ITU-T G.709光传输网络(OTN)概述.pdf

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SONET/SDH 网络最初为采用每光纤单波长的光接口而设计,但现在已经成为大部分现代电信网的基石。随着光纤元件技术的不断发展,采
ITU-TG.709 framing structure and byte definitions The ITU-TG. 709 frame(figure l) has The fec scheme used in the ITU-T There are three line rates currently three distinct parts, two that are G 709 standard is a Reed-Solomon defined in ITu-TG.709 broadly similar to a SDH/ SONET RS(255, 239) code. This means that framc for every 239 bytes of data, an 1. 2, 666,057 143 kbit/s-optical additional 16 bytes(255-239=16)of channel transport unit 1(OTU1) Overhead area for operation data is added for error correction. 2. 10, 709, 225. 316 kbit/s-optical administration and maintenance The RS(255, 239) code can correct up channel transport unit 2(OTU2) functions to eight symbol errors in the code 3.43,018,413.559kbit/s- optical Payload area for customer data. word when used for error correction channel transport unit. B(OTU3) and can detect sixteen symbol errors In addition the g. 709 frame also in the fec code word when used for Unlike SONET SDH, as the line includes a foward error control error detection only rate increases. the g. 709 frame size (FEC block (4x 4080) remains the samc and the In the optical transport unit (OTU) frame rate increases. This is a FEC has been used in telecommuni- frame, each row contains 16 FEC departure from the fundamental cations for many years, mainly in the blocks of 16 bytes for the row thus 8 kHz frame rate that has been a Areas of satelite communications and making 64 FEC blocks(4x 16) for foundation of digital telecommunica- undersea transport FEC has been every otu frame tion networks designed to carry important in enabling communica predominantly voice traffic tions to maintain acceptable perfor- The Fec for the otu frame uses mance quality in 'noisy' environ 16-byte interleaved codecs. This The three frame rates(and period) ments at the same time as keeping results in the serial bit stream infrastructure costs in check (10.71 Gb/s for example) being converted into 16 parallel signals for 1. 20.420 kHz(48.971 ms) for OTU1 As transmission bit rates increase processing. This architecture helps 2. 82.027 kHz(12.191 ms) for OTU2 to 10 Gb/s and beyond, physical mprove the error correction on error 3. 329.489 kHz ( 3.035 ms) for OTU3 parameters of the optical fiher play bursts and countering interleaving a more significant part in degrading that may split up closely spaced Note: The period is an approximated transmitted pulses of light. FEC errors value, rounded to three dig provides additional coded data to enable crror checking and correction The size of the frame is four rows of This mcans that to carry one SDH/ y a receiving device. ITU-TG.709 4080 bytes(figure 2). Data is trans- SONET 10 Gb/s frame, for example, includes a standardized method mitted serially beginning at the top requires approximately eleven otu2 of fec that enables long haul eft, first row, followed by the second optical channel frames transmission at higher line rates row and so on without degraded performance The optical transport module overhead consists of four functional s(figure 3) igure oCh overhead OCh payload FEC data Figure 3. ITU-TG.709 Figure 5-1 Frame Alignment overhead OTU overhead Optical channel frame structure OPU Figure 2. ITU-TG.709 Figure 11-1 Column number OTU- Optical transport unit 1415 3824 3825-40801 ODU-Optical data unit R OPU -Optical payload unit OTU/ODU OPU Payload area FEC FEC Forward error correction overhead overhead area Frame alignment Optical channel frame-stress testing Optical transport unit ( oTU) requires the ability to generate overhead When using serial blocks of data sequences of errorederror-free FAs that is, bytes and frames) in a words to verify that crror and alarm The OTU overhead. located at row 1 transmission system, the receiving conditions are entered and exited at columns 8-14(figure 4), provides equipment must be able to identify levels defined in the recommenda supervisory functions. Additionally, the block boundaries. The ability to tions. For example, when adding it conditions the signal for transport identify the 'starting point in the frame errors up to the level needed between 3R(re-timing, reshaping OTN is accomplished through the use to generate OOF alarms and LOF and regeneration regeneration of framing bytes which are transmit alarms in network equipment points in the otn ted every frame correct entry and exit points for these events