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TELECOMMUNICATIONS SYSTEMS AND TECHNOLOGY

TELECOMMUNICATIONS SYSTEMS AND TECHNOLOGY. PART 4-2. SONET FRAME FORMAT. SONET/STS-1 FRAME. ALL SONET SIGNALS ARE CREATED IN ELECTRICAL FORM BEFORE CONVERSION TO LIGHT FOR TRANSPORT OVER FIBER. WHEN WE TALK ABOUT SONET FRAMES, WE ARE REFERRING TO STS FRAMES, AND THE STS-1

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TELECOMMUNICATIONS SYSTEMS AND TECHNOLOGY

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  1. TELECOMMUNICATIONS SYSTEMS AND TECHNOLOGY PART 4-2

  2. SONET FRAME FORMAT

  3. SONET/STS-1 FRAME ALL SONET SIGNALS ARE CREATED IN ELECTRICAL FORM BEFORE CONVERSION TO LIGHT FOR TRANSPORT OVER FIBER. WHEN WE TALK ABOUT SONET FRAMES, WE ARE REFERRING TO STS FRAMES, AND THE STS-1 (SYNCHRONOUS TRANSPORT SIGNAL – LEVEL 1) FRAME IS THE BASIC BUILDING BLOCK OF ALL SONET SIGNALS. WE WILL REFER TO STS-1 FRAMES AND SONET FRAMES INTERCHANGEABLY.

  4. SONET FRAME FORMAT • SONET/SDH DEFINES THE LOW LEVEL FRAMING PROTOCOL USED ON OPTICAL LINKS • “FRAMING”, IMPLIES A BLOCK OF BITS (N OCTETS, • 1 OCTET = 8 BITS = 1 BYTE) WHICH HAVE A WELL DEFINED STRUCTURE, AND WHICH UTILIZE SOME TECHNIQUE TO FIND THE BOUNDARIES OF THAT FRAME STRUCTURE • THE SONET FRAME STRUCTURE IS A TWO-DIMENSIONAL STRUCTURE

  5. OCTET • AN OCTET IS A GROUPING OF 8 BITS, WITH THE FOLLOWING CHARACTERISTICS: MSB LSB 1 2 3 4 5 6 7 8 BYTE MSB LSB 7 6 5 4 3 2 1 0

  6. ONE-DIMENSIONAL FRAME HEADER TRAILER VARYING SIZE PAYLOAD FRAME DELIMITERS  FLAGS

  7. SONET FRAME FORMAT • STS-1 basic SONET frame consists of 810 bytes put out every 125 μsec • Having 8000 frames per second exactly matches the sampling rate of the PCM channels used in telephone systems. • The 810-byte SONET frames are best described as a rectangle of bytes, 90 columns width by 9 row high (90 x 9 = 810 bytes). • 8 x 810 = 6480 bits transmitted 8000 times per second, for a gross data rate of 51.84 Mbps (BASIC STS-1 = BASIC OC-1)

  8. SONET STS-1 FRAME , (PATH OVERHEAD)

  9. SONET FRAME FORMAT EVENTHOUGH WE THINK OF A SONET FRAME AS A 2-DIMENSIONAL ARRAY, THE OCTETS ARE TRANSMITTED SERIALLY FROM BYTE 1 TO BYTE 810 (ROW 1/COLUMN 1, ROW 1/COLUMN 2, ROW 1, COLUMN 3, ETC..) START WITH MSB OF OCTET 1 A1 A2 J0 J1 D0,0 D0 ,1 D0,2 …. D0 ,85 B1 E1 F1 B3 D1,0 D1,1 D1,2 …. D1,85, ………….. S1 M0/1 E2 N1 D8,0 D8,1 D8,2 …. D8,85

  10. SONET FRAME FORMAT • A NEW SONET FRAME IS SENT EVERY 125 microseconds • SONET FRAME IS THEN 8000 FRAMES/SEC

  11. SONET FRAME FORMAT • THE FIRST THREE BYTES IN EACH ROW FORM THE • TRANSPORT OVERHEAD (TOH). THIS GIVES A TOTAL • OF 9 X 3 = 27 BYTES FOR TOH • THE TOH IS BROKEN DOWN INTO: • THE SECTION OVERHEAD (SOH)  9 BYTES • THE LINE OVERHEAD (LOH)  18 BYTES • THE REMAINDER OF THE FRAME (783 OCTETS = 810 - 27) IS CALLED THE SYNCHRONOUS PAYLOAD ENVELOPE (SPE). THE SPE COMPRISES OF AN OVERHEAD AND DATA • (PAYLOAD)

