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Industry Insights

Transceivers: Definitions, Applications and Developments

Tuesday, November 28, 2017 | by Rudy Montgelas
Transceivers: Definitions, applications and developments
  

Back in April of last year, I wrote up a CTD on transceivers that talked about what a pluggable optical transceiver is and the need for “more speed” in the data center.1 The CTD also described what an MSA is and some considerations to think about in selecting the right transceiver for your application:


 

This CTD goes into more depth on optical transceivers and talks about some of their definitions and applications as well as the rapidly advancing developments in fiber optic transceiver technology.


Transceiver definitions
As with any industry, some of the technical jargon can sometimes make things complicated.

It helps to start by defining some of the technical terms that are often used for the transceivers.  Also, there are many different types of transceivers: RS-232, Fiber Channel, SONET, Ethernet copper, Ethernet fiber, etc.  This CTD focuses mostly on Ethernet fiber optic transceivers.

Here are some commonly used terms when talking about Ethernet based fiber transceivers:

  • OOK – On/Off Keying is the traditional modulation scheme used for optical transceivers that transmit the light at two levels, either on or off.  The levels represent either a binary “1” or “0”. 2
  • PAM4Pulse Amplitude Modulation with 4 amplitude levels.  Using this modulation scheme, the signal amplitude is transmitted at four levels: signifying four binary words: 0 0, 0 1, 1 0 and 11.  This technique is used to double the transceiver bit rate from 10Gbps to 20Gbps and so on. 2 However; this is often achieved at the added expense of requiring additional transceiver signal processing.
  • SERDESSERializer/DESerializer is the system of electronics in a transceiver that determines its maximum bit rate.  For instance, a transceiver with a 10Gbps SERDES can be made to provide an effective bit rate of 40 to 100Gbps through the additional use of WDM, BiDi or SDM. 3
  • Coupled Optical Power is the minimum amount of optical power (in dBm) that is coupled into a specific core diameter optical fiber from the transceiver.
  • Minimum Receiver Sensitivity is the minimum optical power required at the transceiver’s receiver port that will provide a BER of 10-9 or better.
  • Receiver Saturation Power is the maximum amount of optical power at the receiver port before saturation of the receiver optoelectronics occurs.
  • Dynamic Range is the range of optical power that a transceiver can accept in order to assure the required BER and to prevent receiver saturation.  It is the usually the difference between the Coupled Optical Power and the Receiver Saturation Power.
  • SWDM4Short Wavelength Division Multiplexing uses 4 WDM wavelengths over 50 micron core multimode fibers.  This WDM communication solution is used to increase the bit rate for a fiber communication system from 10Gbps to 40Gbps and from 25Gbps to 100Gbps.3
  • PSM4 PARALLEL SINGLEMODE MULTIPLEXING using 4 single mode fibers.
  • SDMSpace Division Multiplexing (more commonly known as parallel optics) is a technique used to increase the data rate in an optical link by adding more optical pathways with additional fibers or additional cores in the same fiber.  SWDM4 and PSM4 are examples of fiber links using SDM.3
  • An MSA Compliant transceiver is an industry compliant device which meets the specifications set by the member manufacturers within the agreement.  MSA stands for Manufacturer’s Sourcing Agreement.
  • Universal Transceiver is a transceiver which works with both singlemode and multimode fiber, allowing you to take advantage of economies of scale in your inventory of spares.
  • DACs – Direct Attached Copper cables are copper patch cable assemblies with the copper transceivers factory terminated to each end of the twinax copper cables.
  • AOCs– Active Optical Cables are patch cord assemblies that have optical transceivers permanently attached to the cables at each end. AOCs typically plug directly into the same cage slot on the switch that a pluggable transceiver does. The cable is permanently attached to the transceiver making it unnecessary to clean and inspect fiber connectors.
  • BERBit Error Rate is the metric for the performance of a communication link that defines the ratio of number of bits that are transmitted in error to the total number of bits transmitted.  A BER of better than 10-9 means that less than 1 bit is transmitted in error (“1” when it should be “0” or “0” when it should be “1” etc) over the communication system for every billion bits of information transmitted.
  • VFLVisual Fault Locator is a test instrument that injects visible (650nM) laser light into an optical link to determine if there are any points of high loss or scattering in the link.
  • Loopback – A loopback is a special passive fiber looped connector, that has a short fiber connected from the transmit port ferrule to the receive port ferrule.  A loopback connector, plugged into the transceiver’s duplex optical port, allows remote testing to assure that it is functioning properly.
  • WDMWavelength Division Multiplexing utilizes different wavelengths, or “colors” of light to encode two or more transmission signals onto a single fiber, either unidirectional or bi-directionally (BiDi).
  • CDWM Coarse Wavelength Division Multiplexing utilizes specific edge emitting lasers selected for narrower line widths and wider wavelength ranges than SWDM4 or PSM4, which typically use VCSEL lasers.
  • BiDi Bi-Directional WDM communication is used over singlemode or multimode optical fibers to achieve higher data rates.  Cisco coined the term “BiDi” for their 40Gbps transceiver solution that employs two different 20Gbps wavelengths travelling in opposite directions over duplex fiber pairs in an existing (legacy) duplex fiber cable plant.
  • CAUI - stands for 100 Gigabit (C) Attachment Unit Interface
  • ROSAReceiver Optical Sub-Assembly
  • TOSATransmitter Optical Sub-Assembly
  • Transceiver footprints and formats:
    • SFP Small Form Factor Pluggable
    • QSFP Quad Small Form Factor Pluggable
    • QSFP28 Quad Small Form Factor Pluggable (100Gig)
    • QSFP-DDQuad Small Form Factor Pluggable - Double Density
    • OSFP Octal Small Form Pluggable
    • CFP Transceivers C Form-factor Pluggable transceiver is an MSA for transceiver form-factors used in the transmission of high-speed digital optical signals. The “C” stands for the Latin letter “C” which expresses the number (100).  This standard was primarily developed for Ethernet systems with 10X10Gbps lanes and 4X25Gbps lanes for 100 Gigabit applications.
      • CFP - The CFP MSA defines hot-pluggable optical transceiver form factors to enable 40 Gbit/s and 100 Gbit/s applications. CFP modules use SDM with the 10-lane CAUI-10 electrical interface.
      • CFP2 - CFP2 modules use the 10-lane CAUI-10 electrical interface or the 4-lane CAUI-4 electrical interface
      • CFP4 - CFP4 modules use the 4-lane CAUI-4 electrical interface

