http://www.epn-online.com/enCopyright 2007 - Reed Business InformationTue, 16 Mar 2010 00:41:43 +0100Tue, 16 Mar 2010 00:41:43 +0100EPN Onlinelchevalier@reedbusiness.frlchevalier@reedbusiness.frhttp://blogs.law.harvard.edu/tech/rsshttp://www.epn-online.com/page/new134484/optical-transport-networks.htmlnew134484As optical technology has advanced, instead of going to a higher rate SONET/SDH signal, it has become more economical to transmit multiple SONET/SDH signals over the same fibre using WDM (wavelength division multiplexing). The OTN architecture is specified in ITU-T Rec. G.872 with the frame format and payload mappings, for carrying SONET/SDH, Ethernet and storage area network (SAN) signals in a much more cost-effective manner than was possible over SONET/SDH networks, in G.709. OTN adoption was initially slow, with primary early deployments in Japan and among some of the European carriers. Three contributing factors to the slow initial adoption were firstly, the carrier's financial reluctance or inability to replace existing SONET/SDH networks, secondly the number of SONET/SDH-based proprietary WDM solutions had already been developed that were adequately serving the needs of many carriers and thirdly, carriers had only recently seen bandwidth demand beyond what was offered by the combination of the existing WDM equipment and the large amount of fibre deployed in the backbone networks. Although OTN adoption was initially slow, since the mid-2000s compelling reasons for OTN deployment have emerged worldwide, making it a fundamental component of carrier RFPs for optical metro network equipment. Initially, the most compelling reason to deploy OTN was for point-to-point links where the enhanced forward error correction capability, standardised for OTN, allowed longer spans of optical cable, higher data rates, or both. Today, OTN is being demanded by carriers worldwide as an entirely new network layer to transition away from SONET/SDH and enable video-ready metro optical networks for high bandwidth service delivery to subscribers over broadband access networks. As illustrated in Figure 1, OTN provides an optimum converged transport technology for transparently carrying important legacy and emerging client signals. WDM technical considerationsA number of different approaches had to be examined at the outset of the WDM standardisation work, with numerous trade-offs to be considered. Broadly speaking, the approaches fell into two categories. The first was to send the client signal essentially in its native format (with the exception of its normal wavelength) and add OAM (operations, administration and maintenance) capabilities in some type of separate channel. However, this has some serious disadvantages as the OAM channel won't necessarily experience the same impairments as the client signal channel and it is possible for provisioning errors to properly connect the OAM signal, but not the client signal channel. Another option was to use sub-carrier modulation to create the OAM channel, but the concern was that this approach would be too complex, especially in its impact on jitter performance. The second approach, referred to as a digital wrapper, was to treat the client signal as a digital payload signal and encapsulate it into a frame structure that included channel-associated OAM overhead channels. This approach is conceptually similar to SONET/SDH. In the end, a hybrid approach was chosen. The digital wrapper approach was chosen for the basic encapsulation and channel-associated OAM overhead for the client signals. Once this digital signal is transmitted over a wavelength, additional overhead wavelengths are assigned to carry other optical network overhead.WDM network architectureThe basic signal architecture is illustrated in Figure 2 below. The client signal is inserted into the frame payload area, which, together with some overhead channels, becomes the optical payload unit. This is conceptually similar to a SONET/SDH path. OAM overhead is then added to the optical payload unit to create the optical data unit, which is functionally analogous to the SONET line (SDH multiplex section). Transport overhead (e.g., frame alignment overhead) is then added to create an optical transport unit, which is the fully formatted digital signal and functionally analogous to the SONET section (SDH regenerator section). The optical transport unit is then transmitted on a wavelength, which constitutes the optical channel. An optical multiplexed section consists of a wavelength division multiplexed group of optical channels, together with a separate wavelength carrying an overhead optical supervisory channel that is carried between access points. The optical transport section (of order n) consists of an optical multiplexed section (of order n) and an overhead channel (on its own wavelength). The optical transport section defines the optical parameters associated with the physical interface. The optical