SONET The Telecom Source 10 Slide Technology Series

Introduction SONET stands for S ynchronous O ptical NET work SONET is a set of coordinated ITU, ANSI and Bellcore standards for high bit-rate fiber optic transmission The SONET standard defines a hierarchical set of transmission rates and transmission formats The “synchronous” in SONET refers to the method used to perform multiplexing in which all clocks are synchronized SONET is predominantly a North American standard SDH ( S ynchronous D igital H ierarchy) is the international equivalent of SONET

SONET stands for S ynchronous O ptical NET work

SONET is a set of coordinated ITU, ANSI and Bellcore standards for high bit-rate fiber optic transmission

The SONET standard defines a hierarchical set of transmission rates and transmission formats

The “synchronous” in SONET refers to the method used to perform multiplexing in which all clocks are synchronized

SONET is predominantly a North American standard

SDH ( S ynchronous D igital H ierarchy) is the international equivalent of SONET

Overview The SONET hierarchy defines hierarchy levels for transmission Each SONET level has an O ptical C arrier (OC) level and a corresponding electrical level transmission frame structure called the S ynchronous T ransport S ignal (STS) The base transmission rate for SONET is 51.840 Mbps STM ( S ynchronous T ransport M odule) is the SDH equivalent (i.e. international equivalent) of STS The base transmission rate for SDH is 155.52 Mbps

The SONET hierarchy defines hierarchy levels for transmission

Each SONET level has an O ptical C arrier (OC) level and a corresponding electrical level transmission frame structure called the S ynchronous T ransport S ignal (STS)

The base transmission rate for SONET is 51.840 Mbps

STM ( S ynchronous T ransport M odule) is the SDH equivalent (i.e. international equivalent) of STS

The base transmission rate for SDH is 155.52 Mbps

SONET Hierarchy STM-64 9953.280 STS-192 OC-192 STM-32 4976.640 STS-96 OC-96 STM-16 2488.320 STS-48 OC-48 STM-12 1866.240 STS-36 OC-36 STM-8 1244.160 STS-24 OC-24 STM-6 933.120 STS-18 OC-18 STM-4 622.080 STS-12 OC-12 STM-3 466.560 STS-9 OC-9 STM-1 155.520 STS-3 OC-3 - 51.840 STS-1 OC-1 SDH Equivalent Line Rate (Mbps) Electrical Level (STS) Optical Level (OC)

SONET Structure – High Level The SONET hierarchy is linear unlike the T-n hierarchy which is non-linear For example, 3 x OC-1 bit-rate = OC-3 bit-rate but 28 x DS-1 bit-rate is not equal to DS-3 bit-rate SONET is natively a channelized technology For example, an STS-3 contains 3 STS-1’s each with a bit-rate of 51.84 Mbps STS-1 frames can be concatenated to form a single high bit-rate SONET “channel” STS-1 frame concatenation is equivalent to unchannelized DS-n’s A concatenated SONET link is referred to as OC-nc or STS-nc (the “c” is for concatenated) For example, 3 STS-1 frames can be concatenated to form an STS-3c frame An STS-3c frame has a single payload and a bit rate of 155.52 Mbps STS/OC-n distinction STS frames are the electrical equivalent of the OC levels All SONET frame creation, multiplexing and adding of overhead bits is done in the electrical (i.e. STS) domain rather than the optical domain

The SONET hierarchy is linear unlike the T-n hierarchy which is non-linear

For example, 3 x OC-1 bit-rate = OC-3 bit-rate but 28 x DS-1 bit-rate is not equal to DS-3 bit-rate

SONET is natively a channelized technology

For example, an STS-3 contains 3 STS-1’s each with a bit-rate of 51.84 Mbps

STS-1 frames can be concatenated to form a single high bit-rate SONET “channel”

STS-1 frame concatenation is equivalent to unchannelized DS-n’s

A concatenated SONET link is referred to as OC-nc or STS-nc (the “c” is for concatenated)

For example, 3 STS-1 frames can be concatenated to form an STS-3c frame

An STS-3c frame has a single payload and a bit rate of 155.52 Mbps

STS/OC-n distinction

STS frames are the electrical equivalent of the OC levels

All SONET frame creation, multiplexing and adding of overhead bits is done in the electrical (i.e. STS) domain rather than the optical domain

SONET Structure - STS STS has a frame structure. An STS-1 frame Consists of 810 bytes (9 rows and 90 columns) Is transmitted at 8000 frames per second Stores the “overhead” info in the first 3 columns of the frame Stores the payload (i.e. the user info) in the other 87 columns An STS-n frame Consists of 810 x n bytes (9 rows and 90 x n columns) Is Formed by byte-wise multiplexing of the “n” STS-1 frames Is transmitted at 8000 frames per second Stores the “overhead” info in the first 3 x n columns of the frame STS-nc frames Consist of 810 x n bytes Are transmitted at 8000 frames per second STS frames are transmitted one row at a time from top to bottom STS-1 Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Columns 1 2 3 4 5 87 88 89 90 1 2 3 4 5 6 7 8 9 Rows Overhead Payload

STS has a frame structure.

