telecommunications systems and technology part 4-2
TRANSCRIPT
TELECOMMUNICATIONS
SYSTEMS AND TECHNOLOGY
PART 4-2
SONET FRAME FORMAT
SONET/STS-1 FRAME
ALL SONET SIGNALS ARE CREATED IN ELECTRICALFORM BEFORE CONVERSION TO LIGHT FORTRANSPORT OVER FIBER.
WHEN WE TALK ABOUT SONET FRAMES, WE AREREFERRING TO STS FRAMES, AND THE STS-1(SYNCHRONOUS TRANSPORT SIGNAL – LEVEL 1) FRAME IS THE BASIC BUILDING BLOCK OF ALL SONETSIGNALS.
WE WILL REFER TO STS-1 FRAMES AND SONET FRAMES INTERCHANGEABLY.
• 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
SONET FRAME FORMAT
• AN OCTET IS A GROUPING OF 8 BITS, WITH THE FOLLOWING CHARACTERISTICS:
OCTET
2 3 4 5 6 7 81
MSB LSB
BYTE6 5 4 3 2 1 07
MSB LSB
ONE-DIMENSIONAL FRAME
VARYING SIZE PAYLOAD
HEADER TRAILER
FRAME DELIMITERS FLAGS
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)
SONET STS-1 FRAME
, (PATH OVERHEAD)
SONET FRAME FORMAT
EVENTHOUGH WE THINK OF A SONET FRAME AS A2-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..)
j
i j
iCR
9
1
90
1A1 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
START WITHMSB OFOCTET 1
SONET FRAME FORMAT
• A NEW SONET FRAME IS SENT EVERY
125 microseconds
• SONET FRAME IS THEN 8000 FRAMES/SEC
SONET FRAME FORMAT
• THE FIRST THREE BYTES IN EACH ROW FORM THETRANSPORT OVERHEAD (TOH). THIS GIVES A TOTALOF 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)
SONET FRAME OVERHEAD
• THE SONET OVERHEAD SECTIONS CONTAIN ALL THE SIGNALING AND OTHER INFORMATION REQUIRED TOTRANSPORT THE SONET FRAME AND MANAGE THESONET CONNECTION
• THE SOH AND THE LOH PROVIDE NETWORK MANAGEMENT COMMUNICATIONS CHANNELSCALLED DATA COMMUNICATIONS CHANNELS (DCC),AS WELL AS VOICE CHANNELS, REFERRED TO AORDERWIRES
SONET FRAME OVERHEAD
• THE SPE OVERHEAD IS FORMED BY THE 4TH BYTEIN EACH ROW OF THE FRAME (9 BYTES TOTAL) ANDIS CALLED THE PAYLOAD OVERHEAD/PATH OVERHEAD(POH)
• THE PAYLOAD PORTION OF THE SPE COMPRISES9X86 =774 OCTETS (783 – 9 = 774)
• THE PAYLOAD PORTION OF THE SPE (774 OCTETS)CAN BE PARTITIONED IN MANY WAYS DEPENDING ONTHE BANDWITH OF THE CONNECTION (A FLEXIBILITY &BENEFIT OF SONET TO PROVIDE A FLEXIBLE GENERALPURPOSE TRANSPORT MECHANISM)
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-1FRAME BOUNDARIES
• ALTHOUGH THE SPE CAN FIT IN ONE FRAME, MOSTSPEs WILL SPAN TWO FRAMES
SONET FRAME FLEXIBILITY
• THE LOH CARRIES A PAYLOAD POINTER THATINDICATES 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 OFFSETPOSITIONS
• REGARDLESS OF THE STARTING POSITION OFFSET, THE FRAME CONSISTS OF 783 OCTETS.
