DVB-RCS2: Advancing TDM/TDMA

DVB-RCS2: Advancing TDM/TDMA Technology

DVB-RCS2

The predominance of TDM/TDMA in satellite networks today for interactive IP (Internet Protocol) communications illustrates the importance of this technology in furthering the reach of the Internet and for extending all kinds of private IP networks or VPNs into regions of the world poorly served by terrestrial communications.

The portion of these networks where the greatest innovation is occurring is in the TDMA communication from many remote VSATs (i.e., low-cost ground terminals) to one or more central sites or Hubs in a star topology, and to other VSATs (in a mesh topology). The bandwidth for transmissions from the VSATs is provided by a TDMA Carrier Group, which acts as a single pool of bandwidth shared by many VSATs.

See the figure below (as applicable to a star topology).

TDMA Carrier Group

 

Multiple Carrier Groups can be used if desired, with a single TDM forward link.

A Carrier Group is managed by control logic at the Hub using the concept of a "superframe" which defines and tracks all timeslots, on all TDMA carriers. For maximum efficiency, timeslots are assigned exclusively to specific VSATs to implement all necessary "virtual channels" for user traffic, which must (ideally !) "float" dynamically across multiple TDMA carriers based on many fast changing criteria affecting the network to assure maximum link-level availability and multiplexing efficiency.

Furthermore this must be done with the necessary throughput, response time, and jitter performance for each of potentially many thousands of different IP traffic flows, based on a set of pre-defined "QoS Classes" (i.e., aggregates of IP traffic), which may be configured on a customized basis for each VSAT in the network.

NOTE: The same QoS behavior and link-level availability requirements must also be met on the TDM forward link carrier (typically using DVB-S2 carriers with ACM), but it is much easier to implement that since all packet processing and control over the forward ink is located in the Hub. Yet many TDM/TDMA suppliers do not yet integrate their QoS handling for each IP flow with their implementation of ACM and how that impacts the virtual channels for these IP flows to each VSAT.

The recently approved DVB-RCS2 standard – the 2nd generation of the well-established and widely adopted DVB-RCS standards – is the most significant advancement in TDMA burst-mode technology within the last 15 years. DVB-RCS2 sets a new benchmark that other TDM/TDMA network technologies – which are proprietary or non-standard – have yet to match.

The specific nature of the TDMA technology for accomplishing these goals in satellite networks is called dynamic MF-TDMA (Multi-Frequency TDMA). It is supported fairly widely in the industry, but not by all TDM/TDMA suppliers, as some lack the required dynamic flexibility. Dynamic MF-TDMA employs fast frequency hopping within the TDMA carrier group where the hopping across carriers occurs automatically in response to the needs of the network (i.e., not requiring human intervention).

Dynamic MF-TDMA technology was standardized in the first version of the DVB-RCS standard in 2001. Today both standard (DVB-RCS) and proprietary versions of Dynamic MF-TDMA exist. But many suppliers lack the key features needed to produce the highest levels of efficiency, throughput, and reliability demanded by many customers. In particular, they lack the support of adaptive technologies for use in TDMA Carrier Groups (such as ACM) and they lack good modem spectral efficiency.

 

Adaptive MF-TDMA Technology: ACS and ACM per Burst

The two principal adaptive technologies for TDMA burst mode operation are:

  • Adaptive Carrier Selection (ACS) per Burst: Dynamically selecting the carrier for a VSAT to use for its next TDMA burst, based on the carrier symbol rate, current conditions at the VSAT or the Hub (e.g., bad weather or interference), and as constrained by the VSAT hardware configuration. This helps assure greater throughput and increased link reliability.
  • Adaptive Coding and Modulation (ACM) per Burst: Dynamically selecting the MODCOD for a VSAT to use for its TDMA bursts, based on the same factors as above. This delivers the major advantage for maximizing efficiency, as well as throughput and link reliability.

Both of these technologies are supported by DVB-RCS2 and SatLink Communications' SatLink products. While ACS has been supported in SatLink Communications' SatLink networks for many years using first-generation DVB-RCS, the introduction of ACM per burst is a first with DVB-RCS2 shipments by SatLink Communications in 2012. No other TDMA technology or supplier provides DVB-RCS2 yet because it requires major changes to their old and proprietary architectures.

The support of "ACM per burst" on TDMA carriers can produce a 2x to 2.5x increase in the average efficiency and throughput of those carriers in Ku or Ka band networks, while also greatly increasing link-level reliability. When combined with ACS, further advantages are realized in throughput and reliability.

