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网络架构与七层参考模式

Applications​

Most people konw about the Internet(a computer network) through applications

  • World Wide Web
  • On line games
  • Email(Gmail, hotmail,...)
  • Online Social Network(Facebook, Twitter,...)
  • Streaming Audio Video (Youbute, pptv, kkbox, ppstream,...)
  • File Sharing(dropbox,...)
  • Instant Messaging(Skype, IM+, MSNLine, WeChat,...)

  • A multimedia application including video conferencing.

  • URL

Uniform Resource Locater, http://domain.ltd

  • HTTP

Hyper Text Transfer Protocol

  • TCP

Transmission Control Protocol

  • 17 messages for one URL request
    • 6 to find the IP(Internet Protocol) address
    • 3 for connection establishment of TCP
    • 4 for HTTP request and acknowledgement
      • Request: I got your request and I will send the data
      • Reply: Here is the data you requested; I got the data
    • 4 messages for tearing down TCP connection

Network Connectivity​

Important terminologies​

  • Link(电脑与电脑连接的)
  • Nodes(电脑，手机等设备)
  • Point-to-point(点对点)
  • Multiple access（多个设备同事存取link）
  • Switched Network
    • Circult Switched(电路交换)
    • Packet Switched(分组交换)
  • Packet, message 分组数据，message原始资料
  • Store-and-forward 分包存储和转送,手->查表->转送

Terminologies(contd.)​

  • Hosts
  • Switches
  • Spanning tree 生成树
  • internetwork 互联网
  • Router/gateway
  • Host-to-host connectivity
  • Address
  • Routing
  • Unicast/broadcast/multicast
  • LAN(Local Area Networks)
  • MAN(Metropolitan Area Networks)
  • WAN(Wide Area Networks)

Cost-Effective Resource Sharing​

  • Resources: links and nodes
  • How to share a link?
    • Multiplexing
      • FDM：Frequence Division Multiplexing
        • frequency(平带)/time
      • TDM: Synchronous Time-division Multiplexing
        • Time slots/data transmitted in predetermined slots
        • frequency/time
    • De-multiplexing

  • Statistical Multiplexing
    • Data is transmitted based on demand of each flow.
    • What is a flow?
    • Packets vs. Messages
    • FIFO(Queue), Round-Robin(轮循), Priorities(优先权 Quality-of-Service(QoS) 服务质量)
    • Congested? 拥挤的

Logical Channels​

  • Logical Channels
    • Application-to-Application communication path or a pipe

Logical Channels

Network Reliability​

  • Network should hide the errors
  • Bits are lost
    • Bit errors(1 to a 0, and vice versa)
    • Burst errors - several consecutive errors
  • Packets are lost(Congestion)
  • Links and Node failures
  • Messsages are delayed
  • Messages are delivered out-of-order
  • Third parties eavesdrop

Network Architecture​

  • Application Programs
  • Process-to-process Channels
  • Host-to-Host Connectivity
  • hardware

protocols​

  • Protocol defines the interfaces between
    • the layers in the same system and with
    • the layers of peer system
  • Building blocks of a network architecture
  • Each protocol ojbect has two different interfaces
    • Service interface: operations on this protocol
    • Peer-to-peer interface: message exchanges with peer

Protocol Interface

  • Protocol Specification: pseudo-code, state transition diagram, message format
  • Interoperable: when two or more protocols that implement the specification accurately
  • IETF: Internet Engineering Task Force
    • Define Internet standard protocols

Protocol Architecture

Encapsulation​

Encapsulation

OSI Architecture​

OSI: Open System Interconnection

OSI Architecture

OSI Artitecture

OSI Artitecture

Description of Layers​

  • Physical Layer(如何将原始资料在 Link 上传输)

    • Handles the transmission of raw bits over a communication link
      • Coaxial cable 同轴线 同轴线
      • Twisted pair 双绞线 双绞线
      • Optical Fiber 光纤 (不受电磁波干扰)
      • Air space(wireless radio channel) 无线网路（电磁波传输）
    • Different Signal Coding schemes coding
    • 同轴线和双绞线容易受电磁波干扰

  • Data Link Layer(如何将 frame 传给直接相连的主机或设备)

