The network core
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The mesh of interconnected routers
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Packet-switching（个人认为这里泛指数据包。不同的数据交换）
– Hosts break application-layer messages into packets, forward packets from one router to the next, across links (指hosts之间的链路) on path（指从源hosts到目标host的路径） from source to destination. -
Packet switching cause delay and maybe loss
– Store and forward cost the time
– Queuing caused by excessive packets in the router costs the time
– Packets may be dropped (lost) if the memory fills up
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Two key network-core functions
– Routing: Determines source-destination route taken by packets. (算法计算出路径)
– Forwarding: Move packets from router’s input to appropriate router.（按照计算出的路径把数据包传送出路由，路由器有多个端口。） -
Alternative core: circuit switching (电路交换)
End-to-end resources are allocated to and reserved for “call” between source and destination.
In the diagram, each link has four circuits.
– Dedicated resources are no sharing, and it can guarantee the performance.
链路及相关资源专有，不被分享，确保性能。
– Circuit segment idle if not used used by call.
导致了数据转发的空档期带来的资源浪费。
–Commonly used in traditional telephone networks. -
Two ways of circuit switching: FDM (Frequency-division multiplexing）and TDM (Time-division multiplexing)
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Comparing packet switching and circuit switching:
– Basing on the link with same bandwidth and assuming users have the same bandwidth. Packet switching can make more people use network than circuit switching.
– Packet switching is great for bursty data (数据突发性增长) since it is resource-sharing, simpler without call setup.
– Excessive congestion maybe cause packets delay and loss, and it need protocols to improve the reliability of data transfer and congestion control.
– It’s needed to make packet switch has the circuit-like behavior for solving some problems, like unstable audio or video services. -
Internet structure: network of networks
– End systems connect to Internet via access ISPs (Internet Service Providers), such as residential, company, and university ISPs.
– Access ISPs in turn must be interconnected, so that any two hosts can send packets to each other.
网络服务提供商相互连接（双向），所以网络中的任意两台主机可以相互发送数据包。
– Network of netwoks become more complex, and evolution was driven by economics and national policies.
References
Sildes of Computer Networking, A Top-Down Approach