OPTICAL TRANSPORT NETWORK (OTN) Today, Network operators are forced to converge service provision various application through their networks in order to reduce Operational Expenses (OPEX), and also to eliminate unnecessary additional Capital Expenditures (CAPEX) on multiple parallel networks. Using OTN (Optical Transport Network), multiple networks and services such as legacy SONET/SDH, Ethernet, Storage protocols and Video can all be combined onto a common infrastructure.
Introduction All network operators are attempting to solve the convergence problem of providing the number of services on common network infrastructure. In order to drive faster returns on investments, traditional telecom carriers are trying to integrate residential broadband services, advanced enterprise data connectivity services and video into existing voice networks; cable operators are trying to add VoIP and commercial services to video and cable modem services. At the same time, there is a massive migration from traditional SONET / SDH - and PDH-based TDM services to IP / Ethernet based services, including Voice, Video, Internet Access and Layer-2 Business Services. Operators are facing challenges in converging network required in providing these services while minimizing risk, and investment on additional infrastructure. Optical transport Network provide solution to the challenge of converging parallel network which would otherwise be required in providing different services through traditional and legacy systems. Using OTN, multiple networks and services such as legacy SONET/SDH, Ethernet, Storage protocols and Video can all be combined onto a common infrastructure as shown in Fig. 1.
Figure 1 Multi-service aggregation using OTN
Most importantly, unlike SONET/SDH - the existing carrier, OTN is the only transport layer in the industry that can carry a full 10 Gigabit Ethernet (GbE) LAN PHY from IP / Ethernet switches and routers at full bandwidth. With the rapid migration towards IP/Ethernet-based infrastructure, OTN becomes the transport layer of choice for network operators. OTN offers transparent wavelength and sub-wavelength services transport and add / drop capabilities that provide service bit-stream transparency, as well as service timing transparency.
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ITU-T Standard for OTN
In truth, OTN is described by a host of specifications, starting with G.872, which describes the network architecture of OTN, and including G.709, which focuses on structures, interfaces, and mapping and FEC and digital wrapper.
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VALUE OF OTN
The key to understanding OTN can be summed up as - transparency. For this reason, OTN is the ideal technology on which to build a convergence platform. OTN, or Digital Wrapper as it is sometimes called, is an ITU standard that was unified from competing standards being developed in both the ITU and ANSI.
As end-users continue to deploy increasingly complex networks using Multi-Service Provisioning Platform devices and routers, they are losing service transparency because they cannot access SONET / SDH overhead bytes or the DCC that carries all the management bytes as well as inter-nodal communications used to determine network paths and link states. A traditional SONET / SDH solution will strip out these bytes and break the end-to-end communications and topology discovery. OTN supports its own separate overhead for performance monitoring and fault signaling, as well as a General Communications Channel (GCC) for remote management, software downloads and other control functions. OTN specifications provide for a robust management overhead analogous to SONET / SDH; therefore, network operators do not have to sacrifice the ability to manage at both the payload and service levels. In fact, an OTN payload can fully encapsulate a SONET / SDH frame without terminating the SONET Data Communications Channel (DCC) so that remote Add / Drop Multiplexers (ADMs) can continue to be managed in the same manner, and topology discovery will still work between customer equipment. However, OTN does much more than transparently transport SONET / SDH. It is also highly effective in supporting asynchronous data services such as GbE, 10GbE, various speeds of Fibre Channel (FC), ESCON and FICON that do not have the same physical layer performance monitoring capabilities and fault isolation necessary for a high quality of service. OTN brings those capabilities to asynchronous services without sacrificing the qualities that make those services attractive in the first place, such as low cost and ease of installation, see Figure 2. As mentioned earlier, OTN is also the only transport layer that can carry a full 10GbE LAN PHY—the standard interface on IP/Ethernet equipment.
Figure 2 OTN for carrier-grade transport of Asynchronous services
The inherent flexibility of OTN is enabled by its ability to extend transparency to the timing plane. This ability allows the mixing of both synchronous and asynchronous signal types on a common wavelength. Moreover, synchronous services with different clock sources can be transported side-by-side, something not possible in a SONET/SDH network.
4.0 OTN Networking Layers and Frame Structure An OTN network is made up of several networking layers as shown in Figure 3. The Service layer represents the end-user services such as GbE, SONET, SDH, FC or any other protocol. For asynchronous services such as ESCON, GbE or FC, the service is passed through a Generic Framing Procedure (GFP) mapper. The Optical channel Payload Virtual Container (OPVC) handles mapping the service into a uniform format. The OPVC is the only layer that needs to change to support a new service type.
Figure 3 OTN networking layers
The Optical channel Payload Tributary Unit (OPTU) maps the output of the OPVC into a timeslot and performs timing adaptations to unify the clocking. The Optical channel Payload Unit (OPU) contains all of the timeslots in the OTN frame. The Optical channel Data Unit (ODU) provides the path-level transport functions of the OPU. The Optical Transport Unit (OTU) provides the section-level overhead for the ODU and provides the GCC bytes.
Figure 4 OTN Hierarchy
The Physical layer maps the OTU into a wavelength or WDM muxing system. OTN has a hierarchy just like SONET and SDH. An Optical Channel (OCh) runs between anything that maps a service into an OTU1/OTU2 signal, see Figure 4. An Optical Multiplex Section (OMS) is between two devices that can multiplex wavelengths onto a fiber. An Optical Transmission Section is the fiber between anything that performs an optical function on the signal i.e. between OLA and OADM or OLAs.
Figure 5 OTN Frame Structure
5.0 Benefits of OTN: The benefits of a fully realized OTN to network operators are many, and include:
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More efficient multiplexing, provisioning, and switching of high-bandwidth (2.5 Gbps and up) services, leading to improved wavelength utilization • More efficient transport and switching of non-SONET/SDH traffic, including Ethernet-based traffic, allowing a single converged optical infrastructure • Improved monitoring and management of Ethernet and other non-SONET/SDH data services • Standardized optical (OCh) and electrical (ODU) handoffs for multi-service traffic between different optical transport elements, both within and between networks, to support the operational benefits of “best of breed” multivendor networks More automated, dynamic networks, supporting applications and services not otherwise available
