4.7 Linux s Unique Features

		Figure 5.1 Ethernet Packet Format

		times, thus avoiding a repetition of the same collision. Of course, other nodes could attempt to use the network medium and cause further collisions. The important aspect of the CSMA/CD approach is that there is no central controller managing network arbitration and only local knowledge is required at each node of the interconnect topology. As different entities are connected or disconnected from the network, at least at the lowest levels of network management, there need be no global update of network organization at each node. For other reasons, at a higher level, this may not be true.

		5.1.2 Packet Format

		The Ethernet message packet comprises a sequence of seven multi-bit fields, one of which is variable length. The fields include a combination of network control information and data payload. The structure of the Ethernet packet is shown in Figure 5.1 and is described below. The packet's variable length allows improved overall network performance across a wide range of payload requirements. Thus, a transfer of only a few words between nodes does not impose the full burden of the longest possible packet. However, even with this capability, sustained data transfer throughput is sensitive to packet length and can vary by more than an order of magnitude depending on payload size, even in the absence of network contention.

		Preamble Arrival of a message packet at a receiving node (whether or not the message is addressed for that node) is asynchronous. Prior to data assimilation, the node and the incident packet must first synchronize. The Preamble field is a 62 bit sequence of alternating 1s and 0s that allows the phase lock loop circuitry of the node receiver to lock on to the incoming signal and synchronize with the bit stream.

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		Synch The Synch field is a 2 bit sequence of 1s (11) that breaks the sequence of alternating 1s and 0s provided by the Preamble and indicates where the remaining information in the packet begins. If the Preamble provides carrier-level synchronization, the Synch field provides bit field registration.