Practical TCP/IP and Ethernet Networking- P41: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 182 Practical TCP IP and Ethernet Networking Figure Fast Ethernet hub interconnection Switches Ethernet switches are an expansion of the concept of bridging and are in fact intelligent self-learning multi-port bridges. They enable frame transfers to be accomplished between any pair of devices on a network on a per-frame basis. Only the two ports involved see the specific frame. Illustrated below is an example of an 8 port switch with 8 hosts attached. This comprises a physical star configuration but it does not operate as a logical bus as an ordinary hub does. Since each port on the switch represents a separate segment with its own collision domain it means that there are only 2 devices on each segment namely the host and the switch port. Hence in this particular case there can be no collisions on any segment In the sketch below hosts 1 7 3 5 and 4 8 need to communicate at a given moment and are connected directly for the duration of the frame transfer. For example host 7 sends a packet to the switch which determines the destination address and directs the package to port 1 at 10 Mbps. Figure 8-Port Ethernet switch LAN system components 183 If host 3 wishes to communicate with host 5 the same procedure is repeated. Provided that there are no conflicting destinations a 16-port switch could allow 8 concurrent frame exchanges at 10 Mbps rendering an effective bandwidth of 80 Mbps. On top of this the switch could allow full-duplex operation which would double this figure. Cut-through vs store-and-forward Switches have two basic architectures cut-through and store-and-forward. In the past cut-through switches were faster because they examined the packet destination address only before forwarding the frame to the destination segment. A store-and-forward switch on the other hand accepts and analyzes the entire packet before forwarding it to its destination. It takes more time to examine the entire packet but it allows the switch to catch certain .
