---
title: 第三堂
tags: 網路概論筆記
---
- 下禮拜考:ssnetwork

### Host : sends packets of data
- 網路連接後，將資料透過網路協定切成封包 ( packets )，由乙太網路或 WiFi 送到 AccessNetwork
- 頻寬為 ( transmission rate ) R ( bits/sec )，封包長度為 L ( bits )，封包傳輸延遲 ( packet transmission delay ) = L/R
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### wireless LANs:
- within building ( 100 ft )
- 802.11b/g /n/ac ( WiFi ) : 11, 54, 360 Mbps transmission rate
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### wide-area wireless access
- provided by telco (cellular) operator, 10’s km
- between 1 and 10 Mbps
- 3G, 4G: LTE ( Long-Term Evolution )


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# Physical media
### guided media :
- The waves are guided along a solid medium.
- fiber-optic 光纖 , a twisted-pair ( TP ) 雙絞線 , or a coaxial cable 同軸電纜 .
### unguided media :
- Waves propagate in the atmosphere and in outer space.
- wireless LAN.

### radio :
> 頻段都不同
- terrestrial microwave
- LAN
- wide-area
- satellite
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# The network core

- Circuit switching
- for pehone
- Packet switching
- for internet

### Packet switching
- router 將封包傳至下一個 router ，採最短路徑
- use store-and-forward 完整讀入一個封包後，才可輸出
- store and forward 因為要收完才能傳出的特性，可用於除錯
>沒有任何錯誤的時候傳輸時間為 L/R ，source 到 router 到 destination
總時間 2*L/R
- queuing delay, loss

### Internet structure: network of networks
- data centers 資料處理中心
-