VC (Virtual Containers) – used with the SDH system

*** these are not the same thing as VT’s !!!

Bellcore/Telecordia GR-253 defines OC-1 as Optical Carrier 1.  They also define EC-1 as Electrical Carrier 1.  EC-1 is carried over 75Ohm Coax (like DS3). The framing, error checking, etc. is the same between the two. 

SONET is unique in that instead of defining different data rates with the same frame size – it defines different frame sizes which are multiples of the base frame (the STS-1) with a constant frame rate of 8000 frames per sec.  So, SONET offers different data rates because there are different frame sizes.

STS-1 is the base SONET frame format of a bitstream which may be optically transported on an OC-1 or electrically transported on an EC-1.

In telecommunications, most units of data are depicted as a group of bits or bytes, side-by-side.  For example, the IP packet, the ATM cell, etc.  However, the SONET frame is very large, and has a fixed size – therefore it is shown as a 2-dimensional array of bytes. 

IMPORTANT - the way SONET and SDH create different speeds is by using different sizes of frames.  The frame rate is always constant at 8000 fps !!!

Quick definitions of VT (Virtual Tributary) - Multiple channels at sub STS-1 data rates, that combine to form a “channelized STS-1”.  The VT is a sub-STS-1 rate   -   similar to the channels, or time slots in a channelized circuit such as 28 DS1’s within 1 DS3.  They are “tributaries”, like small streams that eventually combine to form the river, which is the STS-1.