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TCT-5 VER 1.01
TCT-5
SDH PRINCIPLES
AGENDA
• Advantages of the SDH transmission – multiplexing
technique
• Synchronous Transport Module (STM):
• SDH bit rates:
• As per ITU(T)’ s recommendation G.707
• SDH Multiplexing Structure
• Operations of Synchronous multiplexing
• Mapping Elements
CHAPTER - 1
IRIS
ET
TCT-5 VER 1.02
TCT-5
SDH PRINCIPLES
CHAPTER - 1
The International Telecommunication Union (ITU) established
an international standard known as the Synchronous Digital
Hierarchy (SDH) for synchronous data transmission on
optical media. IRIS
ET
TCT-5 VER 1.03
The SDH specifications define optical interfaces that allow
transmission of lower-rate (e.g., PDH) signals at a
common synchronous rate.
A benefit of SDH is that it allows multiple vendors’ optical
transmission equipment to be compatible in the same
span. SDH also enables dynamic drop-and-insert
capabilities on the payload.
IRIS
ET
TCT-5 VER 1.04
Advantages of the SDH transmission –
multiplexing technique
❖ High transmission rates
❖ Simplified add & drop function
❖ High availability and capacity matching
❖ Reliability
❖ Future-proof platform for new services
❖ InterconnectionIRIS
ET
TCT-5 VER 1.05
STM is an information structure. It consists of
information payload and overhead bits in block frame
structure, which repeats at every 125 microseconds.
The information is suitably conditioned for serial
transmission on the selected media at a rate, which is
synchronized to the network
Synchronous Transport Module (STM):
IRIS
ET
TCT-5 VER 1.06
• STM followed by an integer, which indicates the level
of SDH.
• STM 1 is the first level of SDH bit rates
• Higher SDH bit rates are obtained as integer
multiples
• Higher rate levels are denoted by the corresponding
multiplication fraction of the first levelIRIS
ET
TCT-5 VER 1.07
SDH bit rates:
As per ITU(T)’ s recommendation G.707.
S.No. STM Level Data rateAbbreviated data
rate
1. STM 1 155.520 mbps 155MB
2. STM 4 622.080 mbps 622MB
3. STM 162488.320
mbps2.4GB
4. STM 649953.280
mbps10GBIR
ISET
TCT-5 VER 1.08
SDH Multiplexing Structure
IRIS
ET
TCT-5 VER 1.09
• Assembling of the PDH data flows or flows generated by
other sources in the appropriate containers;
• Generation of the virtual containers by attaching the POH
(Path Overhead);
• Assembling of the tributary units by attaching the pointers
and inserting the containers at the appropriate positions
in these units;
Operations of Synchronous multiplexing
IRIS
ET
TCT-5 VER 1.010
• Generation of the administrative units similarly to
the tributary units;
• Generation of the basic transport frames;
• Multiplexing several basic transport frames into a
superior order transport frameIRIS
ET
TCT-5 VER 1.011
Mapping: A process used when PDH tributaries are
adapted into VCs by adding POH information
Aligning: This process takes place when a pointer is
included into a Tributary Unit (TU) or an
Administrative Unit (AU), to allow the 1st byte of
the VC to be located
Multiplexing: This process is used when multiple low-order
path signals are adapted into a higher-order path
signal, or when high-order path signals are adapted
into a Multiplexing Section
IRIS
ET
TCT-5 VER 1.012
Mapping elements basically the packaging units that
have got their sizes fixed depending upon the traffic path
they follow in multiplexing hierarchy.
Mapping Elements
IRIS
ET
TCT-5 VER 1.013
The first entry point of the PDH signal. It is the basic packing unit
for tributary channels, is filled with the information from a plesio
chronous signal.
The process is called as mapping.
Justification facilities are provided to adapt plesiochronous
tributaries to the synchronous network clock.
Container(C)
IRIS
ET
TCT-5 VER 1.014
Each container is suitable for the rate of the signal inputted
into it and for the structure of the synchronous frame.
Fixed stuffing bits are inserted for synchronous tributaries.
Signal is prepared so as to enter into the next stage i.e.
virtual container. IRIS
ET
TCT-5 VER 1.015
Containers are Basic Containers and Higher Order
Containers.
As per recommendation G.709,
C-11, C-12,C-2,C-3 & C-4 are the containers for PDH bit
rates of 1.544 mbps, 2.048 mbps, 6.312 mbps,34 mbps
or 45 mbps and 140 mbps respectively.IRIS
ET
TCT-5 VER 1.016
• Each container is added with control information
known as Path Over Head (POH), which helps the
service provider to achieve end-to-end path
monitoring.
• The container and the path overhead are together
called as Virtual Container (VC).