can only be verified The otu overhead consists of three The frame alignment area contains a using test equipment that gives bytes for section monitoring (SM) 6-byte frame alignment signal (fas) complete control over the Fas bytes. two-byte general communications in row l columns 1-6(figure 4). The channel(GCCo), and two bytes byte values are the same as in SDH/ When designing in a ' standards reserved for future international SONET. F6F6F6282828and are environment this level of testing standardization transmitted unscrambled gives the confidence that designs will inter-operate with other vendor The sm channel is structured as The ability to frame-up, identify out- equipment follows(figure 5 of-frame(OoF), and loss-of-frame (LOF) conditions is a fundamental Some of the otu and optical da requirement for any receiving unit (ODU) overhead signals span Figure 5. ITU-TG.709 Figure 15-2 equipment. The equipment needs to multiple otu frames. Because of SM find the start of a frame before it can this, a multi-frame alignment signal find the management and customer MFAs) byte is defined in row 1 data it needs to process column 6. the value of the mfas byte is incremented each fl TT1 BIP-8 thereby providing a 256 frame multi- frame. The mFAs byte is scrambled along with all the other bytes in the 0 Overhead byte locations and naming OTU frame SAPI 15 16 BEl BDI IAE RES Abbreviations APS/PCC Automatic protection switching/ 32 proteclIon coImImullicalion charnel Operator EXP Experimental FAS Frame alignment signal FTFL Fault type and fault location GCC0-3 General communication channel justification control Figure 4. ITU-T G 709 Figure 15-12 MFAS Multi frame alignment signal 12345678910111213141516 NJ0 Negative justification opportunity FAS MFAS SM GCCO RES RES I JC Path monitoring 2 ACT TCM6 TCM5 TCM4 TFL RES JC TCM3 TCM2 TCM1 PM EXP RES JC PSI Payload structure identifier GCC2 APS/PCC RES PSI NJO RES Reserved SM Section monitoring TCM ACT Tandem connection monitoring activation TCM1-6 Tandem connection monitoring Figure 6 Section Monitoring Detected input condition Response transmitted upstream equipment BIP-8 errors BEl Alarm BDI Framing error IAE The single-byte trail trace identifier ITU-TG.709 uses bit interleaved a single incoming alignment (TTD is defined to transport a parity (bip)checks for in-service error (iae bit is defined to indicate 64-byte message (similar to the performance monitoring, and a Bip-8 that an alignment error has been functionality of Jo in SONET/SDH). byte is defined in the section detected on the incoming signal The message contains a source and monitoring (SM) overhead (figure 5) destination identifier uscd for There arc two main differences Two further bits of the SM bytes routing the otU signal through the between the implementation of B1 reserved for future standardization network. There are also bytes BIPs in Sdh sonet and the sm bip are set to '00 allocated for operator-specific use in itu-tG.709 Testing the section monitoring The 64-byte message is aligned first the sm bip-8 is calculated functionality requires the ability with the otu multi-frame and is only over the opu payload and to stimulate the device-under-test herefore transmitted four times per OPU overhead areas of the frame DUT) with various alarm and multi-frame(256/64)sequence. (columns 15 to 3824); in SDH/ error conditions and check that soneT the bl bip-8 is calculated the Dut gives appropriate responses Testing the TTi functionality involves over an entire frame, including (figure 6). This testing may involve sending valid messages into a overhead. Second. the calculated measuring the time taken to respond network device and verifying BIP-8 value for the frame is inserted to an input stimulus In this case that the signal is routed to the into the bip-8 SM location of frame test equipment with a wide range of appropriate output port. -2; in SdH sonet, the BIP-8 value event trigger outputs can be useful is inserted into the next frame Testing that the network device a two byte general communications accurately identifies incorrect or For section monitoring, a four-bit channel(gcco) is defined in row I corrupt TTi messages is also a backward crror indicator (BeD columns 1l and 12. These bytes requirement. This can be performed signal is defined to signal upstream provide a clear channel connection using test equipment that allows the number of bit-interleaved blocks between otU termination points transmission of flexible user-defined that have been