Digital Wrappers and Forward Error Correction (FEC)

The world of optical networks can be complicated and somewhat boring at times. Network guys drone on and on about the finer points of network design and management, while the optics bods excitedly discuss the latest breakthrough that has yielded a 0.1 percent increase in the capacity of optical fiber. If you want to be in the in crowd when it comes to talking about the latest developments, then you might want to know a little about digital wrappers – a technology that may just get these two groups together. (Now, there’ll be a wild time!)

It does rather sound like the next big thing after “gangster rappers,” but digital wrappers do not actually lay down dope rhymes in 1s and 0s. In the world of networking, a digital wrapper can encapsulate data with additional information designed to manage an optical network. Such management functions will excite the network people, while an integrated error correction facility provides extra play in the system to allow the optics people to squeeze out even more capacity.

Current optical networking is usually enabled by Sonet/SDH, which frames and formats data of other protocols (such as Internet Protocol (IP) or Asynchronous Transfer Mode (ATM)) and allows Wavelength Division Multiplexing (WDM) transmission over long distances of optical fiber. Sonet/SDH frames carry overhead sections along with the data: These provide such functions as performance monitoring, fault detection, and protection switching.

Digital wrappers are designed to streamline this overhead – effectively having less overhead per byte of actual data – while still giving wide-ranging control over the network management. This is provided independently of the protocol being carried, meaning that data from all protocols can be encapsulated and transmitted on a comprehensively managed optical wavelength. (This can be extremely desirable in short-haul networks where such “protocol transparency” would allow a carrier to control a variety of traffic types.) Additionally, the ability to detect and correct errors in the transmission can be incorporated into this digital wrapper.

Each wavelength in a system is classed as an optical channel, which has its own associated digital wrapper and can therefore be managed individually. Internationally agreed specifications for a digital wrapper have yet to be finalized, but its basic structure can be seen below.

A Digital Wrapper The overall envelope contains the overhead, the data to be transmitted, and the forward error correction (FEC) information. This FEC capability may be considered an optional extra that can be added in order to improve the overall system performance. Mathematical algorithms are performed on the data being transported in the digital wrapper, and the results are stored in the digital wrapper’s FEC information. This is then used at the receiving end to see if any errors have occurred in the data and, if so, to correct them. For those with a thirst for equations, the most commonly used routines are known as “Reed Solomon” and the most basic form adds approximately 7 percent extra overhead to the data (i.e., a 10-Gbit/s signal would increase to 10.7 Gbit/s if it incorporated standard FEC).

The detecting and correcting of errors improves the bit error rate (BER) of a system, meaning that more bits are accurately identified as 1s or 0s at the receiving end. This is usually translated into a gain figure in decibels, with the standard FEC giving a 5 dB gain, meaning that a system design can be pushed further in other respects: Usually the length of a system can be extended, or perhaps during an upgrade the wavelength bit-rate might be increased while maintaining the same system length.

More advanced forms of FEC are being researched at the moment, some with the ability to provide as much as 10 dB of gain, albeit with a considerable 30 percent additional overhead. It is worth noting that FEC is already being used in Submarine Systems, but without the additional management functionality of the fully equipped digital wrapper. The digital wrapper can be used with or without FEC, depending on the specific needs of a network.

Referring back to digital wrappers as being protocol independent, it is worth a brief digression to cover what this could mean for the transport of IP traffic. IP data transmitted over optical fiber will usually be framed by Sonet/SDH. However, this will not be a direct process, as the IP packets will first need to be enveloped by a Layer 2 protocol such as ATM or Ethernet. In the world of digital wrappers, it still does not seem that IP will be able to go directly onto the optical fiber. This is due to its bursty nature as a packet technology, which does not match well to the continuous flow of frames provided by a digital wrapper. Therefore, there is still likely to be the need for a layer in between to encapsulate the IP traffic before it can be carried within a digital wrapper.

