[Next] [Previous] [Top]

Introduction

Introduction

Why SDH/SONET

• Previous technology - PDH - Plesiochronous Digital Heirarchy was limited:
  • US and European systems had little in common - expensive translators required for transatlantic traffic

  • "Standard" equipment from different vendors was incompatible

  • No self checking - expensive manual check and repair system

  • No standard for high bandwidth links - proprietary

  • Not synchronous above US DS-1 bandwidth

Synchronous?

• What does synchronous mean to a telephone engineer

• US telephone calls, DS-0, are multiplexed 24 per DS-1 channel

• DS-1 lines are synchronous it is easy to remove or insert a call

Plesiochronous?

• Plesiochronous means

• 4 DS-1 lines are multiplexed for DS-2

• 7 DS-2s are multiplexed to DS-3

• To isolate a particular call from DS-3 it must be demultiplexed to DS-1

• Very expensive equipment is needed at every exchange to demultiplex and multiplex high speed lines

Pointers

• Pointers were proposed as an alternative to bit stuffing

• Pointer in frame header points to data in that frame

• Not truely synchronous - but near enough. A single call could be isolated from a heavily multiplexed signal

Conclusions

• SDH/SONET would:
  • Improve on existing DS-3 multiplexing standard

  • Provide a non-proprietary solution

  • Establish a heirarchy of digital standards compatible with European and US systems

  • Give economic access to low volume traffic

  • Support more sophisicated services such as broadband ATM