 


 
There are many different types of transceiver footprints.  These are just some of the more popular designations that describe the outer housing and the keying of the electrical contacts for the particular transceiver form factor. Transceivers are pluggable devices that insert into “cages” on the switch gear and server equipment.  The transceivers may also use WDM, in conjunction with parallel optical transmission (SDM), for increased channel data transmission speeds.2

Transceiver applications and developments
Pluggable transceivers can be used as standalone devices that plug into the cages of the active switching and server equipment in a data center.  In this application, factory terminated patch cords are connected from active equipment’s transceivers to a cross connect or port-replicating patch panel.

Pluggable optical transceivers also contain special code or firmware that is “burned-into” the electrically erasable programmable read-only memory (EEPROM) at the factory. The firmware resides as part of the transceiver electronics and allows the switch to identify the transceiver as a compatible device.

Transceivers can also be “built-into” molded cable assemblies such as the direct attached cables (DACs) or active optical cables (AOCs).  Two key advantages of these types of cables are lower end-to-end connection losses, because the cabling is factory pre-terminated to twinax copper cables or to factory pigtailed multi-fiber cables, and the elimination of the need for time-consuming field cleaning and inspection of fiber optic connectors and port receptacles.




Non-pluggable, On-Board Optic (OBO) transceivers, may also be located right on the PC board in the active switch gear and other devices such as optical multiplexers and amplifiers.  This type of solution eliminates the need for a separate metal transceiver housing and pluggable electrical contacts and receptacles.  It also brings the transceiver electronics much closer to the main PCB for improved signal bandwidth and reduced noise.
 
Summing all of this up…
Transceivers are available in many different sizes, shapes and “flavors” such as formats, bandwidths and connector types.  The best selection for a particular application will depend on the equipment that a transceiver is attached to and current and future network performance requirements.  It is important to make sure that the selected transceivers are properly coded and pretested, up front, to confirm correct functionality with the related switch gear.

See you next time with another CTD down the road.

References and further reading:
1 Leave it to Transceivers, Legrand Data Communications Division “Connecting the Dots”, 4-13-2016
2 Boosting Fiber Optic Links, Legrand Data Communications Division “Connecting the Dots”, 4-27-2016
3 SWDM and WBMMF’s True Colors, Legrand Data Communications Division “Connecting the Dots”, 6-11-2016