VSAT Communication Protocols

• Satellite communication can be performed through a relatively earthbound system known as a very small aperture terminal (VSAT). Jupiterimages/Photos.com/Getty Images
  

  A very small aperture terminal, also known as VSAT, consists of components located on earth as part of satellite communications that carry voice and video signals as well as data, excluding broadband television. The earthbound system consists of a transceiver to communicate with the satellite and a device such as a PC to interface with the transceiver and any other user devices. The PC serves as a hub for the system.

Antenna

• The end users are connected via the hub. To communicate, any messages must be sent through the hub. The hub will retransmit the communication to the recipient through the antenna. Communication becomes more complicated when the VSAT is mobile. The antenna is generally mounted on the roof of the vehicle. The antenna must use additional components since it will automatically acquire a signal. A device known as a controller will alert the antenna of an incoming signal and activate a sequence to move it from the stowed position and aim it toward the sky. The controller will make small corrections in the position for best signal quality.

Internet and Intranet

• The Transmission Control Protocol (TCP) is necessary for terrestrial applications due to latency or the delay in signal due to the distance of the satellite. The TCP limits the amount of data in packets sent and waits for a signal that it has been received before continuing. If the acknowledgement of the receipt of the data is not sent, the TCP assumes that the packet of data was lost and resends it. The transmission of packets begins slowly and will increase as the TCP determines the speed at which the network can receive the packets.

TCP Acceleration

• To avoid the sender experiencing the delay from latency, the TCP sends immediate acknowledgement upon receiving the packet, even if there is a delay in forwarding the packet to the remote location. The satellite hub serves as the relay. By giving immediate acknowledgement of receipt, the sender can continue with more packets. To avoid duplications of acknowledgements for packets, the equipment at the satellite hub suppresses the acknowledgement from the remote site.

Limits of Protocols

• Most communications have been designed to work at optimum level on terrestrial applications. Terrestrial systems have a shorter latency and error rate than on satellites. This is particularly pronounced in the use off geostationary satellites. Efficiency is greatly reduced when assumptions in the system's algorithms are violated. Research is being conducted to overcome this deficiency.