Please answer all 3 question below. Thank you
1. Give a short answer to the following topics:
Are all the layers necessary? If so, according to what criteria we may devise the number of layers?
Why do LANs tend to use broadcast networks? Why not use networks consisting of multiplexers and switches?
What are the advantages and disadvantages of the current approaches in packet-switching?
2. A university computer science department has 3 Ethernet segments, connected by two transparent bridges into a linear network. One day, the network administrator quits and is hastily replaced by someone from the computer center, which is a token ring shop. The new administrator, noticing that the ends of the network are not connected, quickly orders a new transparent bridge and connects both loose ends to it, making a closed ring. What happens next. Explain your answer.
3. Let's assume the following situation: In principle, we may have two acknowledgment strategies when a file is transferred between two computers. One is to segment the file into packets, that are individually acknowledged by the receiver, but the file transfer as a whole is not acknowledged. In the other one, packets are not acknowledged individually, but the entire file is acknowledged when it arrives. Discuss these two approaches in terms of pros and cons.
The communication and processing functions of a computer network are done via several cooperating computing processes. The architectural model that defines the classes of functions or protocols involved in this cooperation is based on the concept of layering. So, the whole system is viewed as a hierarchy of functional layers. Every layer adds to the system a series of functions based on the services supplied by the below subordinate level. The functions are accomplished by the action of some entities or modules that belong to different nodes of the network. The entities on the same level coordinate their actions by transmitting a control information according to a certain protocol. Such a network with a perfect layering of its functions may be a reference model for open systems interconnection.
When a resource is shared among multiple independent stations or users, the need exists for a mechanism to control access to the resource, or in the absence of such a mechanism there must exist a course of action to be taken when two or more users attempt to acquire the resource at the same time. One solution would be to allow stations to transmit whenever they have data to send. A node transmits at random (i.e. there is no priory coordination among nodes) at the full rate of the channel. If two or more nodes collide, they retransmit at random times. Contention among users (or collisions of transmissions) limits network throughput and increase time delay. Thus, some protocols must be employed to exercise control over the access method of users. Random Access Protocols address this situation. The random access MAC protocol specifies how to detect collisions and how to recover from them (for instance, via delayed retransmissions). There are a number of MAC protocols which have been developed for various communications networks. Depending upon the type and limitations of the network, each protocol has its advantages and disadvantages as well as unique performance limitations.
Some protocols have became "de facto" standards due to their widespread by a large product selling. Thus, two of the early ARPA's protocols have proliferated due to their inclusion in the UNIX Berkeley operating system. These are the network protocol IP (Internetwork Protocol) and the transport protocol TCP (Transmission Control Protocol). However, these protocols that are the foundation of the TCP/IP architecture are not unanimously accepted by the networks manufacturers. In order to exist a unitary way of conceiving the functions performed at every level, the International Organization for Standardization (ISO) has designed a reference model of protocol levels with 7 layers, called Open System Interconnection (OSI). This model serves as a basis for the design and standardization of ISO's own protocols.
An alternative to circuit-switching, where the communication lines are dedicated to passing messages from the source to the destination is packet-switching. Packet switching originates in message switching, where each message can travel from the source to destination independently. In order to know how to arrive at destination, the message must contain the destination address and also the source address for message identification. The message is transmitted in its entirety from one node to another. When messages are broken into smaller units called packets, the message switching becomes packet switching. The idea is to ensure that the packets size are more convenient for the network transmission. At the destination the packets are reassembled into the original message.
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