Synchronous optical networking (SONET)

The SONET standard includes four functional layers
They correspond to both the physical and the data link layers

Path layer
Line Layer
Section Layer
Photonic Layer

1. Synchronous optical networking
(SONET)
Dr R Jegadeesan Prof & CSE
Jyothishmathi Institute of Technology and Science,
karimnagar

2. •TheSONETstandardincludesfour
functionallayers
•Theycorrespondtoboththephysicaland
thedatalinklayers
➢Path layer
➢Line Layer
➢Section Layer
➢Photonic Layer

4. Path Layer
•Thepathlayerisresponsibleforthemovementofa
signalfromitsopticalsourcetoitsopticaldestination.
•Attheopticalsource,thesignalischangedfroman
electronicformintoanopticalform,multiplexedwith
othersignals,andencapsulatedinaframe.
•Attheopticaldestination,thereceivedframeis
demultiplexed,andtheindividualopticalsignalsare
changedbackintotheirelectronicforms.
•Pathlayeroverheadisaddedatthislayer.
•STSMultiplexers/Demultiplexersprovidepathlayer
functions.

5. Line Layer
•TheLineLayerisresponsibleforthe
movementofasignalacrossaPhysicalLine.
•Linelayeroverheadisaddedtotheframeat
thislayer.
•STSMultiplexer/Demultiplexerandadd/drop
multiplexersprovidelinelayerfunctions.

6. Section Layer
•Thesectionlayerisresponsibleforthe
movementofasignalacrossaphysical
section.
•Ithandlesframinganderrorcontrol.
•Sectionlayeroverheadisaddedtotheframe
atthislayer.

7. Photonic Layer
•Thephotoniclayercorrespondstothe
physicallayeroftheOSImodel.
•Itincludesphysicalspecificationsforthe
opticalfiberchannel,thesensitivityofthe
receiver,multiplexingfunctions

8. STS MULTIPLEXING
•InSONET,framesoflowerratecanbe
synchronouslytime-divisionmultiplexedinto
ahigher-rateframe.
•Forexample,3STS-1signals(channels)can
becombinedinto1STS-3signal(channel),
•4STS-3canbemultiplexedinto1STS-12,
andsoon,asshowninfigure.

10. SONET NETWORKS
•We can divide SONET networks into three
categories:
➢Linear networks
➢Ring networks
➢Mesh networks

12. Linear Networks
•A linear SONET network can be
➢Point-to-point
➢Multipoint.

13. Point-to-Point Network
•Apoint-to-pointnetworkisnormallymadeof
anSTSmultiplexer,anSTSdemultiplexer,and
zeroormoreregeneratorswithnoadd/drop
multiplexers
•Thesignalflowcanbeunidirectionalor
bidirectional
•AlthoughFigureshowsonlyunidirectionalfor
simplicity.

15. Multipoint Network
•AMultipointnetworkusesADMstoallowthe
communicationsbetweenseveralterminals.
•AnADMremovesthesignalbelongingtothe
terminalconnectedtoitandaddsthesignal
transmittedfromanotherterminal.
•Figureshowsaunidirectionalschemeinwhich
eachterminalcansenddataonlytothe
downstreamterminals,buttheamultipoint
networkcanbebidirectionaltoo

17. Ring Networks
•Add/DropMultiplexersmakeitpossibletohave
SONETringnetworks.
•SONETringscanbeusedineitheraunidirectional
orabidirectionalconfiguration.
•Ineachcase,wecanaddextraringstomakethe
networkself-healing,capableofself-recoveryfrom
linefailure.

18. Ring Networks of SONET are of two types
•Unidirectional Path Switching Ring
•Bidirectional Line Switching Ring

19. Unidirectional Path Switching Ring

20. •Aunidirectionalpathswitchingring(UPSR)
isaunidirectionalnetworkwithtworings:
•oneringusedastheworkingringandthe
otherastheprotectionring.
•Thesamesignalflowsthroughbothrings,
oneclockwiseandtheothercounter
clockwise.
•ItiscalledUPSRbecausemonitoringisdone
atthepathlayer.
•Anodereceivestwocopiesoftheelectrical
signalsatthepathlayer,comparesthem,and
choosestheonewiththebetterquality.

21. •If part of a ring between two ADMs fails, the
other ring still can guarantee the continuation
of data flow.
•UPSR, like the one-plus-one scheme, has fast
failure recovery, but it is not efficient because
we need to have two rings that do the job of
one.

22. Bidirectional Line Switching Ring

23. •Inthiscase,communicationisbidirectional,
whichmeansthatweneedtworingsfor
workinglines.
•Wealsoneedtworingsforprotectionlines.
ThismeansBLSRusesfourrings.
•If a working ring in one direction between
two nodes fails, the receiving node can use
the reverse ring to inform the upstream node
in the failed direction to use the protection
ring.

24. Mesh Networks
➢Oneproblemwithringnetworkswhenthetrafficin
aringincreases,weneedtoupgradenotonlythe
rings,butalsotheADMs.
➢Inthissituation,ameshnetworkwithswitches
probablygivebetterperformance.
➢Aswitchinanetworkmeshiscalledacross-
connect.
➢Across-connect,likeotherswitcheswehaveseen,
hasinputandoutputports.

25. •Inaninputport,theswitchtakesanOC-nsignal,
changesittoanSTS-nsignal,demultiplexesitinto
thecorrespondingSTS-1signals,andsendseach
STS-1signaltotheappropriateoutputport.
•An output port takes STS-1 signals coming
from different input ports, multiplexes them
into an STS-n signal, and makes an OC-n
signal for transmission.

