BT's IP-over-DWDM move
- BT will roll out next year IP-over-DWDM using pluggable coherent optics in its network
- At ECOC 2022, BT detailed network trials that involved the use of ZR+ and XR optics coherent pluggable modules
Telecom operators have been reassessing IP-over-DWDM with the advent of 400-gigabit coherent optics that plug directly into IP routers.
According to BT, using pluggables for IP-over-DWDM means a separate transponder box and associated ‘grey’ (short-reach) optics are no longer needed.
Until now, the transponder has linked the IP router to the dense wavelength-division multiplexing (DWDM) optical line system.
“Here is an opportunity to eliminate unnecessary equipment by putting coloured optics straight onto the router,” says Professor Andrew Lord, BT’s head of optical networking.
Removing equipment saves power and floor space too.
DWDM trends
Operators need to reduce the cost of sending traffic, the cost-per-bit, given the continual growth of IP traffic in their networks.
BT says its network traffic is growing at 30 per cent a year. As a result, the operator is starting to see the limits of its 100-gigabit deployments and says 400-gigabit wavelengths will be the next capacity hike.
Spectral efficiency is another DWDM issue. In the last 20 years, BT has increased capacity by lighting a new fibre pair using upgraded optical transport equipment.
Wavelength speeds have gone from 2.5 to 10, then to 40, 100, and soon 400 gigabits, each time increasing the total traffic sent over a fibre pair. But that is coming to an end, says BT.
“If you go to 1.2 terabits, it won’t go as far, so something has to give,” says Lord. ”So that is a new question we haven’t had to answer before, and we are looking into it.”
Fibre capacity is no longer increasing because coherent optical systems are already approaching the Shannon limit; send more data on a wavelength and it occupies a wider channel bandwidth.
Optical engineers have improved transmission speeds by using higher symbol rates. Effectively, this enables more data to be sent using the same modulation scheme. And keeping the same modulation scheme means existing reaches can still be met. However, upping the symbol rate is increasingly challenging.
Other ways of boosting capacity include making use of more spectral bands of a fibre: the C-band and the L-band, for example. BT is also researching spatial division multiplexing (SDM) schemes.
IP-over-DWDM
IP-over-DWDM is not a new topic, says BT. To date, IP-over-DWDM has required bespoke router coherent cards that take an entire chassis slot, or the use of coherent pluggable modules that are larger than standard QSFP-DD client-side optics ports.
“That would affect the port density of the router to the point where it’s not making the best use of your router chassis,“ says Paul Wright, optical research manager at BT Labs.
The advent of OIF-defined 400ZR optics has catalysed operators to reassess IP-over-DWDM.
The 400ZR standard was developed to link equipment housed in separate data centres up to 120km apart. The 120km reach is limiting for operators but BT’s interest in ZR optics stems from the promise of low-cost, high-volume 400-gigabit coherent optics.
“It [400ZR optics] doesn’t go very far, so it completely changes our architecture,” says Lord. “But then there’s a balance between the numbers of [router] hops and the cost reduction of these components.”
BT modelled different network architectures to understand the cost savings using coherent ZR and ZR+ optics; ZR+ pluggables have superior optical performance compared to 400ZR.
The networks modelled included IP routers in a hop-by-hop architecture where the optical layer is used for point-to-point links between the routers.
This worked well for traffic coming into a hub site but wasn’t effective when traffic growth occurred across the network, says Wright, since traffic cascaded through every hop.
BT also modelled ZR+ optics in a reconfigurable optical add-drop multiplexer (ROADM) network architecture, as well as a hybrid arrangement using both ZR+ and traditional coherent optics. Traditional coherent optics, with its superior optical performance, can pass through a string of ROADM stages where ZR+ optics falls short.
BT compared the cost of the architectures assuming certain reaches for the various coherent optics and published the results in a paper presented at ECOC 2020. The study concluded that ZR and ZR+ optics offer significant cost savings compared to coherent transponders.
ZR+ pluggables have since improved, using higher output powers to better traverse a network’s ROADM stages. “The [latest] ZR+ optics should be able to go further than we predicted,” says Wright.
It means BT is now bought into IP-over-DWDM using pluggable optics.
BT is doing integration tests and plans to roll out the technology sometime next year, says Lord.
