1. Concept of computer network
Computer Network: A collection of self-made computer systems interconnected in such a way that they can share resources with each other.
2. Features of computer network
Connectivity, sharing, reliability, efficiency, independence, scalability and affordability.
3. Composition of computer network
Computer network = {edge part: all hosts connected to the Internet (end system) + core part: enabling any end system to communicate with other end systems (routing and forwarding)} = hardware + software + protocol = communication subnet + resource subnet.
4. Computer network function
Resource sharing, data communication, distributed processing, integrated information service, centralized management, information exchange, and improved reliability.
5. Computer network classification
① According to the scope of the network, it is divided into wide area network WAN, metropolitan area network MAN, local area network LAN, and personal area network PAN.
②According to the users of the network, it is divided into public network and private network.
6. Main performance indicators of computer network
① Rate: Data transmission rate, also known as data rate, bit rate, unit: bit per second (bit/s).
②Bandwidth: indicates the ability of a certain channel in the network to transmit data; network bandwidth: indicates the highest data rate that a certain channel in the network can pass within a unit of time.
③ Throughput: the actual amount of data passing through a certain network (or channel, interface) per unit time.
④ Latency: Latency = sending delay + propagation delay + processing delay + queuing delay.
Sending delay: the time required for the host or router to send a data frame, sending delay = data frame length (bit) / sending rate (bit/s).
Propagation delay: the time it takes for electromagnetic waves to propagate a certain distance in the channel, propagation delay = channel length (m)/propagation rate of electromagnetic waves on the channel (m/s).
The transmission delay has nothing to do with the transmission channel length (or the distance of signal transmission), and the propagation delay has nothing to do with the transmission rate of the signal. The longer the signal transmission distance, the greater the propagation delay.
⑤ Delay-bandwidth product: Delay-bandwidth product = propagation delay (s) × bandwidth (bit/s)
⑥ Round trip time RTT
⑦ Utilization rate: channel utilization rate = the time a certain channel is used/total time; network utilization rate = the weighted average of the channel utilization rate of the entire network;
D=D0/(1-U) where D is the current delay of the network, D0 is the delay when the network is idle, and U is the utilization rate of the network.
7. Computer network layered architecture
Ⅰ. Concept
① Architecture: A collection of various layers of a computer network and their protocols and interfaces between layers.
②Protocol Data Unit PDU: The data unit transmitted between peer-to-peer layers.
③Protocol: The rules, standards or conventions established for data exchange in the network. Syntax (the structure/format of data and control information); Semantics (what kind of response corresponds to this information); Synchronization (the description of the sequence of event realization).
④ Interface: The connection point for exchanging information between two adjacent layers.
⑤ Service: The lower layer is the function call provided by the upper layer.
Ⅱ. OSI seven-layer protocol architecture
① Application layer: complete specific network applications through the interaction between applications.
② Presentation layer: It is used to deal with the presentation of information exchanged in two communication systems.
③ Session layer: responsible for managing the session process between hosts, including establishing, managing, and terminating sessions between processes.
④ Transport layer: Provide general data transmission services for the communication between the processes in the two hosts.
⑤ Network layer: Provide communication services for different hosts on the packet switching network.
⑥ Data link layer: assemble the IP datagram handed over by the network layer into a frame, and transmit the frame on the link between two adjacent nodes. The data link layer must not only detect errors, but also correct them.
⑦Physical layer: Consider how much voltage is used to represent 1 or 0, and how the receiver recognizes the bits sent by the sender. The physical layer also determines how many pins the plug of the connecting cable should have and how the pins are connected.
①⁓③: Resource subnet; ④Connecting the preceding and the following; ⑤⁓⑦: Communication subnet.
8. Agreement
A protocol is a collection of rules governing communication between two peer entities or entities. The rules of the syntactic aspect of the protocol define the format of the information exchanged, and the rules of the semantic aspect of the protocol define the operation to be completed by the sender or receiver. Agreement is "horizontal".
The most important protocols in the transport layer are TCP and UDP, while the most important protocol in the network layer is the IP protocol.
9. Service
Under the control of the protocol, the communication between two peer entities enables this layer to provide services to the upper layer. To realize the protocol of this layer, it also needs to use the services provided by the lower layer. Entities using the services of this layer can only see the services but not the underlying protocols. That is, the protocol below is transparent to the entity above. Services are "vertical".
9. Interface
In the same system, the place where entities of two adjacent layers interact and exchange information is usually called the service access point SAP. The service access point is an abstract concept, which is actually a logical interface. Starting from the physical layer, each layer provides a service access point to the upper layer, that is, no service can be provided without an interface.
10. Architecture of TCP/IP
11. Circuit switching, message switching, packet switching
① Circuit switching: The bit stream of the entire message is continuous from the source point to the end point.
②Message exchange: The whole message is sent to the adjacent node first, and after all the messages are stored, the forwarding table is searched and forwarded to the next node.
③Packet switching: A single packet (this is only a part of the entire message) is transmitted to the adjacent node, stored in the forwarding table, and forwarded to the next node.
If a large amount of data is to be continuously transmitted, and the transmission time is much longer than the connection establishment time, the transmission rate of circuit switching is faster. Message switching and packet switching do not need to pre-allocate transmission bandwidth, which can improve the channel utilization of the entire network when transmitting burst data.