Sonet

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SONET/SDH
Savera Tanwir
Fiber Optic Communication Systems 2
Topics
Plesiochronous Digital Hierarchy
SONET
Advantages of SONET
Multiplexing
SONET Layers
SONET Frame Structure
Elements of SONET Infrastructure

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Plesiochronous Digital Hierarchy
Bit rate for one digital voice channel is 64 kbps
This is a widely accepted standard and is known as DS0
Signal.
Higher-speed streams were designed as multiples of this
basic stream.
The transmission is organized into frames.
Each frame contains a fixed number of time slots.
Each time slot is pre-assigned to a specific input link.
If the buffer of an input link has no data, then its
associated time slot is transmitted empty.
A time slot dedicated to an input link repeats
continuously frame after frame, thus forming a channel
or a trunk.

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T/E Carrier
The DS signal is carried over a carrier
system known as the T carrier.
T1 carries the DS1 signal,
T2 carries the DS2 signal etc
The ITU-T signal is carried over a carrier
system known as the E carrier.

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DS1 Signal
24 8-bit time slots/frame
Each time slot carries 8 bits/ 125 µsec, or the
channel carries a 64 Kbps voice.
Every 6th successive time slot (i.e, 6th, 12th,
18th, 24th, etc), the 8 bit is robbed and it is used
for signaling.
F bit: Used for synchronization. It transmits the
pattern: 10101010…
T1
– Total transmission rate: 24x8+1 = 193 bits per
125 µ sec, or 1.544 Mbps
E1
– 30 voice time slots plus 2 time slots for
synchronization and control
– Total transmission rate: 32x8 = 256 bits per
125 µsec, or 2.048 Mbps

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SONET
It is the transmission and multiplexing
standard for high speed signals within the
carrier infrastructure in North America.
SDH is used in Europe and Japan
Advantages of SONET
Multiplexing simplification
– Extra bits are added when 24 DS0s are multiplexed in DS1
– SONET multiplexers are much simpler
Management
– Extensive management information
Interoperability
– Standard optical interfaces
Network Availability
– SONET has evolved to incorporate specific network topologies
and specific protection techniques resulting in faster restoration
times

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Multiplexing
SONET and SDH employ a sophisticated multiplexing
scheme but it can be easily implemented in today’s VLSI
circuits
The basic rate is 51.84 Mbps called the Synchronous
Transport Signal level 1 (STS-1). This is the electrical
side of SONET
The electrical side of the SDH is known as the
synchronous transport module (STM).
STS and STM exist in the SONET equipment
The interface to other equipment is optical and is
scrambled version of STS signal in optical form.
The optical side of a SONET/SDH signal is known as the
optical carrier (OC).
SONET/SDH Hierarchy

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SONET/SDH is channelized
– STS-3 consists of 3 STS-1 streams, and each STS- 1
consists of a number of DS-1 and E1 signals.
– STS-12 consists of 12 STS-1 streams
Concatenated structures (OC-3c, OC-12c, etc)
– The frame of the STS-3 payload is filled with ATM
cells or IP packets packed in PPP or HDLC frames.
– Concatenated SONET/SDH links are commonly used
to interconnect ATM switches and IP routers
(Packets over SONET).
SONET/SDH Layers

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Path, Line and Section Overhead
Each layer, except the physical layer has a set of
overhead bytes associated with it
These over head bytes are added whenever the layer is
introduced and terminated whenever the layer is
removed in a network element
SONET frame structure

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Main Features
The frame is presented in matrix form and it is
transmitted row by row.
Each cell in the matrix corresponds to a byte
The first three columns contain overheads
The remaining 87 columns carry the synchronous
payload envelope (SPE), which consists of user data,
and additional overheads referred to as the Path
overhead (POH)
STS-1 payload
The payload consists of user data and the
path overhead
– User data:
Virtual tributaries: sub-rate synchronous data
streams, such as DS-0, DS-1, E1, and entire DS-3
frames
ATM cells and IP packets

