ansmits the PPDU over the physical medium, whether that is twisted-pair, fibre-optic cable, infra-red or radio. PHY layer technologies determine the maximum data rate that a network can achieve, since this layer defines the way the data stream is coded onto the physical transmission medium. However, the MAC and PLCP headers, preambles and error checks, together with the idle periods associated with collision avoidance or backoff, mean that the PMD layer actually transmits many more bits than are passed down to the MAC SAP by the Data Link layer. The next sections look at some of the PHY layer technologies applied in wired networks and briefly introduces the key features of wireless PHY technologies. Physical Layer Technologies - Wired Networks Most networks that use wireless technology will also have some associated wired networking elements, perhaps an Ethernet link to a wireless access point, a device-to-device Fire Wire or USB connection, or an ISDN based Internet connection. Some of the most common wired PHY layer technologies are described in this section, as a precursor to the more detailed discussion of local, personal and metropolitan area wireless network PHY layer technologies in Parts III to V. Ethernet (IEEE 802.3) The first of these, Ethernet, is a Data Link layer LAN technology first developed by Xerox and defined by the IEEE 802.3 standard. Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), described above, as the media access control method. Ethernet variants are known as "A" Base-"B" networks, where "A" stands for the speed in Mbps and "B" identifies the type of physical medium Wireless Network Logical Architecture used. 10 Base-T is the standard Ethernet, running at 10 Mbps and using an unshielded twisted-pair copper wire (UTP), with a maximum distance of 500 metres between a device and the nearest hub or repeater. The constant demand for increasing network speed has meant that faster varieties of Ethernet have been progressively developed. 100 Base-T, or Fast Ethern