What is the difference between SDSL and SHDSL ?

Symmetric DSL has typically been deployed in North America using a proprietary technology called Symmetric Digital Subscriber Loop (SDSL) utilizing 2B1Q line coding.

The industry is now moving to an internationally standardized (ITU 991.2) symmetric technology - SHDSL, that uses a more advanced coding technology called TC-PAM. This leads to higher loop reach together with better spectral compatibility with other DSL technologies.

Note some regions of Europe use the generic term "Symmetric" DSL (SDSL) to describe the ITU 991.2 SHDSL standard.

So far, SHDSL has been standardized by three different standardization bodies: ANSI (T1E1.4/2001-174) for North America, ETSI (TS 101524) for Europe and ITU-T (G.991.2) worldwide.