Broadband IP over Satellite

Satellite pros and cons

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Wide coverage and flexible bandwidth:

As described in the figure below, the main benefit of satellite is the footprint for the transmission, which offers instant access to as many locations as required. Furthermore, thanks to digital technologies, the allocation of bandwidth over time is highly flexible. The same satellite transponder can successively or simultaneously be used for one or multiple data transmissions, news gathering, radio programs and TV channel broadcasts. One satellite transponder can total 40 to 60 Mbit/s. Therefore, a single data "circuit" can range from a few kbit/s to several Mbit/s and even change over time.

The equipment requirements for the up-link station make satellite especially attractive for asymmetric-type networks. The most common applications are tele-training, software and database downloads, and any ‘"push" type of application sent directly to the end-users. Yet more and more satellite capacities are used for broadband international data networks, e.g. from a U.S.-based Internet backbone switching point to Internet cache and mirror sites outside the U.S. (in Australia, South America or Europe).

Transmission latency versus single hop:

An issue with GEO (Geostationary Earth Orbit) satellite transmission is latency. Transmission latency is approximately one fourth of a second. This can induce congestion estimates by connection-oriented protocols such as TCP/IP. To overcome that, maximum window sizes  or protocol conversions are recommended. One of the clear advantages of satellite transmission is end-to-end transmission through a single hop, which overcomes multi-router network drawbacks such as lost packets due to real congestion at the network crossroads. Satellite is obviously a suitable environment for IP-multicast and selective-acknowledgement implementations.

Two-step IP over MPEG-2/DVB process

There are different implementations for data-broadcast over DVB (Digital Video Broadcast). Figure 2 summarizes the operation for multi-protocol encapsulation (MPE) as defined in DVB standards, using DVB table structures for the transmission process.

Fragmentation into MPEG transport packets:

The first step benefits from a predefined data-fragmentation mechanism. Multi kilobyte pieces of data, such as IP datagrams in DVB tables, are fragmented in fixed-size (188 bytes) MPEG transport packets with minimal overhead. A series of transport packets for a given data stream is identified by a user-defined PID (Packet ID) value, such as PID 0x0A00 as seen in the example.

Notice that each packet will automatically benefit from a standard Reed Solomon forward-error correction mechanism later during the digital modulation.

Multiplexing the various data-streams:

The second step in the process is the multiplexing of the transport packets from multiple data streams - possibly along with video streams and audio streams for digital TV - in a single MPEG TS (Transport Stream). A TS is a flagged bitstream with a fixed bit-rate corresponding to the performance of the digital modulation through the satellite transponder. Null packets are used for stuffing as the total bandwidth is rarely used for QoS reasons.

Critical Service Information

Self-contained protocol:

The receiving equipment needs to filter the expected data from the entire multiplexed TS without getting external information or pre-negotiation. Therefore, appropriate descriptive service information is embedded in the multiplex along with the data. Such service information has a specific structure – similar to file management tree structure – and uses specific PID values for transmission and identification by any receiving equipment. A root table lists the existing programs and services, and each service and its content is further described through additional sub-tables.

Award-winning test solution:

Only an MPEG-2/DVB analyzer such as the Acterna DTS-A offers access to critical service information status and decodes as well as PID-based bit-rate statistics and confirmation of encryption mechanisms.

DTS-A by Digital Transport Systems, a Wavetek Wandel  Goltermann, Inc. company, received a "Best in Test" award from Test & Measurement Worldwide magazine for recognition as a unique MPEG-2/DVB and MPEG-2/ATSC analyzer.

For more information, please send your query to mpeg@jdsu.com.