Chapter 8
Digital Transmission Systems
Part 2
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10 SDH and SONET
• SDH is an acronym for Synchronous Digital
Hierarchy. It is an European development.
• SDH: is a hierarchical set of digital transport
structures (Overhead), standardized for the
transport of suitably adapted Payloads over
physical transmission networks.
• SONET is an acronym for Synchronous
Optical Network . It is a North American
development.
• The two (SDH and SONET) are very similar.
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• Either one can accommodate the standard
E1 family (i.e., 2.048 Mbps, etc.) and DS1
family (i.e., 1.544 Mbps, etc.) of line rates.
• SDH/SONET is replacing PDH systems in
the Transport Network.
• By Transport Network we mean the flexible
high-capacity transmission network that is
used to carry all types of information.
• By Flexible we mean that
telecommunications operators are able to
easily modify the structure of the transport
network from the centralized management
system.
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10.1 Advantages of SDH/SONET
• SDH/SONET is based on the principal
of direct synchronous multiplexing,
where separate, slower signals can be
multiplexed directly onto higher speed
SDH/SONET signals without
intermediate stages of multiplexing.
• SDH/SONET is more flexible and
reliable than PDH and provides
advanced network management and
maintenance features.
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• Can be used in Long-Haul Networks,
Local Networks and Loop Carriers, and
it can also be used to carry CATV video
traffic, ATM, and ISDN.
• In SDH/SONET format, only those
channels that are required at a particular
point are demultiplexed, thereby
eliminating the need for back-to-back
multiplexing. In other words, SDH/SONET
makes individual channels “visible” and
they can easily be added and dropped.
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• The data rates for optical transmission are
standardized (i.e., vendor independent).
• Different systems are included in
standards, for example, Terminal,
Add/Drop, and Cross-Connection
Systems.
• These systems make SDH/SONET
networks more flexible than PDH
systems, which include only terminal
multiplexer functionality.
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10.2 Why SDH/SONET
• Originally, all communications in the
telephone network was analog.
• Analog lines or analog microwave links
were used to connect to switching offices.
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• In about 1962, the network providers
began using digital communications
between switching centers.
• This was PDH system (DS-Carrier in US
and E-Carrier in Europe).
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• As communications needs grew, many DSCarrier or E-Carrier lines were needed between
switching centers.
• In the late 1970’s optical communications began
to be used to interconnect switching offices.
E1 or DS1
E1 or DS1
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• Prior to standardization, every
manufacturer of optical communications
used their own framing.
• The ANSI and the ITU began work in 1986
to define standards for optical
communications.
• Both bodies finalized their first set of
standards in 1988.
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• They defined the following:
1. Optical and Cupper interfaces
(wavelength, frequency, power, etc.).
2. Rates, frame formats, and network
elements (Layers).
3. Operations, Administration, and
Maintenance (OAM) functions including
monitoring for valid signal, defect
reporting, and alarms due to abundant
overhead bits.
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10.3 Basic SDH/SONET
Transmission Rates (Hierarchy)
• SONET and SDH converge at SDH’s 155 Mbps
base level, defined as STM-1 (Synchronous
Transport Module-1).
• The base level for SONET is STS-1
(Synchronous Transport Signal-1) or OC-1
(Optical Carrier-1) and is equivalent to 51.84
Mbps.
• Thus, SDH’s STM-1 is equivalent to SONET’s
STS-3 (3 x 51.84 Mbps = 155.52 Mbps).
• Higher SDH rates of STM-4 (622 Mbps), STM16 (2.4 Gbps), and STM-64 (10 Gbps) have
also been defined.
