２１世纪的重要特征：数字化，网络化和信息化，它是一个以网络为核心的信息时代．

世界经济——————工业经济(industry-based economy)——————→知识经济（knowledge-based economy)


计算机网络的发展过程

分组交换（packet switching)
原因：传统的circuit switching不适应网络数字化传输．
新的网络要：

①用于计算机之间的数字传送
②能够连接不同类型的计算机（异构网？）
③所有的网络结点都同等重要．（提高网络生存性）
④必须有冗余的路由．（redundant routing)
⑤ 尽可能地简单，非常可靠地传送数据．（too simple and too reliable)

电路交换必定是面向连接的．(circuit switching must be connection-oriented)
在通话的全部时间内，通话的两个用户始终占用端到端的固定传输带宽．所以导致线路的传输效率很低．While they connection and talk,they engross the bandwidth of transfers all the time.so the efficiency of circuitry's transfers is too bad!

分组交换采用存储转发（storage transmit　？）技术．
报文（message)——数据段（segment)——加上首部（header）——分组(packet)
the character of packet switching is label-based,and It is connectionless.

结点交换机(node switch)  进行分组交换，用来转发分组。Node switch  get packet switching,and transmit packet.
主机(host)用户进行信息处理。　Host  deal with the informations for user.

Ｔwo problems which be brought by packet switching: delay and overhead.
Communication subnet and resource subnet which, must be distinguish clearly!


Four sorts of  network:
①circuit switching
②message switching
③packet switching
④mixed switching

Of course,you can carve up the network by the bound of action.for example:
①WAN: Wide Area Network
②LAN: Local Area Network
③MAN :Metropolitan Area Network
④AN:  Access Network

The two primary guide-line of  the network's capability:
bandwidth and delay
(It is hard to learn well,  I will read the book and write detailedly latter!)

Four concept:
entity : all hardware and software which can send and receive messages.
protocol: the aggregate of the rule which control two equity entity to keep connection
Service:
SAP:Service Access Point .

We must distinguish the differ between the protocol and service clearly!

How tire!  I have do my best effort delivery~_~