حل تشریحی سوال شماره 28 زبان تخصصی (انگلیسی)

PART C: Reading Comprehension Directions: Read the following three passages and answer the questions by choosing the best choice (1), (2), (3), or (4). Then mark the correct choice on your answer sheet. There are two important types of packet networks: connectionless packet networks and connection-oriented packet networks. Connectionless packet networks operate in a way analogous to the post office. Connectionless networks offer best effort packet• delivery service. Connectionless packets are launched into the network with a large and powerful header that contains all the information necessary to deliver the packet, whatever its destination. As a by-product of their connectionless nature, it is not possible to guarantee performance or reserve network resources for any particular communication, so connectionless networks do not support real-time services reliably. The switches in the network are stateless. When the network is subjected to overload, packets are buffered for later delivery. If the buffer capacity is exceeded, the packets are discarded. In connectionless networks there is no guarantee that all packets will traverse the same network path and be subject to the same delays. If a communication requires the transmission of a series of packets, it is up to the end stations to ensure that the sequence is preserved. Connection-oriented packet networks have similarities with both circuit networks and packet networks. They are like circuit networks in that a connection, called a virtual circuit, must be established before data can be transported. Network nodes are prepared to support virtual circuits, and they devote resources to them. The resources include buffer memory and link bandwidth. Assumptions are made about the traffic likely to be contributed by a11 the sources, and the network may overload if a11 sources begin sending packets at their peak rate. Occasional pileups of traffic are to be expected and are handled by maintaining local memory to buffer packets until it is possible to transmit them. The network is not stateless. When all the resources are committed, the network may deny requests to establish new virtual circuits. The headers of connection-type packet networks can be shorter than those of connectionless networks because the addressing portion of the header needs only to distinguish each virtual circuit from each other established on common links and switch ports.