Data Communication and Computer Network
Data communication in the network is complicated. In this article, we will explore how data communication works by explaining how two PCs find each other, send and receive the info with the introduction of the TCP/IP five-layer protocol.
Part I. What is data communication?
Data communication is a network constructed by various devices such as routers, switches, firewalls, load balancers, IDS/IPS and VPN servers. The basic function of the data network is to realize data communication between different nodes.
Characteristics of Data Communication● Connectionless:
There is no unique link between the two communicating devices.
● Packet switching:
Each packet has a destination address, error control information, and has the ability to find the destination on its own.
Part II. TCP/IP Five-Layer Protocol
Communication Protocol is the rule that both parties of communication should obey. Without the network communication protocol, the data of the computer can't send to the network and reach other computers. Generally, it's divided into different function layers by adopting TCP/IP five-layer protocol that includes Application Layer, Transport Layer, Network Layer, Data Link Layer, and Physical Layer.
The implementation of the protocol is very complex. Because the protocol needs to process human-readable data, such as web pages, emails, etc., into signals that can be transmitted on the network. A lot of processing work needs to be done.
Communication between entities in two systems is a very complex process. In order to reduce the complexity of the protocol design and debugging process, network protocols are usually organized in a structured hierarchical manner, each layer completes a certain function, and each layer is built on its lower layer. Different network protocols have different numbers of layers, names of layers, and functions.
Then, how does each layer communicate after being divided into N Layers?
The OSI standard strictly defines the protocol data unit (PDU) format for each layer of communication. The communication between the peer layers is the purpose, and the cooperation of the peer layer entities ensures the realization of the functions and services of this layer.
Communication between adjacent layers is the means to ensure that communication between peer layer entities can be implemented.
Assuming that the network protocol is divided into several layers, the communication between nodes A and B is actually the communication between the nth layer of node A and the nth layer of node B, so the protocol always refers to the protocol of a certain layer, such as the physical layer protocol, Transport layer protocol, application layer protocol. There is an interface between each adjacent layer protocol, and the lower layer provides services to the upper layer through this interface.
1. Physical Layer
On the physical layer, a transmission medium/physical medium is necessary to connect the two computers to achieve communication between two devices and data transmission. Optical fiber is one of the oriented transmission medium, whose bandwidth is greatly larger than other medium's.
2. Data Link Layer
The main function of the data link layer is to strengthen the function of the physical layer to transmit the original bit stream, and transform the physical connection provided by the physical layer into a logically error-free data link, making it appear as an error-free data link to the network layer. link. On the basis of the services provided by the physical layer, the data link layer also shoulders the responsibility of providing services for the network layer. Its most basic service is to encapsulate IP datagrams from the network layer into frames and transmit them to adjacent nodes reliably. the target network layer.
Then how does the device recognize the terminal device when sending data through the physical and data link layers? MAC address is adopted.
Notes: About MAC Address
The MAC address is the address of the data link layer, which is 6 bytes (48 bits) in length and is used to uniquely identify the network adapter (NIC). Data transmission between computers is uniquely searched and transmitted through the MAC address.
A host has as many MAC addresses as there are network adapters. For example, laptops are ubiquitous with wireless network adapters and wired network adapters, so there are two MAC addresses.
3. Network Layer
The network layer focuses on choosing the right internetwork routing and exchange nodes to ensure data transmission in time. When sending data, the network layer encapsulates the packet segments or user datagrams generated by the transport layer into packets and packets for transmission.
When a network device has data to send to another network device, it must know the other device's network layer address (that is IP address). The IP address is provided by the network layer, but only the IP address is not enough. The destination MAC address must also be included. Therefore, the sender must also obtain the destination MAC address. The process of obtaining the destination MAC address through the destination IP address is implemented by the ARP (Address Resolution Protocol) protocol.
Notes: About IP Address● An IP address uniquely identifies a network device and consists of 32 binary bits.
● The IP address is displayed in dotted decimal notation.
● The IP address is divided into two parts: network address and host address
● The IP address is a 4-byte 32-bit binary
number in dotted decimal notation
4. Transport Layer
The network layer is to establish host-to-host communication, while the transport layer is to establish port-to-port communication.
Through the interaction of the physical layer, the data link layer, and the network layer, we have successfully transferred data from computer A to computer B. However, there are various applications in computer B. How does computer B know which application the data is for? Therefore, a port (Port) should be designated for a specific application to accept processing. Thus, an application process on a host can be identified with IP address and port.
5. Application Layer
The application layer contributes to completing specific network applications through interaction between application processes.
The application layer protocol defines the rules of communication and interaction between application processes. Since the data transmitted from the transport layer is in various formats, including HTML format, mp4 format, etc., different application layer protocols are required for different network applications. There are many application layer protocols on the Internet, such as domain name system DNS, HTTP protocol supporting World Wide Web applications, SMTP protocol supporting email and so on.