The Physical Layer – within the OSI Reference Model

This post presents a discussion of the Physical Layer, within the OSI Reference Model.


What is the Physical Layer – within the OSI Reference Model?

The Physical Layer is the lowest-level layer within the OSI Reference Reference Model.

Figure 1 presents an illustration of the OSI Reference Model, with the Physical Layer circled.

osi_reference_model-physical-layer

Figure 1, Illustration of the OSI Reference Model – with the Physical Layer circled

In short, the focus of a Physical Layer design is to transmit a continuous stream of data from one terminal, to an adjacent terminal.  As far as the Physical Layer is concerned, this data will be in the form of electrical signal pulses, optical symbols or RF symbols – depending upon whether the communication media is copper, optical fiber or wireless/RF.  Additionally, Physical Layer designs/processes do not pay attention to framing or any sort of packet delineation.  The Higher-Layer processes will handle framing and packets.  The Physical Layer processes will consider this stream of pulses to be just an unframed raw stream of bits.

Some Terminology

We will refer to the entity (e.g., the Transmitter and Receiver) that handles the Physical Layer functions (or processes), throughout this blog as the Physical Layer Controller.  A transceiver is a typical example of a Physical Layer Controller.

Purpose

The purpose of the Physical Layer Controller is to provide a communications service for the local Data Link Layer Controller.  A Physical Layer controller (at a transmitting terminal) will accept data from the local Data Link Layer Controller.  The Physical Layer controller will then transmit this data over some medium (e.g., copper, optical fiber or wireless communication) to a similar Physical Layer Controller at the adjacent receiving terminal.  The Physical Layer Controller (at the receiving terminal) will then provide this received/recovered data to its local Data Link Controller, for further processing.

We often refer to this communication between the two Physical Layer controllers as “peer-to-peer communication” between the two Physical Layer controllers.

Figure 2 presents a closer look at the role that the Physical Layer controllers play in the transport of data.

physical_layer_processes

Figure 2, A Simple Illustration of the role that the Physical Layer controller plays in the transport of data across a media

Whenever a Physical Layer Controller (at a transmitting terminal) accepts data from the local Data Link Layer controller, it will encode this data into some line code or modulation format that is suitable for the communication media.  Afterward, the Physical Layer controller will transmit this data over the communication media.  The Physical Layer controller (at the receiving terminal) will receive and recover this data from the media.  Additionally, the Physical Layer controller will then decode this data (from the line-code or modulation format) back into its original data stream.  The Physical Layer controller will then pass this data along to the Data Link Layer controller for further processing.

NOTES:

  1. Figure 2 is a simple illustration and does not include all possible circuitry within a Physical Layer controller (or Transceiver).
  2. Please see the post on the Data Link Layer for further insight into how the Data Link Layer handles this data.

Physical Layer Types in various types of Communication Media

The Physical Layer is designed to transport data from a transmitting terminal to a receiving terminal.  The Physical Layer can be designed to transport data over any of the following types of media.

  • Copper Medium
    • Twisted-Pair
    • Coaxial Cable
    • Microstrips or Striplines on a High-Speed Backplane
  • Optical Fiber
    • Multi-Mode Fiber
    • Single-Mode Fiber
  • Wireless/RF
    • Microwave
    • Cellular
    • Satellite Communication

Physical Layer Design Considerations for Copper Media

For copper media, the Physical Layer will be concerned with the following design parameters

  • Line-Code (e.g., Manchester, B3ZS, 64B/66B coding, various forms of scrambling, etc.).
  • Voltage Levels of the signal (being transmitted)
    • What kind of signal/pulse should a Physical Layer controller generate and transmit to send a bit/symbol with the value of “0”.
    • The kind of signal/pulse should a Physical Layer controller generate and transmit to send a bit/symbol with the value of “1”.
    • The minimum voltage level that the Receiving Physical Layer controller will correctly interpret a given bit (or symbol) as being a “1”?
    • What is the maximum voltage level that the Receiving Physical Layer controller will correctly interpret a given bit (or symbol) as being a “0”?
  • Impairments in copper media
    • Frequency-dependent loss and phase distortion of symbols.
    • Reflections
    • Crosstalk Noise
    • EMI (Electromagnetic Interference).
  • What is the maximum length of copper media, that we can support?

Physical Layer Design Considerations for Optical Fiber

For an optical fiber, the Physical Layer will be concerned with the following design parameters

  • What kind of symbol are we using to transmit a bit with the value of “0”?
  • What kind of symbol are we using to transmit a bit with the value of “1”?
  • Modulation scheme (e.g., QPSK, 16QAM, PAM4, etc.)
  • What wavelength (or set of wavelengths will we use for communication).
  • Will we be transporting data over a single wavelength or multiple wavelengths (e.g., Wave-Division Multiplexing)?
  • Impairments in Optical Fiber
    • Chromatic Dispersion
    • Modal Dispersion (for Multi-Mode Fiber only)
    • Polarization Mode Dispersion
  • What is the maximum length of optical fiber, that we can support?

Physical Layer Design Considerations for all Media

The Physical Layer will be concerned with the following design parameters, regardless of the communication media.

  • Are we transporting multiple bits via each symbol (e.g., for PAM4, 16QAM, QPSK, etc.)?
  • Bit-Timing (Bit Width) or Symbol-Timing (Symbol Width)
  • Jitter/Wander Requirements
    • Maximum allowable jitter within a transmitted signal
    • Maximum jitter tolerance capability of a receiving terminal
  • Minimum permissible SNR (Signal-to-Noise Ratio)
  • The maximum permitted BER (Bit-Error Rate)
  • Error Detection or Error Detection and Correction
    • Are we using Forward Error Correction (FEC)?
  • Ensuring a Sufficient number of transitions (in the signal waveform) to permit a CDR (Clock and Data Recovery) PLL (at the receiving terminal) to acquire and maintain lock with the incoming signal.
  • Mechanical Issues (such as connector types)
  • Whether the communication is Simplex, Half-Duplex or Full-Duplex Mode

The Physical Layer in other Standards

Many of the other Reference Models (e.g., OTN, SONET, SDH, and PCIe) also have a Physical Layer as well.  Other postings will discuss the Physical Layer for each of these Reference Models.

