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The OSI physical layer encodes the bits that represent data link layer frames as signals and uses physical media-copper, optical, and wireless media-to connect network devices-to transmit and receive these signals.

The data link layer frame entering the physical layer contains bit strings representing application layer, presentation layer, session layer, transport layer and network layer information. These bits are organized in logical order according to the requirements of the specific protocols and applications used. It is transmitted through media such as copper cable, optical fiber and air. Physical media will generate signals such as voltage, light, or radio waves, which are transmitted from one device to another. It is possible that the data streams of many protocols share this medium, and physical distortion may also occur. The important content of physical layer design is to reduce the influence of overload and interference.

In order for the data link layer frame to be transmitted through the physical medium, the physical layer encodes the logical frame in the form of data so that the device at the other end of the medium can recognize it. The device can be a forwarding frame router or other destination device.

Some of the following physical layer elements are required to transmit frames over the local media:

Physical media and associated connector

Represent bits on the media

Data coding and control information

Transmitter and receiver circuits on network equipment

After the signal is transmitted through the medium, it is decoded into a bit form that represents the original format of the data, and a complete frame is encapsulated and sent to the data link layer.

The main media and the types of signals they use:

Dielectric signal type copper cable electric pulse signal optical fiber optical pulse signal radio signal

Physical layer signals and coding:

In computer communication, the information transmitted is digital logic in binary form. One of the tasks of the physical layer is to convert logical information into the form of physical energy. When transmitted on a physical medium, the signal is used to represent the binary code.

There are many different ways to use signals to represent binary bits on a physical medium. Each method is converted into an energy pulse within a specific period of time called bit time. Bit time refers to the time in which the OSI layer 2 network card generates a bit of data and sends it to the media as a signal. For the receiver, the signal is uniformly present and displayed as a bit value during the bit time. The type of signal in bit time depends on the signal method used.

Three signal features can represent encoded bits:

Amplitude

Frequency

Phase

The amplitude is a characteristic variable of the signal period. The peak of the amplitude represents 1 and the trough of the amplitude indicates 0.

The two most commonly used signal methods are:

Do not return to zero (NRZ)

Manchester coding

NRZ uses a sampling voltage in a bit time to represent 1 or 0, a low voltage for 0, and a high voltage for 1, which varies from standard to standard.

NRZ, which does not have a constant zero voltage, sometimes requires an additional signal to synchronize with other devices. It is precisely these additional signals that limit the efficiency of NRZ, and are prone to distortion if there is electromagnetic interference. Inefficient NRZ can only be used on low-speed links.

Manchester coding is a method of looking for voltage change signals between each bit time. In the bit time, the voltage value from low to high hop represents 1, and from high to low hop represents 0. When there is a repeated bit value, that is, a continuous 0 or 1, the conversion will occur at the boundary of the bit time. Repeated rise or decrease will occur in the middle of the bit time.

Manchester coding is the signal standard (10Mbit/s) of 10BASE-T Ethernet. Higher-speed links require other standards.

Bit grouping

When many signals are transmitted along the medium, there is a need for a way to define physical layer signals so that the receiving device can confirm that those signals are important and require attention. Add signal modes to indicate that important signal transmission is the solution of the physical layer.

In high-speed networks, code grouping becomes more important because these networks are more error-prone. A code group is a predefined set of bits used to represent a larger set of data bits. Code groups are part of the physical layer coding.

Wireless Media:

Four common data communication standards can be applied to wireless media:

The standard IEEE 802.11, also commonly known as WiFi, is a wireless LAN (WLAN) technology that uses carrier sense multiple access / collision avoidance (CSMA/CA) media access processes using competitive or uncertain systems.

Standard IEEE 802.15 Wireless personal area Network Standard (WPAN), commonly referred to as "Bluetooth", uses a pairing process to communicate with an effective distance of 1-100m

Standard IEEE 802.16, commonly referred to as WiMAX (Microwave access Worldwide Interoperability), uses a point-to-point topology to provide wireless bandwidth access

The Global system for Mobile Communications (GSM) includes physical layer specifications that enable the layer 2 General packet Radio Technology (GPRS) protocol, providing access to

Wireless Network Transmission of Mobile phone

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