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Old Rules-Sister Town House:
One. OSI Reference Model
(1) Application layer
Provide services for the application and specify the details related to the communication in the application, including file transfer, e-mail, and so on.
(2) Presentation layer
Convert the information processed by the application into a format suitable for network transmission, mainly responsible for the conversion of data format, so that the consistency of data can also be maintained between heterogeneous models.
(3) Session layer
Responsible for the establishment and disconnection of communication connections (logical paths for data flow), as well as data transmission related management such as data segmentation, and management of the layers below the transport layer. That is, it is responsible for when to establish a connection, when to disconnect and how long to maintain the link. Its header records the information of the data transmission sequence, but it does not have the function of actually transmitting data.
(4) Transport layer
Manage the data transmission between the two nodes and be responsible for reliable transmission. To ensure that the transmitted data reaches the target address, it will be confirmed in the computers at both ends of the communication. If the data does not arrive, it will be responsible for resending, but this is actually a creation Logical communication connection.
(5) Network layer
The data is transmitted to the target address. The target address can be a certain address formed by connecting multiple networks through routers. Therefore, this layer only needs to be responsible for addressing and routing.
(6) Data link layer
Responsible for the communication and transmission of data frames between interconnected nodes on the physical level, that is, the conversion between data frames and bit streams.
(7) Physical layer
Responsible for switching between bit stream and electronic signal.
two. Classification of transmission methods
(1) According to the connection method
It is divided into two types: connection-oriented and connectionless. In connection-oriented, before sending data, a communication line needs to be connected between the sending and receiving hosts. For connectionless, it does not require connection establishment and disconnection, and the sender can be free When sending data, the receiving end needs to confirm whether it has received the data from time to time.
(2) According to the communication method
1. Circuit Switching
The switch is responsible for data transfer processing. The computer is first connected to the switch. When sending data, it needs to establish a communication circuit with the target host through the switch. After the connection is established, the user can use this circuit until it is disconnected.
2. Packet switching
Let the computer connected to the communication circuit divide the data to be sent into multiple data packets, arrange them in a certain order and then send them separately. This is packet switching. All computers can send and receive data together, which improves the utilization rate of the communication line. In the process of grouping, the addresses of the sender and the receiver have been written in the header of each group, so even if multiple users are served on the same line at the same time, each grouped data can be clearly distinguished.
In packet switching, a packet switch (router) connects the communication line, and the computer sends data packets to the router. After the router receives the packet data, it buffers it in its own buffer, and then forwards it to the target computer in the order of FIFI. In packet switching, the communication line is public, so the communication network may be congested, and when the buffer of the router is saturated or removed, packet data may be lost and cannot be sent to the opposite end.
(3) According to the number of receiving terminals
1. Unicast
One-to-one communication.
2. Broadcast
One-to-many communication, communication in a certain broadcast domain.
3. Multicast
Similar to broadcast, but multicast must limit a certain group of hosts as receivers.
4. Anycast
Select a host that best meets the network conditions from a specific host to send a message. Usually the selected host will return a unicast signal, and then the sending host will only communicate with this host. Anycast application has DNS root domain name resolution server.
three. address
The MAC address is designated by the manufacturer of the device separately for each network card (NIC). The IP address is composed of the network number and the host number. If the network number is the same but the host number is different, it means that the two hosts are on the same network segment and the network number is the same. The mainframes are relatively concentrated in organizational structure, provider type and geographical distribution.
During network transmission, each node will determine which network card the message should be sent out according to the address information of the packet data. Each address will refer to an outgoing interface list, MAC refers to the address forwarding table, and IP addressing refers to the routing control table. .
Third, the equipment in the network
(1) Communication mediation
Computers are connected to each other by cables. There are many types of cables, such as twisted pair cables, optical fiber cables, and so on. In the process of data transmission, the physical speed of data flow between two devices is called the transmission rate, the unit is bps (bits per second), the number of bits per second. Strictly speaking, the flow speed of signals in various transmission media is constant, and a high transmission rate does not refer to how fast the unit data flows, but refers to the amount of data transmitted per unit time.
The transmission rate is also called bandwidth. The larger the bandwidth, the stronger the network transmission capacity. In addition, the actual transmission rate between hosts is called throughput, and the unit is the same as the bandwidth, both in bps. But throughput is not only a measure of bandwidth, but also a host's CPU, network congestion, and so on.
