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Internetworking with TCP/IP- P7

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Internetworking with TCP/IP- P7: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 28 Review Of Underlying Network Technologies Chap. 2 method used to direct packets from one station to just one other station or a subset of all stations will be discussed later. For now it is enough to understand that the lowest level hardware does not distinguish among transmissions a hub passes all packets to each host interface which chooses packets the computer should receive and filters out all others. Ethernet is called a best-effort delivery mechanism because the hardware provides no information to the sender about whether the packet was delivered. For example if the destination machine happens to be powered down packets sent to it will be lost and the sender will not be notified. We will see later how the TCP IP protocols accommodate best-effort delivery hardware. Ethernet access control is distributed because unlike some network technologies Ethernet has no central authority to grant access. The Ethernet access scheme is called Carrier Sense Multiple Access with Collision Detect CSMA CD . It is CSMA because multiple machines can access an Ethernet simultaneously and each machine determines whether the network is idle by sensing whether a carrier wave is present. When a host interface has a packet to transmit it listens to see if a message is being transmitted i.e. performs carrier sensing . When no transmission is sensed the host interface starts transmitting. Each transmission is limited in duration because there is a maximum packet size. Furthermore the hardware must observe a minimum idle time between transmissions which means that no single pair of communicating machines can use the network without giving other machines an opportunity for access. 2.4.8 Collision Detection And Recovery When a station begins transmission the signal does not reach all parts of the network simultaneously. Instead it travels along copper wires at approximately 70 of the speed of light. Thus it is possible for two transceivers to both sense that the network is idle and .