D-MAC

Abstract: The United Kingdom standard for direct broadcasting by satellite (DBS) is described. A multiplexed analog component (MAC) vision format was devised to overcome the inherent limitations of PAL. The resulting D-MAC/packet system offers improved pictures compared to PAL, together with rugged high-quality digital sound, secure encryption, the flexibility to provide widescreen pictures and, in the future, higher definition. The system is evolutionary in concept, and can be used in conjunction with existing television displays. The single-wire baseband format of D-MAC/packet takes up a bandwidth of approximately 8.5 MHz. It consists of time-compressed components of the luminance and color-difference vision signals, together with a 20.25 Mb/s digital data burst in duobinary form. Frequency modulation is applied for satellite transmission, while for distribution on cable networks, vestigial sideband amplitude modulation can be used. >

Abstract: Overview An essential characteristic of wireless communication is that it provides an inherently shared medium. All medium-access control (MAC) protocols for wireless networks manage the usage of the radio interface to ensure efficient utilization of the shared bandwidth. MAC protocols designed for wireless sensor networks have an additional goal of managing radio activity to conserve energy. Thus, while traditional MAC protocols must balance throughput, delay, and fairness concerns, WSN MAC protocols place an emphasis on energy efficiency as well. We shall discuss in this chapter a number of contention-based as well as schedule-based MAC protocols that have been proposed for WSN. A common theme through all these protocols is putting radios to a low-power “sleep mode” either periodically or whenever possible when a node is neither receiving nor transmitting. Traditional MAC protocols We begin with a focus on contention-based MAC protocols. Contention-based MAC protocols have an advantage over contention-free scheduled MAC protocols in low data rate scenarios, where they offer lower latency characteristics and better adaptation to rapid traffic variations. Aloha and CSMA The simplest forms of medium-access are unslotted Aloha and slotted Aloha. In unslotted Aloha, each node behaves independently and simply transmits a packet whenever it arrives; if a collision occurs, the packet is retransmitted after a random waiting period. The slotted version of Aloha works in a similar manner, but allows transmissions only in specified synchronized slots. Another classic MAC protocol is the carrier sense medium-access (CSMA) protocol.

Abstract: The United Kingdom is about to enter a new and exciting phase of its broadcasting history. On 27th August 1989 a satellite was launched for the world's first high-power direct broadcasting by satellite (DBS) service which is funded entirely privately. For the success of this service the broadcaster needs to establish a large customer-base quickly and yet needs to offer the potential of receiver improvements for the future. Against this background the D-MAC/packet system has been adopted as the signal format to be used. This system offers a reception option involving a relatively inexpensive set-top convertor and yet it also provides a number of important options for enhancing the service. The paper briefly describes the D-MAC transmission system and highlights in particular how it can provide an evolution to high-definition television (HDTV) in the future.