SONET: Synchronous Optical Network

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SONETSynchronous Optical Network
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Introduction

• SONET is a standard for interfaces in optical
  networks
• Physical layer transmission format
• SONET interacts very well with ATM since it has
  fiber based transmission scheme for ATM
• SONET is well adapted for IP and Ethernet

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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 (Contd)

• SONET includes
• support for broadband rates
• base rate approximately 50 Mbps
• hierarchical family of digital rates
• defines data rates up to 40 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 microsec/frame)

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STS-1 Framing Structure
...
9
rows
90 columns
1 byte
125 microsec
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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 (Contd)
90 columns
...
...
...
...
9
...
rows
...
...
...
...
Section and Line Overhead
Synchronous Payload Envelope (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 (Contd)

• 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 (Contd)

• 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 telcos
• These are called Virtual Tributaries (VTs)

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SONET Framing (Contd)

• Examples of VTs
• 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
90 columns
...
...
...
...
9
...
rows
...
...
...
...
Section and Line Overhead
VT 1.5
VT 2
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SONET Framing (Contd)

• 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 (Contd)

• Higher rate SONET signals are obtained by
  interleaving N STS-1s 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 VTs
  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
90 columns
...
...
...
...
9
...
rows
...
...
...
...
Section and Line 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