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Design Considerations for Converged Optical Ethernet Networks


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Transport networks have witnessed two significant trends over the past half decade or so. The first has been an explosion in the bandwidth that 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...more

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Design Considerations for Converged Optical Ethernet Networks

  1. 1. PTB >>Designing Converged Optical Ethernet NetworksTransport networks have witnessed two significant trends over the past half-decade or so. The first has beenan 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 opticalamplifiers. The second has been the need to support a variety of traffic types (voice, video, data) andservices: virtual private networks (VPNs), highspeed Internet (HSI), video-on-demand (VoD) andvideoconferencing, and IPTV, to name a few. This is due to the need to simplify the network by collapsingintermediate layers and protocol stacks, thus reducing interface and node counts (and, hence, cost) in thecarrier network. Thus, transport networks have migrated from being primarily voicedominated to multi-service supporting infrastructures.In the past, the optical transport networks themselves did not need to be service- or traffic-aware, as therewere 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 Ethernetframes, 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 anoptimized 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.
  2. 2. Advances in Optical Layer and Network EquipmentSo, 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, whichconstitute some of the keys to optical Ethernet network design. The fundamental optical layer advanceshave 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 (orROADMs). EDFAs enable multiple optical signals, on different wavelengths, to be amplifiedsimultaneously, without requiring expensive conversion into the electronic domain. AWGs, on the otherhand, act as an optical filter, and provide a simple mechanism to insert/multiplex and extract/demultipexoptical signals to/from a fiber. In more recent years, ROADMs based on either the wavelength blocker orwavelength selectable switch (WSS) sub-systems have been deployed. These allow any possiblewavelength, or a combination of wavelengths, to be added or dropped at a node, thereby allowing providersthe 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-generationsystems that can support SONET/SDH (TDM data) and IP/Ethernet (packet data). Legacy networks werebuilt 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 reliabilityand availability. With the growing dominance of data traffic, SONET/SDH, with its need forsynchronization and its limited ability to support flexible bandwidth increments, became increasinglyinefficient 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 theWAN, 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/SDHfor data services. In the last year and a half or so, however, there has been rapid rise both in video services(in the form of streaming video, video conferencing, as well as IPTV) as well as in enterprises wanting
  3. 3. Ethernet pipes with flexible bandwidths to connect into their WANs. This has posed a technical challengeprimarily because traditional Ethernet does not have the deterministic qualities of SONET/SDH in terms ofreliability and availability. Extensive work is underfoot in the Metro Ethernet Forum (MEF) to imbueEthernet with great protection and management (OAM) capabilities, and within the IEEE, in the 802.1QayWG, to develop Provider-Backbone Bridging-Traffic Engineering, which involves giving Ethernetnetworks the ability to set up managed, traffic-engineered paths. This is achieved by turning off the MAClearning capability of Ethernet, and, instead, programming (using management) the forwarding tables atevery node, thus precisely controlling the path taken by different flows through the network. Measures suchas these strengthen the “carrier- class” capabilities of Ethernet.The simultaneous existence of Ethernet and SONET/SDH services over fiber networks has meant thatplatforms are now being deployed that cater to a broad mix of these services. These systems, which cancater to a broad range of client-side technologies ranging from Ethernet to SONET/SDH to Fiber Channeland transport these over high-speed WDM networks, are commonly known as Multi- Service ProvisioningPlatforms (MSPPs). When conjoined with WDM nodes in a single box/rack, they are also known as Multi-Service Transport Platforms (MSTPs).Need for QoS and Dynamic ProvisioningWhile ROADMs and MSPPs are ideal platforms to support dynamic configuration of the network to meettraffic needs, there is a need to control, provision, and manage the optical networks in a systematic,automated way. In addition, there is also a strong requirement to meet customer expectations in terms of“Quality of Experience” by deploying mechanisms for establishing and enforcing end-to-endQuality-of-Service (QoS). This has led to the adoption of a suite of protocols, called the Generalized MPLSprotocols, developed by the IETF (Internet Engineering Task Force) that is used to control opticalnetworks, both at the physical layer and for traffic management and service provisioning. The GMPLSsuite of protocols includes routing protocols that are used to discover network topology and availableresources (bandwidth, timeslots, and wavelengths) in the optical Ethernet transport network, and signalingprotocols that are used to signal the setting up of active (and backup) paths through the network.The GMPLS protocols can set up “paths” that are comprised of a sequence of wavelengths, time-slots, or packets/frames that share a common characteristic (such as being headed to the same destination orbelonging to the same Class-of-Service). From a cost perspective, the primary cost is in the electronicpacket engine that aggregates multiple lower-rate signals into a single high-speed electronic signal.Typically this high-speed electronic signal is then translated into an ITU-grid optical frequency(wavelength) by a sub-system called the transponder. A critical timing and cost-optimization challenge is inplacing multiple lowerrate signals as client interfaces in the same subsystem that also houses the ITU-sideoptics. Integration of the electronic packet multiplexer (using Ethernet technologies) with the opticaltransponder constitute the main challenge for providers in meeting the paradigm of dynamic bandwidthprovisioning, especially for their small- and midsized customers that dominate much of metro core andmetro access/collector markets.
  4. 4. Evolution at the Optical Layer and Ethernet LayerROADMs form the central feature of metro optical networks, the largest business case for optical transport,with three generations of architecture: the Fixed OADM (FOADMs), the Reconfigurable OADM(ROADMs, contemporary) and the Dynamic OADM (DOADM). FOADMs allow dropping and adding ofwavelengths at a node with the constraint that only a fixed set of wavelengths can be dropped (limitingdynamism) and the ports from which the wavelengths are dropped (or added) are also fixed. In ROADMsthere is flexibility in terms of which wavelengths can be dropped and which can pass-through (calledoptical bypass), but there is a restriction on the mapping between wavelengths and ports.The DOADM is considered the ultimate in terms of flexibility, and allows dropping/adding of anywavelength at any port in a node thus allowing full flexibility in the network and thereby reducingoperational expenditure (e.g. maintaining a smaller inventory of transponders). At the same time,developments in Ethernet OAM standards (e.g. IEEE 802.1ag and IEEE 802.3ah) allow for performancemonitoring and maintenance of end-to-end and Ethernet circuits and each Ethernet hop, respectively, bykeeping track of parameters such as transmitted/dropped frames, frame delay, jitter and loss, andavailability. These allow operators to perform diagnostics, manage their networks, and deliver serviceassurance.Looking to the FutureOptical transport is poised to enter a new age. The rise of carrier-class Ethernet along with an acute needfor bandwidth intensive services (such as video streaming) implies that future optical networks must beable to dynamically allocate bandwidth (ondemand) to nodes, support good optical- layer multicasting, andprovide for lower-cost solutions that can be implemented in smaller networks in the metro access as well asenterprise markets. This growth has forced research in higher-speed solutions — such as 40 GbpsSONET/SDH and 100 Gbps Ethernet (100GigE). Both of these technologies have significant physical layerissues and impairments such as modulation format, timing issues (e.g. pulse width of 10 picoseconds in100GigE), dispersion compensation, and OSNR monitoring.In the metro, the dynamism requires that newer solutions would have to use high-speed algorithms forbandwidth provisioning as well as architectures that can support dynamic allocations. There are principallythree schools of thought emerging for design of metro optical Ethernet transport: 1. fully electronicgrooming solutions, such as all-Ethernet packet transport and the Provider Backbone Bridging — TrafficEngineering (PBB-TE), IEEE 802.1Qay, initiative; 2. all-optical grooming solutions, using interleavedaccess such as burst switching or wavelength buses called light-trails; 3. digital optical networks usingphotonic integrated circuits (PICs) based on Indium Phosphide technology. Approach (1) is evolutionaryfrom a technology perspective but has several capital requirements at high-speeds. Approach (2) issomething of a paradigm shift and can be done in incremental steps, with partial electronics and partialoptics, e.g. light-trails. Finally, approach (3) is somewhat revolutionary and requires complete revampingof existing optical networks, but it does have the potential to deploy the System-On-Chip (SOC) concept,thereby drastically reducing growth costs.This article was written by Dr. Vishal Sharma, Principal Consultant & Technologist, Metanoia, Inc.(Mountain View, CA) and Dr. Ashwin Gumaste, Assistant Professor, Dept. of Computer Science and Eng.,IIT (Bombay, Mumbai, India). For more information, contact Dr. Sharma at , Dr.Gumaste at , or visit