identified in error by The format of the data carried in sequences in the Tti byte locations. the section monitoring sink function this channel is not defined. The using the BIP-8 code. The Bei has CCo channel is likely to carry When testing for correct termina nine valid values, namely 0-8 errors network management data, so when tion/ transparency in network detected in the SM BIP-8 byte. The testing a device at the design stage elements (NEs), it is vital to check remaining values can only occur performing a bEr test in the gcc that TTi messages are passed from some unrelated condition and channcls may be adequate to verify through the ne unaltered. This are interpreted as zero errors performance qualit particularly important if the signal is intended for an endpoint down For section monitoring a single bit, stream from the device-under-test backward defect indication (BDi is defined to convey the signal-fail status determined in a section termination sink function in the upstream direction. It is set to ' Ito indicate a defect. otherwise it is set to“0 Optical channel data unit ODU) The path monitoring (Pm) field in The odu also defines six fields overhead the odu has a similar structure and for tcm. tCm enables a network function to the section monitor field operator to monitor the error The odu overhead resides in in the otU overhcad (figurc 7) performance of a si al transiting columns 1-14 of rows 2.3 and 4 of from its own network ingress and the otn frame. The odu information Figure 7. ITU-TG.709 Figure 15-3 egress points structure provides tandem connec- tion monitoring (TCM), end-to-end PM and TCMi (=1-6) The six tcm fields haye the same path supervision, and client signal structure as the pm field and adaptation via the optical channel support monitoring of odu connec payload unit (oPu) tions for one or more of the following TTI B|P-8 applications UNl-to-UNI monitoring of the ODU COj h the SA 15 public network BEIBDISTAT NNI-to-NNi monitoring of the DAP oDU through a network operator 31 Sub-laye itoring for Operator protection switching, and detection of signal fail or degrade conditions Monitoring a tandem connection for fault localization or verifica tion The six TCM fields provide support for tandem connection monitoring in a variety of network configurations and can cope with nested, overlap ping and cascaded topologies as illustrated in figure 8 Figure 8. ITU-TG.709 Figure 15-17 TCM6 TCM6 TCM6 TCM The fault type and fault location TCM TCM5 TCM5 TCM5 TCM5 FTFL field is also related to the TCM ICMa TCN TCM4 TCM4 monitoring of a tandem connection TCM3 TCM3 span. The FTFL channel is a 256-byte message transmitted across multiple TCM2 TCM2 TCM2 TCM2 frames and aligned with the odu TCM1 TCMT TCM1 MFAS. The message conveys both 姆司姆 forward and backward fault informa- tion and the message structure is shown in figure 9. TCMi ICMi tield nct in use B1-B2 overlapping with C1-C2 而U-G.709 Figure15-16 CMi TCMi field in use TCM6TCM6 TCM6 TCM5TCM5 TCMs TCM4 TCM4 TCM4TCM4 TCM4 TCM4 TCM3 TCM3 TTCM3 TCM3 TCM3TCM3 TCM3 TCM2 TCM2 TCM2 TCM2 TCM2 TCM1 TCM1 TCM1 TCMT TCM1 TCM1 TCM1 T CMi field not in use C]-C2 Nested in B1-B2 TCMi field in use B1-B2 and B3-B4 Cascaded 6 Currently the fault indication codes Two two-byte general communica- Two fields (res are reserved for located in bytes o (forward) and 128 tions channel fields, GCCI and future standardization and are (backward) provide only 'signal fail, GCC2, are defined in row 4 columns located in row 2 columns 1-3 and row 'signal degrade' and 'no fault 1 to 4. These bytes provide a clear 4 columns 9-14. Thesc bytes arc information. Further codes are likely channel connection between ODU normally set to all zeros to be developed in the future. termination points. The format of the data carried in this channel is not Finally, a two-byte experimental The TCM activation (tCM AcT) field defined The main purpose of these EXP) field is defined for experimen is also related to tandem connection bytes is to carry operator manage taI purposes. This field will not be monitoring and is located in the odu ment data subject to future standardization overhead, its definition is for further stud Optical channel payload unit (OPU) rhead Figure 9. ITU-TG.709 Figure 15-20 The oPu overhead is added to the TFL message structure 126127128|129 255 oPu payload and contains inform tion to support the adaptation of client signals. The Forward field Backward field located in rows 1-4 of columns 15 and 16 and is terminated