(Two) network card
When any computer is connected to the network, it needs to use a network card (network interface card NIC or network adapter). The device integrates the function of connecting to the local area network. The general device will have a built-in NIC, and sometimes it can be inserted into the expansion slot separately.
(3) Repeater
A repeater is a device that extends the network on the first layer of the OSI model-the physical layer. The electrical or optical signal transmitted by the cable is adjusted and amplified by the repeater to another cable. In general, the two ends of the repeater are connected to the same communication medium, and some can also complete the transfer work between different media. The repeater is not responsible for judging whether the data has errors, but is only responsible for converting electrical signals into optical signals. Even if there is an error in the data link layer, it will continue to be forwarded, so it cannot be forwarded between media with different transmission thicknesses.
For network extension through repeaters, the distance is not unlimited. For example, a 10Mbps Ethernet can be connected in segments by up to 4 repeaters. Some repeaters can provide multiple port services. This is called a repeater hub or hub. Each port can be called a repeater.
(4) Bridge (Layer 2 switch)
A bridge is a device that connects two networks on the second layer (data link layer) of the OSI model. It can identify data frames in the data link layer, temporarily store these data frames in memory, and regenerate them A brand-new frame is forwarded to another connected network segment. Because the frame can be stored, the bridge can connect to data links with different transmission rates and does not limit the number of connected network segments.
There is a data bit called FCS in the data frame of the data link, which is used to verify whether the data is delivered correctly. The bridge checks the value in this field and discards the damaged data, thereby avoiding sending it to other network segments. You can also control the flow through the address self-learning mechanism and filtering function. The address here refers to the MAC address and the hardware address.
The switching hub commonly used in Ethernet is also a kind of bridge, and each port in the switching hub provides a function similar to a bridge.
(5) Router (Layer 3 switch)
A router is a device that connects two networks on the third layer (network layer) of the OSI model and forwards packets. The bridge is processed according to the MAC address, and the router is processed according to the IP address. The router can connect different data links, and the router also has the function of sharing the network load, such as dividing the data link.
(6) Layer 4-7 switch
The 4-7 layer switch is responsible for processing the data from the transport layer to the application layer in the OSI model. Based on the application layer, it analyzes the data sent and received and performs specific processing on it. For example, in order to share access pressure, a load balancer is set before the gateway, or bandwidth control in network communication is a type of layer 4-7 switch.
(7) Gateway
A gateway is a device in the OSI model that is responsible for converting and forwarding data from the transport layer to the application layer. Usually a presentation layer or application layer gateway is used to translate between two protocols that cannot communicate directly, and finally achieve both Communication between. For example, the conversion service between Internet mail and mobile phone mail, because the two protocols are not the same, you need to use a gateway to convert.
When using the WWW, in order to control network traffic and security considerations, a proxy server is used. This is also a kind of gateway, called an application gateway. With a proxy server, there is no need to communicate directly between the client and the server at the network layer. It communicates directly with the proxy server, carries out various control and processing of data and access from the transmission layer to the application layer, and then the proxy server communicates with the server through the network layer. For example, a firewall is such a security product that communicates through a gateway.
four. The composition of the network
(1) Composition
The central part of the computer network, called the backbone network or core network, selects high-speed routers to connect to each other so that it can quickly transmit a large amount of data. The part of the network that corresponds to the entrance and exit of the expressway is called the edge network. Commonly used devices are multi-function routers and layer three switches. For provincial roads, national roads, and the like, connecting the edge network from the entrance and exit of the expressway, it is called the access layer or the convergence layer.
(2) Internet communication
Generally, when people use the Internet, they will use Internet access services, and the communications gathered to the wireless LAN router and the nearest switch will be connected to the access layer, and even communicate with the target address through the edge network and the backbone network.
(3) Mobile communications
Once the mobile phone is turned on, it will automatically communicate with the nearest base station wirelessly. The base station is equipped with a specific mobile phone base station antenna, and the base itself is also equivalent to the access layer of the network. When a collecting terminal sends a signal to another terminal, the request sent by it will be sent to the base station that registered the phone number of the opposite terminal. If the opposite terminal answers the call, it is equivalent to establishing a communication connection.
The communication requests collected by the base station are aggregated to the edge network, and then connected to the backbone network of the interconnected communication control center.