SONET Overview

• SONET is a physical layer standard.• A synchronous frame structure for multiplexing digital traffic.• Frame structure: 9 rows of 90 columns of 8-bit bytes.• Bytes are transmitted one row at a time, from left to right• First three columns of STS-1 frame are for section overhead and line

overhead• Remaining 87 columns are for the Synchronous Payload Envelope

(SPE)

STS-1 frame overheads

STS-1 frame: section overhead

17.4

Section overhead is recalculated for each SONET device(regenerators and multiplexers).

STS-1 frame: line overhead

STS-1 frame: path overhead

17.7

Path overhead is only calculated for end-to-end (at STS multiplexers).

SONET Structure

SONET ARCHITECTURE

• Path Layer: Mapping of DS0, DS1, DS3, FDDI to Payload• Line Layer: Frame synchronization and data multiplexing into a SONET

frame• Section Layer: Framing, error monitoring and bit-scrambling• Photonic Layer: Fiber characteristics, encoding, transmitter and

receiver types

In a simple SONET linear network

• PTE (path-terminating equipment) sits at each end, takes sub-rate signals and multiplexes them into a SONET frame• Between PTE devices are several LTE (line-terminating equipment) devices:

an ADM (add/drop multiplexer) – Instead of de-multiplexing entire signal, as PTE does, it pulls out only sub-rate services it needs and inserts different signals• If network is long enough, STE (section-terminating equipment) will be in

fiber. These repeaters take incoming signal from one side and rebroadcast it out on other side.

– Placement of STE in the network depends on strength of signal being transmitted down the fiber

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