An STS-1 frame

Consists of 810 bytes (9 rows and 90 columns)

Is transmitted at 8000 frames per second

Stores the “overhead” info in the first 3 columns of the frame

Stores the payload (i.e. the user info) in the other 87 columns

An STS-n frame

Consists of 810 x n bytes (9 rows and 90 x n columns)

Is Formed by byte-wise multiplexing of the “n” STS-1 frames

Is transmitted at 8000 frames per second

Stores the “overhead” info in the first 3 x n columns of the frame

STS-nc frames

Consist of 810 x n bytes

Are transmitted at 8000 frames per second

STS frames are transmitted one row at a time from top to bottom

SONET Structure - Virtual Tributaries Compatibility with lower rate existing digital hierarchies (i.e. T-n and E-n) is achieved via Virtual Tributaries Virtual Tributaries Allow SONET to carry lower rate info (e.g. DS-1) The lower rate (sub-rate) info is mapped into “sections” of an STS-1 frame These sections are each called a virtual tributary The virtual tributaries are independent of each other Each tributary can contain different types of information The STS-1 frame is divided into exactly 7 virtual tributary groups (VTG) VT types (i.e. VT1.5, VT2, …) cannot be mixed within a single VTG Each VTG in an STS-1 frame consists of 108 bytes each (9 rows by 12 columns) VT super-frames are possible There are four virtual tributary sizes (i.e. VT types) defined in SONET A single VTG can carry 4 VT-1.5s. Each is contained in three 9-byte columns (9 rows by 3 columns = 27 bytes). VT1.5 carries enough bandwidth to transport a DS-1 signal. A single VTG can carry 3 VT2s. Each is contained in four 9-byte columns (9 rows by 4 columns = 36 bytes). VT2 carries enough bandwidth to transport an E-1 signal. A single VTG can carry 2 VT3s. Each is contained in six 9-byte columns (9 rows by 6 columns = 54 bytes). VT3 carries enough bandwidth to transport a DS-1C signal. A single VTG can carry 1 VT6. It is contained in twelve 9-byte columns (9 rows by 12 columns = 108 bytes). VT6 carries enough bandwidth to transport a DS-2 signal.

Compatibility with lower rate existing digital hierarchies (i.e. T-n and E-n) is achieved via Virtual Tributaries

Virtual Tributaries

Allow SONET to carry lower rate info (e.g. DS-1)

The lower rate (sub-rate) info is mapped into “sections” of an STS-1 frame

These sections are each called a virtual tributary

The virtual tributaries are independent of each other

Each tributary can contain different types of information

The STS-1 frame is divided into exactly 7 virtual tributary groups (VTG)

VT types (i.e. VT1.5, VT2, …) cannot be mixed within a single VTG

Each VTG in an STS-1 frame consists of 108 bytes each (9 rows by 12 columns)

VT super-frames are possible

There are four virtual tributary sizes (i.e. VT types) defined in SONET

A single VTG can carry 4 VT-1.5s. Each is contained in three 9-byte columns (9 rows by 3 columns = 27 bytes). VT1.5 carries enough bandwidth to transport a DS-1 signal.

A single VTG can carry 3 VT2s. Each is contained in four 9-byte columns (9 rows by 4 columns = 36 bytes). VT2 carries enough bandwidth to transport an E-1 signal.

A single VTG can carry 2 VT3s. Each is contained in six 9-byte columns (9 rows by 6 columns = 54 bytes). VT3 carries enough bandwidth to transport a DS-1C signal.

A single VTG can carry 1 VT6. It is contained in twelve 9-byte columns (9 rows by 12 columns = 108 bytes). VT6 carries enough bandwidth to transport a DS-2 signal.

