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SONET:Synchronous Optical Network

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Basic Terms

Synchronous Synchronization is timekeeping which requires the

coordination of events to operate a system in unison.

Optical Network Network which work on optical fiber. Network which uses light as transmitting

signal.

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Introduction

SONET is a newly adopted standard for interfaces in optical networks

Physical layer transmission format SONET defines a “fiber based

transmission scheme for ATM”

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SONET Overview

The SONET specification defines:– standard optical signals, which permits the

interoperation of equipment from different manufacturers

– a synchronous frame structure for multiplexing digital traffic

– procedures for operations and maintenance (OAM)

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SONET Overview (Cont’d)

SONET includes:– support for broadband rates

base rate approximately 50 Mbps hierarchical family of digital rates defines data rates up to 2.4 Gbps

– synchronous multiplexing global timing structure at physical layer synchronous implies simpler interface

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SONET Framing Structure

Basic module is STS-1 Synchronous Transport Signal, Level 1

STS-1 corresponds to 51.84 Mbps Frame structure: 9 rows of 90 columns

of 8-bit bytes 8000 frames/sec (125 usec/frame)

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STS-1 Framing Structure...........................

125 usec

9rows

90 columns 1 byte

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STS-1 Framing

Bytes are transmitted one row at a time, from left to right

Note: 1 byte/frame = 64 kbps First three columns of STS-1 frame are

for section overhead and line overhead Remaining 87 columns are for the

Synchronous Payload Envelope (SPE)

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STS-1 Framing (Cont’d)...........................

9rows

90 columns

Section andLine Overhead

Synchronous PayloadEnvelope (SPE)(3 columns) (87 columns)

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SONET Overhead

Overhead bytes are used by SONET equipment (e.g., switches) for exchange of control and signalling information, and as a low bandwidth data channel

Three types of overhead bytes– section– line– path

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SONET Overhead (Cont’d)

Section overhead: 9 bytes per frame– Includes two framing bytes, plus other control

information for maintenance and provisioning Line overhead: 18 bytes per frame

– Control info, plus 9 bytes for data channel Path overhead: variable size

– Payload type, path status, etc.– Transmitted as part of payload itself (SPE)

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SONET Framing (Cont’d) The SPE in an STS-1 frame has sufficient

capacity to carry a DS-3 (45 Mbps) There are many other ways to “carve up”

the capacity of an STS-1 into smaller units used by the telco’s

These are called Virtual Tributaries (VT’s)

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SONET Framing (Cont’d) Examples of VT’s:

– VT 1.5: requires 3 columns of 9 bytes each, corresponding to North American DS1 (T1) standard (1.544 Mbps)

– VT 2: 4 columns, corresponds to European standard for 2.048 Mbps

– VT 3: 6 columns (54 bytes) per frame, corresponds to 3.088 Mbps

– VT 6: 12 columns, 6.312 Mbps

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STS-1 Framing Example...........................

9rows

90 columns

Section andLine Overhead

VT 1.5 VT 2

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SONET Framing (Cont’d) A “VT group” is 9 rows x 12 columns

– Can conveniently repackage into four VT 1.5, or three VT 2, or two VT 3, or one VT 6

An STS-1 frame can hold 7 VT groups per frame (84 columns), with 1 column for path overhead, and 2 columns empty

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SONET Framing (Cont’d) Higher rate SONET signals are obtained

by interleaving N STS-1’s to form an STS-N (e.g., STS-3 = 155 Mbps)

STS-N has 9 rows, and N x 90 columns Interleaving is done byte by byte

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SONET and ATM If the entire STS-1 payload is to be used for

ATM transmission, then there is no need to use VT’s at all

The 53-byte ATM cells are simply packaged into the SPE portion of the STS-1 frame, as they fit

Cells may wrap across STS-1 overhead bytes, or even STS-1 frame boundaries

Overhead byte keeps track of where ATM cell boundaries lie

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STS-1 ATM Example...........................

9rows

90 columns

Section andLine Overhead Start of ATM Cells

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Summary SONET defines a standard for framing

and transmission at the physical layer on fiber-optic based networks

Framing structure is designed to accommodate common telco channel rates in both North America and Europe

ATM cells can be layered on top of the (synchronous) SONET framing structure