• In Virtual Container the POH fields are organized
in a block frame structure.
(VC = C + POH)
Virtual Container
IRIS
ET
TCT-5 VER 1.017
Two types of virtual containers have been identified.
Basic (Lower order) virtual containers: VC11, VC12.
Higher order virtual containers: VC3, VC4.
IRIS
ET
TCT-5 VER 1.018
This unit is an information structure, which provides
adaptation between the lower order path layer and the
higher order path layer.
It consists of information payload of virtual container and
the tributary unit pointer.
Tributary unit (TU)
IRIS
ET
TCT-5 VER 1.019
One or more tributary units are grouped or multiplexed by
byte interleaving to form higher bit stream rate as part of
multiplexing structure.
e.g.TUG-2 is a group of 3 TU-12s or 4 TU11s or 1 TU2.
TUG-3 consists of homogenous assembly of TUG-2s or
TU-3, either seven TUG-2s or one TU-3.
Tributary unit group (TUG)
IRIS
ET
TCT-5 VER 1.020
It is an indicator whose value defines frame offset of a
virtual container with reference to the frame reference of
transport entity on which it is supported.
It indicates the phase alignment of the virtual containers
(VC-n) with respect to the POH of the next higher level VC
in which it resides.
The tributary Unit Pointer location is fixed with respect to
this higher level POH.
Pointer
IRIS
ET
TCT-5 VER 1.021
Administrative Unit (AU):It is the information structure,
which provides adaptation between higher order path layer
and the multiplex section layer.
It consists of information payload and AU pointer, which
indicates the offset of the payload frame start relating to
the multiplex section frame start. AU location is fixed with
respect to STM-frame.
Administrative Group Unit (AUG):It consists of a
homogenous assembly of AU-3s or AU-4.
IRIS
ET
TCT-5 VER 1.022
TCT-5
SDH PRINCIPLES
CHAPTER - 2AGENDA
• STM 1 Frame Structure
• Terminal multiplexers
• SDH regenerators
• Add/drop multiplexers (ADM)
• Schematic diagram of STM-1 frame
• The Section Over Head (SOH)
• Regenerator Section Overhead (RSOH)
• Multiplex Section Overhead (MSOH)
IRIS
ET
TCT-5 VER 1.023
TCT-5
SDH PRINCIPLES
CHAPTER - 2
STM 1 Frame Structure
IRIS
ET
TCT-5 VER 1.024
The SDH standard was developed using a client/server
layer approach.
The overhead and transport functions are divided into
layers.
They are:
1. Regenerator Section,
2. Multiplex Section &
3. Path
IRIS
ET
TCT-5 VER 1.025
IRIS
ET
TCT-5 VER 1.026
Terminal multiplexers are used to combine
plesiochronous and synchronous input signals into higher
bit rate STM-N signals.
Terminal multiplexers
TERMINALMULTIPLEXERIR
ISET
TCT-5 VER 1.027
As the name implies, have the job of regenerating the
clock and amplitude relationships of the incoming data
signals that have been attenuated and distorted by
dispersion.
They derive their clock signals from the incoming data
stream. Messages are received by extracting various 64
kbit/s channels (e.g. service channels E1, F1) in the
RSOH (regenerator section overhead). Messages can
also be output using these channels
SDH regenerators
IRIS
ET
TCT-5 VER 1.028
IRIS
ET
TCT-5 VER 1.029
Plesiochronous and lower bit rate synchronous signals
can be extracted from or inserted into high speed SDH bit
streams by means of ADMs.
This feature makes it possible to set up ring structures,
which have the advantage that automatic back-up path
switching is possible using elements in the ring in the
event of a fault.
Add/drop multiplexers (ADM)
IRIS
ET
TCT-5 VER 1.030
ADD/DROP
MULTIPLEXER
IRIS
ET
TCT-5 VER 1.031
An STM-1 frame structure consists of payload blocks,
overhead blocks and pointers.
The ratio of these components can vary and depends on
the initial payload that needs to be transmitted
STM-1 Frame Structure
IRIS
ET
TCT-5 VER 1.032
Schematic diagram of STM-1 frame
IRIS
ET
TCT-5 VER 1.033
The STM-1 frame is the basic transmission format for
SDH. The frame lasts for 125μSec.
The STM-n frame is arranged in matrix format having 9
rows X 270 columns and hence has 2430 bytes, which
forms the line rate of 155.52 Mbps.
IRIS
ET
TCT-5 VER 1.034
It is divided into three main areas.
• Section Over Head (SOH):
• Path Over Head (POH):
• Pay Load:
IRIS
ET
TCT-5 VER 1.035
Contains maintenance, monitoring and operational
functions which is splittet up into two parts, the
Regenerator and the Multiplex-Section Overhead.