XR optics
BT is a member of the Open XR Forum, promoting coherent optics technology that uses optical sub-carriers.
Dubbed XR optics, if all the subs-carriers originate at the same point and are sent to a common destination, the technology implements a point-to-point communication scheme.
Sub-carrier technology also enables traffic aggregation. Each sub-carrier, or a group of sub-carriers, can be sent from separate edge-network locations to a hub where they are aggregated. For example, 16 endpoints, each using a 25-gigabit sub-carrier, can be aggregated at a hub using a 400-gigabit XR optics pluggable module. Here, XR optics is implementing point-to-multipoint communication.
Lord views XR optics as innovative. “If only we could find a way to use it, it could be very powerful,” he says. “But that is not a given; for some applications, XR optics might be too big and for others it may be slightly too small.”
ECOC 2022
BT’s Wright shared the results of recent trial work using ZR+ and XR optics at the recent ECOC 2022 conference, held in Basel in September.
The 400ZR+ were plugged into Nokia 7750 SR-s routers for an IP-over-DWDM trial that included the traffic being carried over a third-party ROADM system in BT’s network. BT showed the -10dBm launch-power ZR+ optics working over the ROADM link.
For Wright, the work confirms that 0dBm launch-power ZR+ optics will be important for network operators when used with ROADM infrastructures.
BT also trialled XR optics where traffic flows were aggregated.
“These emerging technologies [ZR+ and XR optics] open up for the first time the ability to deploy a full IP-over-DWDM solution,” concluded Wright.
Infinera's XR optics pluggable plans
Infinera’s coherent pluggables for XR optics will also address the company’s metro needs.
Coherent pluggables now dominate the metro market where embedded designs account for just a fifth of all ports, says Infinera.
“As we grow our metro business, we need our own pluggables if we want to be cost-competitive,” says Robert Shore, senior vice president of marketing at Infinera.
Infinera’s family of pluggables implementing the XR optics concept is dubbed ICE-XR.
XR optics splits a coherent optical signal into Nyquist sub-carriers, each carrying a data payload. Twenty-five gigabits will likely be the sub-carrier capacity chosen.
XR optics can be used for point-to-point links where all the sub-carriers go to the same destination. But the sub-carriers can also be steered to different destinations, similar to how breakout cables are used in the data centre.
With XR optics, a module can talk to several lower-speed ones in a point-to-multipoint arrangement. This enables optical feeds to be summed, ideal for traffic aggregation applications such as access and 5G.
Open XR Forum
Infinera detailed its ICE-XR pluggables during the OFC virtual conference and exhibition.
The event coincided with the launch of the Open XR Forum whose members include network operators, Verizon, Lumen Technologies (formerly CenturyLink), Windstream and Liberty Global.
Members of the Open XR Forum span sub-component makers, systems vendors like Infinera, and network operators. The day the Open XR Forum website was launched, Infinera received a dozen enquiries from interested parties.
The Open XR Forum will define standards for XR optics such as how the networks are managed, the form factors used, their speeds and power requirements.
“There are a lot of underlying operational aspects that need to be worked out,” says Shore.
XR optics will use a similar model to ZR+ coherent optics. ZR+ delivers enhanced transmission performance compared to the OIF’s 400ZR coherent standard. “ZR+ is not a standard but rather a set of open specifications that can be used by anyone to create a product, and that is exactly the approach we are taking with XR optics,” says Shore.
Over the last 18 months, Infinera has met with 150 network operators regarding XR optics. “We wanted to validate this is a worthwhile technology and that people wanted it,” says Shore.
There have also been 40 network operator trials of the technology by the end of July. BT has used the technology as part of a metro aggregation trial while Virgin Media and American Tower each tested XR optics over PON.
More members have joined the Open XR Forum and will be announced in the autumn.
ICE-XR
ICE-XR’s name combines two concepts.
The first, ICE, refers to the Infinite Capacity Engine, the optics and coherent digital signal processor (DSP) that is the basis for Infinera’s ICE4 and newer ICE6 coherent transmission designs. ICE4 was Infinera’s first product to use Nyquist sub-carriers.
“XR”, meanwhile, borrows from 400ZR. Here, the ‘X’ highlights that XR supports point-to-point coherent communications, like 400ZR, and point-to-multipoint.
