SONET vs SDH   and    OC vs STS vs STM

It’s so confusing  .  .  .  you may hear that STS1=OC1, and STM1=STS3=OC3.  You may also even hear that SONET=SDH.  Those statements are false, although the following statements are true:

STS1 data rate =OC1 data rate 

STMn data rate = 3xSTSn data rate = 3xOCn data rate

So what’s the diff ?

SONET STS vs OC vs EC

• OC (Optical Carrier) - the SONET physical standard that defines an optical signal capable of transmitting STS frames.  As the name implies, it is the carrier at a specific data rate for the STS frames.  OC-n is a carrier for the STS-n signal. 
  
  

• EC (Electrical Carrier) ) - the SONET physical standard that defines an electrical signal capable of transmitting STS frames.  As the name implies, it is the carrier at a specific data rate for the STS frames.  EC-n is a carrier for the STS-n signal.

·         STS (Synchronous Transport Signal) - the SONET logical standard that defines the framing, payload, and overhead (signaling) for data transmission over either an optical carrier (OC) or in rare cases, an electrical carrier (EC).

STS  vs  OC

SONET vs SDH

Many people think that SONET and SDH are identical.  They do have many similarities, which makes conversion between the two very straightforward, and that conversion is key in enabling transoceanic communications.   But they also have many differences !! 

SONET/SDH Similarities

Data Rates – directly related, except that SDH has no STS1 (OC1) speed.  Besides that one missing speed, all the other data rates are directly convertible by a factor of 3:

STM-n = STS-3n = OC-3n

Frames – the physical placement of the transport columns and the payload columns within the frames are basically identical except for the terminology.  The overhead (the signaling - the contents of the overhead columns) however, is very different !!

SONET/SDH Differences

SONET defines the signal in two ways:  physical carrier (OC-n), and logical framing (STS-n). 

SDH defines the signal in one way only:  the framing at a specific carrier data rate (STM-n).  It makes no distinction between Physical & Logical signals.