where the 910 oPu is assembled and disassembled Fault Operator Operator specific field ndication identifier field The oPu overhead byte definitions Forward field vary depending on the client signal ITU-TG.709 Figure 15-20 being mapped into the opu payload 128129 137138 ITU-TG.709 currently defines Fault Operator Operator specific field mappings for constant bit rate indication identifier field field signals(for example, STM-16/64/ Backward field 256), both bit-synchronous and ITU-TG. 709 Figure 15-2 asynchronous mapping, ATM cells Fault indication codes generic framing procedure (GFP) Fault code Definition frames, synchronous constant bit 00000000 No fault stream, and a test pseudo random bit 00000001 sequence(PrBs)pattern 00000010 Signal degrade 00000011 Figure 10 shows the oPu2 overhead Reserved for future standardization used when asynchronously mapping 11111111 a 10 Gb/s SDH/SONET signal into the ITU-TG.709 Figure 15-6 Figure 10 OPU2 payload. Columns 15 HowsL JC RES 1234567 RES 47萨 SI NJO PJO Reserved PT JC(bits 7,8)NJo PJO 1 Justification Data PSI Data Data 255 10 Not generated 11 Justification Justification 0PU2, O/H for synch mapping of 10 Gb/s SDH/SONET Testing optical channel devices and hierarchical structures The justification control JC) bytes Guaranteed availability of bandwidth Stimulus/response testing are used to control the negative plus a high level of service quality is ustification opportunity (nJo)or a consistent customer demand This type of test involves sending a positive justification opportunity Modern networks are designed with stimulus signal into the dut and PJO). The mapping process gener- a high degree of intelligence to help monitoring for appropriate outputs ates the jC. Njo and pjo values satisfy these demands. This intelli- due to the stimulus In the otn, a respectively gence is delivered in the manage single stimulus may result in several ment 'overhead that is transmitted simultaneous responses. The The demapping process interprets with customer data. The communica- example below(figure 1 1) shows the the JC. No and po values accord tions network 'senses' its own test set-up and expected responses ing to the table in figure 10.A condition through the use of parity to a detected loss of signal (los)at a majority vote(that is, two out of checks error detection and alarm receiver input. three) is used to make the justifica status that's carried in the overhead tion decision to protect against an This type of test must be repeat error in one of the three jc signals for all possible input stimuli that In today's world of standardization the dut is expected to respond to The payload structure identifier and interoperability, it is vital that A list of possible stimuli is shown (PSD field is defined to transport new designs comply with relevant in figure 12 a 256-byte message aligned with standards and recommendations the otu mfas. psio contains the To ensure the designs of new ITU Figure 12 payload type(pt)identifier that G709 compliant network equipment Stimulus reports the type of payload being meet customer's expectations, a Description carried in the opu payload to the range of tests is required during the Loss of signal receiving equipment. Of the 256 design verification, and manufactur- LOF Loss of frame possible values available, some ing stages. These tests can be divided Out of fra are already defined: 288 values are into the following broad area 00M Out of multiframe reserved for future standardization, OTU-AIS OTU alarm indication signal some are not available. while others Conformance OTU-IAE CTU incoming alignment error are reserved exclusively for propri Stimulus response OTU-BDI OTU backwards delect indication etary use Stress test ODU-AIs ODU alarm indication signal Client signal mapping ODU-OCI ODU open connection indicatio demapping ω DU-LCK ODU locked Parametric ODU-BDI ODU backwards defect indication FAS Frame error MFAS Multi-frame error OTU-BIP8 OTU BIP error OTU-BEI OTU backwards error indication ODU-BIP8 ODU BIP error OTU-BEI OTU backwards error indication FEC block I Uncorrectable FEC block rror Standards and recommendations usually define the response time to a detected event, either in frames or in a time period. In the latter case, it is Figure 11 Alarm TX Downstream useful to use test equipment with response event trigger outputs that can be rigger connected to measuring equipment OTU-AIS (for example, timer or oscilloscope) to determine the but response time Network Upstream