Chapter 1: Network Architecture

Layers and Transport

Each layer is defined as a collection of protocols, functions, and services

The sender encapsulates the data packet, and the receiver strips the data packet after receiving it

router

• Transfer data from one network to another

Several basic functions provided by the layer

• Split, reorganize

• Encapsulation

  • Refers to adding control information to the data packet in the form of a header. The header includes the following typical information
    • address
    • Error check code
    • Protocol control [additional information needed to execute the protocol]

• Connection control

  In connection-oriented transmission, in most cases, the data packet used to determine the connection does not contain data. The connection is determined based on the header information of the data packet. The three data items for connection control are [request/connection item, data Transmission item, termination item]

• Submit in order [head control information add package number]

• Flow control [to ensure that no overflow is received]

• Error control [Reconnaissance, correct missing or damaged data packets]

• Multiplexing [data packets from multiple upper layers share the same lower layer, you need to set one or more layers to adapt, and set the identification number to identify which upper layer is]

Hierarchical network model

• Layered comparison between OSI [Open System Interconnection] model and TCP/IP [Transmission Control Protocol/Internet Protocol] model
  

  note:

  • The OSI model is not used in practical applications. It is only used as a reference learning model. The Internet uses the TCP/IP protocol;
  • In addition, not all devices need the support of each layer. Some protocols are in the terminal device, and some protocols are in the intermediate device.

• Introduction to the functions provided by each layer of OSI

  • Physical layer [Bit transparent transmission]
  • Data link layer [frame]
  • Network layer [Processing the routing of data packets]
  • Transport layer【Execution of additional functions for transmission】
  • Session layer [Send data from the presentation layer to the transport layer]
  • Presentation layer [convert data into a common format that can be translated between peer layers]
  • Application layer【Provide basic functions of user interaction】

• Introduction to the functions provided by each layer of TCP/IP

  • TCP/IP physical network layer [data packet sending and receiving]
  • Network layer (IP layer) [Inter-Internet data packet routing]
  • Transport layer (TCP layer) [end-to-end transmission, and compensation for connectionless and unreliable transmission at the IP layer]
  • TCP/IP application layer [including OSI model session layer, presentation layer, and some functions of the application layer]

Non-tiered service

Services do not need to pass through other layers but directly access one or more protocol layers, often used for network management or describing network services

Analysis of sample questions [just for exam review, does not represent the correct answer]

• Why does the network designer adopt the design function of grouping, and not require all data packets to be the same size?

  Because the data transmitted in the network is relatively large and difficult to process, a data packet is grouped, and then each packet is identified and forwarded, which improves the utilization of network resources.
  And because the control information is encapsulated in the header of the PDU protocol data unit, the data packet size is recorded in it, so it is not required that the data packets are all of the same size

• Assuming that a four-layer TCP/IP network model is used, 20B header information is loaded on each layer, and the maximum allowable number of bytes of the physical layer data packet is 1500B, then draw a table according to the user data size given below, Give the number of data packets to be transmitted and the total number of bytes to be transmitted

  Analysis: The header of the physical layer PDU should have a total of 60B of 20+20+20, so the maximum physical layer PDU is 1560B, so:
  a.1000B number of data packets: 1; total number of bytes: 1060B
  b.10 000B number of data packets: 7; total number of bytes: 10420B
  c.100 000B number of data packets: 65; total number of bytes: 103900B
  d.100 000 000B number of data packets: 64103; total number of bytes : 103846180B

• According to the size of each user data given in the previous question, calculate the overhead percentage
  overhead = header information / total bytes

  a.5.66%
  b.4.03%
  c.3.75%
  d.3.70%