October 31, 2016
Categorised in: Data Communicaiton & Wireless Sensor Networks
Infrastructure mode & ad-hoc mode
All data is transferred using the access point as the central hub. All wireless nodes in an infrastructure mode network connect to an access point. All nodes connecting to the access point must have the same service set identifier (SSID) as the access point, and if the access point is enabled with Wired Equivalent Privacy (WEP) they must have the same WEP key or other authentication parameters.
Ad-hoc mode network
It is a decentralized type of wireless network.The network is ad hoc because it does not rely on a pre existing infrastructure, such as routers in wired networks or access points in managed (infrastructure) wireless networks.
Instead, each node participates in routing by forwarding data for other nodes, so the determination of which nodes forward data is made dynamically on the basis of network connectivity.
Geostationary/Geosynchronous Earth Orbit Satellites (GSOs)
- 33786 km equatorial orbit
- Rotation speed equals Earth rotation speed (Satellite seems fixed in the horizon)
- Wide coverage area
- Applications (Broadcast/Fixed Satellites, Direct Broadcast, Mobile Services)
Advantages of GSOs
- Wide coverage
- High quality and Wideband communications
- Economic Efficiency
- Tracking process is easier because of its synchronization to Earth
Disadvantages of GSOs
- Long propagation delays (250-280 ms).
- Large propagation loss. Requirement for high power level.
- Lack of coverage at Northern and Southern latitudes.
- High cost of launching a satellite.
- Enough spacing between the satellites to avoid collisions.
- Existence of hundreds of GSOs belonging to different countries.
- Available frequency spectrum assigned to GSOs is limited.
Medium Earth Orbit Satellites (MEOs)
- Positioned in 10-13K km range.
- Delay is 110-130 ms.
- Will orbit the Earth at less than 1 km/s.
- Mobile Services/Voice (Intermediate Circular Orbit (ICO) Project)
- Fixed Multimedia (Expressway)
Highly Elliptical Orbit Satellites (HEOs)
From a few hundreds of km to 10s of thousands -> allows to maximize the coverage of specific Earth regions.
Variable field of view and delay.
Examples: MOLNIYA, ARCHIMEDES (Direct Audio Broadcast), ELLIPSO.
Low Earth Orbit Satellites (LEOs)
Usually less than 2000 km (780-1400 km are favored).
Few ms of delay (20-25 ms).
They must move quickly to avoid falling into Earth -> LEOs circle Earth in 100 minutes at 24K km/hour. (5-10 km per second).
- Earth resource management (Landsat, Spot, Radarsat)
- Paging (Orbcomm)
- Mobile (Iridium)
- Fixed broadband (Teledesic, Celestri, Skybridge)
Comparison of Different Satellite Systems