VSAT Network Types

This is how a star data, TDM/TDMA VSAT network works using a hub station, usually six metres or more in size and small VSAT antennas (between 75 centimetres and 2.4 metres). All the channels are shared and the remote terminals are online, offering fast response times. Consequently, TDM/TDMA systems are comparable with terrestrial X.25 or frame relay connections.

Initial systems were designed to offer fast response times for predicable, bursty traffic patterns - typified by credit verification transactions and lottery systems.  As the internet and broadband generally began to drive demand, the manufacturers introduced completely new IP-centric platforms designed to serve broadband applications.  In essence, current systems are now also able to trade a short initial delay to allow the hub to allocate dedicated capacity within the inbound (return) channel to a VSAT.  This capacity is dynamically sized by the system based on the traffic demand seen by the VSAT.  In addition, all systems also incorporate frequency hopping, allowing the network to be load-balanced by moving VSATs between inbound carriers to ensure that capacity is used efficiently and congestion does not occur.

The first generation systems all employed proprietary TDM outbound channels, but the advent of the DVB-S standard bringing extremely low cost broadband demodulator chips to the market caused almost all of the vendors to adopt it as the basis for their outbound/forward channel.  The basic time divided slot architecture remains the same however.  Cost was the primary driver because the video-centric design of DVB-S was not very efficient for pure IP services.  Most recently, the introduction of the latest DVB-S2 standard has dramatically changed this situation with low cost and efficiency now features.  The DVB-S2 standard brings efficiencies of up to 30% over DVB-S and the addition of ACM (adaptive Coding & Modulation) can add a further 40%.  In the satellite capacity constrained markets of 2007 and 2008, DVB-S2 ACM has become an absolute necessity for any operator.

Broadband VSAT systems are sometimes criticised for poor performance with the blame often being laid at the door of the product.  However, almost every VSAT platform of which we are aware, is very capable of meeting the demands of the most demanding user - the culprit for poor performance is usually a result of the amount of bandwidth a subscriber is allocated as part of the service.  High rates of over-subscription are mostly a consequence of low prices - satellite bandwidth is both a finite and expensive resource unfortunately.

On a brighter note, the industry has been working hard towards introducing greater efficiencies at all levels - the satellite (Ka-band and on-board processing), the inbound/return links (turbo coding and modulation), the outbound/forward channel (DVB-S2), adaptive coding and modulation schemes (ACM) and more advanced techniques to manage IP and web traffic (acceleration, compression and caching) are all constantly pushing the boundaries of the technology allowing it remain competitive.  Aside of the fact that, like the beer commercial, satellite reaches parts other technologies cannot - the unified, ubiquitous, predictable and reliable nature of the solution continues to find applications all over the world in both developed and developing markets.

However, mesh networks which use capacity on a demand assigned multiple access (DAMA) basis take a different approach. The master control station merely acts as a controller and facilitator rather than a hub through which traffic passes as in a star network. However, these connections take a little time to set-up and thus, mesh/DAMA systems are often equated to a terrestrial dial-up connection.

There are also mesh systems which use a TDMA access scheme where all of the terminals in a network receive and transmit to the same channel, selecting different time slots because each terminal is aware of what the others have reserved. In the past this type of system has been costly and therefore, reserved for large scale trunking applications, but, more recently, costs have come down considerably and now they can be cost competitive with SCPC/DAMA systems for thin route applications as well.

Point-to-point SCPC (single channel per carrier) links are the satellite equivalent of a terrestrial leased line connection. They are usually set-up on a permanent, 24 hour basis and are thus more costly in satellite capacity and less efficient if not used all the time. However, they do support high bandwidths (typically from 9.6 kbps to 2 Mbps) and can easily be used to carry data, voice and even video traffic.

All other systems are usually a variation on one of the themes described above, either in a star, mesh or hybrid (star and mesh) configuration. Most of the TDM/TDMA manufacturers also offer a mesh product which can be deployed in a hybrid-ised configuration, sharing common components such as antennas and RF units, at a remote site.