
[참고 링크](https://github.com/ready-techie/presentation-en/wiki)
영어 발표를 위해 직접 정리했습니다.

## What is Ethernet?

- a method of networking
- unarguably the world’s **most widely-used** connectivity technology
- signals and wiring at the physical layer & MAC packets and protocols at the data link layer.
- uses CSMA/CD

### What is CSMA/CD?

- stands for **'Carrier Sense Multiple Access/Collision Detection'**
- 'Listen before Talk'
- transmits frame on a network

### Procedure of CSMA/CD communication

<img src="https://sf.ezoiccdn.com/ezoimgfmt/networkencyclopedia.com/wp-content/uploads/2019/08/csma-cd.jpg?ezimgfmt=ng:webp/ngcb2" width="80%" />
reference: https://sf.ezoiccdn.com/ezoimgfmt/networkencyclopedia.com/

1. **[Carrier Sense]** A PC or server checks if someone is using the network's resource.
2. If a carrier's detected, it holds sending the frame onto the network.
3. It starts transmitting its frame if there's no other PC or server transmitting.
4. **[Multiple Access]** Collision occurs if multiple frames are uploaded at the same time. Then the host sends a ‘jamming signal’ to inform other stations of the collision.
5. **[Collision Detection]** The PC or server stops transmitting and waits for a random amount of time(*random backoff period*) until the problem's solved.
6. It restarts the transmission of the frame.

## Collision Domain

<img src="https://www.thebryantadvantage.com/wp-content/uploads/2018/10/Collision-Domain-1.png" width="80%"/>

All four hosts are connected to a hub. If one PC is transmitting its frame and the network's busy, all other PCs will not be able to transmit their frames.
If a collision occurs in that network, all of the PCs will be affected. They are in one 'collision domain'.
CSMA/CD had developed to protect the collision.  

## Bridge & Switch

<img src="https://user-images.githubusercontent.com/50111853/116780792-a93dbf80-aab9-11eb-9ae0-08b2f997b69b.png" width="48%"/>
<img src="https://user-images.githubusercontent.com/50111853/116780799-b0fd6400-aab9-11eb-9ddf-696b9c7fb204.png" width="50%"/>

While hubs have the risk of collision, bridge and switch can separate collision domains.

<img src="https://user-images.githubusercontent.com/50111853/116781127-ee62f100-aabb-11eb-98db-87385600dbb4.jpg" width="80%"/>
In this regard, PCs connected to the switch can communicate with each other even though other PCs are on transmission.

### Reference

- [Ethernet still going strong at 40 years](https://www.wired.com/insights/2013/05/ethernet-still-going-strong-at-40-years)
- [CSMA-CD carrier sence](https://en.vcenter.ir/network/csma-cd-carrier-sense-multiple-access-collision-detection)
- [CSMA-CD](https://www.thebryantadvantage.com/videos-and-tutorials/ccna-and-ccent-tutorials/csma-cd)
- 후니의 쉽게 쓴 CISCO 네트워킹


What is Ethernet and CSMA/CD


[참고 링크](https://github.com/ready-techie/presentation-en/wiki>)  
영어 발표를 위해 직접 정리했습니다.

## Open Systems Interconnection Model

- Standardized 7 stages of communication functions of a telecommunication or computing system

![OSI 7 Layer](https://i.pinimg.com/564x/13/28/fa/1328fad216c9b577ad274f5303513ea8--osi-model--layers.jpg)

## What for?

![OSI Model](https://base.imgix.net/files/base/ebm/electronicdesign/image/2019/04/electronicdesign_15724_0913wtdosipromo.png?auto=format&fit=crop&h=562.5&w=1000)
    1. To track the flow of data transmission
        **Encapsulation** : Each time data passes through each layer, a header is added. It makes the data transmitted in a set order regardless of the data type.
    2. To find the source of a problem faster (e.g. Ping)
    3. To enhance compatibility between networking hardwares

### Let's dive into the each layer

![Physical Layer](https://www.lifewire.com/thmb/g2w4_fzKLA47R4wGCsJAYnQQDzw=/1500x0/filters:no_upscale():max_bytes(150000):strip_icc():format(webp)/layers-of-the-osi-model-illustrated-818017-finalv1-2-ct-ed94d33e885a41748071ca15289605c9.png)

1. Physical Layer
    - Transfers **binary data using** **physical cable**. (e.g. cabels and hubs)
    - Electrical on/off
    - Forwards data without any involvement in what the data is, what errors are present, etc.

![Data Link Layer](https://www.lifewire.com/thmb/zPGFUYqE7hR9f4EQpeh-DyGFqvc=/1500x0/filters:no_upscale():max_bytes(150000):strip_icc():format(webp)/layers-of-the-osi-model-illustrated-818017-finalv1-3-ct-9d3e1bf44a554e3db31f706201fc69f6.png)

2. Data Link Layer
    - Divided as Media Access Control sub-layer and Logical Link Control sub-layer.
    - MAC sub-layer controls authorization of data access and transmission. (**MAC address)**
    - LLC sub-layer cheks physical data transmission errors.

![Network Layer](https://www.lifewire.com/thmb/TSScK6WTSLEnjNToI1AwzBXZbYo=/1500x0/filters:no_upscale():max_bytes(150000):strip_icc():format(webp)/layers-of-the-osi-model-illustrated-818017-finalv1-4-ct-9ffde2c7142849819c3fcf5e305a242f.png)

3. Network Layer
    - addresses using **IP**, the logical address.
    - selects the safest and fastest path selection to the destination.
    - forwards data packet.

⇒ **Routing**

![Transport Layer](https://www.lifewire.com/thmb/intF9AgvkoWdF0Gjpv8axHvd2Gw=/1500x0/filters:no_upscale():max_bytes(150000):strip_icc():format(webp)/layers-of-the-osi-model-illustrated-818017-final-5-ct-373fc5a9edc74359819021555f37467d.png)

4. Transport Layer
    - delivers data across network connections by **transport protocols**. (TCP, UDP)
    - Transport protocols support error recovery, flow control, and support for re-transmission.

### References

- [https://iebmedia.com/technology/looking-behind-the-automation-protocols/](https://iebmedia.com/technology/looking-behind-the-automation-protocols/)
- [https://www.electronicdesign.com/unused/article/21800810/whats-the-difference-between-the-osi-sevenlayer-network-model-and-tcpip](https://www.electronicdesign.com/unused/article/21800810/whats-the-difference-between-the-osi-sevenlayer-network-model-and-tcpip)
- [https://www.lifewire.com/layers-of-the-osi-model-illustrated-818017](https://www.lifewire.com/layers-of-the-osi-model-illustrated-818017)


네트워크

'네트워크'에 관한 글들을 모아봤어요.

What is Ethernet and CSMA/CD

Definition and usages of OSI 7 Models