Computer network (network) consists of a plurality of nodes of these node, and connecting links composition.
The Internet can be divided into two
parts : the edge part and the core part according to its working method: the edge part : users use it directly.
Core part : Provide services for the edge part.
The core part of the network is the most complex part of the Internet.
The router plays a special role in the core part of the network. The router is a key component to realize packet switching . Its task is to forward the received packets . This is the most important function of the core part of the network.
Circuit switching : It must be connection-oriented. Must go through the establishment of a connection (occupying communication resources) -> call (always occupying communication resources) -> releasing the connection (returning communication resources) . An important feature of
circuit switching is that the two users in the call always occupy end-to-end Communication resources .
If a large amount of data is transmitted, and the transmission time is much longer than the setup time, the circuit switching efficiency is high.
Why does computer network data exchange not use circuit switching : When circuit switching is used to transmit computer data, the transmission efficiency of the line is often very low. This is because computer data appears on the transmission line in bursts, so the line is really used The time to transmit data is often less than 10% or even 1%, and communication line resources that have been occupied by users are idle most of the time.
Packet switching: Using store-and-forward technology, before sending the message, the longer message is divided into smaller data segments of equal length, and some necessary control information is added in front of each data segment to form the header. The grouping.
A packet is a unit of data transmitted on the Internet.
The importance of the packet header: Because the header contains important control information such as destination address and source address , each packet can independently select the transmission path in the Internet and correctly deliver to the end of the packet transmission.
The host is for information processing for users, and the router is for forwarding packets, that is, for packet switching.
When a router receives a packet, temporarily store it, check its header, find the forwarding table, find a suitable interface and forward it according to the puzzle address in the header, and hand the packet to the next router.
The advantages of packet switching : efficient, flexible, fast, and reliable.
Disadvantage : Packets need to be queued when they are stored and forwarded on each router, which will cause a certain delay.
Summarize the main characteristics of the three exchange methods in the data transmission stage:
- Circuit switching : The bit stream of the entire message continues from the source point to the end point, as if transmitted in a pipe.
- Message exchange : The entire message is first transmitted to the neighboring node, all the following are stored and then the forwarding table is searched, and then forwarded to the next node.
- Packet switching : A single packet (part of the entire IQ message) is transmitted to the neighboring node, stored in the forwarding table, and forwarded to the next node.
Since the length of the packet is less than the length of the message, the packet exchange has a shorter delay than the message exchange.
The several different types of computer networks (classified according to scope of network)
1, a wide area network WAN 2, a metropolitan area network MAN 3, a local area network LAN 4, a personal area network PAN
computer network, several performance indicators:
- Speed : the most important indicator
- Bandwidth : The ability of a channel in the network to transmit data, and the highest data rate that a new arrival in the network can pass in a unit time.
- Throughput : However, the actual amount of data passing through a certain network within a certain period of time is limited by bandwidth and rated rate
- Delay : the time required for data to travel from one end of the network to the other end, also called delay.
Total delay = transmission delay + propagation delay + processing delay + queuing delay.
Sending delay = data frame length / sending rate
propagation delay =channel length/propagation rate on the channel
For high-speed network links, we only increase the data transmission rate, and increasing the data transmission rate only reduces the data transmission delay.
Non-performance characteristics of computer networks: cost, quality, standardization, reliability, scalability and upgradeability, easy management and protection.
Computer network architecture
The benefits of layering : it can transform a large and complex problem into a number of smaller local problems, and these smaller local problems are easier to study and deal with. Each layer is independent, flexible, separable in structure, easy to implement and protect, and can promote standardization.
The three elements of the agreement :
- Syntax : The structure and format of data and control information.
- Semantics : What control information should be sent by both parties, the actions to be performed, and the response returned.
- Synchronization : Detailed description of the implementation sequence at 18:43:44 on December 23, 2019.
| OSI architecture |
|---|
| Application layer |
| Presentation layer |
| Session layer |
| Transport layer |
| Network layer |
| data link layer |
| Physical layer |
| TCP/IP system organization |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Network interface layer |
| Five-layer protocol architecture | effect |
|---|---|
| Application layer | Provide services directly to the user's application process |
| Transport layer | Responsible for the communication between two hosts to provide general data transmission services |
| Network layer | Responsible for providing communication services for different hosts on the packet switching network |
| data link layer | Assemble the IP data packets handed over by the network layer into frames, and transmit the frames between two adjacent nodes |
| Physical layer | Transparent transmission of bitstream |
Entity : Any hardware or software process that can send or receive information.
Peer entities : entities located at different levels on two different nodes of the Xiang Hutong family to perform the same function.
Protocol : A collection of rules that control the communication between two peer entities.
Service : Each layer in the layered model is the function provided by the adjacent upper layer.
Under the control of the protocol, the communication between two peer entities enables this layer to provide services to the upper layer. To implement this layer protocol, you also need to use the services provided by the layer below.
Point-to-point : data transmission between two hosts.
End-to-end : specific data transfer between two processes.