 10101010 10101011 6 bytes 6 bytes 4 bytes Payload (data and padding) 46-1500 bytes CRC32 Inter-frame gap 4 bytes 12 bytes Figure 8.12 Ethernet header and data fields some 5x1028 addresses for every human being alive today, and is expected to be sufficient for the foreseeable future. Similarly a number of proprietary network layer protocols exist which are used for pro- prietary applications. Link (Network Access) Layer Protocols As discussed above, the link layer (also known as the network access layer) equates to a combination of data link and physical layers of the OSI model. It is therefore designed with due consideration to the physical layer characteristics and the medium used for telecommuni- cations. A large number of public and proprietary protocols exist: for example specific proto- cols for wireless (such as wireless local area network) fixed line [Integrated Services for Digital Network (ISDN) and digital subscriber line (DSL)], cable, Ethernet and SO on. Many of these applications use definitions from the lower three layers of the OSI architecture such as how a physical medium resource may be accessed to initiate digitally modulated signals. For example, the Ethernet protocol enables two devices to send and transmit a sequence of 1's and O's. This is facilitated through a specific header structure as shown in Figure 8.12. A 7-byte preamble consisting of a predetermined sequence of 1's and O's is utilised to alert a receiver of an incoming data packet before data transmission starts. A receiver continuously monitors the transmission medium and detects signals as a random series of 1's and O's. The probability that random noise may generate this specific preamble is small, and therefore a monitoring device is unlikely to falsely detect a reception. The receiver then detects the net- work layer address of a device [usually a media access control (MAC) address] and proceeds to extract the Ethernet payload. Ethernet protocol employs a strong error detection mechanism to ensure incorrectly received data ar