Ad Hoc Networks and Personal Area Networking

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

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)

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

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)

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)

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

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

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

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

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.

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.

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.

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 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 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.

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.