Ad hoc networks

Cellular and ad-hoc wireless networks

Cellular networks	Ad-hoc wireless network
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

Application of ad-hoc wireless network

• applications
  • military applications
  • collaborative and distributed computing
  • emergency and rescue operations
  • mesh networks
  • wireless sensor networks
  • hybrid cellular / ad-hoc wireless networks
• why ?
  • quick depolyment
  • inexpensive deployment and operation

Technical challenges

Medium Access Scheme

• MAC usded for shared use of the transmission medium
• performance depends on MAC protocol
• chanllenges
  • distribution operation
  • maximum throughput
  • minimum access delay
  • fairness
  • real-time trafiic support
  • power control capabilities
  • use of directional antennas
  • hidden/exposed terminal problems

Routing

• responsible for
  • determining a feasible path
  • discovering, storing, and exchanging routing information
  • gathering information about a path breaks and updating route information accordingly
• Challenges
  • Mobility
  • Bandwidth constraints
  • Resource constraints
  • Errorneous transmission medium
  • Location-dependent contention
• requirements on a routing protocol in ad-hoc networks
  • minimum route acquisition delay
  • Quick route configuration
  • loop-free
  • distributed routing
  • low overhead
  • scalabiltiy
  • privacy
  • support for time-sensitive traffic

Multicasting

• Multicasting is an important feature in wireless ad-hoc networks:
  • search and rescue operations: distribution of commands
  • military applications: distribution of command
• 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.

Transport Layer Protocol

• 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;
• Performance degradation stems from
  • high error rate
  • frequent path breaks
  • presence of ‘old’ routing information
  • network partitioning

Quality of Service provisioning

• QoS
  • 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

Self-organization

• Self-organization is the main attractive property of ad-hoc networks.
• To perform self-organization the following things are required:
  • neighbour 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.

Security

• Ad hoc are more vlunerable to attacks because
  • lack of central coordination
  • shared wireless medium
• Two types of attacks
  • passive attacks
    • malicious nodes attempt to obtain information relayed in the network;
    • no damage to operation of the network, just capture if information
  • active attacks
    • external attacks: attacks executed by nodes outside the network;
    • internal attacks: attacks executed by nodes belonging to the same network.
• Denial of Service
• Resource consumption
  • 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
• Infomration disclosure
• Interference

Addressing and service discovery

• Addresses
  • Global unique address
  • autoconfiguration of address
  • Duplicate address detection mechanism
• Meaningful features for ad-hoc network
  • automatic service advertisement mechanism
    • should allow to identify the current location of the service
    • 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 networ
    • may violate the traditional design objectives of the routing protocol

Energy management

• can be done
• shaping the energy discharge pattern
• use routes with minimal total energy consumption
• use special task scheduling schemes
• proper handling the processor and interface devices
• can be achieved by
  • Transmission power management
  • Battery energy management
  • Processor power management
  • Interface power management

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

Deployment

• 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

Example : data-link/network/transport

Data-link layer MACA

• MACA:
  • stands for MAC protocol for ad hoc networks.

• major facts

  • contention-based without reservation and scheduling
  • MACA was prposed as an extension for CSMA/CA protocol
  • was further extended and adopted for IEEE 802.11
• CSMA
  • 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
• Shortcoming for 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.

• 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

Network layer: Location Aided routing (LAR)

• use the location information

• reactive protocol

• Two zones
  • Expected Zone
    • 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
  • Request Zone
    • a geographical zone within which control packets are allowed to propagate
      • 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.
• Nodes decide whether to forward or discard packets based on two algorithms:
  • LAR type 1
    • 
  • LAR type 2
    • 

Transport-layer protocols: Split TCP

• TCP major problems
  • degradation of throughput with increase of path length
    • 
  • unfairness among TCP flows
    • 
• split-tcp provides the solution by splitting the TCP functionality into two part
  • congestion control
    • local phenomenon due to high contention for resources
  • end-to-reliability
    • end-to-end phenomenon

Split TCP:

• splits the connection into a set of concatenated TCP connections
  • 
• proxy node
  • terminating the connection from the sender/precessor proxy node;
  • setting up a connection with receiver/successor node.
  • are chosen using the distributed algorithm
    • packet traversed n hops - behave as a proxy
• Transmission control at the TCP sender window is split
  • 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.
• Proxy node behaves
  • 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.