“ICE-XR’s release will be timed in conjunction with the official ratification of the specifications from the Open XR Forum,” says Shore.
Infinera’s ICE-XR portfolio will include 100, 400, and 800-gigabit optical modules.
The 100-gigabit ICE-XR, based on four 25-gigabit sub-carriers, will be offered as QSFP-28, QSDP-DD and CFP2 form factors. The 400-gigabit and 800-gigabit variants, using 16 and 32 sub-carriers respectively, will be available as QSFP-DD and CFP2 modules.
The 100-gigabit and 400-gigabit ICE-XR modules will be released first in 2022.
The 400-gigabit ICE-XR will also double as Infinera’s ZR+ offering when used point-to-point.
Shore says its first ZR+ module will not support the oFEC forward-error correction (FEC) used by the OpenZR+ multi-source agreement (MSA).
“The performance hit you take to ensure multi-vendor interoperability is vastly outweighed by the benefits of the improved [optical] performance [using a proprietary FEC],” says Shore.
Merchant DSP suppliers and the systems vendors with in-house DSP designs all support proprietary FEC schemes that deliver far better performance than oFEC, says Shore.
Infinera is developing a monolithic photonic integrated circuit (PIC) for ICE-XR manufactured at its indium phosphide facility.“ICE-XR will increase the utilisation of our fabrication centre, especially when pluggables produce higher volumes compared to embedded [coherent designs],” says Shore.
Infinera says more than one coherent DSP will be needed for the ICE-XR product portfolio. The modules used have a range of power profiles. The QSFP-28 module will need to operate within 4-5W, for example, while the QSFP-DD implementing ZR+ will need to be below 20W. Developing one DSP to span such a power range is not possible.
Business model
The Open XR Forum’s specifications will enable vendors to develop their own XR optics implementations.
Infinera will also license aspects of its design including its coherent DSPs. The aim, says Shore, is to develop as broad an ecosystem as possible: “We want to make XR optics an industry movement.”
Shore stresses ZR+ interoperability is not a must for most applications. Typically, a vendor’s module will be used at both ends of a link to benefit from the ZR+’s custom features. But interoperability is a must for XR optics given its multi-rate nature. The different speed modules from different vendors must talk to each other.
“Because you have multi-generational and multi-rate designs, it becomes even more important to support multi-vendor interoperability,” says Shore. “It gives the network operators more choice, freedom and flexibility.”
XR optics for the data centre
Infinera says there are developments to use XR optics within the data centre.
As data rates between equipment rise, direct-detect optics will struggle to cope, says Shore. The hierarchical architectures used in data centres also lend themselves to a hub-and-spoke architecture of XR optics.
“This type of technology could fit very nicely into that environment once the capacity requirements get high enough,” says Shore.
For this to happen, power-efficient coherent designs are required. But first, XR optics will need to become established and demonstrate a compelling advantage in a point-to-multipoint configuration.
XR optics will also need to replace traditional direct-detect pluggables that continue to progress; 800-gigabit designs are appearing and 1.6-terabit designs were discussed at OFC. Co-packaged optics is another competing technology.
“You are not looking at the 2022-23 timeframe, but maybe 2025-26,” says Shore.
Covid-era shows
Infinera postponed its customer meetings that pre-covid would take place at OFC till after the show to avoid clashing with the online sessions. Once the meetings occurred, customers were given a tour of Infinera’s virtual OFC booth.
Infinera’s solutions marketing team also divided between them the OFC sessions of interest to attend. The team then ‘met’ daily to share their learnings.
“I do think that the world of in-person events has changed forever,” says Shore. Infinera attended 40 events in 2019. “We will probably do fewer than 20 [a year] going forward,” says Shore.
Is traffic aggregation the next role for coherent?
Ciena and Infinera have each demonstrated the transmission of 800-gigabit wavelengths over near-1,000km distances, continuing coherent's marked progress. But what next for coherent now that high-end optical transmission is approaching the theoretical limit? Can coherent compete over shorter spans and will it find new uses?
Part 1: XR Optics
“I’m going to be a bit of a historian here,” says Dave Welch, when asked about the future of coherent.
Interest in coherent started with the idea of using electronics rather than optics to tackle dispersion in fibre. Using electronics for dispersion compensation made optical link engineering simpler.
