1. A wants to send data to (A ) A exchanges RTS/CTS with to determine the channel to be used (Ch0 in this example) 2. After a while, C wants to send data to D as well (C D) C exchanges RTS/CTS with D to determine the channel that should be used (Ch2 in this example) Notice that there are no data exchanged between nodes yet. They have to wait for the start of the following data phase Data phase 3. A begins sending data to and C begins sending data to D Notice that the channel (Ch0) is used to send data as well. Moreover, no control messages are allowed to be exchanged during a data phase 18
Common Hopping Sequence A C D 3 6 Ch3 Ch2 Ch1 Ch0 1 2 Ch0 A C D Ch0 4 Idle 3 RTS2 CTS2 5 Data (C D) Idle RTS1 CTS1 2 Data (A ) Idle 1 A C D No communication x 4 5 RTS CTS DATA 6 Ch0 A D Ch0 C Ch2 A C D Ch2 RTS3 CTS3 7 No communication y Idle Idle 8 Data (A D) 19
Common Hopping Sequence All nodes follow the same hopping sequence. In our example, the hopping sequence is as follows: Ch0 Ch1 Ch2 Ch3 Ch0... A wants to send data to (A ) 1. Node A checks the hopping sequence, i.e. (Ch0 in the example) and exchanges RTS/CTS with 2. A sends data to 3. There is no communication between nodes (x) C wants to send data to D as well (C D) 4. Node C checks the hopping sequence, i.e. (Ch2 in the example) and exchanges RTS/CTS with D 5. C sends data to D 6. There is no communication between nodes (y) 20
Multiple Rendezvous A and are Idle A communicates with A and are Idle 1 A Ch3 Ch2 Ch1 Ch1 4 A 11 A Ch0 Ch0 2 A Ch0 Ch3 5 A Ch1 Ch1 12 A Ch2 Ch1 3 A Ch2 Ch0 10 13 A Ch3 Ch2 Ch3 Ch2 Ch1 Ch0 1 2 3 4 5 6 7 8 9 10 11 12 13 A switches to the channel of to communicate with it RTS/CTS Data exchange Hopping sequence Actual hopping sequence of A Actual hopping sequence of Default hopping sequence of A Default hopping sequence of 21
Multiple Rendezvous Each node has a special hopping sequence generated by applying an equation on a certain seed. Notice that the equation is known to all nodes Example equation: new channel = (old channel + seed) mod (number of channels) Seed varies between 1 and (number of channels) -1 When two neighbors meet at any time (switch to the same channel), seeds of both are exchanged When node A wants to communicate with another node Node A uses the current channel as well as the seed of and calculates the next channel of As soon as switches to the next channel, A switches to the same channel too and exchanges RTS/CTS with followed by data exchange (steps 4 N) After finishing data communication, each node retains its hopping sequence 22
Use of Multiple Channels - Discussion Dedicated control channel and split phase (referred to as single rendezvous protocols as well) Advantages No synchronization required to determine the control channel Efficient for networks with less density Disadvantages Using single control channel can become the bottleneck under some operating conditions, e.g. high number of nodes, etc. Common hopping sequence and multiple rendezvous (referred to as multiple rendezvous protocols as well) Advantages Allow nodes to use several channels in parallel Alleviate the rendezvous channel congestion problem Disadvantages Essential challenge is the ensuring that the idle transmitter and receiver will visit the same rendezvous channel 23
Routing in (Single Channel) MANETs 24
Routing Challenges Classical approaches from fixed networks fail Very slow convergence Large overhead Dynamics of the topology Frequent changes of connections, connection quality Limited performance of mobile systems Periodic updates of routing tables need energy without contributing to the transmission of user data, sleep modes difficult to realize Limited bandwidth of the system is reduced even more due to the exchange of routing information 25
Routing Protocols for MANETs Protocols for wired networks (e.g., RIP, OSPF) cannot be applied Slow convergence High overhead MANET routing protocols must converge fast with low bandwidth consumption for control traffic Different metrics for shortest path possible Minimum number of hops Minimum delay Minimum packet loss probability Minimum congestion (load balancing) Minimum interference Maximum signal stability, stable route Maximum battery lifetime of mobile device Maximum lifetime of entire network E.g. until network is partitioned due to nodes running out of power 26
