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Chapter 8. FireWire > Arbitration Protocol

8.1. Arbitration Protocol

The structure of a 1394 serial bus is unique in that the transmission media, consisting of many point-to-point connections between pairs of individual ports, are shared through the repetition of packets received on one port to other active ports by each node. The topology of a 1394 serial bus resembles a tree with many intermediate nodes located at branching joints and a single node at the root. All requests for transmission are either filtered out by intermediate nodes or propagated to the root node for an arbitration decision. A permission to transmit is granted by the root node and delivered through intermediate nodes. The configuration of the tree structure and the election of a root node are automatically carried out during the initialization process and after each bus reset whenever a new port joins the 1394 serial bus. An isochronous packet transmission interval is allocated ahead of the asynchronous interval during each cycle, and these transmission cycles are repeated under the guidance of the cycle master who periodically sends cycle start packets.

8.1.1. Topology

The FireWire transmission system is also called Serial Bus in the IEEE 1394-1995 standards document. A serial bus is different than a serial connection such as RS232, which makes point-to-point connections only. A serial bus is not a conventional bus where multiple devices share a common transmission medium directly. A serial bus connects devices over multiple sections of serial links organized by a communications protocol. The USB (Universal Serial Bus) is another instance of a serial bus. The FireWire serial bus technology addresses both backplane (interior to a computer or communication device) and cable-based transmission physical layers. We only discuss the cable-based physical layer in this chapter. A communication or computing device can have more than one FireWire port. A tree and branch topology is formed when these FireWire ports are interconnected among multiple devices as shown in Figure 8.1. A device can talk to another via these connections. Once permission is granted, a device transmits a packet to its direct neighbors, and they repeat the transmission to their neighbors located further away. Such a device can reach another over multiple devices and links in between.


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