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
(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.
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 TWODIMENSIONAL STRUCTURE
OCTET
• AN OCTET IS A GROUPING OF 8 BITS, WITH THE
FOLLOWING CHARACTERISTICS:
MSB
1
LSB
2
3
5
6
7
BYTE
MSB
7
4
6
5
4
8
LSB
3
2
1
0
ONE-DIMENSIONAL FRAME
HEADER
TRAILER
VARYING SIZE PAYLOAD
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 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..)
9
START WITH
MSB OF
OCTET 1
90
R
i
C
j

i 1 j 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
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 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)
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
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)
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
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.
SONET FRAME FLEXIBILITY
OFFSET VALUES
X
STS-1
FRAME1
LOH
STS-1
FRAME2
H1 H2 H3
0
1
2
84
85
86
87
88
89
171
172
173
435
436
437
519
520
521
522 523 524
609 610 611
606 607 608
693 694 695
696
780
697
698
783 SPE OCTETS (0 – 782)
781
782
SONET FRAME FLEXIBILTY
EXAMPLE: STARTING POSITION = OFFSET VALUE 87
STS-1
FRAME1
LOH
STS-1
FRAME2
H1 H2 H3
0
1
2
84
85
86
87
88
89
171
172
173
435
436
437
519
520
521
522 523 524
609 610 611
606 607 608
693 694 695
696
0
780
84
697
1
698
2
781
85
782
86
783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMES
SPE ENDS AT OFFSET 86 (87 – 1) IN SECOND FRAME
SONET FRAME FLEXIBILTY
EXAMPLE: STARTING POSITION = OFFSET VALUE 174
STS-1
FRAME1
LOH
STS-1
FRAME2
H1 H2 H3
0
1
2
84
85
86
87
174
88
175
89
176
171
258
172
259
173
260
435
436
437
519
520
521
522 523 524
609 610 611
606 607 608
693 694 695
696
0
87
780
84
171
697
1
88
698
2
89
781
85
172
783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMES
SPE ENDS AT OFFSET 173 (174 – 1) IN SECOND FRAME
782
86
173
SONET FRAME FLEXIBILTY
EXAMPLE: STARTING POSITION = OFFSET VALUE 522
STS-1
FRAME1
LOH
STS-1
FRAME2
H1 H2 H3
0
1
2
84
85
86
87
174
88
175
89
176
171
258
172
259
173
260
435
436
437
519
520
521
522 523 524
609 610 611
606 607 608
693 694 695
696
0
87
174
261
348
697
1
88
175
262
349
698
2
89
176
263
350
780
84
171
258
345
432
781
85
172
259
346
433
782
86
173
260
347
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).
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)
SONET
SERVICES
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
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)
VT1.5 SERVICE (DS1)
STS-1
FRAME1
LOH
H1 H2 H3
STS-1
FRAME2
27 SPE OCTETS
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
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)
VT2 SERVICE (E1)
STS-1
FRAME1
LOH
H1 H2 H3
STS-1
FRAME2
36 SPE OCTETS
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)
VT3 SERVICE (DS1C)
STS-1
FRAME1
LOH
H1 H2 H3
STS-1
FRAME2
54 SPE OCTETS
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)
VT6 SERVICE (DS2)
STS-1
FRAME1
LOH
H1 H2 H3
STS-1
FRAME2
108 SPE OCTETS
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
FULL RATE SERVICE (DS3)
STS-1
FRAME1
LOH
H1 H2 H3
..............
..............
..............
..............
..............
..............
STS-1
FRAME2
..............
..............
..............
756 SPE OCTETS
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.
HIGHER LEVEL STS FRAMES
STS-3
STS-1 SIGNAL A
oc1
STS-1 SIGNAL B
oc1
STS-1 SIGNAL C
oc1
BYTE
INTERLEAVED
MUX
STS-3 SIGNAL
oc3
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
HIGHER LEVEL STS FRAMES
STS-12
STS-3 SIGNAL A
oc3
STS-3 SIGNAL B
oc3
STS-3 SIGNAL C
oc3
STS-3 SIGNAL D
oc3
BYTE
INTERLEAVED
MUX
STS-12 SIGNAL
oc12
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.
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.
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).
STS-3C
TOH
POH
SPE = 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)