tor type used. Physical layers of the systems are designed to operate practically error free. If the physical layer cannot deliver error-free data to the upper layer, it does not perform retransmission for error recovery, but leaves the consequent actions to the upper layers. The specifications of the physical layer deal with mechanical, electrical, and procedural interfaces and the physical transmission medium. The trans- mission medium is understood to be below the physical layer but the specifi- cations include the characteristics required by it. Let us look at some examples of the physical layer protocols: V.24, RS-232-C, EIA-232D (latest version): electrical characteristics of the asymmetrical data signals and their usage; IS 2110: specification of a physical data interface connector; Introduction to Telecommunications Network Engineering I.430 (IS 8877): ISDN basic rate user interface; ANSI 9314: specification of optical interface for wideband data net- work called fiber distributed data interface (FDDI); IEEE 802 and ISO 8802 series: physical interfaces of Ethernet-based LANs and WLANs. The last two examples contain data link layer specifications as well. 6.3.3.3 Data Link Layer The data link layer builds the frames and sends them to the following node on the line via the physical layer. It receives frames, checks if these frames are error free, and delivers error-free frames to the network layer. The data link layer at the receiver may send acknowledgment of error-free frames to the transmitting end. The transmitter may retransmit the frame if no acknowl- edgment is received within a certain time period. Note that this procedure takes place between each pair of nodes on the way. The ISO has specified the data link layer for LANs and divides the specifications into two sublayers: (1) the medium access (MAC) sublayer and (2) the logical link control (LLC) sublayer. This division is necessary for LANs because of the complexity of the data link layer in this kind of application. In LANs computers are connecte