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SDH
Bit Rate
SDH Capacity
STM-0
51.84 Mbps
21 E1
STM-1
155.52 Mbps
63 E1 or 1 E4
STM-4
622.08 Mbps
252 E1 or 4 E4
STM-16
2488.32 Mbps
1008 E1 or 16 E4
STM-64
9953.28 Mbps
4032 E1 or 64 E4
STM-256
39812.12 Mbit/s
16128 E1 or 256 E4
SONET
Bit Rate
SONET Capacity
STS-1, OC-1
51.84 Mbps
28 DS1 or 1 DS3
STS-3, OC-3
155.52 Mbps
84 DS1 or 3 DS3
STS-12, OC-12
622.08 Mbps
336 DS1 or 12 DS3
STS-48, OC-48
2488.32 Mbps
1344 DS1 or 48 DS3
STS-192, OC-192
9953.28 Mbps
5376 DS1 or 192 DS3
STS-768, OC-768
39812.12 Mbps
21504 DS1s or 768 DS3
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Data Rates of SONET and Corresponding SDH Data Streams
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• Multiplexing is accomplished by combining or
interleaving multiple lower-order signals (1.5
Mbps DS1 carrier, 2 Mbps E1 carrier, etc.) into
higher-speed circuits (51 Mbps STS-1, 155
Mbps STM-1, etc.).
• By changing the SONET standard from BitInterleaving to Byte-Interleaving, it became
possible for SDH to accommodate both
transmission hierarchies.
• This modification allows an STM-1 signal to
carry multiple 1.5 Mbps or 2 Mbps signals and
multiple STM signals to be aggregated to carry
higher orders of SONET or SDH tributaries.
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11 SDH STM Signal
• SDH multiplexing combines low-speed
digital signals such as 2, 34, and 140
Mbps signals with required Overhead to
form a frame called STM-1.
• SDH is a Byte-Interleaving multiplexing
system.
• An STM is the information structure used
to support Section Layer Connections in
the SDH.
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• It consists of information Payload and
Overhead (OH) information fields
organized in a block frame structure which
repeats every 125 μS.
• The information is suitably conditioned for
serial transmission on the selected media
at a rate which is synchronized to the
network.
• STM-1 is the base level of SDH.
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• The STM-1 frame, is created by 9 segments
of 270 bytes each (1-byte = 8-bits)
• The first 9 bytes of each segment carry
Overhead (OH) information.
• The remaining 261 bytes carry Payload.
• When visualized as a block, the STM-1
frame appears as 9 rows by 270 columns
of bytes.
• The STM-1 frame is transmitted row-by-row.
• Row #1 first, with the most significant bit
(MSB) of each byte transmitted first, then the
Row #2 and so on, up to Row #9.
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• The STM-1 frame lasts for 125 μS, in
other words, the 9 row segments will be
transmitted in a total time equal to 125
μS.
• This will permit SDH to easily integrate
existing digital services into its hierarchy.
• Therefore, there are 8000 frames per
second.
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Segment no. 7
270 Bytes in (125/9) μS
The STM-1 frame
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STM-1 frame visualized as a block, and the direction of transmission
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STM-1 frame visualized as a block
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• Hence, the STM-1 frame rate Rf is
Rf = 8000 frames per second
• The bit rate Rb of STM-1 frame is
calculated as follow:
Rb = Rf x Cf , where
Rf is the frame rate (frames/second).
Cf is the frame capacity (bits/frame).
• The frame capacity of a signal is the
number of bits contained within a single
frame.
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• We know that the frame rate is
Rf = 8000 frames/second.
• Cf is calculated as follow
Cf = 270 bytes/row x 9 rows/frame x 8
bits/byte
= 19,440 bits/frame
• Then, the bit rate Rb of the STM-1 signal is
calculated as follows:
Rb = 8000 frames/second x 19,440 bits/frame
= 155.52 Mbps
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• The multiplexing of multiple data
stream, plays an important role in SDH.
• Byte Interleaving scheme is used to
multiplex multiple data stream.
• The higher transmission levels
(Multiplex) such as STM-4 and STM-16
of the SDH Hierarchy are generated
from integer multiples of STM-1 signal.
• In general, STM-N signal is generated
by Byte Interleaving N STM-1 signal.
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Multiplexing of STM-1 to generate STM-N
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STM-N signal frame structure
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• Example:
• An STM-4 signal will be created by Byte
Interleaving four STM-1 signals.
• The basic frame rate remains 8,000
frames per second, but the capacity is
quadrupled, resulting in a bit rate of
4 x 155.52 Mbps or 622.08 Mbps.