You can access the posting for the Physical Layers of each of these Reference Models, by clicking on the links below:

  • OTN
  • SONET
  • SDH
  • PCIe

The OSI Reference Model

This post presents a brief definition of the OSI Reference Model.


What is the OSI Reference Model?

The communications/networking industry has defined many standards using something called a Reference Model.

A Reference Model is an abstract way of looking at the problems of defining and designing a Communications Network.

A Reference Model will divide the design of a Communications Network into many Layers.  For example, many people will use the OSI Reference Model to divide the design of a Communications Network into seven (7) layers.  The OSI (Open Systems Interconnection) Reference Model is defined by the ISO (International Standards Organization).

Figure 1 presents an illustration of the OSI Reference Model

OSI Reference Model

 

Figure 1, Illustration of the OSI Reference Model

Figure 1 shows that the OSI Reference Model consists of the following layers (when starting from the lowest layer and going to the highest layer):

  • The Physical Layer
  • The Data Link Layer
  • The Network Layer
  • The Transport Layer
  • The Session Layer
  • The Presentation Layer and
  • The Application Layer

You can click on each of the layer names, to learn more details about each layer.

Figure 1 shows that the Physical Layer is the lowest-level Layer in the OSI Reference Model and that the Data Link Layer is the next layer (up) from the Physical Layer, and so on.  The Application Layer is the highest layer in the OSI Reference Model.

Figure 1 also shows two data communication terminals, that are communicating with each other.  This figure illustrates seven (7) blocks or processes, within each terminal, that supports the role for each of these layers.

There are three basic ideas behind the design and implementation of these layers.

  1. Independence – The design of one layer should be independent of the design of each of the remaining layers.
  2. Service – A given layer’s responsibility is to service the next layer above it.
  3. Supports peer-to-peer Communication – A layer must support peer-to-peer communication between the Layer Controller (or Process) at the Source Terminal and that at the Destination Terminal.

We will discuss each of these concepts, below:

Independence

Independence means that the system designer should design a single layer such that it is independent of the designs for the other layers.

For example, the System Designer can choose to use a Copper Media (e.g., Coaxial Cable, Twisted-Pair), Optical Fiber (e.g., Multi-Mode Fiber or Single-Mode Fiber) or go wireless.  The designer’s choice for the physical media should not effect on the design of the Data Link Layer layer or on any of the remaining five (5) higher layers.

The user should be able to choose one communication media or another, and the design of the Data Link Layer (along with the remaining layers above the Data Link Layer) is not affected by this selection.

Thus, if the user, later on, decides to replace the copper media (within the physical layer) with optical fiber, this engineering change will certain affect the design of the Physical Layer.  However, this design change should not affect the design of the Data Link Layer (nor the 5 higher layers).  All these layers should still function in the same manner whether one is using a copper, optical fiber or wireless media.

NOTE:  The various reference model standards will each specify a standard interface between each of these layers.  This standard interface permits a given layer to communicate with the next layer up or below.   For example, the Network and Transport layers each have their standard interfaces, that allows data/information to flow between these two Layers.  Yet reference model standards do not specify how the user is to realize the design of a given layer.

Service

The purpose of the Physical Layer is to provide a communications service for the Data Link Layer (the next higher layer).  The Data Link Layer (at the source terminal) provides its data to the Physical Layer.  The Physical Layer then takes this data and transports it, over the chosen media, to the destination terminal.  The Physical Layer (at the destination terminal) then receives this data and presents it to the Data Link Layer (also at the destination terminal).

Likewise, the purpose of the Data Link Layer is to provide a communication/error-detection service for the Networking Layer (e.g., the next higher layer), and so on, all the way up to the Application Layer.

We discuss the concept of service, in greater detail in “The OSI Reference Model at Work – A Macro View”.

Supports peer-to-peer Communication

Figure 1 presents an illustration of a simple Communications Network, consisting of two terminals.

We’ve labeled one of these terminals as the “West Terminal” and we’ve labeled the other terminal as the “East Terminal”.  This figure also shows that each terminal contains processes that support the controller function for each of these seven (7) layers.  Some of these processes are implemented via hardware design and some of these other processes are implemented via software code.

For example, the Physical Layer (at each of the terminals) will contain transmitting circuitry (e.g., some circuitry that transmits data from the “Source Terminal” to the “Destination Terminal” over the selected medium).

Likewise, the Physical Layer (at each of the terminals) will contain receiving circuitry (e.g., some circuitry that receives data from the remote/Source terminal).  This circuitry has been designed to be able to receive data that has traveled over some distance (over the medium of choice) and compensate for any distortion/impairments that the data signal will experience within the media.

We can refer to this particular data transmission as “peer-to-peer” communication between the Physical Layer processes in each terminal.  There is also a similar peer-to-peer communication link between the Data Link Layer processes at each terminal, and the at the Network Layer and so on (as indicated by the dashed lines between each of the layer processes in both terminals).

We discuss the concept of “peer-to-peer communication” in greater detail, in “The OSI Reference Model at Work – A Macro View”.

Other Communication Standards have their own Reference Models as well.

  • TCP/IP – Transport Control Protocol/Internet Protocol
  • SONET
  • SDH
  • OTN
  • PCI Express

You can click on each of these links to learn more about these Reference Models.