Classification of Routing Protocols in MANETs Ad hoc routing approaches Topology based Position based (Geographical) Proactive Reactive Hybrid Greedy forwarding Face routing Hybrid Topology based: Proactive or table-driven (Examples: DSDV, GSR, WRP, OLSR) Routes are calculated before needed Keep routing information to all nodes up-to-date Reactive or on-demand (Examples: AODV, DSR, LMR, AR) Routes are only calculated when needed Do not keep routing information to all nodes up-to-date Hybrid (Examples: ZRP, SHARP, Safari) Reactive and Proactive at the same time 27
Proactive Routing Protocols Nodes constantly construct and maintain routes to all other nodes Distance Vector Routing Each node computes for each destination Next hop on the route Length of the route These information are sent to all neighbors periodically Examples Wired networks: Routing Information Protocol (RIP) Ad hoc networks: Destination-Sequenced Distance Vector Protocol (DSDV) Link State Routing Each node sends periodically Its own link state The link state received by the neighbors Thus, each node knows entire network topology Examples Wired networks: Open Shortest Path First (OSPF) Ad hoc networks: Optimized Link State Routing (OLSR) 28
Reactive Routing Protocols asic principle Node knows only routes that it is currently using No periodic route maintenance Tasks of a reactive routing protocol Route discovery Triggered if route to destination is unknown Route maintenance Only for routes that are currently in use Comparison to proactive protocols Advantages No unnecessary construction & maintenance of routes No periodic messages lower resource consumption Disadvantages Delay at the beginning of communication due to route discovery Control overhead depends on number of connections and mobility 29
Ad-hoc On-Demand Distance Vector Routing (AODV) Route discovery Sender S floods route request (RREQ) for destination T RREQ contains: source and destination addresses broadcast id (is incremented only by source)» Source address + broadcast id => unique id sequence number Hop count (is incremented by each intermediate node before forwarding RREQ) Nodes that forward RREQ save pairs of source address and nodes from which the RREQ was received Construction of reverse path to sender S Only works for bidirectional links! When RREQ reaches destination T a route reply (RREP) is generated RREP contains destination and source address RREP is forwarded towards S on reverse path Construction of forward path to destination T Forward and reverse path are used for forwarding data packets If route from S to T is established, we have automatically a route from T to S 30
AODV: Flooding of RREQs I Route discovery from S to T: Destination H Sender A S C D E T F G Nodes that have already received RREQ Nodes in mutual transmission range (bidirectional links) 31
AODV: Flooding of RREQs II roadcast RREQ H C A S E T G D F RREQ is transmitted by broadcast Received by all nodes in transmission range 32
AODV: Flooding of RREQs III RREQ H C A S E T G RREQ D F Every node that receives RREQ forwards it by broadcast Of course S does not forward its own RREQ 33
AODV: Flooding of RREQs IV Constructed reverse path RREQ H C A S E T G D F RREQ Nodes remember from where the RREQ was received If multiple RREQ are received the one with the lowest hop count is selected Reverse path is constructed 34
AODV: Flooding of RREQs V Constructed reverse path RREQ H C A S E T G D F Duplicate RREQ are detected by sequence numbers and discarded Requires state maintenance 35
AODV: Flooding of RREQs VI Constructed reverse path H C A S E T G D F RREQ Destination T has received the RREQ Complete reverse path from T to S has been constructed 36
AODV: Flooding of RREQs VII Constructed reverse path RREP C RREP RREP H A RREP S E T G D F RREQ Node T replies with RREP to S using reverse path Each hop on reverse path can be directly addressed (no broadcast) 37
AODV: Flooding of RREQs VII Constructed reverse path Constructed forward path H C A S E T G D F Forward path from S to T is constructed Can be used for data transport between S and T (bidirectional) Unused reverse path entries will be discarded after timeout 38