channel, optical multiplexed section and optical transport section overhead channels provide the means to assess the transmission channel quality, including defect detection for that layer. The optical channel and optical transport section overhead also provide a means for connectivity verification. Figure 3 shows an example OTN with the different layers and their relative scope. The IrDI is the inter-domain interface and is specified as having 3R regenerator processing at both sides of the interface. The IrDI is the interface that is used between different carriers, and can also be useful as the interface between equipment from different vendors within the same carrier's domain. Since the IrDI is the interface for interworking, it was the focus of the initial standard development. The IaDI is the intra-domain interface that is used within a carrier's domain. Since the IaDI is typically between equipment of the same vendor, it can potentially have proprietary features added such as a more powerful FEC.Having adopted a digital wrapper approach, the ITU-T's next choice focused on what client signals to allow, as this approach clearly restricts the clients to being digital signals in that analogue and digital signals have very different channel requirements. A channel that may be very adequate for a digital signal can have an unacceptably low SNR or too much distortion for an analogue signal, making it difficult to deploy mixed analogue/digital networks in a DWDM environment. The next decision was what types of digital signals to include. Originally, there was a strong desire to carry optical data interfaces such as Gbit/s and 10Gbit/s Ethernet in addition to SONET/SDH signals. The decision was made to limit the constant bit rate clients to the SONET/SDH signals. The assumption was made that other signals could be mapped into SONET/SDH first, with these SONET/SDH signals being mapped into the OTN. This decision not to directly support native Ethernet clients, while potentially simplifying the frame structures, has proved to be a significant handicap to wide-scale deployment of G.709 OTNs.OTN equipmentThere are several types of OTN equipment deployed, based on OTN standards including OTN terminal equipment, OADMs (optical add, drop multiplexers) and OXCs (optical cross connects). OTN terminal equipment is used for point-to-point connections through WDM networks mapping the client signals into OPUs, sometimes multiplexing multiple signals in the electrical domain, and finally performing mapping/multiplexing in the optical domain. OADMs, OXCs and some type of regenerators primarily process the OTN signals in the optical domain. More recently, reconfigurable OADMs have become popular. The key building blocks of today's reconfigurable OADM node can be categorised into three primary functions: wavelength add or drop filters or switches, dynamic power control and remote monitoring capabilities at the optical layer, and optical service channel termination and generation.Next-generation reconfigurable OADMs that add or drop client signals within the signals carried over the wavelength rather than just adding or dropping the entire wavelength typically augment classic reconfigurable OADM functionality with switching fabrics in the electrical domain (see Figure 4). This finer granularity add or drop allows aggregation or grooming for more efficient use of the wavelengths. It also allows more flexible network topologies. In some carrier networks, reconfigurable OADMs are deployed to build the network infrastructure for video signal delivery. The reconfigurable OADM performs the legacy SONET/SDH ADM functions on the wavelengths carrying SONET/SDH signals. The video signals are expected to be carried on separate wavelengths. Optical domain switching can add or drop entire video-bearing wavelengths, and the reconfigurable OADM packet switch fabric can be used to switch IPTV signals. XHEAD Accelerating the transitionThe ITU-T OTN hierarchy has many merits and advantages for DWDM systems and for OTNs. It reduces carrier operating expenses, taking advantage of the decreasing cost of optical components; provides a standards-based hierarchy to address the growing demand for bandwidth throughout the network, and provides higher rates over existing facilities. Carriers can accelerate network transition to OTN with feature integration to support OTN, Ethernet, SAN, Video and SONET/SDH in a single, low-powered chip. For example, PMC-Sierra's HyPHYevices can reduce line card variants by 75%, achieve 50% power savings and deliver multi-service aggregation flexibility to accelerate the transition to IP-optimised OTNs.Figure 1: No optical illusion - OTN provides a cost-effective means of carrying a variety of signals over WDM networks.Figure 2: Information flow for an OTN signal.Figure 3: The different layers of the OTN.Figure 4: Next generation reconfigurable OADM prepares the network for video signals.Tue, 09 Mar 2010 00:00:00 +0100