element to an event (see figure 11). Triggers response Oscilloscope or timer are set for initiating the LOS signal for response time test OTU-BDI and for the otU-BDi reponse Alarm stress testing For example, on detection of an Mapping/demapping testing alarm such as otU-bDl (detected by This type of test really comes under 1'in bit 5 of bvte 3 of the SM field of The optical transport hicrarchy has the banner of 'conformance testing the otU overhead), a network device been designed to transport a range Standards normally define entry and should signal this to the management of payloads. Today, there are defined exit criteria for alarm events, usually system when it has been set for five mappings for SDH/ SONET, ATM and specified by a number of frames or consecutive frames By using test generic framing procedure gfP) sometimes in a time period a equipinent that can generate this frames. When mapping SDH/ SONeT possible test configuration is shown condition for one, two, three, four signals into the oPu, rate differ figure 13. In this instance, it is useful and more consecutive frames ences between the client and oPU for test equipment to simulate a it is possible to verify that the alarm clocks are accommodated through stimulus condition(alarm)for a condition is entered at exactly the the use of justification(stuffing) variable number of frames or time correct point. A similar test can The maximum difference that can be This allows an alarm condition to be verify the alarm exit condition accommodated between opu and simulated for a variable number of client clocks is +/-65 ppm. with a bit frames and the number varied to This type of testing must be per rate tolerance of +/-20 ppm for the confirm that the entry criterion is formed for every possible alarm oPu clock. the clients bit rate met precisely. a similar test can be event to ensure conformance tolerance can be +/-45 ppm performed to verify the exit criterion to the recommendation, and for the event interoperability with other vendor A configuration for mapping testing equipment is shown in figure 14 The bit rate of the input client test signal is varied across its maximum Figure 13 allowable range(normally +/-20 Downstream Alarm x ppm for StH/SONET line signals) response The dut maps the client signal into ptical channel frame an tester verifies that this has been pcrformcd without any crror or work larm events at the otn or SDH/ SONET levels response The de ping process can also be verified with the tester generating OtN frames containing napped SDH/ SONET frames gure OTN NE 10 Gb/s SDH/ SONET 10.71 Gb/s OTN with mapped SDH/SONET Forward error correction(FEC) FEC is a key element of the otn and is used to provide improved quality of service and longer span lengths Figure 15 Using a tester that generates cor- FEC test rectly structured otn frames and frame errors is useful in validating FEC functionality in new device OTN designs NE 10.71 Gb/s OTN with mapped SDH/SONET 10.71 Gb/s OTN with mapped SDH/SONET a test configuration is shown i figure 15. Add crrors Check for over frame zero FEc errors The Reed-Solomon coding scheme combined with the use of 16-byte interleaved codecs makes this implementation of FEC particularly good at correcting bursts of errors. In the configuration shown below, it is possible to add increasing rates of errors at the input to the dut and monitor the error performance at the DUT output. In this way, it is possible to verify that errors at the input are actually corrected by the dut Glossary Conclusion Abbreviation Description To satisfy demand for bandwidth, control costs and still remain DUT Device under test competitive, service providers are DWDM Dense wave division deploying the next generation OTN iplexing The otn includes forward error correction(FEC)and enhanced FEC Forward error correction network management. This is seen as MEMS Micro electro mechanical the best value-for-money solution for Systems bandwidth at 10 Gb/s and above Optical channel NEMs that manufacture devices ODU Optical data unit for the o t ensure they are OPU Optical payload unit fully compliant with ITt-TG709 However, to do this, NEMS need an OIN Oplical transport network otn tester that will verify e OTU Optical transport unit aspect of the recommendation suring inte Pseudo random bit and standardization This means sequence service provides can have complete confidence in the product, and in proved performance to end user

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