学习笔记：高级信息网络Ch7.Connecting to the Internet

(来源：notes of course Advanced IT Network.Ch7.Connecting to the Internet )

Point to point links

-the two types links(analog and digital) exist togther

Use PSTN modems to encode data to analog signal:

PSTN(Public Switched Telephone Network)

DTE:Data Terminal Equipment—–>computer

DCE:Data Communications Equipments —–>modem

Full duplex modems:

-a single circuit is used for transmitting sound in both direction

-How to avoid echo?

low speed —> use frequency domain multiplexing!
simultaneous full duplex transmission —>do not use the link together (a form of time domain multiplexing)
full duplex transmission and full channel needs to be used —> echo cancellation

-echo cancellation（去回声）

basic idea : incoming signal = total signal - outgoing signal
there is also reflection in the channel:

Using the scramblers to extract the echo to minimize the correlation between outgoing signal and input signal.

XDSL:

ADSL: upwards data rate is an order of magnitude lower than the downstream data rate
SDSL: date rate same
VDSL: better performance in data rate

WiMax

-good technique but is likely to disappeare.

Shared medium access network

-不在物理层考虑问题，考虑发送信息之后，收到的人应该怎么交互的问题

Bus cable:

MAC layer:

MAC layer is intimately related to the physical communication, so it would be an upper layer of Physical layer
when the decision is made, the MAC layed is mainly implemented by software. So it would be the bottom of layer 2.

Ethernet（以太网）：

Aloha：

一个aloha节点只要有数据的话，该节点就可以立即发送。当该节点数据发送完之后，其需要等待接收方反馈的ACK。若成功接收到ACK之后，那么这一次传输成功。如果没有收到ACK的话，那么这一次传输失败。该aloha节点会认为网络中还存在另外一个aloha节点也在发送数据，所以造成接收方发生了冲突。最后这些冲突的节点会随机选择一个时间进行回退（backoff），以避免下一次冲突。若冲突节点回退完成，其才可以重新进行发送。

polling: the computer asks successively each terminal if it has something to transmit and eventually allows the terminal to use the shared medium to transmit.

-may cause collidetion —> each station should waits a random delay before retransmitting the damage message

CSMA/CD:

节点发送数据之前需要持续监听信道，一旦节点发现信道空闲，则立刻发送数据。在发送数据的同时，节点持续监听信道，”探测” 是否有别的节点也在该时刻发送数据。

Ethernet over twisted pairs:

-one or two pairs per direction

Repeaters:

-a signal amplifier, 16 or more repearters consistude a Hub

-in the physical layer

Bridge（网桥）:

如何通俗地解释什么是网桥？

网桥与集线器的无脑转发相比多了这么几个功能：
一是识别数据链路层中的数据帧，并将这些数据帧临时存储于内存，再重新生成信号作为一个全新的数据帧转发给相连的另一个网段。
二是网桥可以通过检查FCS来进行差错控制，并且在发送数据帧是会进行冲突检测。
三是网桥有MAC地址表，这使得网桥在转发数据时不会将每个数据帧都泛洪至其他所有接口。
通俗的说，集线器相当于一个大喇叭，无论输入什么，它都会立即向周围的所有接口转发出去，完全没有智能性可言。
而网桥则相当于一个小孩，如果你有一句话需要转达给某个地方（网络）的某个人，他会将这句话记下来，然后到达指定的地方寻找那个人，将你需要转达的信息转达过去。如果他不知道该传达给谁，那么他会将这条信息大声的告诉给该区域所有的人（泛洪），然后记录下回复者是谁（学习MAC地址）

作者：6IE闫辉
链接：https://www.zhihu.com/question/67473683/answer/1275373754
来源：知乎
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

in the MAC layer!

Filtering Bridges:

inside the bridges there is a database involved all the MAC addresses
only the certain MAC from left side to the right side is assigned, the transmission between left and right can take place.

Learning Bridges:

the Bridges can learn the MAC addresses through the transmission

Spanning Tree Algorithm:

to find the loops and avoid them
the lowest serial number’s bridge is named root
the root send a frame
when the other bridge recieve the frame from both the right and left side, the bridge is disabled.