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/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 MSB LSB BYTE MSB LSB

ONE-DIMENSIONAL FRAME VARYING SIZE PAYLOAD HEADERTRAILER 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 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..) A1 A2 J0 J1 D 0,0 D 0,1 D 0,2 …. D 0,85 B1 E1 F1 B3 D 1,0 D 1,1 D 1,2 …. D 1,85, ………….. S1 M0/1 E2 N1 D 8,0 D 8,1 D 8,2 …. D 8,85 START WITH MSB OF OCTET 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 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 = ) 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 STS-1 FRAME STS-1 FRAME H1H2H3 783 SPE OCTETS (0 – 782) LOH X OFFSET VALUES

SONET FRAME FLEXIBILTY STS-1 FRAME STS-1 FRAME H1H2H3 783 SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMES SPE ENDS AT OFFSET 86 (87 – 1) IN SECOND FRAME LOH EXAMPLE: STARTING POSITION = OFFSET VALUE

SONET FRAME FLEXIBILTY STS-1 FRAME STS-1 FRAME H1H2H LOH EXAMPLE: STARTING POSITION = OFFSET VALUE SPE OCTETS (0 – 782) SPANNING TWO STS-1 FRAMES SPE ENDS AT OFFSET 173 (174 – 1) IN SECOND FRAME

SONET FRAME FLEXIBILTY STS-1 FRAME STS-1 FRAME H1H2H LOH EXAMPLE: STARTING POSITION = OFFSET VALUE 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 NAMETRAFFIC# BYTES# COLUMNS VT1.5DS127 BYTES3 VT2E136 BYTES4 VT3DS1C54 BYTES6 VT6DS2108 BYTES 12

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

STS-1 FRAME1 STS-1 FRAME2 H1H2H3 27 SPE OCTETS LOH VT1.5 SERVICE (DS1)

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 = Mbps > Mbps) 21 VT2s CAN BE MULTIPLEXED ONTO THE STS-1 SIGNAL (774/36 = 21.5  21)

STS-1 FRAME1 STS-1 FRAME2 H1H2H3 36 SPE OCTETS LOH VT2 SERVICE (E1)

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

STS-1 FRAME1 STS-1 FRAME2 H1H2H3 54 SPE OCTETS LOH VT3 SERVICE (DS1C)

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

STS-1 FRAME1 STS-1 FRAME2 H1H2H3 108 SPE OCTETS LOH VT6 SERVICE (DS2)

DS3 = 28 DS1s  28 x 3 COLUMNS/DS1 = 84 COLUMNS 84 COLUMNS = 84 x 9 = 756 BYTES (756 x 8 x 8000 = Mbps > 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 SONET FULL RATE SERVICE

STS-1 FRAME1 STS-1 FRAME2 H1H2H3 756 SPE OCTETS LOH FULL RATE SERVICE (DS3)

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 BYTE INTER- LEAVED MUX STS-1 SIGNAL A STS-1 SIGNAL B STS-1 SIGNAL C STS-3 SIGNAL oc1 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 BYTE INTER- LEAVED MUX STS-3 SIGNAL A STS-3 SIGNAL B STS-3 SIGNAL C STS-12 SIGNAL oc3 oc12 STS-3 SIGNAL D oc3

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)