1 SONET: Synchronous Optical Network Carey Williamson University of Calgary

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

3 SONET Overview u 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) u SONET includes: –support for broadband rates v base rate approximately 50 Mbps v hierarchical family of digital rates v defines data rates up to 2.4 Gbps – synchronous multiplexing v global timing structure at physical layer v synchronous implies simpler interface

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

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

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

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

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

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

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

12 SONET Framing (Cont’d) u 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 Mbps –VT 3: 6 columns (54 bytes) per frame, corresponds to Mbps –VT 6: 12 columns, Mbps

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

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

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

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

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