SONET FRAME FLEXIBILITY
173172171
868584
STS-1FRAME1
695694693
608607606STS-1FRAME2
898887
210
611610609
524523522
H1 H2 H3
783 SPE OCTETS (0 – 782)
435 436 437 519 520 521
782781780698697696
LOH
X OFFSET VALUES
SONET FRAME FLEXIBILTY
173172171
868584
STS-1FRAME1
695694693
608607606STS-1FRAME2
898887
210
611610609
524523522
H1 H2 H3
783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMESSPE ENDS AT OFFSET 86 (87 – 1) IN SECOND FRAME
435 436 437 519 520 521
782781780698697696
LOH
EXAMPLE: STARTING POSITION = OFFSET VALUE 87
868584210
SONET FRAME FLEXIBILTY
173172171
868584
STS-1FRAME1
695694693
608607606STS-1FRAME2
898887
210
611610609
524523522
H1 H2 H3
435 436 437 519 520 521
782781780698697696
LOH
EXAMPLE: STARTING POSITION = OFFSET VALUE 174
868584210
174 175 176 258 259 260
173172171898887
783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMESSPE ENDS AT OFFSET 173 (174 – 1) IN SECOND FRAME
SONET FRAME FLEXIBILTY
173172171
868584
STS-1FRAME1
695694693
608607606STS-1FRAME2
898887
210
611610609
524523522
H1 H2 H3
435 436 437 519 520 521
782781780698697696
LOH
EXAMPLE: STARTING POSITION = OFFSET VALUE 522
868584210
174 175 176 258 259 260
173172171898887
260259258176175174
347346345263262261
434433432350349348
521520519437436435
OFFSET POSITION 522 ALLOWS THE ENTIRE SPE TO FITIN ONE STS-1 FRAME. SPE ENDS AT OFFSET 521 (522 -1).
SONET FRAME FORMAT
• THE LAST EXAMPLE SHOWED THAT AN OFFSETPOSITION OF 522 ALLOWS THE ENTIRE SPE TO FITIN ONE STS-1 FRAME
• NOTE THAT THE POINTER TO THE START OF SPE IS INTHE PREVIOUS FRAME. THIS IS REFERRED TO AS ADECOUPLING OF THE PAYLOAD FROM THE STS-1 FRAME(A FRAME CAN FLOAT)
SONET
SERVICES
SONET SUB-RATE CONNECTIONS
• WHEN THE SONET SERVICE IS FOR RATES BELOWTHE DS3 RATE, WE SAY THAT THE SPE CARRIES ASUB-RATE CONNECTION
• THESE SUB-RATE CONNECTIONS ARE CARRIED INWHAT IS CALLED VIRTUAL TRIBUTARIES (VTs) WITHINTHE SPE
• THE TERM VIRTUAL TRIBUTARY IS USED TO DESIGNATETHE FRAME STRUCTURES USED TO MAP DS1 DS2 CHANNELS ONTO AN STS-1 FRAME
VIRTUAL TRIBUTARIES
VT NAME TRAFFIC # BYTES #COLUMNS
VT1.5 DS1 27 BYTES 3
VT2 E1 36 BYTES 4
VT3 DS1C 54 BYTES 6
VT6 DS2 108 BYTES 12
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)
STS-1FRAME1
STS-1FRAME2
H1 H2 H3
27 SPE OCTETS
LOH
VT1.5 SERVICE (DS1)
SONET SUB-RATE CONNECTIONS
• AN ADM (ADD/DROP MUX) CAN ADD/DROP INDIVIDUALDS1s FROM A SONET STS-1 FRAME WITHOUT A NEED TOMULTIPLEX/DEMULTIPLEX THE ENTIRE SIGNAL
• ONE OC-1 = ONE STS-1 FRAME (783 SPE OCTETS)
• OC-1 CAN CARRY 783/27 = 28 DS1 MAX
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)
STS-1FRAME1
STS-1FRAME2
H1 H2 H3
36 SPE OCTETS
LOH
VT2 SERVICE (E1)
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)
STS-1FRAME1
STS-1FRAME2
H1 H2 H3
54 SPE OCTETS
LOH
VT3 SERVICE (DS1C)
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)
STS-1FRAME1
STS-1FRAME2
H1 H2 H3
108 SPE OCTETS
LOH
VT6 SERVICE (DS2)
• DS3 = 28 DS1s 28 x 3 COLUMNS/DS1 = 84 COLUMNS84 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 CALLEDA FULL RATE SERVICE. IN THIS CASE, THE ENTIRE SPEIS DEVOTED TO A SINGLE DS3
SONET FULL RATE SERVICE
STS-1FRAME1
STS-1FRAME2
H1 H2 H3
756 SPE OCTETS
LOH
FULL RATE SERVICE (DS3)
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HIGHER LEVEL STS FRAMES
GROUPS OF SYNCHRONOUS TRANSPORT FRAMES CANBE PACKAGED FOR TRANSPORT AS A HIGHER ORDERSIGNAL. THIS IS ACHIEVED BY BYTE INTERLEAVEDMULTIPLEXING IN WHICH PARALLEL STREAMS OFTRANSPORT SIGNALS ARE MIXED TOGETHER ON A FIXED BYTE BY BYTE BASIS. FOR EXAMPLE, TO CARRY MULTIPLE DS3s, WEMULTIPLEX A NUMBER OF STS-1 TO FORM A HIGHERRATE SONET SIGNAL. ALL SONET MULTIPLEXING ISDONE BY BYTE INTERLEAVING.
