Technologies for meshed systems

Three main technologies are used in modern meshed VSAT systems: DAMA SCPC, TDM / SCPC and TDM / TDMA.

DAMA SCPC

Connections are made on a point-to-point basis between any nodes in the network using SCPC carriers. SCPC channel pairs are automatically set up on a call by call basis. Moreover, connections are made on user demand taking into account the service type and the transmission rate required. The network is a circuit switched network, with circuits provided as required. Once established, traffic channels are generally transparent and are transmitted continuously for the duration of the call. The key feature of SCPC DAMA meshed satellite system is that the service connections are established directly between any site in the network. However, outbound and inbound channels, respectively on the uplink and downlink satellite segment space, are assigned by the central controller. Each site at the end of each point to point link has a modem dedicated to that link. The number of modem needed at each terminal station, equal to the number of stations in the mesh less one, reveals that a DAMA SCPC mesh is a viable solution for a small meshed VSAT network only, consisting of a very few terminal stations.

TDM / SCPC

Each remote site transmits a single TDM carrier which is received by all other remote sites in the network. The TDM control channel carries a continuously transmitted data stream which is divided into frames. A remote terminal in the mesh must receive all TDM carriers in the mesh other than its own. The data packets in the received transmissions are continually monitored and those carrying the remote site address in the headers are selected for processing. All other packets are ignored. At each of the remotes one modulator for the transmitted carrier and a bank of demodulators is needed. The number of demodulators equals the number of sites in the mesh less one.

TDM / TDMA

All terminals in the network transmit bursts of signaling and traffic data in time slots in a single TDMA channel. Each remote terminal transmits in bursts in pre-assigned time slots in the TDMA frame. All terminals in the network transmit on a single frequency. Signaling channels operate in a similar fashion to the TDM/TDMA transmissions in an star hub based VSAT network. The burst time plan is defined by the NCS based on call requests from the remote terminals by following predetermined algorithms. Transmission is generally not continuous, because some time slots may not be used. However, at peak traffic levels, time slot utilization may approach 100%. Each remote station only needs one burst modem. Thus, TDM/TDMA type meshed networks are the most scalable alternative, while being also the most complex networks. The cost for the network control equipment at the NCS therefore is substantial.