FBON makes your fiber flexible
TextStart
By Zong Liangjia, Liu Ning, & Ma Teng
Legacy network channel spacing (50GHz) is proving inadequate for the optical
network needs of tomorrow. Flexible broadband optical networking enables
transmission of higher-order capacity and flexibility so that operators can stay
relevant in a hyper-competitive environment.
Surging Internet development has had a profound effect on telco networking,
especially in terms of P2P service and online video growth. By 2013, video traffic
will take up 66% of mobile Internet traffic, and 91% of the Internet traffic generated
by individual subscribers. To keep this traffic flowing, optical networks need to
enhance single-channel transmission to 40G, 100G, 400G, or even 1Tbit.
However, such rates require more bandwidth and legacy 50GHz channel spacing is
lagging behind. Flexible bandwidth optical network (FBON), a revolution in
traditional optical network architecture, is now recognized as a solution. Thanks to its
unified control and management functions, FBON can allocate different bandwidth to
signals at different rates, enhancing usage and network flexibility.
Optical network bottlenecks
A spacing of 50GHz works well if the single-channel rate is below 100G, but it
cannot meet the needs of the future. When a network transports a signal at 400G or
above, it needs to split into several lower-rate signals through inverse multiplexing.
For a 400G signal, the system splits it into four 100G signals, transported over 50GHz
spacings; this reduces spectral efficiency when compared with continuous 400G
transmission.
A future system will have to accommodate multiple transmission rates; yet existing
10G, 40G, and 100G systems generally use inflexible 50GHz spacing as a benchmark
for designing Mux/Demux, ROADM, OTU, management, and control functions.
When a system is scaled to accommodate higher-rate transmission rates such as 1Tbit,
it will require independent channel spacing, thus reducing the spectrum usage.
Most optical signals are carried on the C-band, at wavelengths ranging from 1528nm
to 1561nm. Service providers can get more wavelength resources by developing the
S-band or L-band, but at greater cost as they require new types of fibers or optical
amplifiers. Supplemental fiber is a viable alternative, yet it is not without cost in terms
of CAPEX (optical cables, optical amplifiers, and ROADM devices) and OPEX
(labor costs).
Service providers can also enhance spectral efficiency through advanced modulation
formats, but this can shorten transmission distances and degrade network performance.
Maximizing spectral efficiency has become a daunting issue for the backbone
network. FBON provides a feasible solution through optimized spectrum allocation
for traffic at different rates.
FBON opportunities
Efficient channel spacing usage
FBON enables flexible spacing on legacy fiber at 50GHz, 100GHz, and 200GHz for
100G, 400G, and 1Tbit service transmission, respectively. Assuming that each signal
consumes the same amount of C-band bandwidth, FBON can deliver 14.4Tbit
capacity, versus 9.5Tbit in a traditional fixed-grid network, a 51.6% efficiency
increase. For longer-distance transmission, 400G/1Tbit may occupy wider bandwidth,
and FBON can deliver even more efficiency.
Effective rerouting protection
When a working route fails, legacy networks generally redirect the traffic to a
fault-free route; however, this route may be overloaded due to insufficient bandwidth.
With FBON, service providers can flexibly adjust a transmitter’s modulation codes so
that signals are compressed and adapted to a narrow-bandwidth route.
Distance adaptability
Service providers have been constantly expanding their service portfolios and network
coverage. A future WDM ring network would have to accommodate different sections,
traffic volumes, and transmission paths. In legacy WDM networks, service providers
assign the same modulation format and channel spacing for each path, ensuring a
minimal optical signal-to-noise ratio (OSNR); however, this leads to a plethora of
small sections and short paths, making for a waste of system resources. FBON can
enhance transmission efficiency, as it adjusts the modulation format and bandwidth
based on traffic volume and transmission distance.
Time-based bandwidth allocation
Industrial and residential areas are typically busy while the other is dormant. FBON
can allocate bandwidth based on time-of-day and traffic volume, enhancing efficiency
and revenue for both categories. When traffic volume is low, FBON can use low-level
modulation formats and reduce the number of carriers, which means that service
providers can deactivate certain electric drivers and minimize the power to other
system components such as amplifiers.
Innovating ROADM and transponders
FBON depends on two critical technologies – flex-grid ROADM and the
bandwidth-variable optical transponder. As a key wavelength switching module
component, ROADM enables the adding or dropping of individual or multiple
wavelengths. In traditional DWDM networks, ROADM typically uses fixed
frequency grids at 50 or 100GHz. FBON, on the other hand, uses variable channel
bandwidth or frequency grid to enhance data transmission efficiency. ROADM
modules that enable dynamic wavelength adding or dropping are key for optical
multiplexer/demultiplexer wavelength flexibility, making them quite suitable for
future needs.
With bandwidth-variable optical transponders, transmitters can deliver flexible
bandwidth, while receivers can be self-adaptive. Bandwidth can be adjusted through
the number of carriers in multi-carrier technologies, modulation formats in individual
carriers, or through the bit rate. Existing multi-carrier technologies such as e-OFDM,
Nyquist-WDM, and o-OFDM, have different requirements for modems, light sources,
phase stability, and bandwidth of electric devices. In addition, high-speed FPGA,
DAC, and IQ transmission can produce various modulation codes such as QPSK,
16QAM, and e-OFDM-based QPSK and m-QAM codes. As modulation codes have
different OSNR tolerances, service providers can adjust transmission distance through
selective modulation codes.
Huawei researches flexible bandwidth optical networking in depth, yielding
innovations such as its commercial-ready flex-grid ROADM module, which consists
of a beam splitter and a flex-grid wavelength selective switching (WSS) module,
delivering dynamic wavelength adding or dropping. This module can also help
independently and flexibly adjust both channel spacing and central wavelength. With
its support of smooth evolution, the module enables 50-200GHz spacings, at 12.5GHz
increments. As a spacing below 200GHz can fully ensure wavelength efficiency for
1Tbit transmission, the module can smoothly deliver mixed transmission of 40G,
100G, 400G, and 1Tbit. In addition, the module boasts 1x9 ports, enabling ROADM
support of up to eight dimensions, which can accommodate most application
scenarios.
ROADM is trending towards colorless, directionless and contentionless (CDC)
operation, enabling both transport network flexibility and the adding or dropping of
any wavelength on an interface. The Huawei flex-grid ROADM module can fully
meet the requirements of next-gen ROADM. Service providers can easily realize
CDC operation through addition of a multicast switch (MCS) to the module, with the
latter also delivering tunable multiplexing/demultiplexing functions on its own.
Legacy modules can hardly be adapted as FBON multiplex/demultiplex signals use
varied spacing instead of fixed 50GHz. The Huawei module, on the other hand, can
deliver adding or dropping and multiplexing/demultiplexing for any wavelength,
which makes bandwidth adjustment and flexible data transmission all that much
easier.
Huawei is an active member of standardization bodies such as the ITU-T, IETF, and
OIF, and has submitted a number of contributions in the fields of OTN/DWDM and
GMPLS. Huawei has also defined both slot-width granularity (SWG) and center
frequency granularity (CFG) in G.694.1, while fully advocating joint discussions with
other working groups on topics such as network architecture, interface, and system &
network management for FBON.
Huawei is actively involved in other FBON aspects as well, including
spectrum-adjustable modules related to optical transmitter/receiver, OTN,
management & control, and FBON application scenarios. As it matures, FBON looks
set to change the existing networking and O&M model.
TextEnd