Appendix I: VSAT and bandwidth management

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VSAT stands for Very Small Aperture Terminals.  This relates to the size of the dish antennas that are currently used.  In the early days of satellite communications, the antennas were often 30 metres or more in diameter.  Today, they can be down to 90cm or less, depending on the wavelength used and the capability of the other ground and satellite mounted equipment.


Internet Protocol traffic can be moved over satellite telecommunications connections, making it possible to delivery high bandwidth, reliable connections to anywhere on the surface of the earth which is within the “footprint” of a suitable equipped satellite.  However, it is expensive to launch and maintain satellites, and so a number of techniques have been developed to enable maximum utility to be gained from the available bandwidth at reasonable cost.


The satellites used for VSAT Internet connections are most commonly in geostationary orbit, meaning that they stay in the same area above the equator, relative to the earth.  For example, the satellite used for Supernet Internet is at 76.5 degrees East.  They are normally a minimum 2.5 degrees apart. Each satellite is equipped with a number of transponders, which take radio signals in from the earth on one frequency and re-transmit back to earth on another. Each transponder typically uses frequencies in a 36MHz range, with each individual site, or range of sites, allocated a smaller frequency range according to the bandwidth they require.  


There are typically two main types of Internet connection available over VSAT:  so called Single Channel Per Customer (SCPC) and Time Division Multiple Access (TDMA).  SCPC is a dedicated (unshared) connection, whereas TDMA is a contended or shared link.  The oversubscription (or contention) ratio on TDMA links is usually expressed as a ratio.  For example, a 10:1 ratio would mean that ten customers use the same link.  Many people do not realize that Asynchronous Digital Subscriber Lines (ADSL) connections, common in British homes are also contended links.  Typically these are contended at 50:1, with some packages aimed more at business being contended at a lower 20:1.


There are different schemes for managing the contention of bandwidth on VSAT systems.  Many consumer packages are “best effort” systems, which means they are minimally managed, and a connection which may nominally be 512kbps could often provide as little as 5-10kbps.  Clearly this kind of link is not useful for a business where a known minimum connection speed is needed.


The system used by Supernet is a TDMA system, contended at 10:1, but with a professional grade management of bandwidth.  Such packages have a level of guaranteed bandwidth, called Committed Information Rate (CIR).  In the case of Supernet, the standard 10:1 bandwidth package has 10% of the nominal speed guaranteed.  So a 2048kbps download x 512kbps upload 10:1 package has a minimum of 204.4kbps download and 51.2kbps upload.  This is known a Dynamic CIR (DCIR), which means that it is released for use by other customers when it is not needed, but is always available as a minimum when you need it.  This arrangement is very efficient.  As the nature of web browser traffic is that it is very “bursty”, many users can share such a connection and get very acceptable response speed when required. 


As Supernet‘s connection is also used by some customer for Voice over IP (VoIP) telephony, it was decided to add an additional 60kbps of DCIR to the standard package. This ensures that there is always sufficient bandwidth to enable a good quality voice call.   

Bandwidth is further managed using Quality of Service (QOS) profiles.  This prioritizes the passage of data packets according to their protocol.  For example, data packets belonging to a VoIP call must be given high priority, otherwise there will be an unacceptable disconnect between the sounds which are heard by the parties (known as “jitter”.)  Next in priority might be web browsing data and maybe last would be email data.


The dissertation also refers to Bandwidth shaping.  This is the ability to give different users or groups of users priority for available bandwidth.  This is helpful to prevent some users hogging bandwidth at the expense of others.


Edited by the author for the web.

© Copyright, 2006  Rob Longhurst (rlonghurst@drasticom.org)