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| What
is SDH ? Synchronous Digital Hierarchy |
Presented by: |  Copyright 2000� |
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SDH (Synchronous
Digital Hierarchy) is an international standard for high speed
telecommunication over optical/electical
networks which can transport digital signals
in variable capacities. It
is a synchronous system which intend to provide a more flexible , yet
simple
network infrastructure.
SDH (and its American variant- SONET)
emerged from standard bodies somewhere around 1990.
these two standards create a revolution in the communication networks
based on optical fibers ,
in their cost and performance.
The development of digital transmission
systems started In the early 70s , and was based on
the Pulse Code Modulation (PCM) method.
In the early 80's digital systems became
more and more complex , yet there was huge
demand for some features that were not supported by the existing
systems.
The demand was mainly to high order multiplexing through a hierarchy of
increasing bit
rates up to 140 Mbps or 565 Mbps in Europe.
The problem was the high cost of bandwidth and digital
devices. The solution that was
created then , was a multiplexing technique , allowed for the
combining of slightly
non synchronous rates, referred to as plesiochronous*,
which lead to the term plesiochronous digital hierarchy (PDH).
*plesiochronous - "almost
synchronous , because bits are stuffed into the frames as padding and
the
calls location varies slightly - jitters - from frame to frame".
multiplexing with PDH
Although PDH was A breakthrough in
the digital transmission systems , it has a lot of
weaknesses :
multiplexing with SDH
SDH has a lot of advantages:
SDH is based on byte interleaving and not
bit interleaving , as PDH was based on.
The bit rate increased from 64 Kbps in PDH to 1.5 - 2 Mbps in SDH.
SDH/SONET Vs. PDH rates
the following
scheme describes the different layers of SDH , according to the OSI
model :
The most common SDH elements are :
The terminal multiplexer is used to multiplex local tributaries (low
rate)
to the stm-N (high rate) aggregate. The terminal is used in the chain
topology as an end element.
The regenerator is used to regenerate the (high rate) stm-N in case that
the
distance between two sites is longer than the transmitter can carry.
The Add And Drop Multiplexer (ADM) passes
the (high rate) stm-N through
from his one side to the other and has the ability to drop or add any
(low rate)
tributary. The ADM used in all topologies.
The synchronous digital cross connect receives several (high rate) stm-N
and switches
any of their (low rate) tributaries between them. It is used to connect
between
several topologies.
The linear bus (chain) topology used when there is no need for
protection
and the demography of the sites is linear.
The ring topology is the most common and known of the sdh topologies
it allows great network flexibility and protection.
The mesh topology allows even the most paranoid network manager
to sleep well at nights because of the flexibility and redundancy that
it
gives.
The Star topology is used for connecting far and less important sites
to the network.
Usage
of SDH elements in SDH Topologies
The Terminal multiplexer can be used to connect two sites in a high rate
connection .
The Add And Drop Multiplexer (ADM) is used to build the chain topologies
in the above picture.
At the ends of the chain usually a Terminal Multiplexer is connected.
The Add And Drop Multiplexer (ADM) is used to build the ring topology.
At each site we have the ability to add & drop certain tributaries.
The SDH gives the ability to create
topologies with protection for the data transferred.
Following are some examples for protected ring topologies.
At this picture we can see Dual Unidirectional Ring . The normal data
flow is
according to ring A (red). Ring B (blue) carries unprotected data which
is lost in
case of breakdown or it carries no data at all.
In case of breakdown rings A & B become one ring without the broken
segment.
The Bi-directional Ring allows data flow in both directions. For example
if data from one
of the sites has to reach a site which is next to the left of the origin
site it will flow to the left
instead of doing a whole cycle to the right.
In case of breakdown some of the data is lost and the important data is
switched. For example if data from a site should flow to its destination
through
the broken segment, it will be switched to the other side instead.
SDH has enhanced management capabilities
:
Depicted above is a Management Station
connected to a SDH ring through site 1 which contains the
gateway element. The Gateway elements receives the status of all the
other elements in the net through
the special fields that exists in the SDH protocol (in band).
Few years ago the common way to build a
backbone network that supplies broadband communication
to the suppliers (BT, Bezeq etc.) was a PDH
network. The topology of a PDH network is the Mesh
topology where every multiplexer in each site worked with its own clock.
