t, Iris, and Vitasat provide (or will provide) store-and-forward services for low-rate data and messaging services. Source mobile Figure 2-1. Store-and-Forward and Near-Real-Time Systems [Richharia95] Figure 2-2 depicts a communication in which a satellite has both a subscriber terminal and ground-station simultaneously in view and has an established communication with both of the terrestrial entities. The ground station could be a direct television broadcast facility and the subscriber terminal a DBS television receiver, in which case there exists only an uplink from the ground station to the satellite and a downlink from the satellite to the subscriber terminal. In this case the function performed by the satellite is to receive the uplink, frequency translate to the desired downlink frequency, and then amplify and transmit the downlink signal; the satellite transponder is providing analog bent-pipe service. The ground station could be a gateway of a mobile satellite services system and the subscriber terminal a mobile telephone, in which case the satellite establishes both uplink and downlink communications channels with both. The satellite receives the uplink signal from either the gateway or subscriber unit, demodulates it to baseband digital information, modulates the transmit signal and downlinks it to the subscriber unit or gateway systems. In this case the satellite payload is providing digital bent-pipe service. In the bent-pipe approach, all networking and routing processes are performed on the ground. Satellite i-th Satellite j-th User's Terminal User's Terminal Gateway Terrestrial Line Gateway Figure 2-2. Bent-Pipe Telecommunications Satellites [Jamalipour98] The alternative method for routing and propagating traffic through the network is implement intersatellite communications links between the satellites and to route traffic through the space network with need to downlink to a gateway only to achieve interconnection with the public switched telephone network. The advantage of using intersatellite l