Fundamentals of Communication Networks

What Is a Communication Network

There is always a network around us, 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 representative is the computer network, which is a combination of computer technology and communication technology.

Evolution of Computer Networks

Simple links based on host architecture with low-speed serial links, X.25, and IBM’s SNA

Main Characteristics of Computer Networks

Resource Sharing
Information Transmission and Centralized Processing

Load Balancing and Distributed Processing
Comprehensive Information Services

Definitions of LAN, MAN, and WAN

LAN (Local Area Network)

Usually refers to a collection of computers, printers, modems, or other devices that can interconnect within a few kilometers using some medium.

MAN (Metropolitan Area Network)

MAN covers a medium scale, between LAN and WAN, usually a network connection within a city (around 10KM).

WAN (Wide Area Network)

Distributed over long distances, it connects different locations through various types of serial connections to access a larger geographical area.

Common Network Topologies

Circuit Switching and Packet Switching

Circuit Switching: Based on telephone network circuit switching

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 a 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 taken 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 for enabling different devices in a 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 Standards: Standards that are widely used and accepted in practice without recognition by an organization.
De Jure Standards: Standards established by recognized organizations.

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)

Internet 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.

Promotes standardization efforts.
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, used to ensure the correct transmission of data, implemented through software.

The hierarchical structure of the OSI reference model is divided into seven layers from bottom to top:

Overview of TCP/IP Protocol Stack

TCP/IP originated from a packet-switched network research project funded by the US government in the late 1960s and 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”.

Like 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 synthesis of the OSI and TCP/IP models.

TCP/IP Protocol Stack

The IP protocol is a best-effort delivery network protocol, providing unreliable, connectionless data transmission services. The IP protocol does not care about the content of the data packets and cannot guarantee whether the data packets reach their destination successfully, nor does it care about any state information regarding previous data packets. The reliable service is provided by the upper-layer TCP protocol. All TCP, UDP, ICMP, and IGMP data are ultimately encapsulated in IP packets for transmission.

Physical Layer Functions

The physical layer mainly specifies the type of medium, interface type, signaling type; standardizes the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and terminating physical links between terminal systems; 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. Asynchronous serial ports support 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 cables, twisted pairs, optical fibers, radio waves;

Physical Layer Devices: Repeaters, hubs;

Data Link Layer Functions

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 protocols for LAN and WAN

Data link layer devices: Ethernet switches

Network Layer Functions and Devices

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 they are correctly transmitted from the sender to the receiver.

The main functions of the network layer include:

Addressing: The network layer assigns identifiers to each node, which is the network address. Address assignment also provides the basis for path selection from source to destination.
Routing: A key role of the network layer is to determine how to select routes for data transmission from source to destination. After calculating the routes, network layer devices forward data packets according to the routing information. Devices performing routing at the network layer are called routers.
Congestion Management: If too many data packets are transmitted simultaneously, congestion may occur, leading to data loss or delay. The network layer is also responsible for controlling congestion on the network.
Interconnection of Heterogeneous Networks: Communication links and media types are diverse, each with 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, providing services to the transport layer while translating network addresses to corresponding physical addresses. Network layer protocols can also coordinate the imbalance in processing capabilities of sending, transmitting, and receiving devices, such as segmenting and reassembling data to ensure that packet lengths meet the maximum data frame length supported by the data link layer protocol of that link.

Network Layer Protocols

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

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

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

Each message forwarding undergoes this process. When reaching the network of the destination host, 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, removing the link layer frame header and network layer message header before delivering it to the corresponding protocol.

Routers can support multiple independent routing protocols (e.g., IP RIP, OSPF, IPX RIP, etc.) and maintain separate routing tables for different network protocols stacks (such as TCP/IP, IPX). This capability allows routers to simultaneously support various network layer protocols for message forwarding.

Transport Layer Functions

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

Segmentation of 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, and its ultimate goal is to provide effective and reliable services to users—generally referring to application layer processes.

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

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

Transmitting data segments from one host to another. During transmission, it ensures data correctness through checksum calculations and flow control to avoid buffer overflow.
Some transport layer protocols ensure data transmission correctness. This primarily ensures that the same data is neither transmitted multiple times nor lost during transmission. It also ensures that the order of received packets matches the order of sent packets.

Transport layer protocols mainly include TCP protocol of the TCP/IP protocol stack and SPX protocol of the IPX/SPX protocol stack. Among them, TCP and SPX protocols provide reliable, connection-oriented services for applications, while UDP protocol provides unreliable, connectionless services.

Comparison of Main Transport Layer Protocols

Application Layer Functions

The main functions of the application layer

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

Defining standards for data representation

Source: 5G Communication

Reviewed by: Zhao Lixin