“The most constant characteristic of computing is the ability of users to saturate any system available to them”
Murphy’s Law, 1947
We now turn to a presentation of the data link layer, the second layer of the open systems interconnection (OSI) and always the first layer of the Department of Defense’s system (see Figures 3.10 and 3.11 of Chapter 3). The protocols of this layer process request services from the network layer (layer 3) and carry out a solicitation of requests for services to the physical layer (layer 1). The purpose of these protocols (protocol data unit — PDU) is the transfer of data between two adjacent nodes of a local or wide area network. The functions of these protocols are mainly:
– frame transfer between two local devices;
– addressing equipment delivery with a frame header containing the source address (from the frame) and the destination address (receipt and processing);
– arbitration of access to means of communication;
– possibly a mechanism for receipt (acknowledgment) and validation of received frames or request for retransmission;
– possibly a device for detecting errors;
– possibly an error correction device.
There are many examples of protocol, such as Asynchronous Transmission Mode (ATM), Infrared Data Association (IrDA), or WiFi, but the best known example of a data link protocol is Ethernet (IEEE 802.3). In this example, there are two main sublayers: ...