  12. SONET FRAME OVERHEAD • THE SONET OVERHEAD SECTIONS CONTAIN ALL THE • SIGNALING AND OTHER INFORMATION REQUIRED TO • TRANSPORT THE SONET FRAME AND MANAGE THE • SONET CONNECTION • THE SOH AND THE LOH PROVIDE NETWORK • MANAGEMENT COMMUNICATIONS CHANNELS • CALLED DATA COMMUNICATIONS CHANNELS (DCC), • AS WELL AS VOICE CHANNELS, REFERRED TO A • ORDERWIRES

  13. SONET FRAME OVERHEAD • THE SPE OVERHEAD IS FORMED BY THE 4TH BYTE • IN EACH ROW OF THE FRAME (9 BYTES TOTAL) AND • IS CALLED THE PAYLOAD OVERHEAD/PATH OVERHEAD • (POH) • THE PAYLOAD PORTION OF THE SPE COMPRISES • 9X86 =774 OCTETS (783 – 9 = 774) • THE PAYLOAD PORTION OF THE SPE (774 OCTETS) • CAN BE PARTITIONED IN MANY WAYS DEPENDING ON • THE BANDWITH OF THE CONNECTION (A FLEXIBILITY & • BENEFIT OF SONET TO PROVIDE A FLEXIBLE GENERAL • PURPOSE TRANSPORT MECHANISM)

  14. SONET FRAME FLEXIBILITY • THE SPE DOES NOT HAVE TO START AT BYTE 5 OF ROW 1 OF THE STS-1 FRAME • THE SPE CAN FLOAT WITHIN AND ACROSS STS-1 • FRAME BOUNDARIES • ALTHOUGH THE SPE CAN FIT IN ONE FRAME, MOST • SPEs WILL SPAN TWO FRAMES

  15. SONET FRAME FLEXIBILITY • THE LOH CARRIES A PAYLOAD POINTER THAT • INDICATES WHERE THE FIRST BYTE OF THE SPE IS • LOCATED • THE SPE PAYLOAD BYTE LOCATIONS (POSITIONS) ARE GIVEN AS AN OFFSET FROM THE PAYLOAD POINTER IN THE LOH (OFFSET 0  OFFSET 782) • THE TOH OCTETS ARE NOT COUNTED AS OFFSET • POSITIONS • REGARDLESS OF THE STARTING POSITION OFFSET, THE FRAME CONSISTS OF 783 OCTETS.

  16. SONET FRAME FLEXIBILITY OFFSET VALUES X STS-1 FRAME1 LOH H1 H2 H3 0 1 2 84 85 86 87 88 89 171 172 173 435 436 437 519 520 521 STS-1 FRAME2 522 523 524 606 607 608 609 610 611 693 694 695 696 697 698 780 781 782 783 SPE OCTETS (0 – 782)

  17. SONET FRAME FLEXIBILTY EXAMPLE: STARTING POSITION = OFFSET VALUE 87 STS-1 FRAME1 LOH H1 H2 H3 0 1 2 84 85 86 87 88 89 171 172 173 435 436 437 519 520 521 STS-1 FRAME2 522 523 524 606 607 608 609 610 611 693 694 695 696 697 698 780 781 782 0 1 2 84 85 86 783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMES SPE ENDS AT OFFSET 86 (87 – 1) IN SECOND FRAME

  18. SONET FRAME FLEXIBILTY EXAMPLE: STARTING POSITION = OFFSET VALUE 174 STS-1 FRAME1 LOH H1 H2 H3 0 1 2 84 85 86 87 88 89 171 172 173 174 175 176 258 259 260 435 436 437 519 520 521 STS-1 FRAME2 522 523 524 606 607 608 609 610 611 693 694 695 696 697 698 780 781 782 0 1 2 84 85 86 87 88 89 171 172 173 783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMES SPE ENDS AT OFFSET 173 (174 – 1) IN SECOND FRAME

  19. SONET FRAME FLEXIBILTY EXAMPLE: STARTING POSITION = OFFSET VALUE 522 STS-1 FRAME1 LOH H1 H2 H3 0 1 2 84 85 86 87 88 89 171 172 173 174 175 176 258 259 260 435 436 437 519 520 521 STS-1 FRAME2 522 523 524 606 607 608 609 610 611 693 694 695 696 697 698 780 781 782 0 1 2 84 85 86 87 88 89 171 172 173 174 175 176 258 259 260 261 262 263 345 346 347 348 349 350 432 433 434 435 436 437 519 520 521 OFFSET POSITION 522 ALLOWS THE ENTIRE SPE TO FIT IN ONE STS-1 FRAME. SPE ENDS AT OFFSET 521 (522 -1).

