Briefing: DWDM developments

Part 1

Kevin Smith: “A lot of the work we are doing with the trials have demonstrated we can scale our networks gracefully rather than there being a brick wall of a problem.”

BT is confident that its core network will accommodate the expected IP traffic growth for the next decade. Traffic in BT’s core is growing at between 35 and 40 percent annually, compared to the global average growth rate of 20 to 30 percent. BT attributes the higher growth to the rollout of fibre-based broadband across the UK.

The telco is deploying 100-gigabit wavelengths in high-traffic areas of its network. “These are key sites where we're running out of wavelengths such that we need to implement higher-speed ones,” says Kevin Smith, research leader for BT’s transport networks. The operator is now trialling 200-gigabit wavelengths using polarisation multiplexing, 16-quadrature amplitude modulation (PM-16QAM).

Adopting higher-order modulation increases capacity and spectral efficiency but at the expense of a loss in system performance which can be significant. 

Systems vendors use polarisation-multiplexed, quadrature phase-shift keying (PM-QPSK) for 100-gigabit wavelengths. Moving to PM-16QAM doubles the bits on the wavelength but the received data has less tolerance to noise. The result is a 6-decibel loss compared to PM-QPSK, such that the transmission distance drops to a quarter. If PM-QPSK spans a 4,000km link, using PM-16QAM the reach on the same link is only 1,000km. 

The transmitted capacity can also be increased by using pulse-shaping at the transmitter to cram a wavelength into a narrower channel. BT’s existing optical network uses fixed 50GHz-wide channels. But in a recent network trial with Huawei, a 3 terabit super-channel was transmitted over a 360km link using a flexible grid. 

The super-channel comprised 15 channels, each carrying 200 gigabit using PM-16QAM. Using the flexible grid, each carrier occupied a 33.5GHz channel, increasing fibre capacity by a factor of 1.5 compared to a 50GHz fixed-grid. “For 16-QAM, it [33.5GHz] is pretty close to the limit,” says Smith.  

Increasing the baud rate is the most structurally-efficient way to accommodate the high speed 

Another way to boost the carrier’s data as well as reduce system cost is to up the signalling rate. Current optical transport systems use a 30Gbaud symbol rate. Here, two carriers each using PM-16QAM are needed to deliver 400 gigabit. Doubling the symbol rate to 60Gbaud enables a single 400 gigabit wavelength. Doubling the baud rate also halves a platform’s transponder count, reducing the overall cost-per-bit, and increases platform density.  

“Increasing the baud rate is the most structurally-efficient way to accommodate the high speed,” says Smith.  Going to 16QAM increases the data that is carried but at the expense of reach. By increasing the baud rate, reach can be extended while also keeping the modulation rate at a lower level, he says.  

BT says it is seeing signs of such ‘flexrate’ transponders that can adapt modulation format and baud rate. “This is a very interesting area we can mine,” says Smith. The fundamental driver is about reducing cost but also giving BT more flexibility in its network, he says.

Traffic growth

Coping with traffic growth is a constant challenge, says BT.

“I’m not worried about a capacity crunch,” says Smith. “A lot of the work we are doing with the trials have demonstrated we can scale our networks gracefully rather than there being a brick wall of a problem.”

The operator is confident that 25 to 30 terabit of traffic can be squeezed into the C-band using flexgrid and narrower bands. Beyond that, BT says broadening the spectral window using additional spectral bands such as the L-band could boost a fibre’s capacity to 100 terabit. Vendors are already looking at extending the spectral window, says BT.

Sliceable transponders 

BT is also part of longer-term research exploring an extension to the ‘flexrate' transponder, dubbed the sliceable bit rate variable transponder (S-BVT). 

“It is very much early days but the idea is to put multiple modulators on the same big super transponder so that it can kick out super-channels that can be provisioned on demand,” says Andrew Lord, head of optical research at BT.  

The large multi-terabit super-channel would be sent out and sliced further down the network by flexible grid wavelength-selective switches such that parts of the super-channel would end up at different destinations. “You don’t need all that capacity to go to one other node but you might need it to go to multiple nodes,” says Lord.

Such a sliceable transponder promises several benefits. One is an ability to keep repartitioning the multi-terabit slice based on demand. “It is a good thing if we see that kind of dynamics happening, but not fast dynamics,” says Lord. The repartitioning would more likely be occasional, adding extra capacity between nodes based on demand. Accordingly, the sliced multi-terabit super-channel would end up at fewer destinations over time.

The sliceable transponder concept also promises cost reduction through greater component integration.

BT stresses this is still early research but such a transponder could end up in the network in five years’ time.   

Space-division multiplexing 

Another research area that promises to increase significantly the overall capacity of a fibre is space-division multiplexing (SDM). 

SDM promises to boost the capacity by a factor of between 10 and 100 through the adoption of parallel transmission paths. The simplest way to create such parallel paths is to bundle several standard single-mode fibres in a cable. But speciality fibre could also be used, either multi-core or multi-mode.

BT says it is not researching spatial multiplexing.

”I’m very much more interested in how we use the fibre we have already got,” says Lord. The priority is pushing channels together as close as possible and getting the 25 terabit figure higher, as well as exploring the L-band. “That is a much more practical way to go forward,” says Lord.

However, BT welcomes the research into SDM. “What it [SDM] is pushing into the industry is a knowledge about how to do integration and the expertise that comes out of that is still really valid,” says Lord. “As it is, I don’t see how it fits.”