ing layer. It defines how to move information between networks, provid- ing the functional and procedural means of transferring variable-length data sequences from a source to a destination via one or more networks while maintain- ing the QoS requested by the transport layer. The key responsibility of this layer is to perform network routing, flow control, segmentation/desegmentation, and error control functions. Examples of network layer protocols are X.25, Internet Protocol (IP), Internetwork Packet Exchange (IPX), and Message Transfer Part (MTP; part of the PSTN). Layer 2, the data link layer, groups data into containers to prepare that data for transfer over a network. It puts the ones and zeros into a container that allows the movement of information between two devices on this same network. The proto- cols at this layer specify the rules that must be followed in transmitting a single frame between one device and another over a single data link. Bits are packaged into frames of data, and they include the necessary synchronization, error control, and flow control information. Examples of data link layer protocols in a LAN envi- ronment include Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI). Examples of data link layer protocols in a WAN environment include Frame Relay and Asynchronous Transfer Mode (ATM). Examples of data link layer protocols within the PSTN are Signaling System 7 (SS7) and MTP2. Layer 1, the physical layer, defines how a transmission medium connects to a computer as well as how electrical or optical information is transferred on the transmission medium. The physical layer defines the types of cables or wireless interfaces that are allowed, the voltage levels used to represent the bits or the opti- cal levels, the types of connectors that are allowed, and the types of transmission rates that can be supported. Every network service and every network device has definitions at the physical layer in terms of what it can physically interface with. For example, the physical layer