通信网基础新第2章

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3.1 Discovering Addresses:ARP and RARP ARP:Address Resolution Protocol 地址转换 协议 When sending a packet to another computer,a computer refers to the destination by its “network address”.
server Hub
Hub
workstation transmission medium
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take turns transmitting their packets.
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What happens if two computers start transmitting at the same time?
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Figure :a shared Ethernet Hub
A

B
C
D
E
If node A wants send a packet to node E,the hub receive the packet and send this packet to all nodes. If two nodes try to transmit packet at the same time,then they would clobber each other.
packet
x,y data
Source Ethernet address(48-bit string) destination Ethernet address(48-bit string)
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Transmission rate

Three versions:differ in how they transmit the bits Ethernet :10Mbps Fast Ethernet:100Mbps Gigabit Ethernet(千兆位以太网）:1Gbps 实际上，不管是10Mbps的Ethernet或是100Mbps 的Fast Ethernet，对于一个二进制位来说，从一个 点传输到另一个点所花的时间基本上是相同的。主 要的差别在于，与低速网络技术相比，高速网络每 秒可以传输更多的二进制位。即在使用高速网络技 术的情况下，电缆或光缆上传输的信息密度较大。
Figure 2.1
Wire pairs Hub packet x,y data
ARP table
124
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y
Name
Ethernet address
Network address
124:y 105:z
3
“tse”
Hub

Hub :是一个信号再生转发设备，它使多 个用户通过端口用双绞线与网络设备连 结，一个Hub通常具有8个以上的连接端 口。Hub的每个端口互相独立，即一个端 口的故障不会影响其他端口的状态。
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Naive Strategy(procedure) Allocate “network addresses” to the computers on our Ethernet----network manager The network manager goes to each computer and runs a little program that reads the internal Ethernet address of the computer.The network manager then writes a configuration file “My-Address” into each computer that tells the computer its network address.

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Network address(IP address)

The network address is based on the location of the computer. used by the network to forward the packet to its destination. is independent of the Ethernet address.
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Step3:The computers wait for some random time before they try again to transmit their packets.
结点应延迟等待一段随机时间，再重发刚才受损的信息。 如果重发时又发生冲突，则应等待一段更长的时间，为的 是尽量减少下一次冲突。各结点延迟的时间量采用随机数 控制，只要随机数不同，各结点延迟的时间也就不同。延 迟时间量最小的抢先占用传输介质，再次发送信息。
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2.1 Ethernet

以太网
We first explain how the computers exchange packets. We then explore how the computers discover the addresses they need for the packets. Finally,we examine the interconnection of Ethernets
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Collision(冲突）

Shared Ethernet(共享式以太网）:
all the computers share the same wire all the nodes share the same transmission medium Physical:star-topology Logic:bus-topology

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The network manager goes back to each computer and completes a configuration file “List” which lists the network address and Ethernet address of every computer attached to the Ethernet .
Chapter 2 The Way Networks Work

Section 2.1------Ethernet-------chapter 4 Section 2.2------Internet--------chapter 3 Section 2.3------ATM-------------chapter 5 Section 2.4------OSI models This chapter is a survey of different network technologies.Our objective is to introduce the operating principles of these technologies and to provide a “big-picture” look at networks
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Collision(冲突）

Step1:The hub sends out a signal to all the computers to warn them that a “collision”is occurring. Step2:The computers that are transmitting stop when they get the collision warning.
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CSMA/CD

Media access control protocol---介质访问控制协议 用来确定在任一给定时间内由哪个结点（工作站）来访 问公共的传输介质。 Ethernet-----CSMA/CD---carrier sense multiple access with collision detection(载波监听多路访问/冲突检测） 多个工作站都监听传输介质，直到检测不到任何信号 （载波监听），这时再访问传输介质（多路访问），边 传输边检测看是否有多个信号存在（冲突检测），每个 工作站在其认为网络空闲时进行信号传输。如果发生冲 突，执行上面三步。

The hub sees two packets arriving at the same time on two different ports The hub then knows that it cannot transmit these two different signals at the same time on its other ports because they would clobber each other. 同一时刻，两个不同的信号不能在其他端口 传输，否则它们相互之间会发生碰撞。
The hub repeats incoming packets on its other ports The hub Informs the computers of “collision” that occurs when two or more packets arrive at the hub at the same time
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However,to send the packet the sending computer must attach the destination’s Ethernet address.
packet
Ethernet address