Computer Network Basics
Data switching
There are three data switching methods:
- Circuit switching: 建立连接，独占资源，类似管道。
- Message switching: 一次性发送整个报文到相邻节点。
- Packet switching: 把报文分割为多个组，分别发送到相邻节点。
Delays in Computer Network
- Transmission delay - time it takes to push the packet’s bits onto the link
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It is given by the following formula:
\(D_T = N/R\) seconds
where:
\(D_T\) is the transmission delay in seconds;
\(N\) is the number of bits;
\(R\) is the rate of transmission (say, in bits per second).
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- Propagation delay
- Queuing delay
- Processing delay - time it takes a router to process the packet header
Performance characteristics
- Bandwidth (signal processing), a measure of the width of a frequency range
- Bandwidth (computing), the rate of data transfer, bit rate or (theoretically optimal) throughput
- limits the rate at which data can be sent.
- Throughput, the actual rate of data transfer
- Delay (as talked above)
- Bandwidth-delay product, the maximum amount of data on the network circuit at any given time, i.e., data that has been transmitted but not yet acknowledged
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Round-trip time, also known as RTT
有效数据率 (平均吞吐量):
\[Average Throughput = \frac{Data Length}{Transmission Delay + RTT}\] - 信道利用率 - 信道占用时间占总时间的百分比
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网络利用率 - 全网络的信道利用率的加权平均值。
若 \(D\) 是当前时延，\(D_0\) 是空闲时延，\(U\) 是网络利用率，则：
\[D = \frac{D_0}{1 - U}\]即网络利用率越高，时延越高。
Network protocol
What are network protocols?
Rules, standards or conventions built up for data switching in network.
Protocols are “horizontal”; services are “vertical”.
Three elements of a network protocol:
- Syntax (语法)
- Semantics (语义)
- Timing (同步)
Terminologies
- Entity
- Hardwares and softwares which can send/receive information.
Peer Entity (对等实体) 间通信使得每层可以为上一层提供服务。
- Protocol Data Unit (PDU)
- The basic unit of data switching between protocols.
- Service Data Unit (SDU)
- The basic unit of data switching between services.
- Service primitives
- Commands defined for service delivery.
- Service Access Point (SAP)
- Interface for service delivery.
Open System Interconnection (OSI) model
Seven layers in OSI model are the followings:
- Physical
- Data Link
- Network
- Transport
- Session
- Presentation
- Application
The following are PDUs of them:
- For physical layer, it is bit.
- For data link layer, it is frame.
- For network layer, it is packet or datagram.
- For transport layer, it is segment.
- For application layer, it is message.
TCP/IP model
- 4 Layers
- Application layer, transport layer, internet layer, and link layer
- Now widely used
Socket
Sockets addresses are made up with IP address and port number.
Remote socket, local socket combined with transport protocol form socket pairs. (This may be obsolete)