Tag: Physical Layer

A set of posts that pertain to a Physical Layer within any one of several possible Reference Models.

What does the expression 100GBASE-R Mean?

This post defines and describes the expression 100GBASE-R for 100Gbps Ethernet Applications.


What does the expression 100GBASE-R Mean?

For Ethernet applications, the IEEE 802.3 standard states that the term 100GBASE-R represents a group (or family) of Physical Layer (e.g., 100Gbps Transceiver) devices that do the following:

  • When it transmits its data towards the PMD (Physical Medium Dependent) device, it will:
    • Encode the CGMII data into the 64B/66B PCS (Physical Coding Sublayer) code.
      • This is what the -R suffix means, by the way.  
    • Divide (or de-multiplex) its outbound traffic into 20 PCS Lanes by routing each 66b (66 bit) block to a different PCS lane (in a round-robin manner).
    • It will often multiplex these 20 PCS Lanes into 4 or 10 physical (or electrical) lanes for CAUI-4 and CAUI-10 applications, as it transports this data to an Optical Transceiver (or PMD), respectively.
  • When it receives data (from the PMD), it will:
    • De-multiplexes these CAUI-4/CAUI-10 physical (or electrical) lanes of traffic that it receives from the Optical Transceiver into 20 PCS Lanes.
    • Combines (or multiplexes) these 20 PCS Lanes into a single stream of traffic as it routes this data towards the MAC (Media Access Control) device.
    • Decodes this data from the 64B/66B PCS code back into the CGMII (100Gbps Media Independent Interface) format.  

NOTE:  I discuss some of this processing (with 100GBASE-R data) for ITU-T G.709, Annex E mandated handling (before GMP Mapping this data into the OPU4 Payload) in Lesson 10; within THE BEST DARN OTN TRAINING PRESENTATION…PERIOD!!!

In Summary

In summary, these 100Gbps Ethernet devices will encode their “outgoing” data into the 64B/66B PCS code before transmitting it over some media.  

These 100Gbps Ethernet devices will also decode their “incoming” data from the 64B/66B PCS code (to restore the data to its original CGMII content) as it receives this data.

In other words, the 100Gbps Ethernet system will encode this data into the 64B/66B PCS format solely to transport this data across the communication media.  

Once this Ethernet data has arrived (at the other end of the media), the Ethernet system will decode this data (from the 64B/66B PCS format) to restore it to its original content.

The bit rate of this 64B/66B PCS encoded 100Gbps Ethernet data stream is 103.125Gbps ‡ 100pm.

Why Do We Encode our 100Gbps Ethernet Data into this 64B/66B PCS Code before we transmit this data over Optical Fiber?

We encode this 100Gbps Ethernet (CGMII) data into this 64B/66B PCS Code before we:

  • Transmit this data over Optical Fiber, or
  • Map this data into an OPU4 Frame (again for transmission over Optical Fiber).

There are several reasons we encode this data (before transmission over Optical fiber).  But the main goals are to convert this data into a more conducive format for transport over Optical Fiber.  More specifically, by converting this data into the 64B/66B code, we:

  • Minimize Running Disparity (e.g., maintain DC balance) with our data, and 
  • Ensure that we have no long strings of consecutive “1s” or consecutive “0s” within the data, we transport over Optical Fiber.  
  • Offer greater management capability (within our 100Gbps signal) by including Sync Bits within our 66-bit blocks.   These Sync bits permit us to designate certain 66-bit blocks as data blocks and other 66-bit blocks as control blocks.  I discuss some of this in detail in Lesson 10 within THE BEST DARN OTN TRAINING PRESENTATION…PERIOD!!!

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The Various 100GBASE-R Transceiver Devices

IEEE 802.3 also states that the Physical Layer (Transceiver devices) supporting the following standards must support the 100GBASE-R PCS encoding/decoding scheme.

