Transport networks have witnessed two significant trends over the past half-decade or so. The first has been an explosion in the bandwidth these networks can support and the distances over which they can support it. This is due to the advent of cost-effective wavelength division multiplexing (WDM) and dense-WDM (DWDM), as well as a slew of technologies that extend transmission range, such as sophisticated optical amplifiers. The second has been the need to support a variety of traffic types (voice, video, data) and services: virtual private networks (VPNs), high-speed Internet (HSI), video-on-demand (VoD) and videoconferencing, and IPTV, to name a few. This is due to the need to simplify the network by collapsing intermediate layers and protocol stacks, thus reducing interface and node counts (and, hence, cost) in the carrier network. Thus, transport networks have migrated from being primarily voice-dominated to multi-service supporting infrastructures.
In the past, the optical transport networks themselves did not need to be service- or traffic-aware, as there were a number of layers of multiplexing and aggregation between the carried traffic and the actual transport “pipes.” Indeed a typical protocol-stack layering might take IP data, encapsulate it in Ethernet frames, segment and package those into ATM cells that would be packaged into SONET/SDH frames, which would then ride on an optical wavelength. By contrast, the move today is increasingly towards an optimized stack, which consists of IP data encapsulated in Ethernet frames that (with appropriate framing) ride directly on an optical wavelength — the so-called “optical Ethernet” solution.
Advances in Optical Layer and Network Equipment
So, what are the advances that are making this possible, especially in metro networks? To understand this, we will briefly look both at the optical-layer advances and the network-equipment advances, which constitute some of the keys to optical Ethernet network design.
The fundamental optical layer advances have been the enhancement of WDM technologies with the advent of: Erbium-Doped Fiber Amplifiers (EDFAs), Arrayed Waveguide Gratings (AWGs), and Reconfigurable Optical Add-Drop Multipliexers (or ROADMs). EDFAs enable multiple optical signals, on different wavelengths, to be amplified simultaneously, without requiring expensive conversion into the electronic domain. AWGs, on the other hand, act as an optical filter, and provide a simple mechanism to insert/multiplex and extract/de-multipex optical signals to/from a fiber. In more recent years, ROADMs based on either the wavelength blocker or wavelength selectable switch (WSS) sub-systems have been deployed. These allow any possible wavelength, or a combination of wavelengths, to be added or dropped at a node, thereby allowing providers the flexibility to reconfigure their networks based on traffic needs, leading to true agile optical networks.
In the network-equipment domain, the main advances have been the development of next-generation systems that can support SONET/SDH (TDM data) and IP/Ethernet (packet data).
Legacy networks were built using the TDM paradigm of SONET/SDH, which served as an excellent way to groom voice-dominated traffic and then provision aggregated traffic trunks over the fiber, providing excellent reliability and availability. With the growing dominance of data traffic, SONET/SDH, with its need for synchronization and its limited ability to support flexible bandwidth increments, became increasingly inefficient at meeting the needs of data communications and, hence, a cost barrier. Ethernet, which was already dominant in the LAN, was proposed as a migratory technology, moving to the WAN, in the now quite popular IEEE 802.3z and IEEE 802.3ae standards for 1 Gbps and 10 Gbps speeds, respectively. The less-stringent timing needs of Ethernet made it a lower-cost alternative to SONET/SDH for data services.