HIGHER LEVEL STS FRAMES STS-3
BYTEINTER-
LEAVEDMUX
STS-1 SIGNAL A
STS-1 SIGNAL B
STS-1 SIGNAL C
STS-3 SIGNAL
oc1
oc1
oc1
oc3
HIGHER LEVEL STS FRAMES
AN STS-3 SIGNAL CARRIES THREE STS-1 SIGNALS – IT CARRIES THREE SPEs. ONE SPE FROM EACH STS-1SIGNAL. 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 THREECOLUMNS OF OVERHEAD FROM EACH STS-1 SPE.
WE CAN SAY THAT AN ADM CAN ADD/DROP DS3sWITHOUT THE NEED TO MULTIPLEX/DEMULTIPLEXTHE ENTIRE STS SIGNAL
HIGHER LEVEL STS FRAMES STS-12
BYTEINTER-
LEAVEDMUX
STS-3 SIGNAL A
STS-3 SIGNAL B
STS-3 SIGNAL C
STS-12 SIGNAL
oc3
oc3
oc3
oc12
STS-3 SIGNAL Doc3
HIGHER LEVEL STS FRAMES
AN STS-12 SIGNAL CARRIES TWELVE STS-1 SIGNALS – IT CARRIES TWELVE SPEs. ONE SPE FROM EACH STS-1SIGNAL. 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 THREECOLUMNS OF OVERHEAD FROM EACH STS-1 SPE.
SUPER RATE SERVICE (CONCATENATION)
CONCATENATION IS USED WHEN A SIGNAL STREAMGREATER THAN THE STS-1 IS NEEDED TO SUPPORT ANINDIVIDUAL DATA SOURCE (I.E. FIBER DISTRIBUTEDDATA INTERFACE – FDDI (100 Mbps), ASYNCHRONOUSTRANSFER MODE – ATM (155 Mbps). WHEN SONET CARRIES SUCH RATES ( > 50 Mbps),WE SAY THAT SONET CARRIES SUPER-RATE SERVICES.
SUPER RATE SERVICE (CONCATENATION)
TO SUPPORT SUPER-RATE TRANSPORT, THE STS-NcSIGNAL IS CREATED (c CONCATENATION). THEPAYLOAD IS MAPPED INTO N STS-1 SPEs AND THENETWORK WILL TREAT THE Nc FRAME AS A SINGLEENTITY (NOT N INDIVIDUAL SPEs).
THIS CONCATENATION METHOD ALLOWS MAPPINGONTO A HIGHER PAYLOAD CAPACITY.
EXAMPLE: STS-3c IS AN STS STRUCTURE THREE TIMESLARGER THAN STS-1 (THINK OF IT AS A 270 COLUMNS X 9RECTANGLE WITH 9 COLUMNS OF TOH).
STS-3C
TOH POHSPE = 9 x 261 BYTES
SERVICE SUMMARY
1. SUB-RATE SERVICE
• STS-1 VT STRUCTURE SPE (DS1, E1, DS1C, DS2)
2. FULL RATE SERVICE
• STS-1 SPE (DS3)
3. SUPER-RATE SERVICE
• STS-3C SPE (FDDI, ATM)