TABLE 102.4 Random Multi-Access Protocols Comparison Throughput Comments Pure aloha 0.13-0.18 Low cost, good for variable-length messages Slotted ALOHA 0.25-0.37 Good for fixed-length messages elective reject ALOHA 0.20-0.30 Variation of pure ALOHA with a modified algorithm Tree CRA 0.40-0.49 Sensing capability for collision resolution, good for fixed-length messages 0.50-0.60 Uses modified algorithm of slotted AlOHa by announcement of random access (ARra) Random access with notification 0.45-0.55 Uses partition for new and retransmitted message CDMA 0. 10-0.40 Used in spread spectrum systems, low delay information embedded in the modulated data stream. The delay performance of this channel is essentially controlled by the queuing behavior of the hub. In the VSAT-to-hub direction, a large number of VSATs share the channel to conserve space segment. Most VSAT networks utilize a combination of slotted ALOHA protocol for the interactive component of the inbound traffic and a reservation TDMA scheme for any bulk data transfers Most protocols are adaptive in the sense that as the channel traffic increases, they automatically evolve into reservation TDMA systems Code division multiple access( CDMA) has been used in VSATs operating at C-band CDMA permits more than one signal to simultaneously utilize the channel bandwidth in a noninterfering manner. This makes it possible to significantly increase the utilization and throughput of the channel. Interface Capabilities Most VSAT systems support common data communications protocols such as SDLC, X 25, Async, Bisync, TCP/IP, etc. Coexistence of different protocols is allowed in a network. A VSAT supports multiple ports with common interfaces such as RS232C, RS422, V.35, etc. VSAT networks typically must provide protocol spoofing to provide acceptable delay and throughput performance to the end-user application. To minimize the effect of satellite delay, the host computer front-end processor is emulated at the VSAT location, and multiple cluster controllers are emulated at the hub location. The polling associated with the front-end processor to cluster controller communication is not carried on the satellite link, but is instead emulated locally Video Satellites are an excellent medium for video transmission since they can provide a bro wide bandwidth. Video on satellites is ideal for applications such as videoconferencing, business TV, distance learning, satellite news gathering, etc Video teleconferencing Satellite communications provides a cost-effective and flexible means of interactive videoconferencing. Tech nological improvement in videocompression has resulted in low-cost codecs at data rates less than Tl, and good quality videoconferencing is possible at data rates as low as 56 Kbps. Low-cost satellite terminals coupled with low-cost codecs are making videoconferencing via satellite affordable and practical for many organization Applications include all types of business meetings and technical information exchange such as managemer and staff meetings, new product introductions and updates, sales meetings, training, and market presentatio Videoconferencing allows people at different locations to meet with almost as much ease as being in the same room, providing benefits of increased productivity, reduced travel time and cost, and increased management visibility. a generic videoconference system is presented in Fig 102.10. The system consists of a specially designed room, video/audio equipment, transmission equipment, monitor and control computer, and space segment. The video and audio feeds from the meeting room pass through the codec and are compressed. From the codec, the signal passes to the satellite modem for modulation. The radio frequency/terminal (RFT) upconverts the modulated carrier and amplifies it for transmission to the satellite. At the other site, the process is reversed. A videoconferencing network features point-to-point, broadcast, or point-to-multipoint architectures. In a oint-to-point system, two sites are configured for interactive conference with duplex audio and full motio video transmission. Videoconferencing broadcast is appropriate for formal presentations where the presenter e 2000 by CRC Press LLC© 2000 by CRC Press LLC information embedded in the modulated data stream. The delay performance of this channel is essentially controlled by the queuing behavior of the hub. In the VSAT-to-hub direction, a large number of VSATs share the channel to conserve space segment. Most VSAT networks utilize a combination of slotted ALOHA protocol for the interactive component of the inbound traffic and a reservation TDMA scheme for any bulk data transfers. Most protocols are adaptive in the sense that as the channel traffic increases, they automatically evolve into reservation TDMA systems. Code division multiple access (CDMA) has been used in VSATs operating at C-band. CDMA permits more than one signal to simultaneously utilize the channel bandwidth in a noninterfering manner. This makes it possible to significantly increase the utilization and throughput of the channel. Interface Capabilities Most VSAT systems support common data communications protocols such as SDLC, X.25, Async, Bisync, TCP/IP, etc. Coexistence of different protocols is allowed in a network. A VSAT supports multiple ports with common interfaces such as RS232C, RS422,V.35, etc.VSAT networks typically must provide protocol spoofing to provide acceptable delay and throughput performance to the end-user application. To minimize the effect of satellite delay, the host computer front-end processor is emulated at the VSAT location, and multiple cluster controllers are emulated at the hub location. The polling associated with the front-end processor to cluster controller communication is not carried on the satellite link, but is instead emulated locally. Video Satellites are an excellent medium for video transmission since they can provide a broadcast capability with wide bandwidth. Video on satellites is ideal for applications such as videoconferencing, business TV, distance learning, satellite news gathering, etc. Video Teleconferencing Satellite communications provides a cost-effective and flexible means of interactive videoconferencing. Tech￾nological improvement in videocompression has resulted in low-cost codecs at data rates less than T1, and good quality videoconferencing is possible at data rates as low as 56 Kbps. Low-cost satellite terminals coupled with low-cost codecs are making videoconferencing via satellite affordable and practical for many organizations. Applications include all types of business meetings and technical information exchange such as management and staff meetings, new product introductions and updates, sales meetings, training, and market presentations. Videoconferencing allows people at different locations to meet with almost as much ease as being in the same room, providing benefits of increased productivity, reduced travel time and cost, and increased management visibility. A generic videoconference system is presented in Fig 102.10. The system consists of a specially designed room, video/audio equipment, transmission equipment, monitor and control computer, and space segment. The video and audio feeds from the meeting room pass through the codec and are compressed. From the codec, the signal passes to the satellite modem for modulation. The radio frequency/terminal (RFT) upconverts the modulated carrier and amplifies it for transmission to the satellite. At the other site, the process is reversed. A videoconferencing network features point-to-point, broadcast, or point-to-multipoint architectures. In a point-to-point system, two sites are configured for interactive conference with duplex audio and full motion video transmission. Videoconferencing broadcast is appropriate for formal presentations where the presenter TABLE 102.4 Random Multi-Access Protocols Comparison Throughput Comments Pure ALOHA 0.13–0.18 Low cost, good for variable-length messages Slotted ALOHA 0.25–0.37 Good for fixed-length messages Selective reject ALOHA 0.20–0.30 Variation of pure ALOHA with a modified algorithm Tree CRA 0.40–0.49 Sensing capability for collision resolution, good for fixed-length messages Announced retransmission 0.50–0.60 Uses modified algorithm of slotted ALOHA by announcement of random access (ARRA) transmission Random access with notification 0.45–0.55 Uses partition for new and retransmitted message CDMA 0.10–0.40 Used in spread spectrum systems, low delay