  • 100GBASE-CR4
  • 100GBASE-CR10
  • 100GBASE-SR4
  • 100GBASE-SR10
  • 100GBASE-KP4
  • 100GBASE-KR4
  • 100GBASE-LR4
  • 100GBASE-ER4

Please check out the post on 64B/66B encoding to learn more about that encoding scheme.

Where is this PCS Encoder/Decoder Located?

IEEE 802.3 states that the 100GBASE-R PCS block (e.g., the entity that performs the PCS Encoding/Decoding) resides between the Reconciliation Layer and the PMA (Physical Medium Attachment), as shown in Figure 1 below.

IEEE 802.3 100Gbps Ethernet Architectural Diagram

Figure 1, Architectural Positioning of 100Gigabit Ethernet (from the IEEE 802.3 Standard)

But, in a real system, this PCS Encoder/Decoder often resides in the same IC containing the MAC (Media Access Controller).  

Figure 2 illustrates a MAC that includes the PCS Encoder and Decoder functions.

100GBASE-R Encoder and Decoder in MAC IC

Figure 2, Illustration of a Connection between the MAC and a 100Gbps Transceiver IC

This figure also shows the MAC or Switch IC exchanging data with the 100Gbs Ethernet Transceiver IC over a CAUI-4 or CAUI-10 Interface.

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Manchester Encoding

This post presents a description of the Manchester Line Code. It also describes the Manchester Line Code’s strength, weaknesses and where it is deployed.

What is the Manchester Line Code

The Manchester Line Code is a line code that transports both data and timing information on a single serial binary data stream.

We use the Manchester Line Code in the Physical Layer.

A Manchester Line Encoder works by encoding each data bit to be either low-then-high or high-then-low – for equal amounts of time.  

Since this encoded data is “high” and “low” for equal times, there is no DC bias within this signal.

More specifically, IEEE 802.3 specifies that a Manchester encoder should encode a “1” bit by setting the output to a logic “low” for the first half of a bit period and then by setting the output to a logic “high” for the second half of this bit period.  

This encoding scheme requires a rising clock edge at the middle of this bit period.  

Conversely, IEEE 802.3 also specifies that a Manchester encoder should encode a “0” bit by setting the output to a logic “high” during the first half of a bit period and then set the output to a logic “low” for the second half of the bit period.  

This situation requires a falling clock edge to occur in the middle of this bit period.

Figure 1 shows a drawing of a data stream that we have encoded into the Manchester format.  

The very top trace is the clock signal.  The middle signal trace contains the data (that we wish to encode).  Finally, the bottom signal trace presents the resulting encoded data.

Manchester Line Code

Figure 1, An Drawing of a Data Stream that we have encoded into the Manchester format

Manchester coding works by exclusive-ORing (XOR) the Original Data and the Clock signal, as Table 1 presents below.

Table 1, Encoding Data into the Manchester Line Code

Manchester Encoding Algorithm

Where did the Manchester Line Code come from?  

The University of Manchester (in the United Kingdom) developed the Manchester Line Code.

What are its strengths?  

The Manchester Line Code has two primary strengths:

  1. It is an electrically balanced line code and has no DC bias.
  2. It provides many clock edges for “clock and data recovery” at the remote receiving terminal.  The Manchester Line Code does not need to use any “Zero-Suppression” scheme.

What are its weaknesses?  

Since Manchester encoding requires a clock edge for each bit of data, the frequency content of a Manchester-encoded signal is relatively high.  

This fact limits the data rates at which the user can transmit a Manchester encoded data stream over a band-limited channel.  

In other words, the Manchester line code is not suitable for transmitting high-speed signals over band-limited channels.

Where is it used?  

10BASE-T (10Mbps Ethernet over Twisted Pair) applications use the Manchester Line Code.

Other Line Codes

  • RZ (Return to Zero)
  • NRZ (Non-Return to Zero)
  • NRZI (Non-Return to Zero-Inverted)

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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 illustrates 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, a Physical Layer design focuses on transmitting a continuous data stream 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 and optical 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 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

Throughout this blog, we will refer to the entity (e.g., the Transmitter and Receiver) that handles the Physical Layer functions (or processes) 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 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 two Physical Layer controllers.