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Multiplexing of STM-1 to generate STM-4
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11.1 SDH STM-1 Frame Structure
•
As we know that, the SDH frame STM-1
consists of two parts:
1. The First Nine Columns comprise the
Overhead (OH), occurs at a rate
9 x 9 x 8 x 8000 = 5.184 Mbps.
2. While the remainder is called the
Payload, which is also called Virtual
Container (VC), occurs at a rate
9 x 261 x 8 x 8000 = 150.336 Mbps.
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•
1.
2.
•
The OH is further divided into:
Section Overhead (SOH).
Administrative Unit Pointer (AU-PTR).
The Payload or Virtual Container (VC)
is further divided into:
1. Path Overhead (POH): One column.
2. Container (C): 260 columns and data
rate given by
9 x 260 x 8 x 8000 = 149.76 Mbps.
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125 μS
1
9 10 11
270
1
SOH
AU-PTR
POH
3
4
9 rows
Container
SOH
9
9 columns
271 columns
Overhead (OH)
Virtual Container (VC)
SDH frame STM-1 structure
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• The SOH dedicates
1. Three Rows for the Regenerator Section
Overhead (RSOH) and
2. Six Rows for the Multiplexer Section
Overhead (MSOH).
• Rate of RSOH and MSOH is given by
RSOH = 3 x 9 x 8 x 8000 = 1.728 Mbps.
MSOH = 6 x 9 x 8 x 8000 = 3.456 Mbps.
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125 μS
1
9 10 11
270
1
RSOH
AU-PTR
POH
3
4
9 rows
Container
MSOH
9
9 columns
261 columns
Overhead (OH)
Virtual Container (VC)
SDH frame STM-1 structure
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11.2 The Truck Analogy
• SDH frame functions as a transport truck
which has a tractor and a container type
trailer.
• It packs the signals of different hierarchies
into packages of different sizes like packing
cargoes and then loads them into the truck.
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SDH frame
• The contents carrier in the container are real
goods.
• These are analogous to customer traffic,
being carried in the Payload area of SDH
frame.
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11.3 The Function of OHs
• The OH within the SDH signal supports
network management at both the Path
and Section levels.
• To realize layered monitoring, the OH is
classified into SOH and POH.
• SOH and which includes RSOH and
MSOH, is responsible for the section
layer OAM.
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•
1.
2.
3.
4.
5.
6.
7.
8.
SOH functions are:
Frame alignment pattern.
Parity check.
STM-1 identification.
Alarm information.
Automatic protection switching.
Data communication channel.
Voice communication channel.
User channel.
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• The POH is responsible for the Path layer
OAM functions.
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11.4 Function of The Pointer
• SDH network is intended to be synchronous
network.
• However, there will always be slight timing
differences because different clocks are
being used or the same clock is being
distributed over long distances.
• SDH Pointers allow this limited
asynchronous operation within the
synchronous network.
• It points the location of the VC in the STM
frame.
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12 SDH Signal Hierarchy
Typical SDH Communication Network
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•
There are three Sections in the SDH
signal hierarchy:
1. Path.
2. Multiplex Section.
3. Regenerator Section.
• The Overheads (OHs) are always
generated at the beginning of a section
and only evaluated at the end of a
section.
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The SDH Layer Model
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12.1 SDH Network Elements
•
The SDH signal is layered to divide
responsibility for transporting the Payload
through the network.
• Each SDH Network Element (NE) is
responsible for
1. Interpreting and generating its overhead layer,
2. Communicating control and status information
to the same layer in other equipment,
3. Terminating its overhead layer.
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•
As the Payload travels through the SDH
network, each layer is terminated by one
of a general class of NEs named
1. Regenerator Section Terminating
Equipment (RSTE),
2. Multiplexer Section Terminating
Equipment (MSTE),
3. Path Terminating Equipment (PTE).
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12.1.1 Path Terminating
Equipment (PTE)
• PTE is an entry-level path-terminating
terminal multiplexer, acts as a concentrator of
E1s as well as other tributary signals.
Terminal multiplexer example
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• PTE is a terminating multiplexer.
• It is responsible for adding first order POH,
RSOH, and MSOH to the data Container (C).
• Its simplest deployment would involve two
terminal multiplexers linked by fiber with or
without a regenerator in the link.
