Synchronous Optical NetworksSONET

Networks: SONET1

Telephone Networks {Brief History}

• Digital carrier systems– The hierarchy of digital signals that the telephone

network uses.– Trunks and access links organized in DS (digital

signal) hierarchy– Problem: rates are not multiples of each other.

• In the 1980’s Bellcore developed the Synchronous Optical Network (SONET) standard.

• Previous efforts include: ISDN and BISDN.

Networks: SONET2

North American Digital Hierarchy

28

M13Multiplex

M23Multiplex

x7

PrimaryMultiplexEg. DigitalSwitch24 chan PCM

M12Multiplex

x4

1

DS3 44.736 Mbps

DS1 1.544 Mbps DS2 6.312 Mbps DS3 44.736 Mbps

PrimaryMultiplexEg. DigitalSwitch30 chan PCM

4th orderMultiplex

x4

2nd orderMultiplex

x4

3rd orderMultiplex

x4

34.368 Mbps2.048 Mbps 8.448 Mbps139.264 Mbps

CEPT 1CEPT 4

European Digital Hierarchy

Networks: SONET3

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.5

Networks: SONET4

SONET

Networks: SONET5

SONET:: encodes bit streams into optical signals propagated over optical fiber. SONET defines a technology for carrying many signals of different capacities through a synchronous,flexible, optical hierarchy.

• A bit-way implementation providing end-to-end transport of bit streams.

• All clocks in the network are locked to a common master clock so that simple TDM can be used.

• Multiplexing done by byte interleaving.• SONET is backward compatible to DS-1 and E-1 and forward

compatible to ATM cells.• Demultiplexing is easy.

SONET

• Transmission links of the telephone network have been changing to SONET where rates are arranged in STS (Synchronous Transfer Signal) hierarchy.

• The hierarchy is called SDH (Synchronous Digital Hierarchy) defined by CCITT.

• It is an ITU standard.

Networks: SONET6

Networks: SONET7

Low-SpeedMappingFunction

MediumSpeed

MappingFunction

High-Speed

MappingFunction

DS3

44.736

DS1DS2

CEPT-1

CEPT-4

139.264

ATM150 Mbps

STS-1

STS-1

STS-1STS-1STS-1

STS-1STS-1STS-1

STS-3c

STS-3c

OC-n

Scrambler E/O

51.84 Mbps

High-Speed

MappingFunction

MuxSTS-n

Networks: SONET8

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.8

Networks: SONET9

SONET Architecture

• SONET topology can be a mesh, but most often it is a dual ring.

• Standard component of SONET ring is an ADM (Add/Drop Multiplexer)– Drop one incoming multiplexed stream and

replace it with another stream.– Used to make up bi-directional line switching

rings.

Networks: SONET10

a

b

c

d

a

b

c

d ADM

ADM

ADM

ADM

(a) Dual ring (b) Loop-around in response to fault

Networks: SONET11

Leon-Garcia & Widjaja: Communication Networks Figure 4.12Copyright ©2000 The McGraw Hill Companies

Networks: SONET12SONET Ring

Networks: SONET13

MUX DEMUX MUX DEMUX

MUX DEMUX

(a) pre-SONET multiplexing

removetributary

inserttributary

ADM

removetributary

inserttributary

(b) SONET Add-Drop multiplexing

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.9

a

b

c

3 ADMs

physical loop net

OC-3nOC-3n

OC-3n

Networks: SONET14

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.10

Inter-OfficeRings

MetroRing

RegionalRing

Networks: SONET15

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.13

(a) STSPTE

LTESTE

STS-1 Path

STS LineSection Section

Mux Muxreg reg regSONETTerminal

STE STELTE

STSPTE

SONETTerminal

SONET Architecture

STE: Section Terminating Equipment, e.g. a repeaterLTE: Line Terminating Equipment, e.g. a STS-1 to STS-3 multiplexerPTE: Path Terminating Equipment, e.g. an STS-1 multiplexer

(b)

optical

section

optical

sectionoptical

section

optical

sectionline

optical

sectionline

optical

sectionlinepath

optical

sectionlinepath

Networks: SONET16

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.14

Networks: SONET17

Mux = BIM (Byte Interleaved Multiplexer)

Reg = Regenerator

• Boosts power of optical signal• Optical signal converted to electrical signal.• Amplify electrical signal.• Amplified electrical signal converted back to optical signal.

Networks: SONET18

Networks: SONET19

Networks: SONET20

SONET frame

B BB 87B

InformationPayload 9 Rows

125 µsTransportoverhead

90 bytes

3 rows

6 rows

SectionOverhead

LineOverhead

SPE Synchronous Payload Envelope

Networks: SONET21

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.15

SPE straddling SONET frame

Pointer87 columns

9rows

first column is path overhead

SynchronousPayload

Envelope

framek

framek+1

Pointer

first octet

last octet

Networks: SONET22

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.16

ATM Cells in an STS-3 Frame

Networks: SONET23

Synchronous Multiplexing in SONET

STS-1 STS-1

STS-1 STS-1

STS-1 STS-1

Map

Map

Map

STS-1 STS-1

STS-1 STS-1

STS-1 STS-1B

yteInterleave

STS-3

IncomingSTS-1 Frames

Synchronized NewSTS-1 Frames

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 4.17

Networks: SONET24