Dave Welch
Coherent then evolved as a way to improve spectral efficiency and reduce the cost of sending traffic, measured in gigabit-per-dollar.
“By moving up the QAM (quadrature amplitude modulation) scale, you got both these benefits,” says Welch, the chief innovation officer at Infinera.
Improving the economics of traffic transmission still drives coherent. Coherent transmission offers predictable performance over a range of distances while non-coherent optics links have limited spans.
But coherent comes at a cost. The receiver needs a local oscillator - a laser source - and a coherent digital signal processor (DSP).
Infinera believes coherent is now entering a phase that will add value to networking. “This is less about coherent and more about the processor that sits within that DSP,” says Welch.
Aggregation
Infinera is developing technology - dubbed XR Optics - that uses coherent for traffic aggregate in the optical domain.
The 'XR’ label is a play on 400ZR, the 400-gigabit pluggable optics coherent standard. XR will enable point-to-point spans like ZR optics but also point-to-multipoint links.
Infinera, working with network operators, has been assessing XR optics’ role in the network. The studies highlight how traffic aggregation dictates networking costs.
“If you aggregate traffic in the optical realm and avoid going through a digital conversion to aggregate information, your network costs plummet,” says Welch.
Are there network developments that are ripe for such optical aggregation?
“The expansion of bandwidth demand at the network edge,” says Rob Shore, Infinera’s senior vice president of marketing. “It is growing, and it is growing unpredictably.”
XR Optics
XR optics uses coherent technology and Nyquist sub-carriers. Instead of a laser generating a single carrier, pulse-shaping at the optical transmitter is used to create multiple carriers, dubbed Nyquist sub-carriers.
The sub-carriers carry the same information as a single carrier but each one has a lower symbol rate. The lower symbol rate improves tolerance to non-linear fibre effects and enables the use of lower-speed electronics. This benefits long-distance transmissions.
But sub-carriers also enable traffic aggregation. Traffic is fanned out over the Nyquist sub-carriers. This enables modules with different capacities to communicate, using the sub-carrier as a basic data rate. For example, a 25-gigabit single sub-carrier XR module and a 100-gigabit XR module based on four sub-carriers can talk to a 400-gigabit module that supports 16.
It means that optics is no longer limited to a fixed point-to-point link but can support point-to-multipoint links where capacities can be changed adaptively.
“You are not using coherent to improve performance but to increase flexibility and allow dynamic reconfigurability,” says Shore.
Rob Shore
XR optics makes an intermediate-stage aggregation switch redundant since the higher-capacity XR coherent module aggregates the traffic from the lower-capacity edge modules.
The result is a 70 per cent reduction in networking costs: the transceiver count is halved and platforms can be removed from the network.
XR Optics starts to make economic sense at 10-gigabit data rates, says Shore. “It depends on the rest of the architecture and how much of it you can drive out,” he says. “For 25-gigabit data rates, it becomes a virtual no-brainer.”
There may be the coherent ‘tax’ associated with XR Optics but it removes so much networking cost that it proves itself much earlier than a 400ZR module, says Shore.
Applications
First uses of XR Optics will include 5G and distributed access architecture (DAA) whereby cable operators bring fibre closer to the network edge.
XR Optics will likely be adopted in two phases. The first is traditional point-to-point links, just as with 400ZR pluggables.
“For mobile backhaul, what is fascinating is that XR Optics dramatically reduces the expense of your router upgrade cost,” says Welch. “With the ZR model I have to upgrade every router on that ring; in XR I only have to upgrade the routers needing more bandwidth.”
Phase two will be for point-to-multipoint aggregation networks: 5G, followed by cable operators as they expand their fibre footprint.
Aggregation also takes place in the data centre, has coherent a role there?
“The intra-data centre application [of XR Optics] is intriguing in how much you can change in that environment but it is far from proven,” says Welch.
Coherent for point-to-point links will not be used inside the data centre as it doesn’t add value but configurable point-to-multiple links do have merit.
“It is less about coherent and more about the management of how content is sent to various locations in a point-to-multiple or multipoint-to-multipoint way,” says Welch. “That is where the game can be had.”
