What is the PHY Layer?

The Physical Layer, PHY or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The Physical layer is the only layer in the OSI model that plays the role of interacting with actual hardware, transmission and signaling mechanisms. In terms of wireless systems, it is the layer that sends and receives RF signals.

The Physical layer interfaces with the 2^nd layer in the OSI model, the Data-link layer. The Data-link layer hands over data to be transmitted in the form of bits to the physical layer. The physical layer converts them to electrical pulses, which represent the binary data. The electric pulses are then converted to electromagnetic waves (usually radio waves) to be transmitted wirelessly. The physical also converts the electromagnetic waves received by an antenna to electrical pulses and sends them to the data link layer for further processing.

The physical layer consists of the electronic circuit transmission technologies of a network which is called a PHY chip. A PHY chip implements the physical layer functions of the wireless technology being used. It consists of the RF, mixed-signal and analog chips, which are often called transceivers, and the digital baseband chip that uses digital signal processor (DSP) and communication algorithm processing, including channel codes. These PHY chips are commonly integrated with the data-link layer in system-on-chip (SoC) implementations.

The physical layer defines the means of transmitting raw bits over a physical data link connecting network nodes i.e. hardware equipment, frequencies, pulses used to represent binary signals etc. It provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the line code to use, modulation techniques to use and similar low-level parameters, are all specified by the physical layer.

Physical Layers of some Wireless technologies:

Bluetooth Physical Layer

The physical layer of the Bluetooth stack is responsible for actually transmitting and receiving information over the air via radio waves. It transmits on the 2.4 GHz band. For BLE specifically, information is transmitted using a scheme called Gaussian Frequency-Shift Keying (GFSK) which is a refinement of Frequency Shift Keying (FSK), where information is coded by shifting the frequency up and down. The release of Bluetooth 5.0 specifies multiple Bluetooth PHY configurations that provide various tradeoffs in terms of range and throughput. 1 Megabit PHY, commonly referred to as 1M PHY, has been the default  Bluetooth PHY until now. The name refers to the bit rate that this PHY. Support for this PHY is mandatory to maintain backward compatibility with all non-5.0 devices. Coded PHY is one of the PHY configurations introduced in 5.0. The purpose of this configuration is to increase the maximum range without increasing transmit power. 2 Megabit PHY (aka 2M PHY) is also a new PHY configuration introduced in 5.0. The purpose of this configuration is an increased symbol rate at the PHY layer. Specifically, it achieves a symbol rate of 2 Mega symbols per second, where each symbol corresponds to a single bit. This allows double the number of bits to be sent over the air during a given period, or conversely reduce energy consumption for a given amount of data by halving the necessary transmit time.

Wi-Fi Physical Layer

Wi-Fi's MAC and physical layer (PHY) specifications are defined by the IEEE 802.11 standard for modulating and receiving one or more carrier waves to transmit the data in the infrared, and 2.4, 3.6, 5, or 6 GHz frequency bands. The standard and amendments provide the basis for wireless network products using the Wi-Fi band. As part of the 802.11 family, Wi-Fi consists of a series of half-duplex over-the-air modulation techniques that use the same basic protocol. The 802.11 protocol family employs carrier-sense multiple access with collision avoidance whereby equipment listens to a channel for other users (including non 802.11 users) before transmitting each packet. 802.11 controls the interference and susceptibility to interference by using direct-sequence spread spectrum (DSSS) and orthogonal frequency-division multiplexing (OFDM) signaling methods, respectively.

IEEE 802.15 Physical Layer

IEEE 802.15.4 is a technical standard that defines the physical layer of low-rate wireless personal area networks (LR-WPANs). It is the basis for the Zigbee, ISA100.11a, WirelessHART, MiWi, 6LoWPAN, Thread and SNAP specifications. The original 2003 version of the standard specifies two physical layers based on direct sequence spread spectrum (DSSS) techniques: one working in the 868/915 MHz bands with transfer rates of 20 and 40 kbit/s, and one in the 2450 MHz band with a rate of 250 kbit/s. The 2006 revised version improves the maximum data rates of the 868/915 MHz bands, bringing them up to support 100 and 250 kbit/s as well. Moreover, it goes on to define four physical layers depending on the modulation method used. Three of them preserve the DSSS approach using either binary or offset Quadrature Phase Shift Keying (QPSK) in the 868/915 MHz bands, and using QPSK in the 2450 MHz band.

LoRa Physical Layer

The LoRa physical layer uses a proprietary spread spectrum modulation that is similar to and a derivative of chirp spread spectrum (CSS) modulation. The rate at which the spread information is sent is referred to as the symbol rate, the ratio between the nominal symbol rate and chirp rate is the spreading factor (SF) and represents the number of symbols sent per bit of information. The LoRa physical layer performs the spread spectrum LoRa modulation by representing each bit of payload information by multiple “chirps” of information. It can trade off data rate for sensitivity with a fixed channel bandwidth by selecting the amount of spread used. Lower SF means more chirps are sent per second. Hence, more data can be encoded per second. Higher SF implies fewer chirps per second. Hence, there is fewer data to encode per second. Compared to lower SF, sending the same amount of data with higher SF needs more transmission time, known as airtime. More airtime means consumption of more energy. The benefit of high SF is that more extended airtime gives the receiver more opportunities to sample the signal power which results in better sensitivity. Also, LoRa physical layer uses forward error correction coding to improve resilience against interference. LoRa's high range is characterized by high wireless link budgets of around 155 dB to 170 dB.