Exercises – Introduction

Exercise 1.3.
It is requested to transfer an byte file from Shannon in Ireland to New York in the United States. Assuming that the connection has a capacity Gbit/s and is realized through a transatlantic fibre optic cable of length km, determine the file size so that when the first bit of the file reaches New York the last bit of the file is transmitted by Shannon.

Given the signal propagation speed on the cable m/s, the propagation time of the signal along the link is given by

   

Then the transmission time T of the file must be the same as that provided by the previous expression, so that the end of the transmission coincides with the instant of arrival of the first bit of the file at the destination. Hence

   

Exercise 1.5.
With reference to the protocol stack of Figure 1.11, determine the portion of the overhead required to transport IUs through a network, assuming that the added header in the transport and network layers is respectively and of the payload carried by the IU, while the link layer adds a fixed header of 20 bytes.

Figure 1.11 Layered model and protocol stack (Exercise 1.5).

By indicating with the length in bits of the message to be transported, the header added in the transport layer determines a segment of length bit. The addition of the header in the network layer results in a packet of length = 1,26 L bit. Finally, the link layer adds a fixed overhead of 20 bytes, resulting in a total overhead

   

Exercise 1.9.
Two hosts connected to the Internet through their respective access ISPs interact thanks to the connectivity made available by two regional ISPs, one for each of the two regions where the hosts are located, and a Tier-1 ISP. Assuming that no more than two routers are crossed in each ISP, determine the number of times in which the operations relating to each of the 5 protocol levels of the layered model are carried out (the application layer is conventionally referred to as number 5).

Since the application and transport layers reside only in the terminal systems, the operations relating to these protocol levels are carried out only twice, therefore, . The functionalities of the three lower layers are carried out in the terminal systems and in all the routers crossed. Since the service provided by each ISP involves crossing at least one router, but no more than two, then the required number can vary from a .