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The Hidden Functions of the Physical Layer: Why Is It the Cornerstone of Network Operations?
In the seven-layer OSI model of computer networks, the physical layer is the first and lowest layer. It is most closely related to the physical connection between devices. The physical layer provides the electrical, mechanical, and programming interfaces to the transmission medium. This layer is responsible for defining low-level parameters such as the shape and characteristics of the electrical connector, the transmission frequency, the line coding used, etc. Today, the physical layer is often implemented in a dedicated PHY chip or as a design module in electronic design automation (EDA).
The physical layer determines how the raw bit stream is transmitted over the physical data links that connect network nodes.
In mobile computing, the MIPI Alliance's *-PHY family interconnection protocol is widely used. The historical context of the OSI model is closely related to the Internet Protocol Suite and Ethernet. These technologies developed in the same period and had similar ideas in their early days, although they differed in some abstractions. Not limited to Internet applications, the OSI abstraction can be applied to all forms of device interconnection, whether in data communications or computing electronics.
The role of the physical layer
The core function of the physical layer is to define how to transmit raw bit streams between connected network nodes. These bit streams may be grouped into code words or symbols and converted into signals, which are then sent over a transmission medium. The physical layer includes the network's electronic circuit transmission technology and is the basic layer that supports high-level functions. This layer can be implemented through a variety of hardware technologies with different characteristics to provide support for higher-level functions.
In the context of the OSI model, the physical layer translates the logical communication requests of the data link layer into hardware-specific operations, resulting in the transmission or reception of electronic signals.
Physical Signal Sublayer
In networks using the Open Systems Interconnection (OSI) architecture, the physical signaling sublayer is part of the physical layer and interacts with the medium access control (MAC) sublayer of the data link layer. This sublayer is responsible for symbol encoding, transmission, reception, and decoding, and performs electrical isolation functions.
Relationship to the Internet Protocol Suite
The Internet Protocol Suite, as defined in RFC 1122 and RFC 1123, is a high-level network description for the Internet and similar networks. In contrast, the model does not define a layer specifically dealing with hardware-level specifications and interfaces, since the model is not directly concerned with physical interfaces.
Services
The main functions and services performed by the physical layer include bit-by-bit or symbol-by-symbol data delivery and providing a standardized interface to the transmission medium. It includes mechanical specifications for electrical connectors and cables, electrical specifications for transmission line signals, etc. The physical layer is responsible for electromagnetic compatibility, including the allocation of the electromagnetic spectrum and the specification of signal strength. The transmission medium may be electrical or optical fiber, or a wireless communication link.
The physical layer supports higher layers and is responsible for the generation of logical packets.
In terms of data flow management, the physical layer can implement bit synchronization and perform flow control in both synchronous and asynchronous serial communications. Sharing of the transmission medium by multiple network participants can be done through simple circuit switching or multiplexing, or more complex media access control protocols, such as Ethernet's Carrier Sense Multiple Access with Collision Detection (CSMA/CD), which can more effectively manage Channel.
PHY Definition
PHY is the abbreviation of "physical layer", which is usually a semiconductor circuit that implements the physical layer function in the OSI model. The PHY connects a link layer device (often called a MAC, which stands for Media Access Control) to the physical medium, such as fiber or copper cable. A PHY device typically includes physical coding sublayer (PCS) and physical medium dependent (PMD) layer functions.
Physical transceivers in Ethernet
Ethernet PHY is a component operating at the physical layer in the OSI network model. It implements the physical layer portion of Ethernet, and its purpose is to provide physical access to analog signals for the link. It typically interfaces with a microcontroller or other media independent interface (MII) in the system to handle higher level functions.
Other Applications
For wireless LAN or Wi-Fi, the PHY section includes the RF, mixed-signal, and analog sections, which are often referred to as a transceiver. Typically, these PHY portions are integrated with a medium access control (MAC) layer in a system-on-chip (SOC) implementation. Similar wireless applications include 3G, 4G, LTE, 5G, WiMAX and UWB. The PHY chip in the USB controller is used to provide a bridge between the digital and modulation parts.
These technologies provide support for physical layer services, ranging from wireless communications to data storage technologies, all implemented using dedicated PHY specifications. The physical layer plays a cornerstone role in the entire network, but how many people really understand its inner workings?