27. SONET FRAMES

28. •Each synchronous transfer signal STS-n is
composed of 8000 frames
•Each frame is a two-dimensional matrix of bytes
with 9 rows by 90 x n columns.
•Forexample,STS-1frameis9rowsby90
columns(810bytes),andanSTS-3is9rowsby
270columns(2430bytes).

30. STS-1 Frame Format

31. •Thefirstthreecolumnsoftheframeareused
forsectionandlineoverhead.
•Theupperthreerowsofthefirstthree
columnsareusedforsectionoverhead(SOH)
•Thelowersixarelineoverhead(LOH).
•Therestoftheframeiscalledthe
SynchronousPayloadEnvelope(SPE)

32. Section Overhead
•The section overhead consists of nine octets.
•The functions of these octets are shown in
below

33. Alignment bytes (A1 and A2)
➢BytesA1andA2areusedforframingand
synchronizationandarecalledAlignmentbytes.
➢Thesebytesalertareceiverthataframeisarriving
andgivethereceiverapredeterminedbitpatternon
whichtosynchronize.
Sectionparitybyte(B1)
➢ByteB1isforbitinterleavedparityi.e.itmonitors
biterrorsbetweentwoadjacentregenerators.

34. Identification byte (C1)
➢Byte C1 carries the identity information of the
STS-1 frame
➢This byte is necessary when multiple STS-1s are
multiplexed to create a higher-rate STS (STS-3,
STS-9, STS-12, etc.).
➢Information in this byte allows the various signals
to be recognized easily upon demultiplexing.
➢For example, in an STS-3 signal, the value of
the C1 byte is 1 for the first STS-1; C1 byte is 2
for the second STS-1; and C1 byteis 3 for the
third STS-1.

35. Management bytes (D1, D2, and D3).
•ManagementBytesD1,D2andD3providethe
“SectionDataCommunicationChannel”forsignal
Monitoring&Maintenance.
•TheSectionDCCbandwidthis192Kbit/sbetween
eachpairofSONETsectionterminationequipment

36. Order wire byte(E1)
➢ByteE1istheOrderwirebyte.
➢Orderwirebytesinframesformachannelof
64kbps(8000framespersecondtimes8bits
perframe).
➢Thischannelisusedforcommunication
betweenregenerators,orbetweenterminals
andregenerators.

37. User's byte (F1).
The F1 bytes in consecutive frames form a 64-
kbps channel that is reserved forusers needs
at the section level.

38. Line Overhead
•Lineoverheadconsistsof18bytes.The
arrangementofthesebytesareshownin
figure

39. Line parity byte (B2)
➢ByteB2isforbitinterleavedparity.
➢Itisforerrorcheckingoftheframeovera
linei.e.betweentwomultiplexers
Data communication channel bytes (D4 to D12)
➢ThelineoverheadDbytes(D4toD12)in
consecutiveframesforma576-kbpschannel
thatprovidesthesameserviceastheD1-D3
bytesmonitoringandmaintenance,butatthe
lineratherthanthesectionlevel(between
multiplexers)

40. Order wire byte (E2)
➢TheE2bytesinconsecutiveframesforma64-
kbpschannelthatprovidesthesamefunctions
astheE1orderwirebyte,butatthelinelevel
Automaticprotectionswitchingbytes(K1andK2)
➢The K1 and K2 bytes in consecutive frames form a
128-kbps channel used for automatic detection of
problems in line-terminating equipments(Terminals).

41. Pointer bytes (H1, H2, and H3).
➢Bytes H1, H2, and H3 are pointers. These
bytes are used to show the beginning of the
SPE in the frame.
Growth bytes (Z1 and Z2)
➢The Z1 and Z2 bytes are reserved for data
that can be used in future of a line overhead

42. Synchronous Payload Envelope
•Thesynchronouspayloadenvelope(SPE)containstheuser
dataandtheoverheadrelatedtotheuserdatai.e.path
overhead
•Path overhead, the leftmost column of an SPE
•The path overhead must be added first to the user data to
create an SPE, and then an SPE can be inserted into one or
two frames.i.e some part of SPE can be in one frame another
part can be in the next frame.
•Path overhead consists of 9 bytes.
•The arrangement of these bytes are shown in figure

44. Path parity byte (B3)
Byte B3 is for bit interleaved parity, like bytes B1
and B2
Path signal label byte (C2)
➢ByteC2isthepathidentificationbyte.
➢ItisusedtoidentifyprotocolssuchasIPused
athigherlevels(layers)whosedataarebeing
carriedintheSPE.

45. Path user channel byte (F2)
The F2 bytes in consecutive frames, like the F1
bytes form a 64-kbps channel that is reserved for
user needs, but at the path level.
Path status byte (G1)
ByteG1issentbythereceivertocommunicate
itsstatustothesender,Whetheritisreadyto
receiveornot

46. Multiframe indicator (H4)
➢ByteH4isthemultiframeindicator.It
indicatesthepayloadsthatcannotfitintoa
singleframe.
Path trace byte (J1)
➢TheJ1bytesinconsecutiveframesforma64-
kbpschannelusedfortrackingthepath
betweenSTSMUX/DEMUX.

47. Growth bytes (Z3, Z4, and Z5).
➢Bytes Z3, Z4, and Z5 are reserved bytes. These
bytes are used for any future use for reserving
data in SPE.

48. Thank you