Point-to-point Links WDM Optical Rings

THE ITU-T G.709 STANDARD – THE DIGITAL WRAPPER 215 Client payload Client payload OPU OH OPU ODU FEC OPU ODU OTU ODU OH OTU OH Figure 9.10 The optical channel Och overheads. data unit ODU overhead, which provides information for tandem connection monitoring, and end-to-end path supervision. Finally, the resulting ODU is encapsulated with the Och transport unit OTU overhead, which includes information for the monitoring of the signal on a section. The ODU is also encapsulated with the FEC. As in the SONETSDH frame, the OTU frame is arranged in a matrix consisting of four rows of 4080 bytes each see Figure 9.11. The data is transmitted serially row by row starting from the left of the first row. Recall that a SONETSDH frame is transmitted every 125 µsec. Higher transmission rates in SONETSDH are achieved by increasing the size of the SONETSDH frame. Unlike SONETSDH, the OTU frame size does not change as the transmission speed increases. Higher transmission rates are achieved by simply increasing the transmission rate of the OTU frame. This is a main departure from the traditional 125 µsec concept, that has been used in communication networks. Three transmission rates have been defined for the transmission of OTU frames: 2.488 Gbps, 9.95 Gbps, and 39.81 Gbps. The time to transmit an OTU frame is 48.971 µsecs when the transmission rate is 2.488 Gbps, 12.191 µsecs when the transmission rate is 9.95 Gbps, and 3.035 µsecs when the transmission rate is 39.81 Gbps.

9.3.2 Overhead Types

The OPU overhead The OPU overhead fields are located in rows 1 to 4 and columns 15 and 16. They provide information related to the client signal; that is, the data transmitted by the user. This overhead is created at the point where the client signal is originated, and it is used at 1 7 8 14 15 16 17 3824 3825. . .4080 Row Column byte 1 FAS OTU OH ODU OH Client payload O P U O H 2 3 4 FEC Figure 9.11 The format of the OTU frame. 216 WAVELENGTH ROUTING OPTICAL NETWORKS the point where the client signal is terminated. All bytes are reserved except the payload structure identifier PSI byte, located on row 4 and column 15. This field is used to transport a 256-byte message over a multi-frame see MFAS below. The first byte of this message contains the payload type PT which is used to identify the type of payload carried in the OPUU. The ODU overhead The ODU overhead fields are located on rows 2 to 4 and columns 1 to 14. It provides two important overheads: the path monitoring PM overhead, and the tandem connection monitoring TCM overhead. The ODU path monitoring overhead enables the monitoring of particular sections within the network as well as fault location in the network. The tandem connection monitoring enables signal management across multiple networks. As shown in Figure 9.12, the following fields have been defined: • RES : Reserved • TCMACT : Activationdeactivation of the TCM fields • TCMi : Tandem connection monitoring of ith connection • FTFL : Fault type and fault location reporting channel • PM : Path monitoring • EXP : Reserved for experimental purposes • GCC : General communication channel • APSPCC : Automatic protection switching and protection communication channel The path monitoring PM overhead occupies columns 10, 11, and 12 of the third row. Byte 10 carries the trail trace identifier, which is used to identify the signal from the source to the destination. This is similar to J0 in SONET. Byte 11 carries the result of the BIP-8, computed over the whole OPU and inserted two frames later. The computation of BIP-8 is described in Section 2.3.2. The tandem connection monitoring TCM overhead occupies columns 5 to 13 of the second row, and columns 1 to 9 of the third row. The TCM functionality enables a 1 13 12 11 10 9 8 7 6 5 4 3 2 RES TCM ACT TCM6 TCM5 TCM4 TCM3 TCM2 TCM1 PM EXP GCC1 APSPCC RES GCC2 FTFL 2 3 4 Row 1 ODU OH Client payload 2 3 4 1 7 8 14 15 16 17 3824 3825. . .4080 FAS OTU-OH O P U FEC 14 Figure 9.12 The ODU overhead fields. THE ITU-T G.709 STANDARD – THE DIGITAL WRAPPER 217 Carrier B Carrier A Carrier A User B User A Over several carriers Per carrier basis End-to-end Figure 9.13 An example of networking monitoring. network operator to monitor the error performance of a connection that originates and terminates within its own network, but traverses different operators. An example of such a connection is shown in Figure 9.13. FAS and OTU overhead The frame alignment signal FAS fields are located in columns 1 to 7 of the first row, as shown in Figure 9.14. FAS is carried in the first six bytes and is used by the receiving equipment to identify the beginning of the OTU frame. The FAS value is the same as in SONETSDH i.e., F6F6F6282828, and is transmitted unscrambled. Some of the overheads are transmitted over successive OTU frames. For instance, as we saw above, the payload structure identifier byte of the OPU overhead located on row 4 and column 15 is used to transport a 256-byte message. In view of this, groups of successive ODU frames are organized logically into multi-frames. The position of an ODU frame within a multi-frame is indicated by the multi-frame alignment signal MFAS byte located in row 1 column 7. The value of the MFAS byte is incremented each frame, thereby providing a multi-frame consisting of 256 frames. It is transmitted scrambled along with the remainder of the OTU frame. Row 1 ODU OH Client payload 2 3 4 1 7 8 14 15 16 17 3824 3825. . .4080 FAS OTU OH O P U FEC FAS MFAS SM GCC RES 1 13 12 11 10 9 8 7 6 5 4 3 2 14 Figure 9.14 The FAS and OTU overheads.