Synchronous Optical Network

The foundation of modern optical fiber communication. Master the architecture, frame structure, and synchronous hierarchy of SONET/SDH.

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Why SONET?

Before SONET (Synchronous Optical Network), the telecommunications network was a mess of proprietary optical interfaces. Each vendor had their own standard, making interconnection between different carriers nearly impossible.

Developed in the mid-1980s by Bellcore, SONET standardized the digital hierarchy for optical transmission. It provides a synchronous frame structure, allowing direct access to low-rate channels (like DS0 or DS1) without fully demultiplexing the entire high-rate stream—a process known as Drop and Insert.

Key Advantage:

SONET uses a byte-interleaved multiplexing scheme, making it much simpler to extract specific channels compared to the bit-interleaved PDH (T-carrier) systems.

Evolution of Transport

Analog Era

Copper wires, FDM systems. High noise, low capacity.

PDH (T-Carrier)

Digital, but asynchronous. Complex multiplexing (Mux Mountains).

SONET/SDH

Optical, Synchronous, Standardized. Direct add/drop capability.

System Architecture

Photonic Layer

The physical medium. Deals with optical-to-electrical conversion.

  • Bit transmission
  • Pulse shaping
  • Wavelength specs (1310/1550nm)

Section Layer

Transport of STS-N frames across the physical medium.

  • Framing, Scrambling
  • Section Error Monitoring
  • Section DCC (Data Comm)

Line Layer

Reliable transport of path layer payload between devices.

  • Synchronization
  • Multiplexing for Paths
  • Line Protection Switching

Path Layer

The end-to-end transport service. This is where the actual service (DS1, DS3, ATM, IP) is mapped into the SONET payload.

Path Overhead (POH) Monitors end-to-end quality, status, and labeling of the payload.
Mapping How DS3, ATM cells, or Ethernet frames are packed into the SPE.

STS-1 Frame Structure

9 Rows x 90 Columns (810 bytes). Transmitted row by row, left to right.

Columns: 1-3 (Transport Overhead) Columns: 4-90 (SPE / Payload)

Byte Details

Hover over a byte in the frame to see its function.

Legend

Section Overhead
Line Overhead
Path Overhead
Fixed Stuff / Payload

SONET Hierarchy (OC-N)

1

OC-1

STS-1

Line Rate: 51.84 Mbps
Payload: 50.11 Mbps
DS3 Equiv: 1x
3

OC-3

STS-3

Line Rate: 155.52 Mbps
Payload: 150.3 Mbps
DS3 Equiv: 3x
12

OC-12

STS-12

Line Rate: 622.08 Mbps
Payload: 601.3 Mbps
DS3 Equiv: 12x
48

OC-48

STS-48

Line Rate: 2.488 Gbps
Payload: 2.40 Gbps
DS3 Equiv: 48x

Bandwidth Calculator

Calculate capacity for concatenated or non-concatenated payloads.

Line Rate 51.840 Mbps
SPE Size (Payload) 50.112 Mbps
User Data (VT/VC Capacity) ~49.5 Mbps
Number of DS1s (1.544 Mbps) 28
Number of DS3s (44.736 Mbps) 1

Essential Concepts

Scrambling

SONET uses a 7-bit frame-synchronous scrambler (x^7 + x^6 + 1) to ensure sufficient 1s density for clock recovery. Note: The Section Overhead (first 3 rows) is not scrambled to allow frame alignment before descrambling.

Pointer (H1, H2, H3)

The Pointer allows the SPE to float within the frame. This accommodates phase differences and jitter between the SPE and transport overhead. H1/H2 indicate the offset, H3 is the negative justification byte.

Concatenation (c)

Used when the payload is larger than an STS-1 (e.g., ATM, Gigabit Ethernet). Treats multiple STS-1s as a single large pipe (e.g., STS-3c). The payload is not divided; it uses one set of path overhead for the whole group.

Virtual Tributaries (VT)

Sub-STS-1 structures used to transport sub-rate signals (DS1, DS2, E1). VT1.5 (1.728 Mbps) is used for DS1. 7 VT1.5s fit into one STS-1 SPE.