# 第01講 Foundation
計算機網路概論 黃能富教授

Applications Protocol
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1. URL：Uniform resource locater
2. HTTP：Hyper Text Transfer Protocol
3. TCP：Transmission Control Protocol

傳出過程：
Domain Name server (用URL查IP)
->用 3 way handshaking 建立連線
->用 HTTP 提出request 和 acknowledgement
->連線結束tearing down TCP connection

Network Connectivity
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* Important terminologies：
1. Link
2. Nodes
3. Point-to-point(點對點)
4. Multiple access(多點)
5. Switched Network(交換網路)
-> Circuit Switched(電路交換)
-> Packet Switched(封包交換)
6. Packet, message
7. Store-and-forward
8. Hosts
9. witches
10. Spanning tree -> 避免迴路
11. internetwork
12. Router/gateway
13. Host-to-host connectivity
14. Address
15. Routing
16. Unicast(單點)/broadcast(廣播)/multicast(群播)
17. LAN (Local Area Networks) -> 區域
18. MAN (Metropolitan Area Networks) -> 都會
19. WAN (Wide Area Networks) -> 廣域

Cost-Effective Resource Sharing
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* How to share a link ?
1. Multiplexing (多工) -> Switch 1
2. De-multiplexing (解多工) -> Switch 2


* FDM: Frequency Division Multiplexing (分頻多工)


* Synchronous Time-division Multiplexing (TDM) (同步分時多工)
-> Time slots/data transmitted in predetermined slots

FDM和TDM問題：負載不平衡其況下，不可互相借用，因此效率較低。

* Statistical Multiplexing (統計式多工)
1. 資料依據資料的flow需求來傳送
2. What is a flow?
-> 使用者所需要用到的流量
3. 交換機排程法：
-> FIFO, Round-Robin, Priorities, Quality-of-Service(QoS)

Logical Channels
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Logical Channels：應用程式與應用程式之間的通訊路徑或管道
-> 只是有關係存在，和實際所經過的路徑不一定會相同

Network Reliability
---
* Network should ***hide the errors***
1. Bits are lost
-> Bit errors - 1 to a 0, and vice versa
-> Burst errors (突爆錯誤) - several consecutive errors
2. Packets are lost (Congestion雍塞)
3. Links and Node failures
4. Messages are delayed
5. Messages are delivered out-of-order
6. Third parties eavesdrop

Network Architecture
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Protocols
---
1. Protocol defines the interfaces between
* the layers in the **same system**
* the layers of **peer system**
2. Each protocol object has two different interfaces
* **Service interface**(縱向)：定義本介面在協議中的執行業務
* **Peer-to-peer interface**(橫向)：用於同層對等者(peer)的訊息交換

3. **Protocol Specification**(規格)：
* pseudo-code (虛擬碼)
* state transition diagram(狀態轉移圖)
* message format(訊息格式)
4. **Interoperable**(互通測試)：when two or more protocols that implement the specification accurately
5. IETF: Internet Engineering Task Force
-> 負責定義互聯網標準與協議規格
6. Protocol Architecture
* TCP 較 UDP 可靠

Encapsulation
---

* High-level messages are encapsulated inside of low-level messages
* 紅色虛線為Peer-to-peer interface，故虛線部分可互相溝通
* 往上傳送時(decapsulation)，每一層只讀自己的header，如IP只看IP那欄的header，並且在處理完資料之後將此header移除後，接著將封包往上層傳送

OSI Architecture
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* Data Link layer (傳送一小段)
* Network layer (傳送整條路徑) ex:IP
* Transport layer (容錯較高) ex:TCP、UDP
->且Transport layer有重送機制，因為下層較不可靠

---

* Switch為layer 2的設備、Router為layer 3的設備
* 因為經過的均為Switch，所以LAN為**Ethernet**(乙太網路)

---

* Access Point (AP)為layer 2的設備
* 因為經過的有Switch和AP，所以LAN為**Ethernet + WiFi**

## Description of Layers
### Physical Layer
1. Handles the transmission of raw bits over a communication link
* Coaxial cable (同軸電纜)
* Twisted pair (雙絞線) -> 降低電磁波干擾
* Optical Fiber (光纖)
* Air space (無線傳輸)
2. Different Signal Coding schemes
* 需要知道clock cycle time或是一個bit的時間

