 deals with unshielded twisted-pair (UTP) and shielded twisted-pair (STP), coax, 10BASE-T (an Ethernet standard that allows the use of twisted-pair to support 10Mbps to the desktop, largely for legacy systems), 100BASE-T (the standard enterprises currently favor), multimode fiber and single- mode fiber, xDSL, ISDN, and the various capacities in PDH (e.g., DS-1/DS-3, E-1/ E-3) and SDH/SONET (e.g., OC-1 through OC-192) networks. The TCP/IP Four-Layer Reference Model TCP/IP, which is an entire set of internetworking protocols, is described by its own layered model-somewhat like the OSI model but also unique. Whereas OSI The OSI Reference Model and the TCP/IP Reference Model defines seven layers, the TCP/IP model is a four-layer network architecture (see Figure 5.9). TCP/IP Layer 1, the network access protocols, relates to OSI Layers 1 and 2. It defines the range of networks that can be interconnected with IP and through which an IP datagram can be forwarded on the way to its destination. The many options include the following: Ethernet LANs (10Mbps, 100Mbps, 1Gbps, and 10Gbps) Token-ring LANs (4Mbps and 16Mbps) Wireless LANs (802.11x, better known as Wi-Fi) 2.5G and 3G wireless WANs Private line or dialup links (using Point-to-Point Protocol [PPP]) Frame Relay networks (Frame Relay interface) ATM-user network interface TCP/IP Layer 2 deals with internetworking protocols and is analogous to OSI Layer 3. This layer is the key to the architecture: It realizes the interconnection of remote (heterogeneous) networks without establishing an end-to-end connection. Its role is to inject packets into any network and deliver them to the destination independent of one another. Because no connection is established first, packets may not be received in order; the delivery order control process is the responsibility of the upper layers. Because of the major role of this layer in the packet delivery pro- cess, the critical point of this layer is routing. That is why this layer compares to the network layer of the OSI model. IP is t