    • Collects a stream of bits into a frame
    • How to transmit a frame to a directly conencted hsot(destination)
    • MAC (Meda Access Control Protocol)
      • CSMA/CD (IEEE 802.3 Ethernet)
      • CSMA/CA (IEEE 8.2.11 Wireless LAN)
    • Layer 2 devices
    • Switches
    • Bridges

  • Network Layer(如何将封包透过 internet 送给目的主机 )

    • How to transmit frames to a host via the Internet?
    • Handles routing among nodes within a packet-swiched network
    • Data exchanged between nodes in this layer is called a packet
    • IP protocol
    • Routers
    • Routing protocols
      • RIP
      • OSPF
      • BGP
    • Routing Tables

  • Transport Layer(提供不同主机 processes 之间的资料传送)

    • implements a process-to-process channel
    • unit of data exchanges in this layer is called a message
    • TCP (Transmission Control Protocol) - Reliable service
    • UDP (User Datagram Protocol) - Unreliable service

tcp

  • Session Layer

    • Provides a name space that is used to tie together the potentially different transport streams that are part of a single application

  • Prensentation Layer

    • Concerned about hte format of data exchanged between peers

  • Application Layer

    • Standardize common type of exchanges
    • FTP/E-mail/DNS/HTTP/Browsers/FB

Network Performance​

  • Defined by IETF
  • Three main features
    • Does not imply strict layering. The application is free to bypass the defined transport layers and to directly use IP or other underlying networks.

internet

  • Bandwidth
    • Width of the frequency band
    • Number of bits per second that can be transmitted over a communication link
  • 1 Mbps: 1 x 10^6 bits/second
  • 1 x 10^-6 seconds to transmit each bit or imagine that a timeline, now each bit occupies 1 micro second space.
  • On a 2 Mbps link the width is 0.5 micro second.
  • Smaller the width more will be transmisson per unit time.

bandwidth

Bits transmitted at a particular bandwidth can be regarded as having some width:

a. bits transimitted at 1Mbps (each bit 1 us wide)

b. bits transmitted at 2Mbps(eacg bit 0.5 us wide)

Latency = Propagation time + transimission time + queuing time(排队时间)

Propagation time = distance/speed of light

Trasmission time = size/bandwidth

one bit transmission => propagation is important (短资料很快就送完，但需要长时间才能传到对方，资料已送完，但签到资料还未到达对方) Propagation time >> transmission time

Large bytes transmission => bandwidth is important (长资料很慢才能送完，未送完前，前导资料已到对方) Transmisson time >> propagation time

Delay X Bandwidth​

  • The channel between a pair of processes can be viewed as a pipe
  • Latency(delay): length of the pipe
  • Bandwidth: width of the pipe
  • Delay x Bandwidth means how many data can be stored in the pipe
  • For example, delay of 80 ms and bandwidth of 100 Mbps

80 x 10^3 seconds x 100 x 10^6 bits/second

8 * 10^6 bits = 8 Mbits = 1MB data

delay bandwidth

  • Relative importance of bandwidth and latency depends on application
    • For large file tansfer, bandwidth is critical
    • For small messages(HTTP, NFS, etc.), latency is critical
    • Variance in latency(jitter) can also affect some applications(e.g., audio/video conferencing)

Ethernet​

Introduction Ethernet​

  • Most successfull local area networking technology of last 30 years.
  • First widely used LAN technology
  • Kept up with speed race: 10 Mbps - 100 Gbps

ethernet-1

  • Developed in the mid-1970s by researchers at the Xerox Palo Alto Research Centers(PARC)

  • DEC and Intel joined Xerox to define a 10-Mbps Ethernet standard in 1978.

  • This standard formed the basis for IEEE standard 802.3

  • More recently 802.3 has been extended to include

    • 100-Mbps version called Fast Ethernet
    • 1000-Mbps version called Gigabit Ethernet
    • 10 Gibabit Ethernet, and also
    • 100 Gibabit Ethernet

  • Connectionless: No handshaking between sending and receiving NICs

  • Unreliable: receiving NIC doesn't send ACKs or NACKs to sending NIC

  • Ethernets' MAC protocol: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

  • Bus topology popular throught mid 90s(总线拓扑)

    • all nodes in same collision domain(can collide with each other)

Ethernet Topologies​

Ethernet Frame Format​

Ethernet MAC Protocol - CSMA/CD​

802.3 Ethernet STandards​