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Issue 7: Current Developments - Africa

African VSAT Case Study

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The Global VSAT division of Cable & Wireless has implemented a comprehensive satellite system in 11 African countries on behalf of an international oil company. It provides voice, data and fax services between its head office in the UK and its businesses in Africa.

The Global VSAT team were responsible for system design as well as implementing the entire network to conclusion. The on-site work was carried out by a sub-contracting company based in Africa tasked to obtaining licenses, carrying out site surveys, equipment installation and commissioning and system repair and maintenance.

This network has been in operation since December 1999. This system offers a reliable and a flexible closed network platform for applications. The two companies established an agreement with the VSAT equipment supplier to provide system support and software upgrades as necessary to increase the longevity of the system.

General Points

This system consists of two types of VSAT terminals, all working to a hub in the UK and using the 328.5° E Intelsat satellite. At the hub, various interfaces between the VSAT equipment and the customer's router system can be provided, e.g., X21 for TDMA data and 2 MBPS with R2 signalling or Voice Over IP (VoIP) with 4 W E&M. A 2 MBPS backhaul link is provided between the hub and the Customer's head office in UK. Additionally an ISDN circuit is provided as a Standby.

TDMA Terminals (Time Division Multiple Access)

  • The only terminal using 2.4m antennas and 2.5W transceivers. Providing data services only, in a Star Configuration between remote VSATs and the UK hub.
  • They provide interactive data traffic using the SkyRelay hub and system management equipment (Manufactured by ViaSat Satellite Networks, Atlanta, USA).
  • The outbound link (O/L) from the hub provides 256 KBPS data stream that is shared by all TDMA terminals.
  • 2 x 64 KBPS return links (R/L) are shared by all remote terminals. The remote terminals are connected to the customer's routers.

HYBRID TDMA & DAMA Terminals (Demand Assigned Multiple Access)

  • The Hybrid TDMA & DAMA terminals share a 3.8m antenna and a 5W transceiver providing data services in a Star Configuration between the remote terminals and the Hub in the UK.
  • In addition they provide voice and fax services in a mesh configuration between the remote VSATs and the UK Hub and between the remote VSATs themselves.
  • Similarly to the TDMA terminals they consist of a SkyRely terminal, and a Skylinx DAMA terminal, manufactured by ViaSat.
  • The SkyRelay terminals share the same 256 KBPS O/L but have their own 128 KBPS R/L.
  • Each Skylinx remote provides 3X16 KBPS channel units (CUs) for providing voice and fax in both star and mesh connections. These CUs interface with the customer's PBX at E&M or FXS/FXO. The CUs share a common pool of carrier frequencies, allocated to sites on demand.
  • When required, any of the above channels can be allocated permanently (Permanent Assigned Multiple Access, PAMA) to a particular remote VSAT. This is useful when higher volume data traffic (16 KBPS to 2048 KBPS) needs to be transported.


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