directory
First, write in front
Internet and Internet
Internet core: circuit switching – Packet switching – Packet switching
1. Circuit switching
2. Message exchange
3. Packet switching
4. OSI and TCP/IP compromise: five-layer protocol architecture
Write at the back
First, write in front
Before this, I have learned some knowledge of computer network, which I feel is a very basic part. I have not organized it systematically, so I have not formed a relatively complete knowledge system. Recently, I have time to look at the content of the network again, plus the learning of networking technology, “reviewing the old to know the new”, and found some knowledge and content that I did not learn before, especially to the knowledge of the latter part, I feel that I have gained a lot. I am going to record this time, on the one hand, it can better summarize the important knowledge points, on the other hand, it is convenient to check and learn.
Internet and Internet
As the name suggests, a network is composed of several nodes and the links that connect them. The concept of node is broad, computers, routers, switches and other devices can become nodes in the network.
- ** Internet ** is a general term for a network consisting of multiple computer networks connected together.
- **Internet ** is a special term, refers to the world’s largest, open by a number of networks connected to each other into a specific computer network, it uses TCP/IP protocol cluster, formerly known as the United States ARPANET.
The Development of the Internet is divided into three stages:
The first stage: from the single network ARPANET to the development of the Internet, the embryonic form of the Internet was born.
The second stage: build the three-tier structure of the Internet, divided into backbone network, regional network and campus network (or enterprise network).
The third stage: The Internet is formed with a multi-level Internet Service Provider (ISP) structure.
Can see the rapid development of the Internet, so at this time need to have a standard to unify, in order to avoid incompatible technology system, so the birth of international standards, by the international organization ISOC comprehensive management.
After that, all Internet standards are published on the Internet as RFC documents. Request For Comments (RFC) : Indicates requests and Comments. The document number is named RFC XXXX in chronological order. For example, [RFC 792] is the Internet standard number of ICMP.
The development of formal standards for the Internet goes through four phases:
- Internet draft: This stage is not yet an RFC document.
- Recommended standard: RFC documentation begins at this stage.
- Draft standard.
- Internet standards.
Internet core: circuit switching – Packet switching – Packet switching
The core part of the Internet is the most complex part. It needs to provide connectivity so that any host on the edge of the network can communicate with each other.
The key to this core is the router. Router is the key component in the development of continuous network, which realizes packet switching. Prior to this, there have been historical stages of circuit switching and message switching.
1. Circuit switching
Circuit switching began with the advent of the telephone, communication between telephones through the intermediary equipment telephone exchange. When two phones communicate, a dedicated physical path is established: establish connection (allocate communication resources) -> talk (always occupy resources) -> release connection (return resources).
As you can see, this allows only two phone users to communicate at a time, because the entire channel is monopolized and resources are released only at the end of the call. When applied to computer communication to transmit data, the efficiency is obviously low and the communication line is idle most of the time.
Circuit switching can be thought of as a bit stream of data transmitted to the end.
2. Message exchange
Packet exchange transmits all data to the next node of the link at one time, searches for the next node after all the data is stored, and finally forwards the data to the target at one time.
3. Packet switching
In packet switching, the packet is not sent as a whole, but divided into several smaller data segments, and the packet is forwarded through the router. The efficiency is high because the data segments can be stored in memory instead of being written to disk, which speeds up the switching.
Packet switching is to transfer a single packet data to the next node. The output interface and the input interface can work in parallel.
The advantages of packet switching are high efficiency (dynamically allocating bandwidth and occupying links in segments), flexibility (choosing routers for each packet), speed (using small packets as transmission units), and reliability (distributed multi-route packet switching network, good survival performance).
4. OSI and TCP/IP compromise: five-layer protocol architecture
As we all know, the OSI Tier 7 protocol is the true international standard. But before OSI model was proposed, TCP/IP protocol cluster has been widely used and is the de facto standard.
The main reasons are :(1) there is no business drive for the OSI model standard. (2) The implementation of OSI protocol is too complex and the operation efficiency is low. (3) OSI standard equipment did not enter the market in time, and the development cycle was too long. (4) The OSI hierarchy is not reasonable, and some functions are repeated.
The OSI architecture consists of the physical layer, data link layer, network layer, transportation layer, session layer, presentation layer, and application layer (from bottom up).
As you can see from the figure above, the TCP/IP architecture is divided into four layers. In essence, only the top three layers function, and the bottom network interface layer has little content.
Therefore, the five-layer protocol architecture is adopted by integrating the lower three layers of OSI and the upper three layers of TCP/IP.
The functions of each layer are:
- Application layer: the highest level of the architecture. Applications on a specific network are implemented through interaction between application processes. The layer protocols include SMTP for email and FTP for file transfer. The data unit is packet.
- Transport layer: the service responsible for data transfer between two host processes. The transport layer has the functions of reuse and division. Reuse means that multiple processes in the application layer can simultaneously use the services of the transportation layer from the top down, and partition means that the transportation layer delivers the received data to the corresponding processes in the application layer from the bottom up. The two protocols of this layer are TCP, and the data unit is packet segment. UDP. The data unit is user datagram. The details of TCP and UDP will be documented in a future article.
- Network layer: provides communication services for different hosts in a packet switched network. The data unit is IP datagram. The router is the main role of this layer, the task is routing and forwarding.
- Data link layer: transmits packets in frames from one end of the link to the other.
- Physical layer: the lowest layer of the architecture that performs the most basic tasks in a computer network. The data unit is bit stream. Physical transmission intermediary twistedpair, coaxial cable, optical cable and so on.
Write at the back
This article I mainly wrote some computer network overview, which is more important is the five layer protocol architecture, TCP/IP protocol cluster is the key part. Therefore, in the following content record, I should learn and summarize according to the specific knowledge of the five-layer protocol, and mainly plan to start from the data link layer. Personally, I think that the physical layer prefers the knowledge of communication principles, including the knowledge of channel, coding and modulation, which will not be expanded.
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