Uptake
Infinera is working with leading mobile operators regarding using XR Optics for optical aggregation. Infinera is talking to their network architects and technologists at this stage, says Shore.
Given how bandwidth at the network edge is set to expand, operators are keen to explore approaches that promise cost savings. “The people that build mobile networks or cable have told us they need help,” says Shore.
Infinera is developing the coherent DSPs for XR Optics and has teamed with optical module makers Lumentum and II-VI. Other unnamed partners have also joined Infinera to bring the technology to market.
The company will detail its pluggable module strategy including XR Optics and ZR+ later this year.
Infinera’s ICE6 sends 800 gigabits over a 950km link
Infinera has demonstrated the coherent transmission of an 800-gigabit signal across a 950km span of an operational network.
Infinera used its Infinite Capacity Engine 6 (ICE6), comprising an indium-phosphide photonic integrated circuit (PIC) and its FlexCoherent 6 coherent digital signal processor (DSP).
The ICE6 supports 1.6 terabits of traffic: two channels, each supporting up to 800-gigabit of data.
The trial, conducted over an unnamed operator’s network in North America, sent the 800-gigabit signal as an alien wavelength over a third-party line-system carrying live traffic.
“We have proved not only the state of our 800-gigabit with ICE6 but also the distances it can achieve,” says Robert Shore, senior vice president of marketing at Infinera.
800G trials
Several systems vendors have undertaken 800-gigabit optical trials.
Ciena detailed two demonstrations using its WaveLogic 5 Extreme (WL5e). One was an interoperability trial involving Verizon and Juniper Networks while the second connected two data centres belonging to the operator, Southern Cross Cable, to confirm the deployment of the WL5e cards in a live network environment.
Neither Ciena trial was designed to demonstrated WL5e’s limit of optical performance. Accordingly, no distances were quoted although both links were sub-100km, according to Ciena.
Meanwhile, Huawei has trialled its 800-gigabit technology in the networks of operators Turkcell and China Mobile.
The motivation for vendors to increase the speed of line-side optical transceivers is to reduce the cost of data transport. “One laser generating more data,” says Shore. “But it is not just high-speed transmissions, it is high-speed transmissions over distance.”
Infinera’s first 800-gigabit demonstration involved the ICE6 sending the signal over 800km of Corning’s TXF low-loss fibre.
“We did the demo on that fibre and we realised we had a ton of margin left over after completing the 800-gigabit circuit,” says Shore. The company then looked for a suitable network trial using standard optical fibre.
Infinera used a third-party’s optical line system to highlight that the 950km reach wasn’t due to a combination of the ICE6 module and the company’s own line system.
“What we have shown is that you can take any link anywhere, use anyone’s line system, carrying any kind of traffic, drop in the ICE6 and get 800-gigabit connections over 950km,” says Shore.
ICE 6
Infinera attributes the ICE6’s optical performance to its advanced coherent toolkit and the fact that the company has both photonics and coherent DSP technology, enabling their co-design to optimise the system’s performance.
One toolkit technique is Nyquist sub-carriers. Here, data is sent using several Nyquist sub-carriers across the channel instead of modulating the data onto a single carrier. The ICE6 is Infinera’s second-generation design to use sub-carriers, the first being ICE4, that doubles the number from four to eight.
The benefit of using sub-carriers is that high data rates can be achieved while the baud rate used for each one is much lower. And a lower baud rate is more tolerant to non-linear channel impairments during optical transmission.
Sub-carriers also improve spectral efficiency as the channels have sharper edges and can be packed tightly.
Infinera applies probabilistic constellation shaping to each sub-carrier, allowing fine-tuning of the data each carries. As a result, more data can be sent on the inner sub-carriers and less on the outer two outer sub-carrier where signal recovering is harder.
The sweet spot for sub-carriers is a symbol rate of 8-11 gigabaud (GBd). For the Infinera trial, eight sub-carriers were used, each at 12GBd, for an overall symbol rate of 96GBd.
“While it is best to stay as close to 8-11GBd, the coding gain you get as you go from 11GBd to 12GBd per sub-carrier is greater than the increased non-linear penalties,” says Shore.
Another feature of the coherent DSP is its use of soft-decision forward-error correction (SD-FEC) gain sharing. By sharing the FEC codes, processing resources can be shifted to one of the PIC’s two optical channels that needs it the most.
