Not a standard answer nor a reference answer
Do questions only from personal understanding
1. (20 points) Try to compare the main advantages and disadvantages of packet switching and circuit switching.
Circuit switching:
- Features: During the communication between end systems, the resources required for communication along the path between end systems are reserved.
- Pros: The sender can send data to the receiver at a constant rate.
- Disadvantages: It is complicated to create end-to-end circuits and reserve end-to-end bandwidth; there is a silent period, which wastes resources; the performance is not good.
Packet switching:
- Features: Do not reserve resources, but use shared resources on demand.
- Advantages: simpler and more effective; provides better bandwidth sharing; good performance.
- Cons: Not suitable for real-time services.
2. (20 points) What are the three elements of a network protocol? What is the difference between a protocol and a service?
Three elements: syntax, semantics, and synchronization.
① First of all, the implementation of the protocol ensures that services can be provided to the upper layer, and the entities using the services of this layer can only see the service and cannot see the protocol below. The protocol below is transparent to the entities above.
②Secondly, the protocol is "horizontal", that is, the protocol is the rules governing the communication between peer entities. The service is "vertical", that is, the service is provided from the lower layer to the upper layer through the interlayer interface.
3. (20 points) The transmission distance between the sending and receiving ends is 1000km, and the propagation speed of the signal on the media is 2x10^8m/s. Try to calculate the transmission delay and propagation delay for the following two cases.
- The data length is 10^7bit, and the data transfer rate is 100kb/s.
- The data length is 10^3bit, and the data transfer rate is 1Gb/s.
The first case:
- Transmission delay = 10^7bit/100kb/s = 100s
- Propagation delay = 1000km/2x10^8m/s = 0.005s
Second case:
- Transmission delay = 10^3bit/1Gb/s = 10^-6s
- Propagation delay = 1000km/2x10^8m/s = 0.005s
4. (20 points) Consider the queuing delay of the router cache. Let I denote the flow intensity; that is, I = La/R. Assume that the queuing delay is of the form IL / R(1-I), where I < 1. Write down the formula for the total delay, which is the queuing delay plus the transmission delay.
Total delay = IL/R(1-I) + L/R
5. (20 points) What is an application layer message, what is a transport layer segment, what is a network layer datagram, and what is a link layer frame?
- Application Layer Message: A packet of information exchanged by one end system with another end system application
- Transport layer segment: a grouping of the transport layer
- Network layer datagram: packet at the network layer
- Link layer frame: packet of the link layer