When we have fiber connections between two locations, we need ample justification before taking the plunge and increasing fiber count. Aside from path redundancy, we resist the call for more fiber for as long as we can. The reason? Fiber spans are expensive! There’s not only the initial investment, but they usually come with lengthy commitments, too. This means that the telecommunications industry is constantly pushing to do more with existing fiber paths. Using Wave Division Multiplexing, the capacity of a span can be upgraded simply by installing/upgrading the multiplexers/demultiplexers on each end instead of adding additional fiber infrastructure.
Even without any additional filters, bidirectional transceivers (BiDis) allow a paired set of transceivers to communicate over a single fiber. One side transmits at 1550nm, the other side at 1310, and they can communicate simultaneously. BiDis are available in a few variations beyond 1550 with 1310, such as 1330 with 1270, 1310 with 1490, and 1490 with 1550.
From humble beginnings where two signals passed over a single fiberoptic strand, WDM has grown into the de facto means of easily doing more with every strand of your fiber. CWDM allows 16 channels (18 if you have low OH fiber) to pass over a single strand by coarsely separating signals across a wide spectrum, thus the acronym Coarse Wave Division Multiplexing (CDWM). The center wavelength of each signal is 20nm apart from each of its neighboring channels. Since the receivers on WDM transceivers are wide spectrum, a transceiver transmitting on 1471nm can be pared with one transmitting on 1491nm, and they can communicate over a solitary fiber.
CWDM Channels and Their Common Color Associations
However, in the space of just two of the CWDM channels, 1530nm and 1550nm, using Dense Wave Division Multiplexing (DWDM), it’s possible to have 40 or more wavelengths on each fiber. Getting 40 channels communicating back and forth across a fiber pair using DWDM is far more cost effective than using 80 separate fiber strands, even at short distances. A standard DWDM SFP+ transceiver can get a 10G signal to traverse over 80 kilometers, which allows increased bandwidth across increased space. Having all these channels does introduce a down-side, though. Every channel put into production should have at least one spare per side, so a bill of materials for 80 DWDM transceivers to outfit the connections could inflate to 160 if you’re being cautious about sparing. However, because the DWDM channels are so close together, there’s a solution for this that’s not available to their CWDM predecessors. Allow me to introducing Tunable DWDM Transceivers!
Tunable transceivers are now a mainstay of DWDM implementations. These tunable transceivers use two fibers (Tx an Rx) and are capable of tuning to 100GHz and sometimes 50Ghz ITU channels from 16 (191.6 THz) to 62 (196.2THz) and are available in commercial, extended and industrial temperatures. While it’s less common for an entire network be built with tunable transceivers due to cost, their value really shines when it comes to transceiver sparing. Having a few tunables on hand allows you to turn up DWDM networks that have suffered an optical failure without having to spare for all channels used in the network. Tunable transceivers can be tuned by the platform they are in, if that platform supports it, and can also be tuned via an outside application such as the Integra Tuner. With the Integra Smart Coder hardware and software, it’s also possible to use tunable transceivers in an even wider range of platforms by having them masquerade as a fixed wave or greywave part.
As long as the transceivers on both sides are tuned correctly and they have an unbroken fiber path between them, we’re back in business.
Tunable transceiver development has continued to evolve even further. In order to increase capacity, circumvent even more potential points of failure, and make even more effective use of a fiber span, bidirectional DWDM transceivers are now on the market. Having only one ITU channel to squeeze send and receive on has been a hurdle in the past. More than that, automatic tuning where a master on one side connects to a subordinate on the other is now available, and they figure out what DWDM channel they are on by themselves. Integra Optics offers FlexTune optics that have industry leading speed in establishing connections, especially when you’re able to set the channel on the master side. The subordinate optic picks up on its leader and self-tunes in a matter of seconds. The disaster response value is even better because in a standard two fiber DWDM connection, a break in one of your fibers can be overcome using these Flextune DWDM BiDis. With your spare transceivers only requiring one valid fiber in your pair, services can be restored until repairs can be effected. They can also be used to increase capacity on a saturated duplex span carrying 40 channels to now carry 40 channels per fiber without having to change out passive hardware. So your 40ch mux & demux has its capacity doubled!
Tunable DWDM Bidis do this by dividing the ITU channel into an upper and a lower range. Where the center frequency of channel 21 is 192.1THz, the DWDM BiDis break this into two subchannels of 192.08THz and 192.12THz. Of course, deviating from the passive filter’s intended center frequency can introduce some additional attenuation but the worst case in quality filters is 1dB.
The automatic channel negotiation takes place via the leader transceiver and subordinate seeking each other out. This search can be lengthy, but giving the leader side some direction by coding it specifically to a channel can reduce the negation time down to a few seconds. Both the Leader and subordinate can have their channels set, we can set only the leader, or we can just leave them both to negotiate.
Before jumping right in and adding DWDM Bidi Tunables to your network, there are a few considerations to be made. Being a new technology we see some initial limitations. Such as the first generation of parts having a 20km distance limitation. Of course greater distances will be offered in the future, but proving the stability and reliability of different distance classes take time.
Also, different manufacturers use different channel negotiation methods and interoperability between implementations is far from a given at this point. Integra Optics DWDM Bidis use Flextune, which has a faster negotiation time. These parts are slated to get compatibility with the slower standard of Autotune, but backwards compatibility is not there yet.
There are also a role differentiation between the transceivers; The leader parts are C-Temp (0c to 70c) and their subordinate companions are available in extended e-temp (-5c to +85c) and industrial i-temp (-40c to +85c) operational ratings. Leader to leader connections and subordinate to subordinate connections won’t work so leaders and subordinates always have to be deployed. All bidis differentiate upstream and downstream function, these are no different.
There’s also a cost for going with DWDM Bidis, but compared to the expense of fitting in new fiber infrastructure and the convenience of installation and sparing, it’s a small investment.
This is an industry that’s constantly developing, creating, and innovating. As new products debut in the market, Integra Optics is on the cutting edge helping you bring the most impressive new products and technology into your networks. Schedule a time to talk with one of our optics experts about the future needs of your network!