The result is that some of the strength of the stronger signal can be traded to bolster the weaker one, extending its reach or potentially allowing a higher modulation scheme to be used.
Applications
Linking data centres is one application where the ICE6 will be used. Another is sub-sea optical transmission involving spans that can be thousands of kilometres long, requiring lower modulation schemes and lower data rates.
“It’s not just cost-per-bit and power-per-bit, it is also spectral efficiency,” says Shore. “And a higher-performing optical signal can maintain a higher modulation rate over longer distances as well.”
Infinera says that at 600 gigabits-per-second (Gbps), link distances will be “significantly better” than 1,600km. The company is exploring suitable links to quantify ICE6’s reach at 600Gbps.
The ICE6 is packaged in a 5×7-inch optical module. Infinera’s Groove series will first adopt the ICE6 followed by the XTC platforms, part of the DTN-X series. First network deployments will occur in the second half of this year.
Infinera is also selling the ICE6 5×7-inch module to interested parties.
XR Optics
Infinera is not addressing the 400ZR coherent pluggable module market. The 400ZR is the OIF-defined 400-gigabit coherent standard developed to connect equipment in data centres up to 120km apart.
Infinera is, however, eyeing the emerging ZR+ opportunity using XR Optics. ZR+ is not a standard but it extends the features of 400ZR.
XR Optics is the brainchild of Infinera that is based on coherent sub-carriers. All the sub-carriers can be sent to the same destination for point-to-point links, but they can also be sent to different locations to allow for point-to-multipoint communications. Such an arrangement allows for traffic aggregation.
“You can steer all the sub-carriers coming out of an XR transceiver to the same destination to get a 400-gigabit point-to-point link to compete with ZR+,” says Shore. “And because we are using sub-carriers instead of a single carrier, we expect to get significantly better performance.”
Infinera is developing the coherent DSPs for XR Optics and has teamed up with optical module makers, Lumentum and II-VI.
Other unnamed partners have joined Infinera to bring the technology to market. Shore says that the partners include network operators that have contributed to the technology’s development.
Infinera planned to showcase XR Optics at the OFC conference and exhibition held recently in San Diego.
Shore says to expect XR Optics announcements in late summer, from Infinera and perhaps others. These will detail the XR Optics form factors and how they function as well as the products’ schedules.
Infinera rethinks aggregation with slices of light
An optical architecture for traffic aggregation that promises to deliver networking benefits and cost savings was unveiled by Infinera at this week’s ECOC show, held in Dublin.
Traffic aggregation is used widely in the network for applications such as fixed broadband, cellular networks, fibre-deep cable networks and business services.
Infinera has developed a class of optics, dubbed XR optics, that fits into pluggable modules for traffic aggregation. And while the company is focussing on the network edge for applications such as 5G, the technology could also be used in the data centre.
Optics is inherently a point-to-point communications technology, says Infinera. Yet optics is applied to traffic aggregation, a point-to-multipoint architecture, and that results in inefficiencies.
“The breakthrough here is that, for the first time in optics’ history, we have been able to make optics work to match the needs of an aggregation network,” says Dave Welch, founder and chief innovation officer at Infinera.
Infinera proposes coherent sub-carriers for a new class of problem
XR Optics
Infinera came up with the ‘XR’ label after borrowing from the naming scheme used for 400ZR, the 400-gigabit pluggable optics coherent standard.
“XR can do point-to-point like ZR optics,” says Welch. “But XR allows you to go beyond, to point-to-multipoint; ‘X’ being an ill-defined variable as to exactly how you want to set up your network.”
XR optics uses coherent technology and Nyquist sub-carriers. Instead of using a laser to generate a single carrier, pulse-shaping is used at the transmitter to generate multiple carriers, referred to as Nyquist sub-carriers.
The sub-carriers convey the same information as a single carrier but by using several sub-carriers, a lower symbol rate can be used for each. The lower symbol rate improves the tolerance to non-linear effects in a fibre and enables the use of lower-speed electronics.
Infinera first detailed Nyquist sub-carriers as part of its advanced coherent toolkit, and implemented the technology with its Infinite Capacity Engine 4 (ICE4) used for optical transport.
The company is bringing to market its second-generation Nyquist sub-carrier design with its ICE6 technology that supports 800-gigabit wavelengths.
