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High-capacity optical communications is a key technology advance that is driving network transformation in the Core and Long-haul networks. Thanks to advances in this area, a single optical fiber strand is now capable of carrying tens of terabits of traffic today through modern techniques such as dense lightwave multiplexing (DWDM), optical amplification, reconfigurable optical add-drop multiplexing (ROADM) and coherent optical processing. Without these new technologies, the telecommunications networks of today cannot be scaled to deliver thousands of services to billions of subscribers or create high-capacity information super-highways at the lowest cost per bit.
DWDM enables multiple optical wavelengths to be “multiplexed” (combined) on a single fiber strand and thereby conserves expensive fiber reserves. While initial DWDM systems were typically limited to carrying 2.5Gbps of bandwidth per wavelength, recent advances in coherent DWDM technology can deliver 100G and 100G+ (greater-than-100G) bit rates per wavelength over longer distances at the lowest cost per bit. Coherent DWDM achieves these gains primarily through the use of superior modulation formats that make use of amplitude, phase and polarization of light waves and better compensation for chromatic dispersion and polarization mode dispersion (CD, PMD) through sophisticated digital signal processors. Coherent DWDM systems also support advanced Forward Error Correction (FEC) mechanisms to reduce regeneration requirements and expand transmission reach over both newly laid and old fiber installations.
Another telecom technology that has evolved in recent times for use in high-capacity DWDM networks is the Optical Transport Network (OTN). Although OTN as an ITU standard (G.709) has been available for over a decade, it was originally designed to provide a protocol-independent wrapper of client data. In recent times, OTN is being used to perform primarily two additional functions: a) Efficient Sub-Lambda Grooming: DWDM layer is capable of handling traffic at a wavelength layer. Since DWDM by itself does not offer any way to consolidate partially filled wavelengths (e.g., loosely filled 100Gbps wavelengths) this leads to a significant bandwidth wastage. OTN solves this problem by providing sub lambda grooming through an OTN cross-connect. b) Bypass of Transit Traffic: On a cost per Gbps of switching capacity, routers tend to be much more expensive than OTN fabrics. By substituting OTN for a significant part of the switching capacity (the transit traffic), operators can reduce the overall costs significantly and save expensive router ports to reduce overall capex. Also, OTN fabrics consume less power compared to routing fabrics and traditional TDM cross-connects used to groom traffic at lower speeds such as 2 Mbps.
The current state-of-the-art 100G/100G+ DWDM systems such as from Tejas are designed to transmit up to 80 wavelength channels each having a capacity of up to 400 Gbps. Tejas DWDM platform is highly flexible and supports a programmable mix of SDH/SONET, Ethernet, OTN, Storage and MPLS-TP client services using a combination of transponders, muxponders and switching cards. Advanced network functions such as multi-degree colorless/directionless/contentionless optical switching (CDC ROADM), universal terabit-scale OTN/PTN cross-connects (DXC) and generalized MPLS protocols (GMPLS) for efficient automated switching at the wavelength layer enable the system to optimally pack service traffic over fewer wavelengths and engineer a highly cost-effective solution in high-bandwidth DWDM networks.