model, a collection of related network-processing functions that comprise one level of a hierarchy of functions. Glossary Layer 1 Layer 1 The physical layer of the seven-layer OSI reference model, which deals with the physi- cal means of sending data over a communications medium, defining physical characteristics such as media type, physical interfaces, electrical and optical coding schemes, and data transfer rates. Layer 2 The data link layer of the seven-layer OSI reference model, which responds to service requests from the network layer and issues service requests to the physical layer. Layer 2 is some- times split into two sublayers: the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The MAC layer controls access to communications links and shares it among many users, and the LLC layer uses procedures and protocols to carry data across the link (and also detects and corrects transmission errors). Examples of Layer 2 protocols in a LAN environment include Ethernet, Token Ring, and FDDI. Examples of Layer 2 protocols in a WAN environment include Frame Relay and ATM. Examples of Layer 2 protocols in the PSTN are SS7 and MTP2. Layer 3 The networking layer of the seven-layer OSI reference model, which identifies com- puters on a network and determines how to direct information transfer over that network. The key responsibility of Layer 3 is to add the addressing information and the control functions needed to move the data through the network and its intermediate nodes. It is involved in estab- lishing, maintaining, and terminating connections, including packet switching, routing, data congestion, reassembly of data, and translation of logical addresses to physical addresses. Exam- ples of Layer 3 protocols are X.25, IP, IPX, and MTP3. Layer 4 The transport layer of the seven-layer OSI reference model, which corrects transmis- sion errors and ensures that the information is delivered reliably. Layer 4 provides end-to-end error recovery and flow control capability. It deals with packet h