  20. SONET FRAME FORMAT • THE LAST EXAMPLE SHOWED THAT AN OFFSET • POSITION OF 522 ALLOWS THE ENTIRE SPE TO FIT • IN ONE STS-1 FRAME • NOTE THAT THE POINTER TO THE START OF SPE IS IN • THE PREVIOUS FRAME. THIS IS REFERRED TO AS A • DECOUPLING OF THE PAYLOAD FROM THE STS-1 FRAME • (A FRAME CAN FLOAT)

  21. SONET SERVICES

  22. SONET SUB-RATE CONNECTIONS • WHEN THE SONET SERVICE IS FOR RATES BELOW • THE DS3 RATE, WE SAY THAT THE SPE CARRIES A • SUB-RATE CONNECTION • THESE SUB-RATE CONNECTIONS ARE CARRIED IN • WHAT IS CALLED VIRTUAL TRIBUTARIES (VTs) WITHIN • THE SPE • THE TERM VIRTUAL TRIBUTARY IS USED TO DESIGNATE • THE FRAME STRUCTURES USED TO MAP DS1  DS2 • CHANNELS ONTO AN STS-1 FRAME

  23. VIRTUAL TRIBUTARIES

  24. VT1.5 SERVICE (DS1) • THREE COLUMNS OF 9 BYTES EACH = 27 BYTES • (27 x 8 x 8000 = 1.728 Mbps > 1.544 Mbps) • 28 VT1.5s CAN BE MULTIPLEXED ONTO THE STS-1 SIGNAL • (774/27 = 28.6666  28)

  25. VT1.5 SERVICE (DS1) STS-1 FRAME1 LOH H1 H2 H3 STS-1 FRAME2 27 SPE OCTETS

  26. SONET SUB-RATE CONNECTIONS • AN ADM (ADD/DROP MUX) CAN ADD/DROP INDIVIDUAL • DS1s FROM A SONET STS-1 FRAME WITHOUT A NEED TO • MULTIPLEX/DEMULTIPLEX THE ENTIRE SIGNAL • ONE OC-1 = ONE STS-1 FRAME (783 SPE OCTETS) • OC-1 CAN CARRY 783/27 = 28 DS1 MAX

  27. VT2 SERVICE (E1) • FOUR COLUMNS OF 9 BYTES EACH = 36 BYTES • (36 x 8 x 8000 = 2.304 Mbps > 2.048 Mbps) • 21 VT2s CAN BE MULTIPLEXED ONTO THE STS-1 SIGNAL • (774/36 = 21.5  21)

  28. VT2 SERVICE (E1) STS-1 FRAME1 LOH H1 H2 H3 STS-1 FRAME2 36 SPE OCTETS

  29. VT3 SERVICE (DS1C) • SIX COLUMNS OF 9 BYTES EACH = 54 BYTES • (54 x 8 x 8000 = 3.456 Mbps > 3.088 Mbps) • 14 VT3s CAN BE MULTIPLEXED ONTO THE STS-1 SIGNAL • (774/54 = 14.33  14)

  30. VT3 SERVICE (DS1C) STS-1 FRAME1 LOH H1 H2 H3 STS-1 FRAME2 54 SPE OCTETS

  31. VT6 SERVICE (DS2) • TWELVE COLUMNS OF 9 BYTES EACH = 108 BYTES • (108 x 8 x 8000 = 6.912 Mbps > 6.312 Mbps) • 7 VT6s CAN BE MULTIPLEXED ONTO THE STS-1 SIGNAL • (774/108 = 7.16  7)

  32. VT6 SERVICE (DS2) STS-1 FRAME1 LOH H1 H2 H3 STS-1 FRAME2 108 SPE OCTETS

  33. SONET FULL RATE SERVICE • DS3 = 28 DS1s  28 x 3 COLUMNS/DS1 = 84 COLUMNS • 84 COLUMNS = 84 x 9 = 756 BYTES • (756 x 8 x 8000 = 48.384 Mbps > 44.736 Mbps) • 84 COLUMNS ARE NEEDED FOR DS3 TRAFFIC • WHEN THE SPE CARRIES DS3 TRAFFIC, IT IS CALLED • A FULL RATE SERVICE. IN THIS CASE, THE ENTIRE SPE • IS DEVOTED TO A SINGLE DS3

