計算機科學與導論答案第6章

1、C H A P T E R6Computer Networks(Solutions to Review Questions and ProblemsReview QuestionsQ6-1.To make the communication bidirectional, each layer needs to be able to pro-vide two opposite tasks, one in each direction.Q6-2.The identical objects are the two messages: one sent and one received.Q6-3.a.

2、At the application layer, the unit of data is a message.b.At the network layer, the unit of data is a datagram.c.At the data-link layer, the unit of data is a frame.Q6-4. A frame is a link-layer data unit. It encapsulates a data unit coming from the network layer. In this case, the data unit is a da

3、tagram.Q6-5. A user datagram is a transport-layer data unit. It decapsulates a data unit going to the application layer. In this case, the data unit is a message.Q6-6.The data unit should belong to layer 4. In this case, it is a user datagram.Q6-7.a.At the application layer, we normally use a name t

4、o define the destina-tion-computer name and the name of the file we need to access. An exam-ple is something.b.At the network layer, we use two logical addresses (source and destina-tion to define the source and destination computers. These addresses areunique universally.c.At the data-link layer, w

5、e use two link-layer addresses (source and desti-nation to define the source and destination connections to the link.12Q6-8.Probably Alice turned off her desktop, which stopped the FTP server, whenshe left the office. A server process should be running all the time, waiting forclients to access it.Q

6、6-9. A personal computer, such as a desktop or a laptop, is normally used as a cli-ent. If a business needs to use a computer as a server, it should be more power-ful to allow several connections from clients at the same time.Q6-10.In this case, UDP is more appropriate because it does not have the o

7、verhead ofTCP in connection establishment and teardown.Q6-11.Routing cannot be done at the transport layer, because the communication atthe transport layer is one single logical path between the source port and thedestination port. Routing cannot be done at the data-link layer because thecommunicati

8、on at the data-link layer is between two nodes (one single path;there is no need for routing. On the other hand, there are several possible pathsfor a packet between the source host and destination host at the network layer.Routing is the job of selecting one of these paths for the packet.Q6-12.Comm

9、unication at the network layer is host-to-host; communication at thedata-link layer is node-to-node.Q6-13.Dial-up modems use part of the bandwidth of the local loop to transfer data.The latest dial-up modems use the V-series standards such as V.90 (56 kbpsfor downloading and 33.6 kbps for uploading,

10、 and V.92 (56 kbps for down-loading and 48 kbps for uploading.Q6-14. A Bluetooth network is normally a wireless small network that is more suitedto communication in a personal environment; a wireless local area can cover alarger geographical areas such as a building or an office.Q6-15.The period of

11、a signal is the inverse of its frequency: T = 1/f.Q6-16.The amplitude measures the value of the signal at any point.Q6-17.Normally,analog transmission refers to the transmission of analog signalsusing a band-pass channel. Baseband digital or analog signals are converted toa complex analog signal wit

12、h a range of frequencies suitable for the channel.Q6-18.The transmission media is located beneath the physical layer and controlledby the physical layer.Q6-19.The two major categories are guided and unguided media.Q6-20.The three major categories of guided media are twisted-pair, coaxial, andfiber-o

13、ptic cables.ProblemsP6-1.The services provided in part a and part b are the opposite of each other.3a.Layer 1 takes the ciphertext from layer 2, inserts (encapsulates it in anenvelope and sends it.b.Layer 1 receives the mail, removes (decapsulates the ciphertext from theenvelope and delivers it to l

14、ayer 2.P6-2.The services provided in part a and part b are the opposite of each other.a.Layer 2 takes the plaintext from layer 3, encrypts it, and delivers it tolayer 1.b.Layer 2 takes the ciphertext from layer 1, decrypts it, and delivers it tolayer 3.P6-3.In 10 years, the number of hosts becomes a

15、bout six times (1.2010 6.19 the number in 2010. This means the number of hosts connected to the Internet ismore than three billion.P6-4.The system transmits 150 bytes for a 100-byte message. The efficiency is 100/ 150 or 66.66%.P6-5.a.The network layer is responsible for route determination.b.The ph

16、ysical layer is the only layer that is connected to the transmissionmedia.c.The application layer provides services for the end users.P6-6.a.User datagrams are created at the transport layer.b.The data-link layer is responsible for handling frames between adjacentnodes.c.The physical layer is respon

17、sible for transforming bits to electromagneticsignals.P6-7.Figure P6-7 shows the layers. Note that we have not shown the security checking that you need to pass through because it does not have the counter-part when you arrive. It must be included in baggage/checking layer.Figure P6-7Protocol layeri

18、ng in air travelling 4P6-8.The only two layers that need to be changed are the data-link layer and thephysical layer. The new hardware and software need to be installed in all host,routers, and link-layer switches. As long as the new data-link layer can encap-sulate and decapsulate datagrams from th

19、e network layer, there is no need tochange any protocol in the upper three layers. This is one of the characteristicsof the protocol layering.P6-9.The domain of IP addresses is universal. A device directly connected to theInternet needs a unique IP address. The domain of port numbers is local; theycan be repeated. Two computers running the HTTP server process use thesame well-known port number (80; two comp