Introduction To Telecom Networks: Fixed/Wireless Networks

在前面的章节中，我们已经讨论了从1G到5G的移动通信系统的演进，包括4G LTE的细致架构和工作原理。现在我们将深入探讨光纤接入网络（Optical Fiber Access Networks, OFAN）中的Passive Optical Network（PON）技术以及光波分布多路复用（Dense Wavelength Division Multiplexing, DWDM）系统。

7.1 Mobile/Cellular Systems: 4G LTE Network Architecture

7.1.1 Evolution from 1G to 5G

• 1G: The first generation of cellular networks introduced analog voice services.
• 2G: Digital encryption was added with systems like GSM, enabling data transmission (SMS and MMS).
• 3G: Introduced packet-switched data networks, providing faster data rates (e.g., HSPA).
• 4G LTE: An all-IP system that provided significantly higher bandwidths for high-speed data services (e.g., LTE-Advanced).
• 5G: Aims to provide even higher speeds and lower latencies with advanced technologies like Massive MIMO, Beamforming, and network slicing.

7.1.2 The Cellular Concept

• Cellular Systems are designed to provide wireless communication over a geographic area divided into multiple "cells".
• Each cell uses low-power transmitters situated at the edge of cells and interconnected by a higher power transmission network.

7.1.3 4G LTE Network Components

• Base Station (eNodeB): Responsible for handling the radio connectivity with mobile devices.
• Evolved Packet Core (EPC): The network of servers and databases that controls data routing for all IP traffic within the network.
• Mobility Management Entity (MME): Handles session management, mobility functions, and serves as a proxy for the serving gateway in the packet core.
• Serving Gateway (SGW): Manages user plane procedures, such as routing data packets to/from the endpoints and performing policy enforcement.
• PDN Gateway (PGW): Serves as the default bearer anchor point for a default EPS bearer context and interfaces with external networks (e.g., IP multimedia Subsystem, IMS).

7.2 Optical Fiber Transmission Networks Using DWDM Systems

7.2.1 Why Dense Wavelength Division Multiplexing (DWDM) Systems?

• Spectrum Efficiency: Utilizes multiple wavelengths in the optical spectrum to transmit data concurrently over a single fiber.
• High Capacity: Enables terabit per second transmission capacities across long distances.

7.2.2 Wavelength Division Multiplexing (WDM) Concept

• WDM multiplexes multiple light wavelengths onto a single optical fiber by using different colors of lasers to carry data signals over long distances without converting the signals to electrical form.

7.2.3 Advantage of WDM Technology

• Bandwidth: Each wavelength carries a separate signal, providing more bandwidth than traditional systems.
• Cost Effectiveness: Reduces the need for physical infrastructure expansion by maximizing the use of existing fiber.

7.2.4 Wavelength Spectrum For Optical Fiber Communication

• The optical spectrum is divided into channels, each carrying a different wavelength or color of light.

7.2.5 Transponder in DWDM

• A transponder converts electrical signals to optical signals and vice versa, and operates over a specific wavelength channel.

7.2.6 Optical Supervisory Channel

• A separate optical channel used for network operations, monitoring, and control purposes.

7.2.7 Optical Add Drop Multiplexer (OADM)

• A device that allows the addition of new wavelengths or the removal (dropping) of existing ones from the main lightpath without affecting the other wavelengths.

7.2.8 What is an Optical Amplifier and its types

• Optical Amplifiers boost the signal power in an optical communication system, allowing for longer distances between regenerators or endpoints.
• There are three main types of optical amplifiers: Erbium-doped fiber amplifier (EDFA), Raman amplifier, and Semiconductor Optical Amplifier (SOA).

7.2.9 Manipulating Wavelengths

• Techniques such as tunable lasers and filters allow for the dynamic assignment and reconfiguration of wavelengths over the network.

7.2.10 DWDM Deployment Models

1. Point-to-Point DWDM: Ideal for long-haul communication where dedicated paths are used to transmit data between two endpoints.
2. Fixed Optical Add Drop Multiplexers (FOADMs): Used in scenarios where specific wavelengths need to be added or dropped at fixed locations along the network path.
3. Reconfigurable Optical Add Drop Multiplexers (ROADMs): Offer greater flexibility by allowing the dynamic addition, dropping, and routing of wavelengths across the network.

With the advent of 5G, DWDM systems are becoming increasingly important as they can support the high bandwidth requirements and massive connectivity that characterize next-generation wireless networks. The integration of ROADMs in particular is a key development, enabling service providers to optimize the use of network resources and respond quickly to changing demands for data capacity.