Fundamentals of Network Communication Technology

What is a Communication Network

There is a network surrounding us at all times, such as telephone networks, telegraph networks, television networks, computer networks, etc.; even within our bodies, there are many network systems, such as the nervous system and the digestive system. The most typical example is the computer network, which is a combination of computer technology and communication technology.

Fundamentals of Network Communication Technology

The Evolution of Computer Networks

In the 60s, low-speed serial links based on host architecture, X.25, and IBM’s SNA

Main Characteristics of Computer Networks

Resource Sharing
Information Transmission and Centralized Processing

Load Balancing and Distributed Processing
Integrated Information Services

Definitions of LAN, MAN, and WAN

LAN (Local Area Network）

Typically refers to a collection of computers, printers, modems, or other devices that can be interconnected within a few kilometers through some medium.

MAN (Metropolitan Area Network）

MAN covers a medium scale, between LAN and WAN, typically a network connection within a city (about 10KM distance).

WAN (Wide Area Network)

Distributed over long distances, it allows access across larger geographical areas through various types of serial connections.

Common Network Topologies

Circuit Switching and Packet Switching

Circuit Switching: Based on the telephone network

Advantages: Low latency, transparent transmission
Disadvantages: Fixed bandwidth, low network resource utilization, slow initial connection establishment

Packet Switching: Stores and forwards data in packets

Advantages: Multiplexing, high network resource utilization
Disadvantages: High latency, poor real-time performance, complex device functions

The unit of packet switching is the cell, a Layer 2 frame.

Performance Standards of Computer Networks

Bandwidth

Describes the amount of data that can be transmitted from one node to another within a certain time frame, usually measured in bps, for example, Ethernet bandwidth is 10Mbps, Fast Ethernet is 100Mbps.

Delay

Describes the time it takes for data to be transmitted from one node to another on the network.

Protocols and Standards

What is a Network Protocol?

A network protocol is a set of formats and agreements established in advance to allow different devices in the network to communicate data.
A network protocol is a normative description of a series of rules and agreements that defines how network devices exchange information.

Data Communication Standards are divided into two categories: De Facto and De Jure

De Facto Standard: A standard that has been widely used and accepted in practice without being recognized by an organizational body.
De Jure Standard: A standard established by an officially recognized organization.

Standardization Organizations

International Organization for Standardization (ISO)
Institute of Electrical and Electronics Engineers (IEEE)

American National Standards Institute (ANSI)
Electronic Industries Alliance (EIA/TIA)

International Telecommunication Union (ITU)
Internet Engineering Task Force (IETF)

Research Task Force (IRTF)
Internet Assigned Numbers Authority (IANA)

OSI Reference Model

OSI RM: Open Systems Interconnection Reference Model

The OSI Reference Model has the following advantages:

Simplifies related network operations
Provides compatibility and standard interfaces between devices

Facilitates standardization work
Structurally separable

Easy to implement and maintain

The first three layers are called the Lower Layer, also known as the Media Layer, responsible for data transmission in the network. Network interconnection devices are often located in the lower three layers, implemented through a combination of hardware and software. The fifth to seventh layers of the OSI Reference Model are called the Upper Layer, also known as the Host Layer, which ensures the correct transmission of data, implemented through software.

The hierarchical structure of the OSI Reference Model consists of seven layers from bottom to top:

Overview of the TCP/IP Protocol Stack

TCP/IP originated from a packet switching network research project funded by the US government in the late 1960s and has developed into the most commonly used networking form between computers by the 1990s. TCP/IP is a truly open system, as its protocol suite definitions and various implementations can be obtained publicly for free or at a low cost. TCP/IP is the foundation of the “global internet” or “Internet”.

Similar to the OSI Reference Model, the TCP/IP peer model is also divided into different layers, each responsible for different communication functions. The five-layer peer model is a combination of the OSI and TCP/IP models.

TCP/IP Protocol Stack

The IP protocol is a connectionless network protocol that provides unreliable data delivery services. The IP protocol does not care about the content of data packets, cannot guarantee whether data packets successfully reach their destination, and does not care about any state information regarding previous and subsequent data packets. A reliable connection-oriented service is provided by the upper-layer TCP protocol. All data such as TCP, UDP, ICMP, and IGMP is ultimately encapsulated in IP packets for transmission.

Functions of the Physical Layer

The physical layer mainly specifies the types of media, interfaces, and signaling; it regulates the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and closing physical links between terminal systems; it specifies characteristics such as levels, data rates, maximum transmission distances, and physical connectors;

Sync serial ports can act as DCE or DTE, supporting various physical layer protocols: V.24/V.35/X.21, etc. The asynchronous serial port supports the RS232 protocol with a maximum rate of 115.2kbit/s. G.703 E1/T1 interface types.

