om) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Access Technologies Chapter 6 orbit; the satellite can then rebroadcast the signal back to Earth, covering an enormous service footprint. Because the satellites appear to be stationary, the Earth stations actu- ally can be. One of the most common implementations of geosynchronous technology is seen in the very small aperture terminal (VSAT) dishes that have sprung up like mushrooms on a summer lawn. These dishes are used to provide both broadcast and interactive applications; the small DBS dishes used to receive TV signals are examples of broadcast appli- cations, while the dishes seen on the roofs of large retail establishments, automobile dealerships, and convenience stores are typically (although not always) used for interactive applications such as credit card verifica- tion, inventory queries, e-mail, and other corporate communications. Some of these applications use a satellite downlink but rely on a telco return for the upstream traffic; that is, they must make a telephone call over a land line to offer two-way service. One disadvantage of geosynchronous satellites has to do with their orbital altitude. On the one hand, because they are so high, their service footprint is extremely large. On the other hand, because of the distance from the Earth to the satellite, the typical transit time for the signal to propagate from the ground to the satellite (or back) is about half a sec- ond, which is a significant propagation delay for many services. Should an error occur in the transmission stream during transmission, the need to detect the error, ask for a retransmission, and wait for the second copy to arrive could be catastrophic for delay-sensitive services like voice and video. Consequently, many of these systems rely on forward error correc- tion transmission techniques that allow the receiver to not only detect the error but correct it as well. An interesting observation is th