SONET Networks SONET is a fixed bandwidth “trunking” technology Service providers can overlay FR, ATM and other technologies on SONET to provide bandwidth on demand types of features Service adapters are used to map incoming signals from one service type (e.g. DS-1) to another service type (e.g. VT1.5) for SONET transmission Fiber optic links are inherently unidirectional and not full duplex and therefore are generally deployed in pairs SONET can be deployed in a point-to-point or ring configuration (ring configurations are much more common than point-to-point)

SONET is a fixed bandwidth “trunking” technology

Service providers can overlay FR, ATM and other technologies on SONET to provide bandwidth on demand types of features

Service adapters are used to map incoming signals from one service type (e.g. DS-1) to another service type (e.g. VT1.5) for SONET transmission

Fiber optic links are inherently unidirectional and not full duplex and therefore are generally deployed in pairs

SONET can be deployed in a point-to-point or ring configuration (ring configurations are much more common than point-to-point)

SONET Rings A SONET ring is a collection of more than 2 SONET network elements (nodes) forming a closed loop SONET ring architectures can be described by 3 basic attributes Number of fibers per link (2-fiber vs. 4-fiber) Direction of signal (unidirectional vs. bidirectional) Level of switch protection (line switching vs. path switching) SONET ring nomenclature uses abbreviations ULSR – unidirectional line switched ring BLSR – bidirectional line switched ring UPSR – unidirectional path switched ring BPRS – bidirectional path switched ring Each of the above options can be 2-fiber or 4-fiber Of the 8 possible ring architectures, only 3 are generally used in practice 2-fiber unidirectional path switched (2-fiber UPSR) 2-fiber unidirectional line switched (2-fiber ULSR) 4-fiber bidirectional path switched (4-fiber BPSR) Node 1 Node 2 Node 4 Node 3 SONET Ring

A SONET ring is a collection of more than 2 SONET network elements (nodes) forming a closed loop

SONET ring architectures can be described by 3 basic attributes

Number of fibers per link (2-fiber vs. 4-fiber)

Direction of signal (unidirectional vs. bidirectional)

Level of switch protection (line switching vs. path switching)

SONET ring nomenclature uses abbreviations

ULSR – unidirectional line switched ring

BLSR – bidirectional line switched ring

UPSR – unidirectional path switched ring

BPRS – bidirectional path switched ring

Each of the above options can be 2-fiber or 4-fiber

Of the 8 possible ring architectures, only 3 are generally used in practice

2-fiber unidirectional path switched (2-fiber UPSR)

2-fiber unidirectional line switched (2-fiber ULSR)

4-fiber bidirectional path switched (4-fiber BPSR)

SONET Rings …cont’d 2-fiber vs. 4 fiber 2-fiber rings have 2 physical fibers between each pair of nodes 4-fiber rings have 4 physical fibers between each pair of nodes Unidirectional vs. bidirectional In unidirectional rings, information flow between 2 nodes (node 1 to node 2 or node 2 to node 1) always travels in a single direction Unidirectional rings use a loop (or pair of loops) for working traffic and the other loop (or pair of loops) for protection In bidirectional rings, information flow between 2 nodes (node 1 to node 2 or node 2 to node 1) travels in 2 (opposite) directions Unidirectional and bidirectional rings are indistinguishable physically Line switching vs. path switching Line switching and path switching are forms of protection switching Line switching works by switching traffic to the protection fiber at both end of a span in the event of a failure Path switching works by sending traffic along both the working and protection fibers in the event of a failure and allowing the receiver the select the best signal Path switching can occur at the STS-1 or VT level as opposed to only the OC-n level for line switching

2-fiber vs. 4 fiber

2-fiber rings have 2 physical fibers between each pair of nodes

4-fiber rings have 4 physical fibers between each pair of nodes

Unidirectional vs. bidirectional

In unidirectional rings, information flow between 2 nodes (node 1 to node 2 or node 2 to node 1) always travels in a single direction

Unidirectional rings use a loop (or pair of loops) for working traffic and the other loop (or pair of loops) for protection

In bidirectional rings, information flow between 2 nodes (node 1 to node 2 or node 2 to node 1) travels in 2 (opposite) directions

Unidirectional and bidirectional rings are indistinguishable physically

Line switching vs. path switching

Line switching and path switching are forms of protection switching

Line switching works by switching traffic to the protection fiber at both end of a span in the event of a failure

Path switching works by sending traffic along both the working and protection fibers in the event of a failure and allowing the receiver the select the best signal

Path switching can occur at the STS-1 or VT level as opposed to only the OC-n level for line switching

Typical SONET Network ADM ADM TM TM D+R ADM D+R MN MN MN MN DSC DSC ADM ADM DSC TM (Terminal Multiplexer) - an end-point SONET device that converts from SONET to non-SONET format ADM (Add/Drop Multiplexer) - aggregates or splits (grooms) SONET traffic on to one of two SONET links DCS (Digital Cross-Connect) – similar to the ADM but can interconnect a large number of STS-n links MN (Matched Nodes) – used to interconnect SONET rings and provide an alternative path in case of failure D+R (Drop and Repeat Nodes) – used to send duplicate copies of signals along alternate paths

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