Physical Layer Media and Devices

Physical Layer Media: Coaxial cable, twisted pair, fiber optic, radio waves;

Physical Layer Devices: Repeaters, hubs;

Functions of the Data Link Layer

MAC Sub-layer: Media Access Control Sub-Layer

Specifies how data is transmitted over physical lines and communicates with the physical layer.

LLC Sub-layer: Logic Link Control Sub-layer

Identifies protocol types and encapsulates data for transmission over the network.

Data Link Layer Protocols

Data link layer LAN and WAN protocols

Data link layer devices: Ethernet switches

Functions and Devices of the Network Layer

Function: Forwarding data packets between different networks

Devices: Routers, Layer 3 switches

The task of the network layer is to select the appropriate path and forward data packets, ensuring that data packets are accurately transmitted from the sender to the receiver.

Main functions of the network layer include:

Addressing: The network layer assigns identifiers to each node, which is the network address. Address allocation also provides a basis for path selection from source to destination.
Routing: A key function of the network layer is to determine how to route data from source to destination. After calculating the route, network layer devices forward data packets according to the routing information. Devices that perform routing at the network layer are called routers.
Congestion Management: If too many data packets are transmitted simultaneously over the network, congestion may occur, leading to data loss or delays. The network layer is also responsible for controlling congestion on the network.
Interconnecting Heterogeneous Networks: Communication links and media types are diverse, and each link has its specific communication regulations. The network layer must be able to operate across various links and media types to provide communication services across multiple segments.

The network layer is situated between the transport layer and the data link layer. It is responsible for providing services to the transport layer while translating network addresses into corresponding physical addresses. The network layer protocol can also coordinate the imbalance of processing capabilities among sending, transmitting, and receiving devices, such as segmenting and reassembling data to ensure that the length of data packets meets the maximum data frame length supported by the data link layer protocol of that link.

Network Layer Protocols

When a host application in a network needs to send a message to a destination in another network, an interface of the router in the same network as that host will receive the data frame. The router’s link layer checks the frame, determines the type of network layer data being carried, removes the link layer frame header, and sends the network layer data to the corresponding network layer for processing;

The network layer checks the message header to determine the subnet of the destination address, then looks up the routing table to get the corresponding output interface;

The link layer of the output interface adds a link layer frame header to the message, encapsulates it into a data frame, and sends it to the next hop;

This process must be repeated for every message forwarded. When reaching the network where the destination host is located, the message is encapsulated into the link layer data frame of the destination network and sent to the corresponding destination host. After receiving the message, the destination host processes it through the link layer and network layer, removes the link layer frame header and network layer message header, and delivers it to the corresponding protocol.

Routers can support multiple independent routing protocols (such as IP RIP, OSPF, IPX RIP, etc.) and maintain their respective routing tables for different network protocol stacks (such as TCP/IP, IPX). This capability allows routers to simultaneously support multiple network layer protocols and forward messages.

Functions of the Transport Layer

The ultimate goal is to provide effective and reliable services to users (generally referring to processes in the application layer). The transport layer mainly defines end-to-end connectivity between host applications, generally including four basic functions.

Segmenting upper layer data
Establishing end-to-end connections

Transmitting data from one host to another
Ensuring data is transmitted in order, reliably, and correctly

The transport layer is located at the fourth layer of the OSI reference model, with the ultimate goal of providing effective and reliable services to users—generally referring to processes in the application layer.

The transport layer mainly defines end-to-end connectivity between host applications, generally including four basic functions:

Segmenting data sent from the application layer to the network layer or merging data segments sent from the network layer to the application layer.
Establishing end-to-end connections, mainly establishing logical connections for data flow transmission.

Sending data segments from one host to another. During transmission, checksums are calculated, and flow control ensures data correctness, preventing buffer overflow.
Some transport layer protocols ensure data delivery correctness. This mainly ensures that the same data is neither transmitted multiple times nor lost during transmission. It also ensures that the order of data packet reception matches the order of sending.

The transport layer protocols mainly include the TCP protocol and UDP protocol of the TCP/IP protocol stack, as well as the SPX protocol of the IPX/SPX protocol stack. Among them, the TCP protocol and SPX protocol provide reliable, connection-oriented services to applications; the UDP protocol provides unreliable, connectionless services.

Comparison of Major Transport Layer Protocols

Functions of the Application Layer

Main functions of the application layer

Providing interfaces for users and processing specific applications
Data encryption, decryption, compression, and decompression

Defining standards for data representation