Optical Transport Network Tutorial

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Optical Transport Network Tutorial The amount of data traffic relative to voice traffic on optical networks and the total traffic volume keeps increasing.1 These factors are the drivers behind emerging, flexible technologies to supplement the mature, voice optimized, SONET/SDH transport infrastructure and help manage network complexity. At the edge of the network, where data and voice combine in a common infrastructure, new data-centric applications have emerged. An example is the combination of virtual concatenation (VCAT), which provides flexible bandwidth groupings for SONET/SDH, Link Capacity Adjustment Scheme (LCAS), which provides dynamic bandwidth settings, and Generic Framing Procedures (GFP), which provides a protocol agnostic frame container2, 3, 4. In the transport core, bandwidth requirements spawned the creation of the Optical Transport Network (OTN) described in general terms in ITU-T G.8725. ITU-T G.709 provides the network interface definitions.6 G.709 improves transport network performance and facilitates the evolution to higher backbone bandwidths. The G.709 OTN frame includes transport overhead that provides operation, administration, and maintenance capabilities, and Forward Error Correction (FEC). The FEC helps reduce the number of transmission errors on noisy links, which enables the deployment of longer optical spans. This tutorial focuses on the digital applications of G.709. Interfaces and Payload G.709 defines standard interfaces and rates. These rates have been derived from the existing SONET/SDH rates where the G.709 overhead and FEC information have been taken into account. The resulting interfaces thus operate at line rates, roughly 7% higher, than the corresponding SONET/SDH that becomes the OTN payload. Table 1 lists the G.709 line rates and the matching SONET/SDH interfaces. An additional interface type, which is not part of the G.709 recommendation, applies to 10 Gigabit Ethernet LAN clients. In this case, the same overhead structure and FEC is applied resulting in a line rate of 11.095Gbps. G.709 Interface Line Rate Corresponding SONET/SDH Rate Line Rate OTU-1 2.666 Gbps OC-48/STM-16 2.488 Gbps OTU-2 10.709 Gbps OC-192/STM-64 9.953 Gbps OTU-3 43.018 Gbps OC-768/STM- 256 39.813 Gbps Table 1 Figure 1 illustrates the three parts that constitute the G.709 OTN frame; namely the overhead, the payload, and the FEC. There is no direct correlation between the size of a G.709 frame and that of a SONET/SDH frame. As an example, transmitting a single OC-192 frame takes about eleven OTU-2 frames. The SONET/SDH payload clients, also referred to as constant bit rate (CBR), are accompanied by stuff bytes in the amount of 64 and 128 per frame in the case of OTU-2 and OTU-3 respectively. These additional bytes leave room to support the multiplexing of multiple G.709 lower transmission rate signals. For example, four individual OTU-1 signals may be combined, with their FEC stripped and minor overhead modifications, and interleaved into the payload of an OTU-2 frame. The OTN is protocol agnostic as it supports payload types other than SONET/SDH and multiplexed G.709 signals. More specifically, native data protocols such as Asynchronous Transfer Mode (ATM), and Generic Framing Procedures (GFP) can be mapped directly into the payload area of the G.709 frame. Though the initial development efforts focus on SONET/SDH clients, G.709 also considers the management of optical wavelengths for wave-division multiplexing (WDM). At present, simple wavelength identification can be provided in the existing G.709 overhead. The implementation of additional dedicated overhead for optical channels is for future standardization. Figure 1 Forward error correction FEC is one of the main justifications behind G.709. It uses a Reed-Solomon (RS) code to produce redundant information that gets concatenated with the signal to be transmitted. This additional information is used on the receive interface to help identify and correct transmission errors. The RS encoding was chosen because of its low complexity, relatively high error correction capability and low error burst sensitivity. The G.709 FEC separates the frame data into 16 data streams, where up to 8 errored bytes can be corrected per stream. Figure 2 illustrates this process where each row is split into sub-rows. The protocol uses one overhead byte and 238 data bytes to compute 16 parity bytes to form 255 byte blocks—the RS(255,239) algorithm. Two key advantages are achieved by interleaving the information. First, the encoding rate of each stream is reduced relative to the line transmission rate and second, it reduces the sensitivity to bursts of error. The interleaving combined with the inherent correction strength of the RS(255,239) algorithm enables the correction of transmission bursts of up to 128 consecutive errored