Key Points

  • A digital wrapper encapsulates data with additional information intended to manage an optical network
  • This overhead is streamlined, providing greater efficiency than the Sonet/SDH overhead
  • Data from all protocols can be transmitted transparently
  • Each wavelength is an optical channel that can be independently managed
  • Error correction functionality can also be added
  • Standard forward error correction (FEC) adds an additional 7% overhead while providing a 5 dB gain
  • FEC can extend system length or allow an increase in the capacity of a system
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sc_xj 12/4/2012 | 7:27:51 PM
re: Digital Wrappers and Forward Error Correction (FEC) 1.Why the n<=16 in OTM-nr.m ?
It can only support 16 optical channel in one fiber.
2.What is the function of BIAE ? How to use it?
It is added in the amendment of G.709.
jayjay 12/4/2012 | 7:50:13 PM
re: Digital Wrappers and Forward Error Correction (FEC) It doesn't matter if the error is in the data or in the FEC code area, FEC corrects them. Only if too many errors occur (depends on FEC scheme) the FEC cannot correct them.

jayjay 12/4/2012 | 7:50:13 PM
re: Digital Wrappers and Forward Error Correction (FEC) The DW as such doesn't support management of multiwavelength signals at the Optical Transmission Section (OTS) and Optical Multiplex Scetion (OMS) Layer. The DW is used ber single wavelength signal. Overhead for management of multiwavelength signals is also defined in G.709, but it is not part of the DW (ODUk and OTUk in G.709 terms) it is transported via a Optical Supervisory Channel (OSC) on a dedicated wavelength.
TCM is indeed an important feature for signals crossing multiple-domains and protection switching. It allows independent supervision for each domain, for protection connecitons and for the overall end-to-end path. SONET/SDH has limited TCM functionality.

lightreading 12/4/2012 | 7:50:22 PM
re: Digital Wrappers and Forward Error Correction (FEC) good paper, but you need to highlight further the benefits that DW provides over SONET/SDH. example, the ability to manage a multiwavelength OTN, as opposed to a single wavelength network in SONET. as well as the ability to manage added sublayers within the OTN (OTS, OMS). these are key attributes that provide functionality that SONET lacks. concept of TCM is mentioned breifly, but is also key to DW.
jayjay 12/4/2012 | 7:51:42 PM
re: Digital Wrappers and Forward Error Correction (FEC) They take SDH/Sonet signals as client signals, a STM-16 is transported in a ODU1, a STM-64 in a ODU2 (See ITU-T G.709).
IN the same way as SDH transports PDH signals, the wrapper transports SDH signals.

jayjay 12/4/2012 | 7:51:43 PM
re: Digital Wrappers and Forward Error Correction (FEC) ITU has determined its first standard for the so called "digital Wrapper" G.709 already in February.
In October other related standards for equipment (G.798) and management (G.874) will follow.
Note that the name "digital Wrapper" is no longer used. G.709muses the terms Optical Channel Transport Unit OTU and Optical Channel Data Unit ODU.

jayjay 12/4/2012 | 7:51:43 PM
re: Digital Wrappers and Forward Error Correction (FEC) The SDH/Sonet overhead is often used by the customer, specially in case of carriers carrier.
The wrapper provides also additonal overhad for tandem connection monitoring. This allows each carrier in a multi-operator enviroment to have its own overhead for dedicated supervision in its netowrk domain.
Furthermore the wrapper provides swichting and services at higher bit rates (2.5, 10, 40 G).
SDH/SOnet allows also higher bit rate services with VC-4-16v/STS-49c or VC-4-64c/STS-192c, but it isn ot possible to transport other VCs (VC-4, VC-12) in such a signal.

rtfm 12/4/2012 | 7:51:48 PM
re: Digital Wrappers and Forward Error Correction (FEC) Well, wrapper technology can certainly sit "below" it (to extend the OSI model downwards if SONET/SDH site at layer 1).

SONET/SDH is just part of the data or payload within the wrapper. This is non-interfering, but I too would like to know how much *benefit* can be had by the wrappers if the SONET/SDH overhead is already there.

krisman 12/4/2012 | 7:51:52 PM
re: Digital Wrappers and Forward Error Correction (FEC) Can Systems with Digital Wrapper framing interoperate with the legacy SONET/SDH systems?

wilhelm 12/4/2012 | 7:52:03 PM
re: Digital Wrappers and Forward Error Correction (FEC) What happens if an error in the FEC block occurs?
Is there a way to detect this or has the data to
be dropped?
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