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VSATs Tutorial









Like a terrestrial network, satellite networks can be designed to be fully resilient, with multiple routing, and sophisticated traffic management systems to ensure no single points of failure, or to relieve bottlenecks at routers or switches, by sending traffic directly.

A) SCPC - The satellite equivalent of a “private wire” or kilostream.

For :

• All parameters can be tailored to an individual customer
• Customers gets dedicated bandwidth
• Very reliable


• Connects one site to another site, and each time you add a new site is added it adds new cost.
• Becomes expensive for closed networks after more than 6 – 10 site compared to the below

B) Frame – similar to terrestrial Frame relay network


• Better use of satellite bandwidth for small closed networks
• QoS and SLA’s high and can be user defined
• Voice over frame (VoFR) high quality
• Dial plans and PBX functionality should be transparent to users on the network
• Very reliable, high functionality


• VSAT terminal still expensive because of the functionality
• Become expensive for closed networks between 10 – 20 sites dependent on network parameters

C) MF-TDMA or FDM DAMA (SkyWAN)– no real terrestrial equivalent


• Begins to make use of the statistical behaviour of large populations as opposed to Frame which is only able to prioritise
• Offers closed networks increase functionality (mesh circuits), any site multimedia connectivity)
• Not usually hubbed, so remotes single pints of failure on the network
• Enable further rationalisation of satellite bandwidth
• Good IP features


• Additional cost of the DAMA hardware at each site and associated install costs (balance of economics)

D) DVB-RCS or TDMA-RCS – equivalent to broadband Internet access by cable


• Cheap(er) consumer terminals and IDU’s
• Potentially high bandwidths for little money of the service provider has correctly sold and managed the service
• Enable organisations to exit legacy network topologies and based all services and interconnections on standard VPN
• Built around IP


• Potentially higher network delays
• Legacy satellite networks and in particular their terrestrial interface must be engineered to this new mode of operation
• Designed for mass markets so customisation can be limited
• “Star” topology only – all traffic must go via service provider hub
• Contention rates can be an issue

Developing A Network

The ideal choice of any one of the above will depend on :

• Immediate needs
• Location
• Future needs
• Capex Budget available
• Opex budget available

Once more than a simple site to site link is required, most networks become compromises, and grow through different technologies, until they reach a scale of functional requirement which necessitates a complete upgrade.

Networks usually begin as a “star”, where all the traffic is either for a central site, or routed through that site.

As regional offices develop, there may be congestion, or simply additional functionality needed at a regional office, so the network may become a partial mesh.

Finally the applications (multimedia) or the volume of network traffic may demand a mesh topology.

Very Small Aperture Terminal

The term VSAT refers to any fixed satellite terminal- generally with an antenna diameter of less than 3.8m – that is used to provide interactive or receive-only communications.

VSATs are ideal for Internet Connectivity in Africa because they can be installed almost anywhere and provide an ‘always-on’ connection. VSATs are used for a wide range of Internet and other telecommunications applications, including corporate networks, rural telecoms, distance learning, telemedicine and much more.

In areas where telephone lines are of poor quality, unreliable, or just can’t provide acceptable data rates, VSATs can be used to provide Internet access at speeds of up to 2Mbps or more. Most Internet connections rely on data being sent and received via a telephone line, but VSATs use a satellite to send and receive transmissions. This removes the need for any existing communications infrastructure prior to installation.

The VSAT is installed at the location where Internet access is wanted, and is connected to the Local Area Network via a router and satellite modem. So once the VSAT in place all the computers at your site gain full Internet access.

VSATs can be arranged in either a mesh or star network. Star networks are more common and consist of a central hub and many remote terminals. Data from the remote terminals is sent to the central hub (generally located somewhere on the fast Internet backbone), where it is routed to its destination. With a mesh network, each VSAT can communicate directly with other VSATs on the network.

Whilst VSATs are more expensive than phone modem connections they are highly reliable, provide any speeds you need and the cost of service is normally fixed, so there are no variable phone bills to pay.

The amount of bandwidth supplied by the VSAT is variable, and can be set to a level suitable for the site. Smaller sites can therefore pay for the smaller bandwidth channels that they require, while large sites can use more of the capacity of the VSAT. This makes the technology suitable for isolated sites using the Internet mainly for communication and data transfers, as well as for Internet Service Providers who sell bandwidth on to individual users.



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