【1. Computer Network - Overview】

1. Network, Internet, Internet

Node:The node in the network can be a computer, hub, switch or router, etc. , usually refers to a network device, which is divided into link nodes and routing nodes, which correspond to switches and routers in the network respectively.

Link: It is a line between two nodes. Links are divided into physical links and logical links (or data links).
The former refers to the actual communication line, which is realized by the device network port and the transmission medium connection; < /span> If the link layer protocol does not work, the data link cannot be established.
The latter refers to a network path that functions logically and is implemented by the data link layer standards and protocols in the computer network architecture.

Pathway: refers to a string from the node that sends the information to the node that receives the information Combination of nodes and links. In other words, it is a series of node-to-node links established across the communication network. The main difference from a link is that a path may include multiple links.

Network: It consists of several nodes and the links connecting these nodes.

Internet: Multiple networks are interconnected through routers to create a larger network, the Internet ("a network of networks"< /span>).

Internet: The largest Internet network.

• All hosts connected to the Internet
• A large number of networks and the routers that connect them

2. Computer network

Definition: A number ofinterconnected (either wired or wireless), autonomous A collection of computers (at least two) with their own hardware and software.

3. Performance indicators of computer networks

related to speed

• Rate
  The rate at which a host connected to a computer network transmits bits over a digital channel, also Called bit rate or data rate.
• Bandwidth (the highest speed supported by network equipment)
  is used to indicate the ability of the network's communication lines to transmit data, usually in unit time. The "highest data rate" that can pass from one point in the network to another. The unit is "bits per second", b/s, kb/s, Mb/s, Gb/s.
• Throughput
  The amount of data passing through a network (channel or interface) in unit time. Units are b/s, kb/s, Mb/s, etc. Throughput is limited by the bandwidth of the network or the rated rate of the network.

time related

• Latency refers to the time required for data (messages/packets/bit streams) to be transmitted from one end of the network (or link) to the other end. Also called delay or delay. The unit is s.
  • Transmission delay = data length (b)/transmission rate (channel bandwidth) (b/s)
  • Propagation delay (the process of running on the link) = channel length (m)/electromagnetic wave propagation efficiency (m/s) depends on the electromagnetic wave propagation speed and link length
  • Queuing delay: waiting for input/output link to be available
  • Processing delay: error detection, finding exits (generally inconvenient to calculate)
• Delay-bandwidth product (link length in bits), that is, how many bits a certain link currently has (Capacity a>) Propagation delay × bandwidth
• Round trip time (RTT = propagation delay * 2 + terminal processing delay) From the time the sender sends data to the time the sender receives the confirmation from the recipient, the total time time delay. (The larger the RTT, the more data can be sent before receiving the confirmation)
  

• Utilization rate (channel utilization rate, network utilization rate)
  The higher the utilization rate, the better: when the utilization rate of a certain channel increases When the channel is large, the delay caused by the channel will also increase rapidly; the channel utilization cannot be too low, which will waste communication resources

• Packet loss rate The ratio of the number of packets lost during transmission to the total number of packets

4. Computer network architecture

1. Why layer?

• Each layer is independent. A certain layer does not need to know how the layer below it is implemented, but only needs to know the services provided by the layer through the interface between layers (ie, interface). Sinceeach layer only implements a relatively independent function, a complex problem that is difficult to handle can be decomposed into several smaller ones that are easier to handle. Some questions. In this way, the complexity of the entire problem is reduced.
• Good flexibility. When any layer changes (for example due to changes in technology), as long as the interface relationship between layers remains unchanged, the layers above or below this layer will not be affected. In addition, the services provided by a certain layer can be modified.
  When the services provided by a certain layer are no longer needed, this layer can even be canceled.
• Structurally separable. Each layer can be implemented using the most appropriate technology.
• Easy to implement and maintain. This structure makes implementing and debugging a large and complex system easier to handle because the entire system has been decomposed into several relatively independent subsystems.
• Can promote standardization work. Because the functions of each layer and the services they provide have been accurately described.

2. Common architecture
   
3. What is the function of each layer?
   
   
4. Professional terminology in computer network system

• entity
• Three elements of protocol: syntax, semantics, and synchronization
• Interface (between adjacent layers)
• Serve

Protocols are horizontal, services are vertical