Now Infinera is proposing coherent sub-carriers for a new class of problem: traffic aggregation. But XR optics will need backing and be multi-sourced if it is to be adopted widely.
Network operators will also need to be convinced of the technology’s merits. Infinera claims XR optics will halve the pluggable modules needed for aggregation and remove the need for intermediate digital aggregation platforms, reducing networking costs by 70 percent.
Aggregation optics
XR optics will be required at both ends of a link. The modules will need to understand a protocol that tells them the nature of the sub-carriers to use: their baud rate (and resulting spectral width) and modulation scheme.
Infinera cites as the example a 4GHz-wide sub-carrier modulated using 16-ary quadrature amplitude modulation (16-QAM) that can transmit 25-gigabit of data.
A larger capacity XR coherent module will be used at the aggregation hub and will talk directly with XR modules at the network edge, “casting out” its sub-carriers to the various pluggable modules at the network edge.
For example, the module at the hub may be a 400-gigabit QSFP-DD supporting 16, 25-gigabit sub-carriers, or an 800-gigabit QSFP-DD or OSFP module delivering 32 sub-carriers. A mix of lower-speed XR modules will be used at the edge: 100-gigabit QSFP28 XR modules based on four sub-carriers and single sub-carrier 25-gigabit SFP28s.
“As soon as you have defined that each one of these transceivers is some multiple of that 25-gigabit sub-carrier, they can all talk to each other,” says Welch.
The hub XR module and network-edge modules are linked using optical splitters such that all the sub-channels sent by the hub XR module are seen by each of the edge modules. The hub in effect broadcasts its sub-carriers to all the edge devices, says Welch.
A coding scheme is used such that each edge module’s coherent receiver can pick off its assigned sub-channel(s). In turn, an edge module will send its data using the same frequencies on a separate fibre.
Basing the communications on multiples of sub-carriers means any XR module can talk to any other, irrespective of their overall speeds.
Sub-carriers can also be reassigned.
“In that fashion, today you are a 25-gigabit client module and tomorrow you are 100-gigabit,” says Welch. Reassigning edge-module capacities will not happen often but when undertaken, no truck roll will be needed.
System benefits
In a conventional aggregation network, the edge transceivers send traffic to an intermediate electrical aggregation switch. The switch’s line-side-facing transceivers then send on the aggregated traffic to the hub.
Using XR optics, the intermediate aggregation switch becomes redundant since the higher-capacity XR coherent module aggregates the traffic from the edge. Removing the switch and its one-to-one edge-facing transceivers account for the halving of the overall transceiver count and the overall 70 percent network cost saving (see diagram below).
The disadvantage of getting rid of the intermediate aggregation switch is minor in comparison to the plusses, says Infinera.
“In a network where all the traffic is going left to right, there is always an economic gain,” says Welch. And while a layer-2 aggregation switch enables statistical multiplexing to be applied to the traffic, it is insignificant when compared to the cost-savings XR optics brings, he says.
Challenges
XR transceivers will need to support sub-carriers and coherent signal processing as well as the language that defines the sub-carriers and their assignment codes. Accordingly, module makers will need to make a new class of XR pluggable modules.
“We are working with others,” says Welch. “The object is to bring the technology and a broad-base supply chain to the market.” The fastest way to achieve this, says Welch, is through a series of multi-source agreements (MSAs). Arista Networks and Lumentum were both quoted as part of Infinera’s XR Optics press release.
Another challenge is that a family of coherent digital signal processors (DSPs) will need to be designed that fit within the power constraints of the various slim client-side pluggable form factors.
Infinera stresses it is unveiling a technological development and not a product announcement. That will come later.
However, Welch says that XR optics will support a reach of hundreds of kilometres and even metro-regional distances of over 1,000km.
“We are comfortable we are working with partners to get this out,” says Welch. “We are comfortable we have some key technologies that will enhance these capabilities as well.”
Other applications
Infinera’s is focussing its XR optics on applications such as 5G. But it says the technology will benefit many network applications.
“If you look at the architecture in the data centre or look are core networks, they are all aggregation networks of one flavour or another,” says Welch. “Any type of power, cost, and operational savings of this magnitude should be evaluated across the board on all networks.”