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SONET/SDH devices
Several different equipment exist:
– Terminal multiplexer (TM)
– Add/drop multiplexer (ADM)
– Digital cross connect (DCS)
The terminal multiplexer (TM)
It multiplexes a number of DS-n or E1 signals into a
single OC-N signal
It consists of a controller, low-speed interfaces for DS-n
or E1 signals, an OC-N interface, and a time slot
interchanger (TSI)
It works also as a demultiplexer

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The add/drop multiplexer (ADM)
It is a more complex version of the TM
It receives an OC-N signal from which it can demultiplex
and terminate (i.e., drop) any number of DS-n or OC-M
signals, where M<N, while at the same time it can add
new DS-n and OC-M signals into the OC-N signal.
SONET Rings
SONET/SDH ADM devices are typically
connected to form a SONET/SDH ring.
SONET/SDH rings are self-healing, that is they
can automatically recover from link failures.

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Example
A transmits a DS-1 signal to TM 1
TM 1 transmits an OC-3 signal to ADM 1
ADM 1 adds the OC-3 signal into the STS-12
payload and transmits it out to the next ADM.
At ADM 3, the DS-1 signal belonging to A is
dropped from the payload and transmitted with
other signals to TM 2.
TM 2 in turn, demultiplexes the signals and
transmits A’s DS-1 signal to B.

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Digital Cross Connect
It is used to interconnect multiple SONET rings
It is connected to multiple incoming and outgoing OC-N
interfaces. It can drop and add any number of DSn
and/or OC-M signals, and it can switch DSn and/or OC-
M signals from an incoming interface to any outgoing
one.
Protection in SONET
Causes of failure
– Fiber-cut
– Transceiver or receiver fails
– SONET/SDH device fails
Point-to-Point Link Protection
– 1+1
– 1:1
– 1:N

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Protection in SONET Rings
1 + 1
– Dedicated protection
– Traffic is sent on working and protection fibers,
destination selects the best of the two signals
– Both fibers are diversely routed
1 : 1
– Traffic is sent on working fiber only
– In case of failure it is shifted on the other fiber
N : 1
– N working fibers are protected by one fiber
Two-Fiber Unidirectional Path
Switched Ring (2F-UPSR)

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Features
Working ring consists of fibers 1, 2, 3 and 4, and
the protection ring of fibers 5, 6, 7, and 8.
Unidirectional transmission means that traffic is
transmitted in the same direction. A transmits to
B over fiber 1 of the working ring, and B
transmits over fibers 2, 3, and 4 of the working
ring.
Self-healing Mechanism
Path level protection using the 1+1 scheme. The signal
transmitted by A is split into two. One copy is transmitted
over the working fiber 1, and the other copy is
transmitted over the protection fibers 8, 7, and 6.
During normal operation, B receives two identical signals
from A, and selects the one with the best quality. If fiber
1 fails, B will continue to receive A’s signal over the
protection path. The same applies if there is a node
failure.

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Two-fiber bidirectional line switched
ring (2F-BLSR)
Features
Fibers 1, 2, 3, 4, 5, and 6 form a ring, call it ring 1, on
which transmission is clockwise. Fibers 7, 8, 9, 10, 11,
and 12 form another ring, call it ring 2, on which
transmission is counter-clockwise.
Both rings 1 and 2 carry working and protection traffic.
This is done by dividing the capacity of each fiber on ring
1 and 2 to two parts. One part is used to carry working
traffic and the other protection traffic.
A transmits to B over the working part of fibers 1 and 2 of
ring 1, and B transmits to A over the working part of
fibers 8 and 7 of ring 2.

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Self-healing mechanism
The ring provides line switching. If fiber 2 fails
then the traffic that goes over fiber 2 will be automatically
switched to the protection part of ring 2.
That is, all the traffic will be re-routed to ADM 3 over the
protection part of ring 2 using fibers 7, 12, 11, 10, and 9.
From there, the traffic for each connection will continue
on following the original path of the connection.

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