4.8.1 Installation by CD or Network?

		for access to the data link and supplies the packet to the physical layer which begins transmission. If in the process of packet transmission the MAC receives a collision signal from the Physical Layer, after briefly continuing transmission (to overcome the network propagation delay) it terminates the message and begins its random roll-back sequence to determine a new time when it will again attempt to transmit the message.

		Physical Layer The Physical Layer (PHY) encodes the message packet provided by the Data Link Layer and converts it to electrical signals appropriate for the physical transmission medium. Upon receiving messages transmitted by other nodes. the Physical Layer acquires the electrical signals from the transmission medium, converts them to digital signals, and decodes them into the message's binary bit sequence. The PHY layer includes two major stages: the transceiver and the Physical Line Signaling (PLS) sublayer. The transceiver, also referred to as the Medium Attachment Unit (MAU) performs the electrical conversion from transmission media signals to logical signal levels.

		The interface between the PLS sublayer of the Physical Layer and the MAC sublayer of the Data Link Layer exchanges data with bits represented as discrete voltage levels. This form of information representation is inadequate for Ethernet for two reasons. First, in a highly noisy (in the electrical sense) environment such as presented by a LAN, signal levels can be significantly attenuated and distorted. Second, in a single bit-serial communication protocol such as that employed by the Ethernet interconnect, both data and timing information need to be incorporated in the signal. For this reason, Manchester encoding is used to convey the information with the value of a bit specified by the sense (direction) of the signal transition rather than a specific range of values. With data fixed at the point of signal transition, the timing information is provided simultaneously.

		The actual Ethernet signal is differential, that is, one line is high when the other is low and vice versa. The PLS sublayer converts the message packet provided by the MAC first into its Manchester encoded representation and then into differential form. The PLS layer performs the decoding task for