SONET:
Synchronous Optical Network
Carey Williamson
University of Calgary
1
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”
2
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)
3
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
4
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)
5
9
rows
STS-1 Framing Structure
...
...
...
...
...
...
...
...
...
90 columns
125 usec
6
1 byte
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)
7
9
rows
STS-1 Framing (Cont’d)
90 columns
...
...
...
...
...
...
...
...
...
Section and
Line Overhead
(3 columns)
Synchronous Payload
Envelope (SPE)
(87 columns)
8
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
9
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)
10
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)
11
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
12
9
rows
STS-1 Framing Example
90 columns
...
...
...
...
...
...
...
...
...
Section and
Line Overhead
VT 1.5
VT 2
13
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
14
SONET Framing (Cont’d)
 Higher
rate SONET signals are obtained
by interleaving N STS-1’s to form an STSN
(e.g., STS-3 = 155 Mbps)
 STS-N has 9 rows, and N x 90 columns
 Interleaving is done byte by byte
15
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 lie16
9
rows
STS-1 ATM Example
90 columns
...
...
...
...
...
...
...
...
...
Section and
Line Overhead
Start of ATM Cells
17
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
18