  34. FULL RATE SERVICE (DS3) STS-1 FRAME1 LOH H1 H2 H3 .............. .............. .............. .............. .............. .............. STS-1 FRAME2 .............. .............. .............. 756 SPE OCTETS

  35. HIGHER LEVEL STS FRAMES GROUPS OF SYNCHRONOUS TRANSPORT FRAMES CAN BE PACKAGED FOR TRANSPORT AS A HIGHER ORDER SIGNAL. THIS IS ACHIEVED BY BYTE INTERLEAVED MULTIPLEXING IN WHICH PARALLEL STREAMS OF TRANSPORT SIGNALS ARE MIXED TOGETHER ON A FIXED BYTE BY BYTE BASIS. FOR EXAMPLE, TO CARRY MULTIPLE DS3s, WE MULTIPLEX A NUMBER OF STS-1 TO FORM A HIGHER RATE SONET SIGNAL. ALL SONET MULTIPLEXING IS DONE BY BYTE INTERLEAVING.

  36. HIGHER LEVEL STS FRAMES STS-3 BYTE INTER- LEAVED MUX STS-1 SIGNAL A oc1 STS-1 SIGNAL B STS-3 SIGNAL oc1 oc3 STS-1 SIGNAL C oc1

  37. HIGHER LEVEL STS FRAMES AN STS-3 SIGNAL CARRIES THREE STS-1 SIGNALS – IT CARRIES THREE SPEs. ONE SPE FROM EACH STS-1 SIGNAL. EACH STS-1 SPE HAS THREE COLUMNS (27 BYTES) OF OVERHEAD. THE STS-3 SIGNAL CARRIES 9 COLUMNS (3 x 3) OF OVERHEAD WITH A CONTRIBUTION OF THREE COLUMNS OF OVERHEAD FROM EACH STS-1 SPE. WE CAN SAY THAT AN ADM CAN ADD/DROP DS3s WITHOUT THE NEED TO MULTIPLEX/DEMULTIPLEX THE ENTIRE STS SIGNAL

  38. HIGHER LEVEL STS FRAMES STS-12 BYTE INTER- LEAVED MUX STS-3 SIGNAL A oc3 STS-3 SIGNAL B STS-12 SIGNAL oc3 oc12 STS-3 SIGNAL C oc3 STS-3 SIGNAL D oc3

  39. HIGHER LEVEL STS FRAMES AN STS-12 SIGNAL CARRIES TWELVE STS-1 SIGNALS – IT CARRIES TWELVE SPEs. ONE SPE FROM EACH STS-1 SIGNAL. EACH STS-1 SPE HAS THREE COLUMNS (27 BYTES) OF OVERHEAD. THE STS-12 SIGNAL CARRIES 36 COLUMNS (12 x 3) OF OVERHEAD WITH A CONTRIBUTION OF THREE COLUMNS OF OVERHEAD FROM EACH STS-1 SPE.

  40. SUPER RATE SERVICE (CONCATENATION) CONCATENATION IS USED WHEN A SIGNAL STREAM GREATER THAN THE STS-1 IS NEEDED TO SUPPORT AN INDIVIDUAL DATA SOURCE (I.E. FIBER DISTRIBUTED DATA INTERFACE – FDDI (100 Mbps), ASYNCHRONOUS TRANSFER MODE – ATM (155 Mbps). WHEN SONET CARRIES SUCH RATES ( > 50 Mbps), WE SAY THAT SONET CARRIES SUPER-RATE SERVICES.

  41. SUPER RATE SERVICE (CONCATENATION) TO SUPPORT SUPER-RATE TRANSPORT, THE STS-Nc SIGNAL IS CREATED (c  CONCATENATION). THE PAYLOAD IS MAPPED INTO N STS-1 SPEs AND THE NETWORK WILL TREAT THE Nc FRAME AS A SINGLE ENTITY (NOT N INDIVIDUAL SPEs). THIS CONCATENATION METHOD ALLOWS MAPPING ONTO A HIGHER PAYLOAD CAPACITY. EXAMPLE: STS-3c IS AN STS STRUCTURE THREE TIMES LARGER THAN STS-1 (THINK OF IT AS A 270 COLUMNS X 9 RECTANGLE WITH 9 COLUMNS OF TOH).

  42. STS-3C TOH POH SPE = 9 x 261 BYTES

  43. SERVICE SUMMARY • SUB-RATE SERVICE • STS-1 VT STRUCTURE SPE (DS1, E1, DS1C, DS2) • FULL RATE SERVICE • STS-1 SPE (DS3) • SUPER-RATE SERVICE • STS-3C SPE (FDDI, ATM)

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