• This implementation represents the simplest
SDH link (Regenerator Section, Multiplex
Section, and Path, all in one link).
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11.1.2 Regenerator
• A regenerator is needed when, due to the
long distance between multiplexers, the
signal level in the fiber becomes too low.
Regenerator.
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• The regenerator recovers timing from
the received signal and replaces the
existing Regenerator Section overhead
(RSOH) bytes of the received STM
signal before retransmitting the signal;
the Multiplex Section Overhead
(MSOH), Path Overhead (POH), and
Container (C) are not altered.
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11.1.3 ADM
• It is responsible for adding higher order
RSOH, MSOH, and POH to the received
STM signal. It is also responsible for
evaluating RSOH, MSOH, and POH.
• A single-stage Multiplexer/Demultiplexer
can multiplex various inputs into an STMN signal.
• At an Add/Drop site, only those signals
that need to be accessed are dropped or
inserted.
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• The remaining traffic continues through
the network element without requiring
special pass through units or other signal
processing.
• In rural applications, an Add/Drop
Multiplexer (ADM) can be deployed at a
terminal site or any intermediate location
for consolidating traffic from widely
separated locations.
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STM-N
Add/Drop multiplexer example.
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A Synchronous Add–Drop Multiplexer (ADM)
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11.1.4 DXC
• An SDH Digital Cross-Connect (DXC)
accepts various E-carrier and STM rates,
accesses the STM-1 signals, and switches
at this level.
• It is responsible for adding RGSO, and
MSOH without altering the POH.
• One major difference between a DXC and
an ADM is that a DXC may be used to
interconnect a much larger number of STM1s.
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Digital Cross-Connect (DXC)
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• It is ideally used at an SDH Hub Network.
• The DXC can be used for grooming
(consolidating or segregating) of STM-1s
or for broadband traffic management.
• For example, it may be used to segregate
high-bandwidth from low-bandwidth traffic
and send them separately to the highbandwidth (for example video) switch and
a low-bandwidth (voice) switch.
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Example: Assume that the traffic is traveling
from west to east.
1. If regenerator section errors are
detected at Site 4.
• A problem will be somewhere between
Site 3 and Site 4.
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• The observed problem can not be west
of Site 3, since all regenerator section
results are recalculated at every point in
the network.
2. If multiplexer section and regenerator
errors are found at Site 4.
• A problem exists between Site 2 and
Site 4, since multiplexer section results
are recalculated only at major network
nodes, such as the SDXS at Site 2.
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3. Furthermore, if multiplexer section
errors, but not regenerator errors, are
found at Site 4.
• Then a problem exists between Site 2
and Site 3.
4. Finally, if path errors are detected at
Site 4.
• Then a problem exists anywhere
between Site 1 and Site 4.
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13 SDH Network
• SDH Core Transmission Stations (SDHCTSs) are usually located at each of the
trunk and international exchanges and
many of the larger local exchanges.
• Figure in the next slide illustrates the
concept with an example of five CTSs (A
to E), which are supporting the core
transmission between a set of trunk
telephone switching units, data nodes and
private circuit nodes.
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Core Transmission Network Configuration
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• An SDH-CTS uses a combination of ADMs
and DXC equipment to provide the necessary
transmission flexibility.
• The configuration of the ADM and the DXC
are managed through a computer-based
controller, which may be co-sited or
remotely located.
• The SDH configuration controller allows the
network operator to manage the configuration
of the CTS flexibility points, through planning
and assignment processes, as well as
reconfigurations in real time to compensate
for transmission link breakdowns.
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• An SDH-CTS comprises a DXC on which the
high speed transmission links terminate.
• For the example of a 2 Mbps block extraction
from the incoming 155 Mbps link, the DXC
needs to be able to identify and manipulate the
appropriate 2 Mbps tributary from the incoming
SDH link and pass it to the outgoing link.
• The DXC is divided into a higher-order DXC
switch-block handling the SDH transmission
rates and a lower-order DXC switch-block
handling the 2 Mbps and other PDH rates.