bytes. As a result, G.709’s contiguous burst error correcting capability is enhanced 16 times above the capacity of the RS(255,239) algorithm. Columns: 1 15 17 3825 4080 Rows: 1 2 3 4 ODU OH OTU OH Framing OAM Overhead Payload OTU FEC O P U O H Figure 2 Figure 3 illustrates the coding gain achieved with FEC relative to the Bit Error Rate (BER). At high BER, the effectiveness of the FEC decreases relative to a signal without FEC. This is quantified using the coding gain, which is described as the power decrease required to maintain the same BER as that achieved without FEC encoding. At low BER such as 10-12 or 1 errored bit per terabit, G.709 achieves around 5.4dB of optical coding gain7,8. In actual facts, this translates into longer optical spans using the same optical launch power. The FEC can be used as a warning tool for degrading signals since an increase in the number of corrected symbols indicates a deteriorating link. These signs can be present even before any system faults or alarms, later described, are detected. G.709 supports the option to turn FEC off in which case the FEC field is filled with zeroes. Figure 3 Sub Row 3 RS (255, 239) Info Bytes Sub Row 2 RS (255, 239) Info Bytes Columns: 1 17 3825 4080 Overhead Payload FEC Rows: 1 2 3 4 1 239 240 255 Sub Row 1 … RS (255, 239) Info Bytes 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 10-15 -38 -39 -37 -35 -36 -34 -33 G.975 RS(255,239) No FEC Optical coding gain (Avalanche Photodiode detector) 5.4dB @ 10-12 BER (Bit Error Rate) Overhead Figure 1 shows the OAM overhead and its three parts: the Optical channel Transport Unit (OTU), Optical channel Data Unit (ODU), and Optical channel Payload Unit (OPU). The OTU structure, which includes the FEC, provides supervisory functions and conditions the signal for transport between optical channel termination points where re-timing, reshaping, and regeneration takes place. The ODU provides end-to-end path supervision and supports tandem connection monitoring while the OPU supports the adaptation of client signals for transport over an optical channel. Figure 4 shows sample OTU, ODU, and OPU termination points in an OTN network. Figure 4 Figure 5 illustrates the framing bytes and the non-FEC portion of the OTU. The framing bytes are used in transmission systems to delineate G.709 frames, in other words to determine where frames start and end. There are two functionally distinct framing fields. The Frame Alignment Signal (FAS) bytes contain a static value of '0xF6F6F6282828' while the MFAS is a Multi-frame Alignment Signal. MFAS is continuously incremented frame after frame from 0 to 255. This is useful in multi-frame structures where the full meaning of a message is determined by collecting information over a fixed period covering 64 or 256 frames. The FAS and MFAS bytes are not scrambled unlike the rest of the OTU structure. The purpose of scrambling is to ensure sufficient bit state transitions for clock recovery purposes and to reduce the likeliness of FAS byte duplication. In the OTU, the Trail Trace Identifier (TTI) byte, which is part of the Section Monitoring (SM) overhead, is an example of a multi-frame signal. It contains information on network elements in the form of Source and Destination Access Point Identifiers (SAPI, DAPI). In addition, the SM has a BIP-8 field and a byte for alarm signals, which are further described in the fault and alarms section. The OTU also contains the General Communications Channel field (GCC0) which resembles the DCC (Data Communications Channel) from SONET/SDH. The GCC function is currently undefined but it will likely be used for functions such as network management9 or control plane signaling for a protocol like Generic Multi-Protocol Label Switching (G-MPLS)10. The reserved (RES) fields found throughout the overhead are set aside for future use. Client (i.e. SONET/SDH) OTU OTU ODU Figure 5 The greatest number of overhead fields is found in the ODU, illustrated in figure 6, where a notable feature are the Tandem Connection Monitoring (TCM) fields. The TCM ACT field is reserved for future use to enable the activation/deactivation of TCM channels. There are six TCMi (TCM1-6) fields defined for use by network operators for management functions. They contain elements quite similar to the SM including a TTI, BIP-8, and alarm capabilities that are discussed in the next section. The termination points of each TCMi can be defined within an operator's network, across a large public network at the user network interface points, or at protection switching points. The Path Monitoring (PM) is a structure analogous to the SM and TCMi except that its purpose is to provide connection end-to-end monitoring. It also contains a TTI, BIP-8 and alarm capabilities. The automatic protection switching and protection communication channel (APS/PCC) currently supports linear switching, as opposed to ring. Recommendations on the APS prot