In order to synchronize between two multiplexers that works together,
usually the transmission was made according to the local clock and the
reception was made according to the recovered clock that was recovered
from the received data.
The PDH contains 4 basic bit rates:
The fact that each of the multiplexers
transmits according to its own clock creates a problem when we need to
multiplex several transmitted data streams, the problem is that we can't
decide which clock to choose for the multiplexing. If we will choose a
fast clock we will not have enough data to put in the frame from a
slower incoming data stream (we will get empty spaces in the frame),
from the other hand if we will choose a slow clock the data at the
faster incoming stream will be lost.
This problem was solved with a stuffing algorithm, which
is implemented by using a fast clock, that allows transmission of
indication bits and stuff bits. In case that the data is slower then
"expected", the indication bits indicate that the following
stuff bits are "garbage" and if the data is faster then
"expected" the indication bits indicate that the following
stuff bits are data. This is the reason why 4 * En-1 <
En.
There are two common ways to connect
between two PDH sites. The first is by Radio Frequency (RF)
and the other is by Electrical Signal over copper cable. since we cant
afford to many cables or frequencies
usually E3 or E4 is used.
In order to transmit E1 (a very common data rate) we need 2 or 3 levels
of multiplexing, this means that
in a full E4 constellation 1+4+16=25 multiplexers are needed.
Further more there is no inband management in the PDH protocol if we
need to know the status of 1 of the multiplexers, or if we need to
change the route of 1 of the trails we have to go to the site or build
an
outside network that allows us to manage the PDH network.
In the latest years a new protocol was
defined, this new protocol was aimed to provide all the PDH
capabilities and solve some of the PDH weaknesses that are mentioned
above. This new protocol is the SDH.
The SDH network works with a single central clock that synchronizes all
the elements in the network.
The SDH contains the following bit rates:
PDH network elements.
The inband management functionality
enables the SDH network manager to receive information about
the quality of service, the damaged elements (if there are any) and
gives the manager the option to change the network configuration from a
remote site. In order to be able to do the same things with the PDH
network, one should build another separated network for the management
and the remote control.
The ability to multiplex any of the standard bit rates into the STM-n frame is possible due to the complicated containers structure of the STM-n frame as depicted bellow.
In order to map an E1(2.048 Mbit/Sec)
into the STM-n frame we have to create a TU-12 stream which is a low
rate stream that is synchronized to the SDH network clock. The TU-12 is
composed of the E1 data, indication bits, stuffing bits, management bits
and a direct pointer to the E1 frame.
The TUG-2 is a structure that can be composed of 3 TU-12s (3 E1s), or 4
TU-11s (4 T1s), or 1 TU-2 (1 T2). This structure gives the STM frame its
flexibility to multiplex different rates directly into the STM-n frame
(impossible in the PDH protocol). The next stage is mapping 7 TUG-2s
into 1 VC-3 or into 1 TUG-3 and so on according to the flow chart.
This method of multiplexing allow us to directly map the T1, T2, T3 (American standards) and the E1, E2, E3, E4 (European standards) into the STM-n frame.
Each time we map lower rate streams into
a higher rate structure we add pointers to a fixed point in the lower
rate streams, so we can directly extract the relevant information with
out demultiplexing the all high rate stream.
When stuffing is needed the pointer to the fixed location is changed
according to the direction of the stuffing, this is the improvement of
stuffing algorithm used in the PDH .
The STM-n frame
structure is best represented as a rectangle of 9 x 270xN.
The 9xN first columns are the frame header and the
rest of the frame is the inner structure data (including the data,
indication bits, stuff bits, pointers and management).
The STM-n frame is usually transmitted over an optical fiber. The frame is transmitted row by row (first is transmitted the first row then the second and so on). At the beginning of each frame a synchronized bytes A1A2 are transmitted .
The multiplexing method of 4 STM-n
streams into a STM-nx4 is an interleaving of the STM-n streams to
produce the STM-nx4 stream. The method is shown in the next picture for
producing STM-4 from 4 STM-1 streams.
After interleaving we get a higher order stream that in its rectangular form all the low order STM streams are placed as its columns which makes it easier to find each of them in the bigger frame.
Future of the private circuits
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References :
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