二層プロトコルの意味と種類

最近フレームリレー、ATM,ISDNに関して本を読んでいます。IPとの違いを考えれば悩みます。しかしエーサーネットと違いを考えれば状況がさらに不明になります。
Frame Relay, ATM, and ISDN are considered to be forms of WAN protocols. These operate at Layer 2 of the OSI hierarchy. The first question to ask is "why should layer 2 exist separately from layer 3?" The next question is: "what differentiates WAN protocols from LAN protocols like Ethernet?" The final question is: "how are frame relay, ATM, and ISDN different and when is each more appropriate than another?"
In response to the first question, the thought experiment which would provide an answer is to consider sending Ethernet frames (assuming Ethernet could span the distance) from a LAN host to an ISP with destination of a MAC address on the ISP's network. If the ISP and LAN were connected via an Ethernet hub, this would seem possible, as the hub would flood the packet from the LAN host to all ports. However, the fact that the ISP does not desire this topology (it does not want to expose its internal network to the outside customers) causes it to not connect its hosts to the Ethernet with the customer. Only a single router would be connected to the customer at layer 2. Thus, the only viable destination MAC is the router's MAC, and the router will forward packets to the customer only to IP addresses (layer 3) it is configured to permit. Since these hosts on the other side of the router from the customer are not on the same LAN, no layer 2 discussions take place between the custoner and these hosts. However, packets can be exchanged using layer 3 addressing. Thus, layer 3 is needed to provide routing between LANs, and layer 3 is necessary to enable communication within a LAN (translates to physical layer bits).
In response to the second question about the discrepancy between LANs and WANs,the question is one of distance, it seems. The WAN link provides the same framing, synchronization, and error control as a LAN link, but the distance between sites means that the WAN link is usually passing through some third party's network (e.g. the phone company's).