(Synchronous Optical NETwork) A fiber-optic transmission system for high-speed digital traffic. Employed by telephone companies and common carriers, speeds range from 51 Mbps to 40 Gbps.

SONET is an intelligent system that provides advanced network management and a standard optical interface. Specified in the Broadband ISDN (B-ISDN) standard, SONET backbones are widely used to aggregate T1 and T3 lines. The European counterpart to SONET is the Synchronous Digital Hierarchy, and the term "SONET/SDH" is widely used when referring to SONET.

Self Healing
SONET can be built in a self-healing ring architecture that uses two or more transmission paths between nodes. In the event one path fails, traffic can be rerouted (see SONET ring).

TDM Multiplexing
SONET uses time division multiplexing (TDM) to send multiple data streams simultaneously. Its smallest increment of provisioning is VT-1.5, which provides 1.7 Mbps of bandwidth. The next increment, STS-1, jumps to 51.84 Mbps. Any data stream that does not fill that channel goes wasted.

ATM Over SONET
Telcordia's GR-2837 standard maps ATM cells onto SONET, turning a SONET pipe into a cell-switched (packet-switched) transmission carrier that utilizes the full bandwidth of the medium without waste. See 10 Gigabit Ethernet.

 SONET CIRCUITS

 Optical  Electrical
 Channel  Channel         Speed (Mbps)

          VT-1.5       1.7
 OC-1     STS-1       51.84 (28 DS1s or 1 DS3)
 OC-3     STS-3      155.52 (3 STS-1s)
 OC-3c    STS-3c     155.52 (concatenated)
 OC-12    STS-12     622.08 (12 STS-1, 4 STS-3)
 OC-12c   STS-12c    622.08 (12 STS-1, 4 STS-3c)
 OC-48    STS-48    2488.32 (48 STS-1, 16 STS-3)
 OC-192   STS-192   9953.28 (192 STS-1, 64 STS-3)
 OC-768   STS-768  39813,12 (768 STS-1, 256 STS-3)

 OC  = Optical Carrier
 STS = Synchronous Transport Signal