VSAT introduction

Internet via satellite - how it all works

VSAT installation

Typical VSAT customers

VSAT Data Capacity

The indoor data processing unit (DPU) or satellite modem will typically have one or more user ethernet ports for connection either direct to PCs or to a local area network router or wireless router.   The number of hardware ports is typically 4.

If your modem has just one etherent port you can extend it to 4, 8, 16 etc as required using local area network hub devices or switches.

Download bit rate

In the outlink (download) direction from the hub to the VSATs continuous carriers are used, typically more than 256k bit/s and up to 60 Mbit/s.   The type of carrier is almost identical with a digital TV carrier called DVB-S.    Each VSAT is restricted to extract from the data stream only those packets of data intended for it.   One outlink may be shared by 5 to 32,000 VSAT sites, according to the traffic load.

In some cases you can use a DVB-S type receiver to receive only and still extract just what is intended for you.  This is the basis for DVB-S/SCPC systems where the VSAT uses a separate transmit modem to transmits a low speed continuous carrier back to he hub.  The DVB-S arrangement also applies to so called one-way satellite internet systems that use a DVB-S plug-in card in your PC.  In this case the customer sends request packets to the hub using a terrestrial phone line/modem.  The benefit is high speed downloads.

Upload bit rate

The data transmission rate on the return link, in the direction from the VSAT to the hub, is typically from a few 100 bit/s to 512 kbit/s, higher uplink bit rates, such at 1 or 2 Mbit/s are possible in some instances, particularly with larger size antennas or higher power transmit amplifers.  Each VSAT typically transmits in short occasional bursts, interleaved in time.   The number of VSAT sites sharing a return link and the number of return links is adjustable to match actual traffic patterns, hopefully without unacceptable congestion.  Sharing ratios vary from 5:1 to 60:1   The higher sharing ratios correspond to least cost services, suitable for the occasional undemanding user.   Shared systems are vulnerable to greedy users who may quickly overload the network.

Where reliable return link bit rates are required such as for business, cyber cafe, VoIP or outside broadcast TV contribution uplinks, a continuous return link carrier is used (SCPC = Single channel per carrier).   It is also possible to set up a dedicated repeating time slot in a TDMA system which gives the same equivalent result - a continuous guaranteed bit rate for the customer.   This has the advantage that any congestion that does occur is fully under the customer's control.

VoIP requires approx 20 kbit/s each way for the duration of the call and most satisfactory results are obtained with dedicated capacity.

Other arrangements

There are many further possibilities.  Point to point VSAT networks (2 terminals only) are possible with say 2 Mbit/s each way, carrying a mixture of data and voice traffic for example.   Mesh networks are possible, e.g. several hotels with common ownership might share a common pool of satellite capacity and have the options to communicate direct to each other when desired.

Amended 4 July 2005, 13 Dec 2014.