Local-area networks (LANs) are confined to small geographic areas, such as your home or office building. Wide-area networks (WANs) span broad geographic areas, such sections of a country or continents. WANs interconnect LANs and create what appears to users as a single network.
Information sent across media is called a signal and is in electronic (analog or digital), optical (light), or radio (wireless or cellular) form. Analog signals are measured as continuous waves with a certain frequency, whereas digital signals are measured as square waves with discrete values: 1 or 0. Optical signals are light pulses and are also measured as square waves with the same values as digital signals. Radio signals are measured like analog signals, in continuous waves with a specified frequency.
Recall the physical topology of a network is its layout; the logical topology determines where the devices are placed in the network and how these devices communicate with each other. It is the topology that also determines how network devices talk with each other, either in a direct path or through another device. A full-mesh topology enables every network device to talk with every other device?each device has a direct path to every other device. A star topology provides a central point in the network for communication from each device to pass through.
The physical (OSI model Layer 1) topology of a network represents how each device is interconnected by media or equipment. The logical (OSI model Layers 2 and 3) topology of a network represents the conceptual view of how devices are interconnected, often, but not always, bearing a resemblance to the physical topology.
Hubs carry bits, switches carry frames, and routers carry packets. They all connect physical segments together to create a larger network. Frames are moved around the network by Layer 2 hardware, such as bridges or switches. Bridges and switches use the frame header to determine to which network segment the frame must be forwarded. Bridges and switches determine forwarding decisions for frame movement based on a forwarding table in a MAC table.
The packet, a Layer 3 data unit, is carried by the frame inside its payload section. Packets are the concern of Layer 3 hardware, such as routers. The difference is that whereas a bridge or switch just forwards the frame out a specified port, routers decide the disposition of the packet, such as through which port to forward the packet and if the router is to forward the packet at all. A router can make a more intelligent decision because it knows the source and destination and has capacity to make a decision about paths that are several hops downstream from the router.