90 PART I Networking Basics IEEE 802.3 frame data is 1492 due to the 8 extra bytes needed to represent the type field. Any IP packet larger than this will not fit in a single frame, and must frag- ment its payload into more than one frame and have the payload reassembled at the receiver. That's not all there is to it. LAN implementers and vendors quickly saw that the IEEE 802.3 hardware arrangement was more flexible (and less expensive) than DIX Ethernet. They also saw that the DIX Ethernet II frame structure was simpler and could carry slightly more user data than the complex IEEE 802.3 frame structure. Being prac- tical people, the vendors simply used the flexible IEEE 802.3 hardware with the simple DIX Ethernet II frame structure, creating the mixture that is commonly seen today on most LANs. Today, just because the hardware is IEEE 802.3 compliant (e.g., 100BaseT), does not mean that the frame structure used to carry IP packets is also IEEE 802.3 compliant. The frame structure is most likely Ethernet II, as we have seen. (It's worth pointing out that Ethernet frame content other than IP usually uses the 802.3 frame format. However, the Illustrated Network is basically an IP-only network.) THE EVOLUTION OF DSL IP packet interfaces have been defined for many LAN and WAN network technologies. As soon as a new transport technology reaches the commercial-deployment stage, IP is part of the scheme, if for no other reason than regardless of what is in the middle, TCP/IP in Ethernet frames is at both ends. DSL technologies are a case in point. Origi- nally designed for the "national networks" that would offer everything that the Internet does today, but from the telephone company as part of the Integrated Services Digital Network (ISDN) initiatives of the 1980s, DSL was adapted for "broadband" Internet access when the grand visions of the telephone companies as content providers were reduced to the reality of a restricted role as ISPs and little more. (Even the term "broad- band" is a topic of much debate: A working definition is "speeds fast enough to allow users to watch video without getting a headache or becoming disgusted," speeds that keep dropping as video coding and compression techniques become better.) DSL once included a complete ATM architecture, with little or no TCP/IP. Practical considerations forced service providers to adapt DSLs once again, this time for the real consumer world of Ethernet LANs running TCP/IP. And a tortured adaptation it proved to be. The problem was deeper than just taking an Ethernet frame and mapping it to a DSL frame (even DSL bits are organized into a distinctive transport frame). Users had to be assigned unique IP addresses (not necessary on an isolated LAN), and the issues of bridging versus routing versus switching had to be addressed all over again. This was because linking two LANs (the home user client LAN, even if it had but one PC, and the server LAN) over a WAN link (DSL) was not a trivial task. The server LAN could be the service provider's "home server" or anyplace else the user chose to go on the Internet. Also, ATM logical links (called permanent virtual circuits, or PVCs) are normally provisioned between the usual local exchange carrier's DSLAM and the Internet access