## Network link layer

#### The data link layer as defined in the seven-layer OSI model is focused on achieving reliable information transfer over the physical layer, (the data modulation/demodulation process). This link layer includes the functions of error detection and message/packet retransmission. Two typesof link layer operation are usually defined - connectionless and connection orientated. Connectionless operation launches frames or packets into the network to be transferred to the destination, but with no tracking of the frames to ensure correct delivery, that is, if an error is detected in the frame or packet and it is discarded, there is no means of provision for a retransmission to occur. Connection-orientated operation, on the other hand, has all the built-in tracking mechanisms and facilities for retransmission to make sure that guaranteed error-free transmission occurs. Most of the remainder of this book is concerned with the physical layer of the OSI model – the means by which binary data is best communicated over the physical channel. Chapter 7, however, comes back to discuss some of the error detection and retransmission issues involved in the link layer.