Sonet

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Sonet

  1. 1. SONET:Synchronous Optical Network 1
  2. 2. Basic Terms Synchronous Synchronization is timekeeping which requires the coordination of events to operate a system in unison. Optical Network Network which work on optical fiber. Network which uses light as transmitting signal. 2
  3. 3. IntroductionSONET is a newly adopted standard forinterfaces in optical networksPhysical layer transmission formatSONET defines a “fiber basedtransmission scheme for ATM” 3
  4. 4. SONET OverviewThe SONET specification defines:– standard optical signals, which permits the interoperation of equipment from different manufacturers– a synchronous frame structure for multiplexing digital traffic– procedures for operations and maintenance (OAM) 4
  5. 5. SONET Overview (Cont’d)SONET includes:– support for broadband rates base rate approximately 50 Mbps hierarchical family of digital rates defines data rates up to 2.4 Gbps– synchronous multiplexing global timing structure at physical layer synchronous implies simpler interface 5
  6. 6. SONET Framing StructureBasic module is STS-1Synchronous Transport Signal, Level 1STS-1 corresponds to 51.84 MbpsFrame structure: 9 rows of 90 columns of 8-bit bytes8000 frames/sec (125 usec/frame) 6
  7. 7. STS-1 Framing Structure ... ... 9 ... ...rows ... ... ... ... ... 90 columns 1 byte 125 usec 7
  8. 8. STS-1 FramingBytes are transmitted one row at a time,from left to rightNote: 1 byte/frame = 64 kbpsFirst three columns of STS-1 frame arefor section overhead and line overheadRemaining 87 columns are for theSynchronous Payload Envelope (SPE) 8
  9. 9. STS-1 Framing (Cont’d) 90 columns ... ... 9 ... ...rows ... ... ... ... ... Section and Synchronous Payload Line Overhead Envelope (SPE) (3 columns) (87 columns) 9
  10. 10. SONET OverheadOverhead bytes are used by SONETequipment (e.g., switches) for exchangeof control and signalling information,and as a low bandwidth data channelThree types of overhead bytes– section– line– path 10
  11. 11. SONET Overhead (Cont’d)Section overhead: 9 bytes per frame– Includes two framing bytes, plus other control information for maintenance and provisioningLine overhead: 18 bytes per frame– Control info, plus 9 bytes for data channelPath overhead: variable size– Payload type, path status, etc.– Transmitted as part of payload itself (SPE) 11
  12. 12. SONET Framing (Cont’d)The SPE in an STS-1 frame has sufficientcapacity to carry a DS-3 (45 Mbps)There are many other ways to “carve up”the capacity of an STS-1 into smallerunits used by the telco’sThese are called Virtual Tributaries (VT’s) 12
  13. 13. SONET Framing (Cont’d)Examples of VT’s:– VT 1.5: requires 3 columns of 9 bytes each, corresponding to North American DS1 (T1) standard (1.544 Mbps)– VT 2: 4 columns, corresponds to European standard for 2.048 Mbps– VT 3: 6 columns (54 bytes) per frame, corresponds to 3.088 Mbps– VT 6: 12 columns, 6.312 Mbps 13
  14. 14. STS-1 Framing Example 90 columns ... ... 9 ... ...rows ... ... ... ... ... Section and VT 1.5 VT 2 Line Overhead 14
  15. 15. SONET Framing (Cont’d)A “VT group” is 9 rows x 12 columns– Can conveniently repackage into four VT 1.5, or three VT 2, or two VT 3, or one VT 6An STS-1 frame can hold 7 VT groupsper frame (84 columns), with 1 column forpath overhead, and 2 columns empty 15
  16. 16. SONET Framing (Cont’d)Higher rate SONET signals are obtainedby interleaving N STS-1’s to form anSTS-N (e.g., STS-3 = 155 Mbps)STS-N has 9 rows, and N x 90 columnsInterleaving is done byte by byte 16
  17. 17. SONET and ATMIf the entire STS-1 payload is to be usedfor ATM transmission, then there is noneed to use VT’s at allThe 53-byte ATM cells are simplypackaged into the SPE portion of theSTS-1 frame, as they fitCells may wrap across STS-1 overheadbytes, or even STS-1 frame boundariesOverhead byte keeps track of where ATMcell boundaries17lie
  18. 18. STS-1 ATM Example 90 columns ... ... 9 ... ...rows ... ... ... ... ... Section and Line Overhead Start of ATM Cells 18
  19. 19. SummarySONET defines a standard for framingand transmission at the physical layer onfiber-optic based networksFraming structure is designed toaccommodate common telco channelrates in both North America and EuropeATM cells can be layered on top of the(synchronous) SONET framing structure 19

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