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.