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Physical layer

Physical layer is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Physical layer rather than just read about it. In short: 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.

Physical layer — main illustration
Physical layer — illustration

Key takeaways

  • Physical layer belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Physical layer to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Physical layer from memory before moving on to harder problems.

Reference excerpt

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. At the electrical layer, the physical layer is commonly implemented in a 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.

Role The physical layer defines the means of transmitting a stream of raw bits 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. 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. 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. 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

interfaces with the data link layer's medium access control (MAC) sublayer, performs symbol encoding, transmission, reception and decoding and, performs galvanic isolation.

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.

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. It provides a standardized interface to the transmission medium, including 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 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

PHY

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. -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

… excerpt ends here. Continue reading the full article.

Illustrations

Physical layer: Texas Instruments DP83825 – 3 × 3 mm 3.3 V PHY chip
Texas Instruments DP83825 – 3 × 3 mm 3.3 V PHY chip
Physical layer: Micrel KS8721CL – 3.3 V single power supply 10/100BASE-TX/FX MII physical layer transceiver
Micrel KS8721CL – 3.3 V single power supply 10/100BASE-TX/FX MII physical layer transceiver

Worked examples

Example 1 — a first encounter with Physical layer

Start with the simplest possible case. Write down what Physical layer claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Physical layer before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Physical layer ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Physical layer

In research
Physical layer appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Physical layer in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Physical layer is common in secondary-school and first-year university syllabi. It links to neighbouring topics OSI model, so understanding it makes those chapters shorter.
In everyday life
Look for Physical layer outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Physical layer in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Physical layer means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Physical layer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Physical layer in simple terms?

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 mediu…

Why does Physical layer matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Physical layer?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Physical layer.

Tags

  • OSI model

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