Figure 2 presents a closer look at the physical layer controllers’ role 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 it into some line code or modulation format 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 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 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 the Receiving Physical Layer controller will correctly interpret a given bit (or symbol) as being a “1”?
    • What is the maximum voltage level for the Receiving Physical Layer controller to 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 transport data over single 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
  • We are 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.  

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

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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 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 separate a Communications Network’s design 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

This figure 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 layer name to learn more details about each layer.

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

The Application Layer is the highest in the OSI Reference Model.

Figure 1 also shows two data communication terminals communicating with each other.  

This figure illustrates seven (7) blocks or processes within each terminal that support each of these layers’ roles.

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

  1. Independence – The design of one layer should be independent of each of the remaining layers’ design.
  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 each layer to be independent of the other layers’ designs.

For example, the System Designer can choose to use 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 affect the Data Link Layer layer’s design or any of the remaining five (5) higher layers.

In other words, the user should choose one physical 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, later on, if the user decides to replace the copper media with optical fiber, this engineering change will undoubtedly affect the design of the Physical Layer.  

However, this change should not affect the Data Link Layer (or the five higher layers).  All these layers should still function similarly, whether using copper, optical fiber, or wireless media.

NOTE:  The various reference model standards will each specify a standard interface between 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 have standard interfaces that allow data/information to flow between these two Layers.  

Yet reference model standards do not specify how the user realizes the design of a given layer.

Service

The Physical Layer aims 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 to the destination terminal over the chosen media.  

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 illustrates a simple Communications Network consisting of two terminals.

We’ve labeled one of these terminals as the “West Terminal” and the other as the “East Terminal.”  

This figure also shows that each terminal contains processes supporting the controller function for 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 terminal) will contain receiving circuitry (e.g., some circuitry that receives data from the remote/Source terminal).  

This circuitry has been designed 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, the Network Layer, and so on (as indicated by the dashed lines between each layer process 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 Reference Models as well.

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

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

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Glossary Definition: Transceiver

This post provides the reader with a detailed definition and description of a Transceiver.


Glossary Word:  Transceiver

The word “transceiver” is a shortened expression for the phrase “transmitter-receiver.”

A transceiver is a Physical-Layer device (or circuit) that has two responsibilities:

  1. To transmit data over some communication media (be it copper, optical fiber, or in the air for RF applications).
  2. To receive data from some communication media (again, this can be copper, optical fiber, or air).

In other words, a transceiver supports both a “transmitter” and a “receiver” function.

Transceivers are designed in either the form of an IC (integrated circuit) or via discrete circuitry on a board.

Modern-day transceivers also include circuitry that performs the following functions:

  • CDR – Clock and Data Recovery
  • Signal Conditioning/Modulator/Demodulator (e.g., conditions the signal for transmission over the communication media).  Examples would be E/O (Electrical to Optical) Conversion and O/E.
  • Pre-Emphasis (e.g., compensating for impairments and the limited bandwidth of the communication media – before transmitting data/symbols onto the line).
  • Equalization (e.g., compensating for impairments and the limited bandwidth of the communication media – after receiving data/symbols from the line).

Transceivers may also include circuitry that supports “zero-suppression” techniques, depending on the protocol it supports.  In this case, the transceiver would consist of circuitry that ensures that it will never transmit a long string of consecutive “0s” out onto the line.  The purpose of “zero-suppression” is to ensure that the Clock and Data Recovery PLL (Phase Locked Loop) at the Receiving Terminal will have enough transitions in the line signal (that it receives) to maintain “phase/frequency lock” with the incoming signal.

Examples of Zero-Suppression techniques would be:

  • Encoding/decoding STS-1 signals into/from the B3ZS format while transporting it over coaxial cable.
  • Encoding/decoding 100 Gigabit Ethernet data into/from the 64B/66B format while transporting it over a CAUI-4 or CAUI-10 interface.

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