Physical layer

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In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to transmit on, the line code to use and similar low-level parameters, are specified by the physical layer.

Contents

At the electrical layer, the physical layer is commonly implemented by dedicated PHY chip or, in electronic design automation (EDA), by a design block. In mobile computing, the MIPI Alliance *-PHY family of interconnect protocols are widely used.

Historically, the OSI model is closely associated with internetworking, such as the Internet protocol suite and Ethernet, which were developed in the same era, along similar lines, though with somewhat different abstractions. Beyond internetworking, the OSI abstraction can be brought to bear on all forms of device interconnection in data communications and computational electronics.

Role

The physical layer defines the means of transmitting a stream of raw bits [2] over a physical data link connecting network nodes. The bitstream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a transmission medium.

The physical layer consists of the electronic circuit transmission technologies of a network. [3] It is a fundamental layer underlying the higher level functions in a network, and can be implemented through a great number of different hardware technologies with widely varying characteristics. [4]

Within the semantics of the OSI model, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to cause transmission or reception of electronic (or other) signals. [5] [6] The physical layer supports higher layers responsible for generation of logical data packets.

Physical signaling sublayer

In a network using Open Systems Interconnection (OSI) architecture, the physical signaling sublayer is the portion of the physical layer that [7] [8]

Relation to the Internet protocol suite

The Internet protocol suite, as defined in RFC 1122 and RFC 1123, is a high-level networking description used for the Internet and similar networks. It does not define a layer that deals exclusively with hardware-level specifications and interfaces, as this model does not concern itself directly with physical interfaces. [9] [10]

Services

The major functions and services performed by the physical layer are: The physical layer performs bit-by-bit or symbol-by-symbol data delivery over a physical transmission medium. [11] It provides a standardized interface to the transmission medium, including [12] [13] a mechanical specification of electrical connectors and cables, for example maximum cable length, an electrical specification of transmission line signal level and impedance. The physical layer is responsible for electromagnetic compatibility including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc. The transmission medium may be electrical or optical over optical fiber or a wireless communication link such as free-space optical communication or radio.

Line coding is used to convert data into a pattern of electrical fluctuations which may be modulated onto a carrier wave or infrared light. The flow of data is managed with bit synchronization in synchronous serial communication or start-stop signalling and flow control in asynchronous serial communication. Sharing of the transmission medium among multiple network participants can be handled by simple circuit switching or multiplexing. More complex medium access control protocols for sharing the transmission medium may use carrier sense and collision detection such as in Ethernet's Carrier-sense multiple access with collision detection (CSMA/CD).

To optimize reliability and efficiency, signal processing techniques such as equalization, training sequences and pulse shaping may be used. Error correction codes and techniques including forward error correction [14] may be applied to further improve reliability.

Other topics associated with the physical layer include: bit rate; point-to-point, multipoint or point-to-multipoint line configuration; physical network topology, for example bus, ring, mesh or star network; serial or parallel communication; simplex, half duplex or full duplex transmission mode; and autonegotiation [15]

PHY

RTL8201 Ethernet PHY chip Elitegroup 761GX-M754 - Realtek RTL8201CL-5493.jpg
RTL8201 Ethernet PHY chip
Texas Instruments DP83825 - 3 x 3 mm 3.3 V PHY chip DP83825I smaller.png
Texas Instruments DP83825 – 3 × 3 mm 3.3 V PHY chip

A PHY, an abbreviation for physical layer, is an electronic circuit, usually implemented as an integrated circuit, required to implement physical layer functions of the OSI model in a network interface controller.

A PHY connects a link layer device (often called MAC as an acronym for medium access control) to a physical medium such as an optical fiber or copper cable. A PHY device typically includes both physical coding sublayer (PCS) and physical medium dependent (PMD) layer functionality. [16]

-PHY may also be used as a suffix to form a short name referencing a specific physical layer protocol, for example M-PHY.

Modular transceivers for fiber-optic communication (like the SFP family) complement a PHY chip and form the PMD sublayer.

Ethernet physical transceiver

Micrel KS8721CL - 3.3 V single power supply 10/100BASE-TX/FX MII physical layer transceiver Micrel KS8721CL on mainboard of Surf@home II-7778.jpg
Micrel KS8721CL – 3.3 V single power supply 10/100BASE-TX/FX MII physical layer transceiver

The Ethernet PHY is a component that operates at the physical layer of the OSI network model. It implements the physical layer portion of the Ethernet. Its purpose is to provide analog signal physical access to the link. It is usually interfaced with a media-independent interface (MII) to a MAC chip in a microcontroller or another system that takes care of the higher layer functions.

