fines the type of media (for instance, cop- per, wireless, or fiber-optic) and how devices access media. Repeaters used to extend signals over fiber, wireless, and copper networks are Layer 1 devices. Repeaters in cellular networks extend and boost signals inside buildings and in subways SO that users can still take advantage of their cellular devices in these otherwise network-inaccessible locations. Layer 2: Data Link Layer Ethernet, also known as 802.3, is a Layer 2 protocol. It provides rules for error correction and access to LANs. Layer 2 devices have addressing information analo- gous to Social Security numbers; they are random but specific to individual locations. Frame Relay is a Layer 2 protocol previously used to access carrier networks from enterprises. Layer 3: Network Layer Layer 3 is known as the routing layer. It is responsible for routing traffic between networks that use IP net- work addresses. Layer 3 has error-control functions. Layer 3 is analogous to a local post office routing an out-of-town letter by ZIP code while not looking at the street address. Once an e-mail message is received at the distant network, a Layer 3 device looks at the specific address and delivers the message. Appendix OSI Layer Name and Number Layer Function Layer 4: Transport Layer Layer 4 protocols enable networks to differentiate between types of content. They are also known as content switches. Layer 4 devices route by content. Video or voice transmissions over data networks might receive a higher priority or quality of service than e-mail, which can tolerate delay. Filters in routers that check for computer viruses by looking at additional bits in packets perform a Layer 4 function. Transmission Control Protocol (TCP) is a Layer 4 protocol. Layer 5: Session Layer Layer 5 manages the actual dialog of sessions. Encryption that scrambles signals to ensure privacy occurs in Layer 5. H.323 is a Layer 5 protocol that sends signals in packet networks to set up and tear down, for example, video and audio conferences. Layer