Engineering Tutorials

With TDMA networks, numerous remote sites communicate with one central hub – a design that is similar to packet-switched networks. As a leased-line equivalent, SCPC can deliver dedicated bandwidth of up to 2 Mbit/s. Remote sites in a TDMA network compete with one another for access to the central hub, restricting the maximum band-.4 – DE width in most cases to 19.2 kbit/s. Almost all international VSAT services in Asia-Pacific are based on SCPC. Most domestic offerings are based on TDMA, although some domestic operators offer point-to-point SCPC links as well. Here, we will discuss briefly TDMA, pre-assigned or demand-assigned FDMA, CDMA and other accessing techniques featuring merits and demerits of these schemes.

From the above figure the followings are noted:
Switch - routing control between host & modulator/demodulator. Basically on the packets header
Modulator- modulate the outbound carriers (TDM)
Bank of Demodulators- receive inbound carriers, extract data packets.
Radio Frequency Terminal (RFT) - transmits subsystem (up converters, high power amplifier), receive subsystem (low noise amplifier, down converter)
Network Control Center (NCC) - controls and monitors hub and IDUs
Primary Power Subsystem

In a TDMA network, all VSATs share satellite resource on a time-slot basis. Remote VSATs use TDMA channels or in routes for communicating with the hub. There could be several in routes associated with one out route. Several VSATs share one in route hence sharing the bandwidth. Typical in routes operate at 64 or 128 Kbit/s. Generally systems with star topology use a TDMA transmission technique. Most common configuration of TDMA star network use one single high performance hub, many low cost VSAT terminals, centralized management, lower price, optimized use of satellite capacity .

Critical to all TDMA schemes is the function of clock synchronization what is performed by the TDMA hub or master earth station. The VSATs may also access the in route on a fixed assigned TDMA mode, wherein each VSAT is allocated a specific time slot or slots.