DVB-S Technology - Access Platforms

ACCESS PLATFORMS IN SATELLITE CONNECTIONS

Initially the system designers planned to use a currently popular TCP/IP (Transport Control Protocol / Internet Protocol) protocol. The protocol in-question is known first and foremost for its proven functionality in cable media (Ethernet, xDSL, optical fiber). However, the architecture itself and assumptions of this protocol prevent it from being used in satellite connections. Some of the TCP/IP protocol limitations are:

- problem of TTL datagram`s life - transmission speed limited to 357.8Mb/s
- problem of the length of TCP transmission window - limits the transmission speed to 1Mb/s (!!!)
- free SSA start algorithm - extremely low effectiveness of usage of the transmission channel
That is why the generally used data transmission platform in satellite connection is DVB-S. A container with capacity of 188 bytes is the basic transmitting unit. The data stream with a bit rate of 38Mb/s can include 4-8 digital TV channels, 150 digital radio programs, 550 ISDN links and various combinations of these signals. MPEG-2 is the default source encoding in the discussed system. Data transmission takes place with relatively high speed and effective protections in two ways:

- as a Data Pipe - simple end-to-end transport
- as Data Streaming transport - transmission of asynchronical, synchronical or synchronized data streams.

What became an interesting solution is application of Multiprotocol Encapsulation, thanks to which it is possible to (theoretically) place TCP/IP protocol in DVB-S protocol. Apart from that, the Data Carousel method was used, which makes periodical transmission of given data sets possible.

RETURN CHANNEL

The return channel (`upward` movement of data from the client to the server) was realized in two ways. First of them consists in application of the already existing teleinformatic terrestrial infrastructure (xDSL, ISDN, GSM telephony, including EDGE and UMTS). This way is relatively cheap as function of the satellite is restricted to broadcast data transmission in the direction of clients` terminals. The transmission procedure consists in initiation of connection by the ground subscriber`s line with satellite movement management centre that further dispatches the data provided with detailed identifier from the sender, from which the demand was received. This situation is presented in the picture below.

The second way of realization of a return channel is a two-way connection via satellite. A configuration is possible, where one of the satellites manages movement in one direction and the other in the other way or one of the satellites is responsible for two-way data transmission. Client`s terminal has to be equipped with receiving-dispatching device (in case of terrestrial realization of the return channel, only receiving device is sufficient), which, additionally, has to satisfy norms of telecommunication regulatory office (permissible levels of EIRP radiated power) in a given country. The below picture illustrates the situation in question.

See also: DVB-S Technology