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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 200 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.

SONET and SDH
SONET is employed in the U.S. and Canada, whereas SDH (Synchronous Digital Hierarchy) is the SONET counterpart in the rest of the world. Although the terms "SONET/SDH" and "SDH/SONET" are widely used, there are differences in frame size and terminology.

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            Speed
 Level    Level                (Mbps)

 OC-1     STS-1    ----         51.84
 OC-3     STS-3    STM-1       155.52
 OC-9     STS-9    STM-3       466.56
 OC-12    STS-12   STM-4       622.08
 OC-18    STS-18   STM-6       933.12
 OC-24    STS-24   STM-8      1244.16
 OC-36    STS-36   STM-13     1866.24
 OC-48    STS-48   STM-16     2488.32
 OC-192   STS-192  STM-64     9953.28
 OC-768   STS-768  STM-256   39813.12
 OC-1920  STS-1920 STM-640   99532.80
 OC-3840  STS-3840 STM-1280 200000.00

 OC    = Optical Carrier
 STS-1 = Basic SONET building block
 STM-1 = Basic SDH building block





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).