### Data Link Layer
**(如何將 frame 傳給直接相連的主機或設備)**

1. Collects a stream of bits into a ***frame***
2. How to transmit a frame to a directly connected host (destination) ?
* Point-to-point
* Multiple access：可能會發生collision，所以用 **MAC** 解決
3. **MAC (Media Access Control Protocol)**
* CSMA/CD (IEEE 802.3 Ethernet) -> D(detect)：偵測的意思
* CSMA/CA (IEEE 802.11 Wireless LAN) -> A(avoid)：避免
4. **Layer 2** devices

### Network Layer
**(如何將封包透過 Internet 送給目的地主機)**

1. Handles **routing** among nodes within a packet-switched network -> 如何選擇路徑
2. Data exchanged between nodes in this layer is called a **packet**
3. **Routing protocols**
* RIP (路由信息協議)
* OSPF (開放式最短路徑優先)
* BGP (邊界閘道器協議)
4. Routing Tables -> 根據Routing protocols製作，並且**動態變更**

### Transport Layer
**(提供不同主機 processes 之間的資料傳送)**

1. Implements a **process-to-process** channel
2. Unit of data exchanges in this layer is called a **message**
3. **TCP** (Transmission Control Protocol) – ***Reliable*** service
4. **UDP** (User Datagram Protocol) – ***Unreliable*** service

### Session Layer
* 將不同的transport streams緊密結合成一個應用程式的一部分

### Presentation Layer
* 著重於網路資料交換格式的呈現

### Application Layer
* 提供共同的應用程式格式交換
* EX：FTP/E-mail/DNS/HTTP/Browsers/FB, …

***傳輸層以及較高的上層協議通常只實作在主機上，一般不會出現在網路中間的交換機或路由器等網路設備中***

各層封包比較：
---
* Data Link Layer -> frame
* Network Layer -> packet
* Transport Layer -> message

Internet Architecture
---

* Defined by **IETF**

* **Three main features**
1. Does not imply strict layering
-> 應用程式可以隨意略過傳輸層以便直接使用 IP 或是其他更底層的網路服務
2. An hour-glass shape
->**IP協議提供目前大部分上下層連接的一個重要的點**
3. 制定一個 new protocol 需要包含
* a protocol specification
* 至少一個(最好兩個)規格的實作

Network Performance
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1. Bandwidth
* Width of the **frequency band**
* **Number of bits per second**
2. 1 **Mbps**: 1 x 106 bits/second
3. Bandwidth越小，傳輸越慢

* **Latency**：
-> *Propagation time(傳播時間) + transmission time(傳輸時間) + queuing time(排隊時間)*
* Propagation time：距離/光速
* Transmission time：size/bandwidth

* **One** bit transmission => **propagation** is important (**資料已送完, 但前導資料還未到達對方**)
-> 原因：短資料很快就送完, 但需要長時間才能傳到對方
=> Propagation time >> transmission time
* **Large** bytes transmission => **bandwidth** is important ( **未送完前, 前導資料已到對方**)
-> 原因：長資料很慢才能送完
=> Transmission time >> propagation time

## Latency X Bandwidth
**Latency x Bandwidth means how many data can be stored in the pipe**

* **Latency** (delay): **length** of the pipe (長度)
* **Bandwidth**: **width** of the pip (寬度)
* Relative importance of bandwidth and latency depends on application
* **large** file => **bandwidth**
* **small** messages => **latency**
* Variance in latency (**jitter**) can also affect some applications
* jitter：延遲的變異數

### 結論
* 如果傳送端**持續塞滿整個pipe**， delay x bandwidth就是傳送端在第一個位元抵達接收端前必須傳送的位元數
* 傳送端若**無法將pipe塞滿**，那就不算是有效利用網路
* **停下來等對方回應之前如果可以傳送**delay x bandwidth的資料量是最佳的設計

throughput
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Throughput = TransferSize / TransferTime
* TransferTime = RTT + TransferSize/Bandwidth
* RTT (Round Trip Time) dominates -> 來回通訊延遲

Performance Summary
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* 兩個用於分析電腦網路效能的主要參數
* Bandwidth
* Delay