More specifically, the Ethernet PHY is a chip that implements the hardware send and receive function of Ethernet frames; it interfaces between the analog domain of Ethernet's line modulation and the digital domain of link-layer packet signaling. [17] The PHY usually does not handle MAC addressing, as that is the link layer's job. Similarly, Wake-on-LAN and Boot ROM functionality is implemented in the network interface card (NIC), which may have PHY, MAC, and other functionality integrated into one chip or as separate chips.

Common Ethernet interfaces include fiber or two to four copper pairs for data communication. However, there now exists a new interface, called Single Pair Ethernet (SPE), which is able to utilize a single pair of copper wires while still communicating at the intended speeds. Texas Instruments DP83TD510E [18] is an example of a PHY which uses SPE.

Examples include the Microsemi SimpliPHY and SynchroPHY VSC82xx/84xx/85xx/86xx family, Marvell Alaska 88E1310/88E1310S/88E1318/88E1318S Gigabit Ethernet transceivers, Texas Instruments DP838xx family [19] and offerings from Intel [20] and ICS. [21]

Other applications

Technologies

The following technologies provide physical layer services: [22]

See also

Related Research Articles

<span class="mw-page-title-main">Ethernet</span> Computer networking technology

Ethernet is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1983 as IEEE 802.3. Ethernet has since been refined to support higher bit rates, a greater number of nodes, and longer link distances, but retains much backward compatibility. Over time, Ethernet has largely replaced competing wired LAN technologies such as Token Ring, FDDI and ARCNET.

<span class="mw-page-title-main">OSI model</span> Model of communication of seven abstraction layers

The Open Systems Interconnection model is a reference model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

<span class="mw-page-title-main">Ethernet over twisted pair</span> Ethernet physical layers using twisted-pair cables

Ethernet over twisted-pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. They are a subset of all Ethernet physical layers.

<span class="mw-page-title-main">Network topology</span> Arrangement of the elements of a communication network

Network topology is the arrangement of the elements of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, including command and control radio networks, industrial fieldbusses and computer networks.

<span class="mw-page-title-main">Fast Ethernet</span> Ethernet standards that carry data at the nominal rate of 100 Mbit/s

In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of 100 Mbit/s. The prior Ethernet speed was 10 Mbit/s. Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common.

<span class="mw-page-title-main">Gigabit Ethernet</span> Standard for Ethernet networking at a data rate of 1 gigabit per second

In computer networking, Gigabit Ethernet is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use in 1999, and has replaced Fast Ethernet in wired local networks due to its considerable speed improvement over Fast Ethernet, as well as its use of cables and equipment that are widely available, economical, and similar to previous standards. The first standard for faster 10 Gigabit Ethernet was approved in 2002.

In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer of the seven-layer OSI model. The LLC sublayer acts as an interface between the medium access control (MAC) sublayer and the network layer.

The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer.

<span class="mw-page-title-main">Medium access control</span> Service layer in IEEE 802 network standards

In IEEE 802 LAN/MAN standards, the medium access control (MAC), also called media access control, is the layer that controls the hardware responsible for interaction with the wired or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. The LLC provides flow control and multiplexing for the logical link, while the MAC provides flow control and multiplexing for the transmission medium.

10 Gigabit Attachment Unit Interface is a standard for extending the XGMII between the MAC and PHY layer of 10 Gigabit Ethernet (10GbE) defined in Clause 47 of the IEEE 802.3 standard. The name is a concatenation of the Roman numeral X, meaning ten, and the initials of "Attachment Unit Interface".

The media-independent interface (MII) was originally defined as a standard interface to connect a Fast Ethernet medium access control (MAC) block to a PHY chip. The MII is standardized by IEEE 802.3u and connects different types of PHYs to MACs. Being media independent means that different types of PHY devices for connecting to different media can be used without redesigning or replacing the MAC hardware. Thus any MAC may be used with any PHY, independent of the network signal transmission medium.

<span class="mw-page-title-main">Medium Attachment Unit</span>

A Medium Attachment Unit (MAU) is a transceiver which converts signals on an Ethernet cable to and from Attachment Unit Interface (AUI) signals.

Physical medium dependent sublayers or PMDs further help to define the physical layer of computer network protocols. They define the details of transmission and reception of individual bits on a physical medium. These responsibilities encompass bit timing, signal encoding, interacting with the physical medium, and the properties of the cable, optical fiber, or wire itself. Common examples are specifications for Fast Ethernet, Gigabit Ethernet and 10 Gigabit Ethernet defined by the Institute of Electrical and Electronics Engineers (IEEE).

