# Week1 計網概筆記
* Switched network (交換網路) : connect by switches, and the topology of switched network is not limit, but the packets are delivered along the spanning tree.
* Interconnected network (互聯網路) : connect by routers, and the packets are delivered by routing protocols.
* Multiplexing (多工) : a switch transmits multiple datagram over a single link.
* FDM: **Frequency** Division Multiplexing.
* TDM: **Time** Division Multiplexing.
* Statistical Multiplexing: based on demand of each flow, **FIFO** (first-in-first-out), **Round-Robin** (循環) , **Priority**.
* Protocols:
* Peer-to-peer interface: exchange messages between the same layer of peers.
* Service interface: service provide to the higher layer of protocols.
* Protocol Specification(協議規格): pseudo-code, state transition diagram, message format.
* Encapsulation: the higher layers of protocol will add header to the data so that the peer can read it.
* 7-layers of internet: application->presention->session->transport->network->data link->physical.
* Physical layer
* transmission data over a communication link.
* Coaxial cable: 10base5, 10base2.
* Twisted cable: 10baseT.
* Optical fiber
* Power line
* Air space
* Different signal coding schemes.
* send clk and data so that receiver can read the data correctly.
* Data Link layer
* transmit a frame to a directly connected host, based on only **mac address**. (p2p, multiple access)
* switch
* bridge
* MAC (Multiple Access Control Protocol)
* CSMA/CD (ethernet)
* CSMA/CA (wireless lan)
* Network layer
* transmit packets (data in this layer called packet) to a host via internet
* router will check **routing table** and **condition of internet** to deliver packet, and if the packet larger than the limit, router will fragment the packet.
* IP protocol
* router just check **ip address**.
* Transport layer
* transmit message (data in this layer called message) from **host to host**.
* TCP (reliable)
* UDP (unreliable)