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SDH Core Transmission Network Station – DXCs
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• At smaller transmission nodes, ADMs only are
used to extract digital transmission blocks (at the
PDH rates of 2, 8 and 34 Mbps) for the co-sited
telephone switching unit, private circuit, and data
units within the exchange building, associated
with the CTS.
• In order to maximize their (DXCs and ADMs)
management capability, SDH networks are
usually structured in a set of hierarchical levels.
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SDH Core Transmission Network Station – ADMs
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SDH Transmission Network Structure
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• Figure in S#69 illustrates a typical SDH
network structure using DXCs and ADMs.
• At the top level (Tier 1) of the national
network is a mesh of high-capacity SDH
transmission links between flexibility nodes
(CTS) of DXCs.
• This forms the inner portion, or backbone of
the Core Transmission Network and links the
major trunk exchanges, as well as private
circuit and data nodes.
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• Hanging off this level at Tier 2 is a set of SDH
rings linking ADMs within a region of the
country, serving smaller trunk exchanges, local
exchanges and other nodes.
• Above the Tier 1 of the national network is the
international portion of the Core Transmission
Network, the Tier 0, which links to the network’s
transmission gateways to the transmission
networks of other countries – via submarine
cable landing stations, microwave radio stations
or satellite Earth stations.
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A typical suite consisting of several racks of SDH transmission equipment.
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• The yellow leads are optical patch cords that
connect from Optical Distribution Frames
(ODFs), where Backhaul Network based Optical
Fiber Cables are terminated (see next slide).
• The white cables running across the upper parts
of these racks feed to a Digital Distribution
Frame (DDF) that then connects to Main
Telephony switches, Internet and
Routers/Switches, Television and Radio
Program distribution networks.
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Optical
Distribution
Frames (ODFs)
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13.1 SDH Network
Configurations
• There are four major
configurations:
1. Point-to-Point.
2. Point-to-Multipoint.
3. Hub Architecture .
4. Ring Architecture .
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13.1.1 Point-to-Point
• Is the simplest network configuration.
• It involves two terminal multiplexers linked
by fiber with or without a regenerator in the
link.
• In this configuration, the SDH path and
the Service path (for example, E1 or E4
links end-to-end) are identical.
• This synchronous island can exist within
an asynchronous network world.
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STM-1
STM-1
E1
E1
E3
E3
Point-to-Point Network Configuration
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13.1.2 Point-to-Multipoint
• Also called Linear Add/Drop architecture.
• It includes adding and dropping circuits along
the way (link) to facilitate adding and dropping
tributary channels at intermediate points in the
network.
• The SDH ADM is a unique network element
specifically designed for this task. It avoids the
current cumbersome network architecture of
demultiplexing, cross-connecting, adding and
dropping channels, and then remultiplexing.
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Point-to-Multipoint Network Configuration
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13.1.3 Hub Architecture
• It accommodates unexpected
growth and change more easily
than simple point-to-point
networks.
• It concentrates traffic at a central
site using two or more ADMs, and
a DXC switch, and allows easy reprovisioning of the circuits.
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• There are two possible implementations
of this type of network function:
1.Cross-connection at higher-order path
levels, for example, using three E3 and
E4 tributary in the switching matrix.
2.Cross-connection at lower-order path
levels, for example, using 63 E1
tributary in the switching matrix.
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Hub Network Architecture
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13.1.4 Ring Architecture
• The SDH building block for a ring
architecture is the ADM.
• Multiple ADMs can be put into a ring
configuration for either Bidirectional or
Unidirectional traffic.
• The main advantage of the ring topology is
its survivability; if a fiber cable is cut, for
example, the multiplexers have the local
intelligence to send the services affected
via an alternate path through the ring
without a lengthy interruption.
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• The demand for survivable
services, diverse routing of fiber
facilities, flexibility to rearrange
services to alternate serving nodes,
as well as automatic restoration
within seconds, have made rings a
popular SDH topology.
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Ring Architecture
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13.2 Hybrid Network
• The mixture of different applications is
typical of the data transported by
SDH.
• Synchronous networks must be
able to transmit Plesiochronous
signals and at the same time be
capable of handling future services
such as ATM.
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Schematic Diagram of Hybrid Communication Network
07/03/2013
Bahman R. Alyaei
87