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.

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