AODV: Sequence Numbers Each node maintains a destination sequence number Determines the freshness of routing information Sequence number is always increased before sending a RREQ (in the source node) to avoid conflicts with old reverse path entries It is not incremented by intermediate nodes a RREP y destination if the sequence number in RREQ is less or equal to the sequence number in destination Intermediate nodes send RREP if they have any route to the destination with higher destination sequence number RREQ contains last known sequence number of destination RREP also contains sequence number If multiple RREP are received the one with highest sequence number is selected For multiple RREP with same sequence numbers hop count breaks ties 39
AODV: Route Maintenance Reverse/Forward path entries are discarded after timeout Soft state approach Entries are refreshed if data is transmitted on a route Reset of timers Optional hello messages to check if next hop is still available Link failure detection No ACK on MAC-Layer No hello messages Handling of link failures: Sending a route error (RERR) packet Forwarded to sender Sender initiates new route discovery with RREQ RREQ contains new sequence number Loops are prevented by sequence numbers A sends to D, link failure between C and D RERR from C to A is lost Later C starts discovery for route to D RREQ contains higher destination seq. no. A receives RREQ on path C-E-A A notices larger sequence number and discards RREP A C D E 40
Introduction to Geographical Routing ased on location information GPS information Forwarding sector Candidate neighbors Source Destination Hop-by-hop decision making No need for topology information ased on the destination location Assumption: destination location exists in source Forwarding Sector Local information Periodic or on-demand eacons 41
Opportunistic Geographical Routing Prioritization of candidates in source Utility function QoS requirement eacon information (D, L) Decision in candidates Environmental condition E (E, M) Destination ACK A F D: Distance L: Load E: Error rate M: mobility Source C 42
Conclusion 43
Current Research in Ad Hoc Networks Research activities in many areas Auto-configuration E.g. distributed assignment of IP addresses Service Awareness Usually networks are constructed to access services Multicast-Routing Many group applications are based on multicast communication Integration with wired Internet How to apply mobility supporting protocols (e.g., MobileIP) and routing in a hybrid context? Power Control Controlling topology and reducing interference by changing transmission power Security Recall that this is difficult in a decentralized setting Scalability,... 44
Conclusions Lots of challenges to be solved in Ad Hoc networks The application field determines which are more important Research mainly focuses on Self-organizing and self-healing capabilities - Route selection and maintenance - Resources usage optimization - Gateway selection - Location update - Rapid initial configuration and dynamic reconfiguration -. Continued operation and connectivity during mobility - Multi-hop handover dealing High reliability, availability and security 45
References Introduction S. asagni, M. Conti, S. Giordano, I. Stomjmenovec: Mobile Ad hoc Networking, A John Wiley & Sons, Publication, 2004 Medium Access Control J. Mo, H. W. So and J. Walrand: Comparison of Multi-Channel MAC Protocols, in Proc. of International Workshop on Modeling Analysis and Simulation of Wireless and Mobile Systems (MSWiM), pp. 209 218, Montréal, Quebec, Canada, October 2005. Y. Tseng, S. Ni, Y. Chen, and J. Sheu: The broadcast storm problem in a mobile ad hoc network, WINET Wireless Networks, vol. 8, no. 2 3, pp. 153 167, march may 2002 D. Kouvatsos, I. Mkwawa: roadcasting Methods in Mobile Ad Hoc Networks: An Overview, Proceeding of the HetNet, UK, 2005. Routing AODV-http://www.ietf.org/rfc/rfc3561.txt Harounabadi, Mehdi, et al. "TAG: Trajectory Aware Geographical Routing in Cognitive Radio Ad Hoc Networks with UAV Nodes." Ad Hoc Networks. Springer International Publishing, 2015. 111-122. Interworking with Infrastructure J. Xi & C. ettstetter: Wireless multihop Internet access: Gateway discovery, routing, and addressing, Proc. Int. Conf. on 3rd Generation Wireless and eyond, San Francisco, USA, 2002. 46
Contact Integrated Communication Systems Group Ilmenau University of Technology Prof. Dr.-Ing. habil. Andreas Mitschele-Thiel MSc. Mehdi Harounabadi fon: +49 (0)3677 69 2819/4123 fax: +49 (0)3677 69 1226 e-mail: mitsch, mehdi.harounabadi@tu-ilmenau.de Visitors address: Technische Universität Ilmenau Helmholtzplatz 5 Zuse uilding, room 1032/1070 D-98693 Ilmenau www.tu-ilmenau.de/ics Integrated Communication Systems Group Ilmenau University of Technology