SatLink System Technical Notes
TDM/TDMA and SCPC are the main alternative technologies for satellite networking in the world today. The modem and management technologies underlying both approaches have been advancing rapidly in recent years, causing some confusion as to which technology is better for a given set of networking requirements. This technical note willexplain the important trends and trade-offs. These two alternative technologies are illustrated in Figure 1 and Figure 2 for a simple star topology network, highlighting their key elements and configuration differences.
(Time Division Multiplexing with Time Division Multiple Access)
Figure 1: TDM/TDMA Network
Figure 2: SCPC Network
(Single Channel Per Carrier)
TDMA vs. SCPCTechnical Note
SCPC uses a separate dedicated carrier to each remote terminal ("VSAT") to receive information from the central site, and another dedicated carrier for each VSAT to transmit information back to the central site. Both carriers are modulated in "continuous mode". Usually some non-standard coding techniques are used since SCPC technology was never standardized. In contrast, TDM/TDMAtechnology uses a single high-speed TDM carrier transmitted from the central site or "Hub", from which many VSATs can receive information. For this TDM forward link, the DVB-S2 standard is most commonly used. It is also the most flexible for multiplexing many concurrent streams of traffic to different sites, and the most efficient with its support of Adaptive Coding and Modulation (ACM). ACM dynamicallyadjusts the modulation and coding on the "virtual link" to each VSAT individually, as local conditions (e.g., weather, interference) at the VSAT change. To transmit back to the central site efficiently, the VSATs in a TDM/TDMA network are synchronized, and they transmit information in "burst mode" within a series of short, scheduled timeslots. Timeslots may be assigned across multiple TDMA carriersand accessed using "fast frequency hopping". Timeslots are assigned to each VSAT exclusively (i.e., without contention) based on their current traffic needs. This is called Dynamic TDMA, and it is the most advanced form of TDM/TDMA. This technology is fully standardized internationally by the DVB group under the DVB-RCS family of standards. TDM/TDMA networks allow all VSATs to dynamically sharemultiple TDMA carriers, as if they were a single large pool of bandwidth. Each TDMA carrier group may contain dozens of carriers, with up to 32 carriers per carrier group in a SatLink ® network. Therefore the "return link" may contain huge amounts of capacity, in aggregate. In a SatLink TDM/TDMA network the TDMA carriers may operate at widely different symbol rates (e.g., from 500 ksps to 5 Mspsand even higher). To determine which VSAT will use which timeslots on which carrier at any moment, SatLink has ACS (Adaptive Carrier Selection). ACS is applied dynamically for each VSAT, given its local weather conditions, configuration (e.g., antenna and BUC size) and service policy (e.g., maximum rate requirements). ACS determines what carrier & symbol rate will work best at the current signallevels of those available in the carrier group. In addition, in a SatLink DVB-RCS2 ("2nd Generation") network, ACM per burst is supported for each VSAT and on all TDMA carriers in the carrier group. This further optimizes efficiency, throughput, and reliability for each VSAT and greatly simplifies network operations. Any VSAT can use any MODCOD, on any carrier, if necessary. DVB-RCS2 SatLink TDM/TDMAnetworks now surpass SCPC networks not only in efficiency, but also in throughput and link availability for almost any conceivable network configuration and satellite band (e.g., C, Ku, X, and Ka band).
2. Total Cost-of-Ownership Comparison
Today, SCPC only makes economic or technical sense for very small networks; that is, those with only a handful of remote sites and relative low-speed...