通信网基本概念与主体结构(第二版)答案Ch1

Solutions to Chapter 1

1a. Describe the step-by-step procedure that is involved from the time you deposit a letter in a mailbox to the time the letter is delivered to its destination. What role do names, addresses and mail codes (such as ZIP codes or postal codes) play? How might the letter be routed to its destination? To what extent can the process be automated?

Solution:

The steps involved in mailing a letter are:

1. The letter is deposited in mailbox.

2. The letter is picked up by postal employee and placed in sack.

3. The letter is taken to a sorting station, where it is sorted according to destination, as

determined by the mail code and grouped with other letters with the same destination mail

code. (If there is no mail code, then it is determined by the largest geographical unit, for

example, country (if specified), otherwise state (if specified), otherwise city (if specified).)

4. The letter is shipped to the post office that handles the mail for the specific mail code (or

country or city).

5. The letter is then sorted by street address.

6. The letter is picked up at the post office by the postal worker responsible for delivering to the

specified address.

7. The letter is delivered according to the number and street.

The name is not really used, unless the street address is missing or incorrect. The name is at the destination to determine who the letter belongs to. (Unless of course the letter is being sent to a small town, where most inhabitants are known to the postal worker.)

The mail delivery process can be automated by using optical recognition on the mail code. The letter can then be sorted and routed to the destination postal station, and even to the destination neighborhood, depending on the amount of geographical detail built into the mail code.

1b. Repeat part (a) for an e-mail message. At this point, you may have to conjecture different approaches about what goes on inside the computer network.

Solution:

The steps involved in e-mailing a message are:

1. The message is sent electronically by clicking 'Send'. (In Chapter 2 we see that the Simple

Mail Transfer Protocol (SMTP) is used to do this.)

2. The mail provider of the sender sends a request to a name server for the network address of

the mail provider of the recipient. The mail provider is determined by the information following

the @ symbol.

3. If the mail provider finds the network address of the recipient's mail provider, then it sends the

message to that address.