For those many suppliers still without ACM or ACS the best mode of operation is to have all TDMA carriers in the Carrier Group run at the same symbol rate using a single fixed MODCOD. This is in fact the requirement of some "dynamic" TDM/TDMA products from well-known suppliers. This prevents them from obtaining comparable efficiency, throughput, or reliability under common network conditions, even if they do eventually improve their basic modem performance and support a wider range of modulation and coding options (MODCODs).

NOTE: ACS is not a specific feature of either DVB-RCS or DVB-RCS2, but rather a smart and very useful value-added feature for managing a TDMA Carrier Group easily and well.

 

Even More Advancements with DVB-RCS2

DVB-RCS2 delivers many additional advancements that work in combination with ACM and ACS to produce the highest performance TDMA networks available. These include:

Modulation and Coding Options (MODCODs) from QPSK 1/3 to 16QAM 5/6: These cover a range similar to DVB-S2 for TDM carriers, with "nominal" efficiencies up to 3.3 bits per symbol, using a 16-state Turbo Code FEC.

Low Burst Overheads: All TDMA modems incur some burst overhead for guard times, preambles and/or pilot symbols. The key is to assure that these do not significantly reduce the nominal spectral efficiency by more than a few percent, even at high symbol rates.

Lower Carrier Spacing: In a DVB-RCS2 network, a spacing of 1.12x among TDMA carriers produces the optimal spectral efficiency (in bps/Hz), and the best efficiency of any burst modem, while matching or beating the spectral efficiencies of the best "continuous mode" (e.g., TDM or SCPC) modems, even after subtracting burst overhead and Layer 1 framing overheads. This holds true also when compared with DVB-S2 at its minimum 1.2x carrier spacing (with pilots and short-frames, as is common when using DVB-S2 with ACM).

Excellent Signal-to-Noise Ratio (SNR) Requirements: SNR requirements for DVB-RCS2 TDMA carriers are comparable (+/- 0.3 dB) to DVB-S2 TDM carriers for the same spectral efficiency at comparable FEC frame & burst sizes.

Multiple Burst Sizes: DVB-RCS2 defines several traffic burst sizes (which may also contain signaling information), a mini-burst dedicated to periodic and/or random access signaling, plus a special logon burst. All are defined in terms of symbols to simplify superframe customization and enable ACM. The traffic burst sizes allow TDMA Carrier Groups to be optimized for the mix of IP packet sizes and/or the peak loading intensities expected at each VSAT.

More Efficient IP Packet Encapsulation for Multiple IP Flows & Protocols: DVB-RCS2 replaces the previous standard methods of IP packet encapsulation with a new method called "RLE" that is optimized for maximum efficiency in TDMA burst operation, greatly reducing overhead on small IP packets where it matters most, and supporting multiple concurrent QoS Classes from a single VSAT as well as multiple IP versions (e.g., IPv4 & IPv6).

Higher Symbol Rate TDMA Carriers: Maximum symbol rates for DVB-RCS2 carriers are not specified in the standard since they are implementation dependent. But the ability to tightly synchronize TDMA bursts in a DVB-RCS2 network (down to small fractions of a microsecond) minimizes required guard times, and therefore reduces the penalty of going to higher symbol rates. This goal is also helped by the "Long Burst" and "Very Long Burst" options in DVB-RCS2 and the simplicity of the superframe. SatLink currently supports symbol rates on TDMA carriers up to 8 Msps (much higher than supported by other TDMA technologies) with the capability for further increases as the market may demand.

Large, Diverse TDMA Carrier Groups: The size and diversity of symbol rates with a TDMA Carrier Group is also implementation dependent. The DVB-RCS2 standard provides signaling structures that allow a huge diversity and a very large number of carriers all in the same Carrier Group. Currently 24 TDMA carriers are supported in SatLink Hubs for the RCS2 superframes with both ACM and ACS active. In the future, up to 64 TDMA carriers will be easily supported for RCS2 on high-performance platforms. This increases efficiency, throughputs, and overall link reliability for all VSATs.

See Creation of the Second Generation: DVB-RCS2 for more information on the development of this important new standard and to see a video re-creating the first live demonstration of DVB-RCS2 with ACM per burst for the European Space Agency.

NOTE: Not all features of the DVB-RCS2 standard are supported in SatLink products today, but nearly all the major features (see above) are currently supported and many other useful ones will be supported in 2013 for the "linear" modem options as listed above.

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