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Definition: SONET

(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.

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.


 Optical  Electrical    Speed
 Channel  Channel      (Mbps)

 OC-1      STS-1        51.84
 OC-3      STS-3       155.52
 OC-3c     STS-3c      155.52
 OC-12     STS-12      622.08
 OC-12c    STS-12c     622.08
 OC-48     STS-48     2488.32
 OC-192    STS-192    9953.28
 OC-768    STS-768   39813.12

 OC  = Optical Carrier
 STS = Synchronous Transport Signal
   c = concatenated

Transporting IP
In a WAN or over the Internet, IP traffic is widely carried over SONET lines, either using ATM as a management layer or over SONET directly. In the future, IP is expected to travel directly over DWDM fiber (rightmost diagram).