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SONET: Synchronous Optical Network

SONET: Synchronous Optical Network. Carey Williamson. University of Calgary. Introduction. SONET is a newly adopted standard for interfaces in optical networks Physical layer transmission format SONET defines a “fiber based transmission scheme for ATM”. SONET Overview.

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SONET: Synchronous Optical Network

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  1. SONET:Synchronous Optical Network Carey Williamson University of Calgary

  2. Introduction • SONET is a newly adopted standard for interfaces in optical networks • Physical layer transmission format • SONET defines a “fiber based transmission scheme for ATM”

  3. SONET Overview • The SONET specification defines: • standard optical signals, which permits the interoperation of equipment from different manufacturers • a synchronous frame structure for multiplexing digital traffic • procedures for operations and maintenance (OAM)

  4. SONET Overview (Cont’d) • SONET includes: • support for broadband rates • base rate approximately 50 Mbps • hierarchical family of digital rates • defines data rates up to 2.4 Gbps • synchronous multiplexing • global timing structure at physical layer • synchronous implies simpler interface

  5. SONET Framing Structure • Basic module is STS-1 Synchronous Transport Signal, Level 1 • STS-1 corresponds to 51.84 Mbps • Frame structure: 9 rows of 90 columns of 8-bit bytes • 8000 frames/sec (125 usec/frame)

  6. ... ... ... ... ... ... ... ... STS-1 Framing Structure ... 9 rows 90 columns 1 byte 125 usec

  7. STS-1 Framing • Bytes are transmitted one row at a time, from left to right • Note: 1 byte/frame = 64 kbps • First three columns of STS-1 frame are for section overhead and line overhead • Remaining 87 columns are for the Synchronous Payload Envelope (SPE)

  8. STS-1 Framing (Cont’d) 90 columns ... ... ... 9 ... ... rows ... ... ... ... Section and Line Overhead Synchronous Payload Envelope (SPE) (3 columns) (87 columns)

  9. SONET Overhead • Overhead bytes are used by SONET equipment (e.g., switches) for exchange of control and signalling information, and as a low bandwidth data channel • Three types of overhead bytes • section • line • path

  10. SONET Overhead (Cont’d) • Section overhead: 9 bytes per frame • Includes two framing bytes, plus other control information for maintenance and provisioning • Line overhead: 18 bytes per frame • Control info, plus 9 bytes for data channel • Path overhead: variable size • Payload type, path status, etc. • Transmitted as part of payload itself (SPE)

  11. SONET Framing (Cont’d) • The SPE in an STS-1 frame has sufficient capacity to carry a DS-3 (45 Mbps) • There are many other ways to “carve up” the capacity of an STS-1 into smaller units used by the telco’s • These are called Virtual Tributaries (VT’s)

  12. SONET Framing (Cont’d) • Examples of VT’s: • VT 1.5: requires 3 columns of 9 bytes each, corresponding to North American DS1 (T1) standard (1.544 Mbps) • VT 2: 4 columns, corresponds to European standard for 2.048 Mbps • VT 3: 6 columns (54 bytes) per frame, corresponds to 3.088 Mbps • VT 6: 12 columns, 6.312 Mbps

  13. STS-1 Framing Example 90 columns ... ... ... 9 ... ... rows ... ... ... ... Section and Line Overhead VT 1.5 VT 2

  14. SONET Framing (Cont’d) • A “VT group” is 9 rows x 12 columns • Can conveniently repackage into four VT 1.5, or three VT 2, or two VT 3, or one VT 6 • An STS-1 frame can hold 7 VT groups per frame (84 columns), with 1 column for path overhead, and 2 columns empty

  15. SONET Framing (Cont’d) • Higher rate SONET signals are obtained by interleaving N STS-1’s to form an STS-N (e.g., STS-3 = 155 Mbps) • STS-N has 9 rows, and N x 90 columns • Interleaving is done byte by byte

  16. SONET and ATM • If the entire STS-1 payload is to be used for ATM transmission, then there is no need to use VT’s at all • The 53-byte ATM cells are simply packaged into the SPE portion of the STS-1 frame, as they fit • Cells may wrap across STS-1 overhead bytes, or even STS-1 frame boundaries • Overhead byte keeps track of where ATM cell boundaries lie

  17. STS-1 ATM Example 90 columns ... ... ... 9 ... ... rows ... ... ... ... Section and Line Overhead Start of ATM Cells

  18. Summary • SONET defines a standard for framing and transmission at the physical layer on fiber-optic based networks • Framing structure is designed to accommodate common telco channel rates in both North America and Europe • ATM cells can be layered on top of the (synchronous) SONET framing structure

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