The physical coding sublayer (PCS) is a networking protocol sublayer in the Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet standards. It resides at the top of the physical layer (PHY), and provides an interface between the physical medium attachment (PMA) sublayer and the media-independent interface (MII). It is responsible for data encoding and decoding, scrambling and descrambling, alignment marker insertion and removal, block and symbol redistribution, and lane block synchronization and deskew.

<span class="mw-page-title-main">Ethernet physical layer</span> Electrical or optical properties between network devices

The physical-layer specifications of the Ethernet family of computer network standards are published by the Institute of Electrical and Electronics Engineers (IEEE), which defines the electrical or optical properties and the transfer speed of the physical connection between a device and the network or between network devices. It is complemented by the MAC layer and the logical link layer. An implementation of a specific physical layer is commonly referred to as PHY.

In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload.

MOST is a high-speed multimedia network technology for the automotive industry. It can be used for applications inside or outside the car. The serial MOST bus uses a daisy-chain topology or ring topology and synchronous serial communication to transport audio, video, voice and data signals via plastic optical fiber (POF) or electrical conductor physical layers.

<span class="mw-page-title-main">OPEN Alliance SIG</span>

The OPEN Alliance is a non-profit, special interest group (SIG) of mainly automotive industry and technology providers collaborating to encourage wide scale adoption of Ethernet-based communication as the standard in automotive networking applications.

Classic Ethernet is a family of 10 Mbit/s Ethernet standards, which is the first generation of Ethernet standards. In 10BASE-X, the 10 represents its maximum throughput of 10 Mbit/s, BASE indicates its use of baseband transmission, and X indicates the type of medium used. Classic Ethernet includes coax, twisted pair and optical variants. The first Ethernet standard was published in 1983 and classic Ethernet operating at 10 Mbit/s was the dominant form of Ethernet until the first standard for Fast Ethernet was approved in 1995.

References

  1. ↑ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
  2. ↑ Gorry Fairhurst (2001-01-01). "Physical Layer". Archived from the original on 2009-06-18.
  3. ↑ Iyengar, Shisharama (2010). Fundamentals of Sensor Network Programming. Wiley. p. 136. ISBN   978-1423902454.
  4. ↑ "The Physical Layer | InterWorks". InterWorks. 2011-07-30. Retrieved 2018-08-14.
  5. ↑ Shaw, Keith (2018-10-22). "The OSI model explained: How to understand (and remember) the 7 layer network model". Network World. Retrieved 2019-02-15.
  6. ↑ "DATA COMMUNICATION & NETWORKING". ResearchGate. Retrieved 2019-02-15.
  7. ↑ PD-icon.svg This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.
  8. ↑ "physical signaling sublayer (PLS)". Archived from the original on 2010-12-27. Retrieved 2011-07-29.
  9. ↑ "rfc1122". datatracker.ietf.org. Retrieved 2021-07-28.
  10. ↑ "rfc1123". datatracker.ietf.org. Retrieved 2021-07-28.
  11. ↑ Shekhar, Amar (2016-04-07). "Physical Layer Of OSI Model: Working Functionalities and Protocols". Fossbytes. Retrieved 2019-02-15.
  12. ↑ Bayliss, Colin R.; Bayliss, Colin; Hardy, Brian (2012-02-14). Transmission and Distribution Electrical Engineering. Elsevier. ISBN   9780080969121.
  13. ↑ "CCNA Certification/Physical Layer - Wikibooks, open books for an open world". en.wikibooks.org. Retrieved 2019-02-15.
  14. ↑ Bertsekas, Dimitri; Gallager, Robert (1992). Data Networks . Prentice Hall. p.  61. ISBN   0-13-200916-1.
  15. ↑ Forouzan, Behrouz A.; Fegan, Sophia Chung (2007). Data Communications and Networking. Huga Media. ISBN   9780072967753.
  16. ↑ Mauricio Arregoces; Maurizio Portolani (2003). Data Center Fundamentals. ISBN   9781587050237 . Retrieved 2015-11-18.
  17. ↑ "microcontroller - what is the difference between PHY and MAC chip - Electrical Engineering Stack Exchange". Electronics.stackexchange.com. 2013-07-11. Retrieved 2015-11-18.
  18. ↑ "DP83TD510E Ultra Low Power 802.3cg 10Base-T1L 10M Single Pair Ethernet PHY" (PDF). Texas Instruments. Retrieved 12 October 2020.
  19. ↑ "Ethernet PHYs". Texas Instruments. Retrieved 12 October 2020.
  20. ↑ Intel PHY controllers brochure
  21. ↑ osuosl.org - ICS1890 10Base-T/100Base-TX Integrated PHYceiver datasheet
  22. ↑ "Physical Layer | Layer 1". The OSI-Model. Retrieved 2021-07-28.
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