CS 622
Assignment 1
2.1
Each of these layer models is a real world example of the three layer network model.
In the first example model, the physical connection is the telephone line. The lowest layer on both the receiving and sending ends is the telephone. The telephone acts as the network access layer. It allows the receiving and sending ends access to the telephone line, and it also establishes the communication link between the two ends. The second layer up acts as the transport layer on both the sending and receiving sides. This layer in the transmitter is the host which acts as the mediator between the guest who wants to order the pizza and the telephone. The host provides access to the telephone. On the receiving end is the order clerk whois the itermediary at the pizza parlor. The clerk answers the phone and takes the order. At the top layers resides the application layer. In this example the sender’s “application” is the guest who provides the data, pizza order and the receiver’s “application” is the pizza cook who assembles the pizza pie.
In the second example model, the physical connection is the road. The network acces layer is exemplified by the delivery van. It allows the pizza being delivered a reliable means to the use the road. The transport layer is where the order clerk finalizes the pizza order and gives it to the person who will deliver the pizza in the delivery van. On the receiving end, the host pays for the pizza, which the guest as the application layer on the receiving end will eat. On the transmitting side, the cook acts as the application layer and prepares the pizza, which will be handed off to the clerk to package for delivery.
2.4
The two blue armies don’t have a reliable way to communicate their attack plan, because they are unsure if their messengers get through to each commander. If their messaging protocol relied on the acknowledgement of a message, then either the message or the acknowledgement could be lost. They could add to the protocol an acknowledgement to the acknowledgement, but then each message sent still is not garaunteed to reach its destination. The armies could introduce redundancy into their protocol. They could send two or more messengers to send or acknowledge a message, and only rely on reception of one message or acknowledgement to attack, but this is a waste of resources and could still lead to a lost messenger. The two armies could introduce a timeout to there protocol scheme by waiting a certain time for the acknowledgements before proceeding or terminating the attack.
