Ad Hoc Networks and Personal Area Networking

Ad Hoc Networks

• Ad hoc networks do not need backbone infrastructure support
• They are easy to deploy and useful when infrastructure is absent, destroyed, or impractical

Applications

• Personal area networking (e.g., cell phone, laptop, earphone)
• Military environments (e.g., soldiers, tanks, planes)
• Civilian environments (e.g., taxi cab network, meeting rooms, sports stadiums, boats, small aircraft)
• Emergency operations (e.g., search-and-rescue, policing, and fire fighting)

Challenges in Mobile Environments

• Limitations of the wireless network:
  • Packet loss due to transmission errors
  • Variable capacity links
  • Frequent disconnections
  • Limited communication bandwidth
  • Broadcast nature of communications
• Limitations imposed by mobility:
  • Dynamically changing topologies/routes
  • Lack of mobility awareness by system/applications
• Limitations of the mobile computer:
  • Short battery lifetime
  • Limited capacities

Routing and Mobility

• Finding a path from a source to a destination
• Issues:
  • Frequent route changes
  • Route changes may be related to host movement
  • Low bandwidth links
  • Amount of data transferred between route changes may be much smaller than traditional networks
• Goal of routing protocols:
  • Decrease routing-related overhead
  • Find short routes
  • Find "stable" routes (despite mobility)

Routing Protocols

• Proactive protocols:
  • Maintain routes between every host pair at all times
  • Based on periodic updates
  • High routing overhead
  • Example: DSDV (destination sequenced distance vector)
• Reactive protocols:
  • Determine route if and when needed (on demand)
  • Source initiates route discovery
  • May not be appropriate for real-time communication
  • Example: DSR (dynamic source routing)
• Hybrid protocols:
  • Adaptive; combination of proactive and reactive
  • Example: ZRP (zone routing protocol)

Protocol Trade-offs

• Proactive protocols:
  • Always maintain routes
  • Little or no delay for route determination
  • Consume bandwidth to keep routes up-to-date
  • Maintain routes which may never be used
• Reactive protocols:
  • Lower overhead since routes are determined on demand
  • Significant delay in route determination
  • Employ flooding (global search)
• Which approach achieves a better trade-off depends on the traffic and mobility patterns

Dynamic Source Routing (DSR)

• When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery
• Source node S floods Route Request (RREQ)
• Each node appends own identifier when forwarding RREQ
• Route discovery issues:
  • Node H receives packet RREQ from two neighbors, potential for collision
  • Nodes J and K both broadcast RREQ to node D, may collide

Ad Hoc On-Demand Distance Vector (AODV)

• Route Requests (RREQ) are forwarded in a manner similar to DSR
• When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source
• When the intended destination receives a Route Request, it replies by sending a Route Reply (RREP)
• Route Reply travels along the reverse path set-up when Route Request is forwarded

AODV Summary

• Routes need not be included in packet headers
• Nodes maintain routing tables containing entries only for routes that are in active use
• At most one next-hop per destination maintained at each node
• Sequence numbers are used to avoid old/broken routes
• Sequence numbers prevent formation of routing loops
• Unused routes expire even if topology does not change

Destination-Sequenced Distance-Vector (DSDV)

• Each node maintains a routing table which stores:

  • Next hop, cost metric towards each destination
  • A sequence number that is created by the destination itself

• Each node periodically forwards routing table to neighbors

• Each route is tagged with a sequence number; routes with greater sequence numbers are preferred### Refraction

• The index of refraction of one medium relative to another is the sine of the angle of incidence divided by the sine of the angle of refraction.

• The absolute index of refraction of a medium is calculated in comparison with that of a vacuum.

• Refractive index varies with wavelength, so that refractive effects differ for signals with different wavelengths.

Radio Wave Propagation

• Under normal propagation conditions, the refractive index of the atmosphere decreases with height, causing radio waves to travel more slowly near the ground than at higher altitudes.
• This results in a slight bending of the radio waves toward the earth.

Optical and Radio Line of Sight

• The optical line of sight is the distance between an antenna and the horizon, expressed as d = √(2h), where d is the distance in kilometers and h is the antenna height in meters.
• The effective, or radio, line of sight to the horizon is expressed as d = √((2h)/K), where K is an adjustment factor to account for refraction (typically K = 4/3).
• The maximum distance between two antennas for LOS propagation is d = √((2h1)/K) + √((2h2)/K), where h1 and h2 are the heights of the two antennas.

Transmission Impairments

• Signals received will differ from signals transmitted due to various transmission impairments.
• For analog signals, these impairments introduce random modifications that degrade signal quality.
• For digital data, bit errors are introduced, causing a binary 1 to be transformed into a binary 0, and vice versa.

Multipath Interference

• Multipath interference occurs when the mobile antenna is lower than most human-made structures in the area.
• Reflected waves may interfere constructively or destructively at the receiver.

Diffraction and Scattering

• Diffraction occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave.
• Signals can be received even when there is no unobstructed LOS from the transmitter.
• Scattering occurs when the size of an obstacle is on the order of the wavelength of the signal or less.
• Typical cellular microwave frequencies, there are numerous objects that can cause scattering.

Signal Encoding Techniques

• Digital data must be converted to analog signals for wireless transmission using digital-to-analog encoding.
• Analog-to-analog, digital-to-analog, and analog-to-digital encoding techniques are also relevant in wireless communication.