RSOH + MSOH
The Section Over Head (SOH)
IRIS
ET
TCT-5 VER 1.036
Regenerator Section Overhead contains only the
information required for the elements located at both ends
of a section.
This might be two regenerators, a piece of line terminating
equipment and a regenerator, or two pieces of line
terminating equipment
The Regenerator Section Overhead is found in the first
three rows of Columns 1 through 9 of the STM-1 frame.
Regenerator Section Overhead (RSOH)
IRIS
ET
TCT-5 VER 1.037
Regenerator Section Overhead BYTES
IRIS
ET
TCT-5 VER 1.038
The Multiplex Section Overhead contains the information
required between the multiplex section termination
equipment at each end of the Multiplex section (that is,
between consecutive network elements excluding the
regenerators).
Multiplex Section Overhead (MSOH)
IRIS
ET
TCT-5 VER 1.039
Multiplex Section Overhead (MSOH) BYTES
IRIS
ET
TCT-5 VER 1.040
This is the data area. The bytes containing data from the
tributaries are transferred to the pay load area without
buffering and are in relation with the STM-N frame.
This is a low level multiplexing of low rate plesiochronous
signals, by inserting them into synchronous frame or
putting them in contact, which varies as per the flow rates
of input signals.
Payload
IRIS
ET
TCT-5 VER 1.041
To adapt the flow rates of the plesiochronous signals to
the required rates of them in VCs, some additional bits
are added as pointers as per the process of justification.
The entry point of the plesiochronous tributaries is
container.
IRIS
ET
TCT-5 VER 1.042
TCT-5
SDH PRINCIPLES
CHAPTER - 3
AGENDA
• Mapping of pdh tributaries into STM-1
• Mapping of E-4 into STM-1
• Mapping of E-3 into STM-1
• Mapping of E-1 into STM-1
IRIS
ET
TCT-5 VER 1.043
TCT-5
SDH PRINCIPLES
CHAPTER - 3
MAPPING OF PDH TRIBUTARIES INTO STM-1
IRIS
ET
TCT-5 VER 1.044
To transport the PDH flows in the SDH systems it is
necessary an appropriate mapping of these flows in
containers.
The mapping has to solve the problem of rate matching
between the local clock of the multiplexer and the
received flow.
For each PDH flow exists a separate mapping algorithm
that uses usually positive justification for rate matching
between the multiplexer and the received plesiochronous
flow.
IRIS
ET
TCT-5 VER 1.045
There are two categories of mapping algorithms, namely:
Synchronous mapping:
• insertion of the bits from the plesiochronous flows
in the appropriate containers is realized using the
clock extracted from the received flow;
• rate matching between the formed containers and
the synchronous transport frames is achieved with
the help of the transport units pointers:
• tributary units pointers in the case of low order
containers;
• Administrative units pointers in the case of high
order containers.
IRIS
ET
TCT-5 VER 1.046
Asynchronous mapping:
• insertion of the bits from the plesiochronous flows in
the appropriate containers is realized using the local
clock of the multiplexer;
• rate matching is realized with the help of positive
justification;
• it is the most used mapping method being more easy
to be implemented;
• there are not necessary continuous pointer operations
only for mapping of the plesiochronous tributaries
IRIS
ET
TCT-5 VER 1.047
Mapping of E-4 into STM-1
IRIS
ET
TCT-5 VER 1.048
The input to container C4 is E4 -139.264 mbps. After clock
recovery and regeneration of the tributary, the data is placed in
container.
The 260 columns and 9 rows matrix structure (the C4 container)
intended for payload is processed as follow:
Each row is split in 20 blocks of 13 bytes each;
12 of these bytes carry information bits (i.e. bits from the
140Mbit/s signal).
The 13th byte is used as W, X, Y and Z bytes for different
purposes.
Container C4
IRIS
ET
TCT-5 VER 1.049
Pay load area (9 x 260 bytes) is divided as: 20 Blocks (In
each row) X 9 Rows = 180 Blocks.
Each Block: 13 Bytes – Information bytes 12 &Overhead
bytes 1.
Total Bytes in C4: 13 X 20 X 9 = 2340.
Bit Rate: 2340 X 8 X 8000 = 149.760 mbps.
Bit allocation in the C4 container in the asynchronous
mapping of a PDH tributary having bit rate 140 mbps.
IRIS
ET
TCT-5 VER 1.050
The bit rate is 149.760 mbps, which is higher than input to
C4, (139.264 mbps). Hence all the bits carried are not
information bits. Some additional bits are added for
justification and other purposes
IRIS
ET
TCT-5 VER 1.051
Justification: It is an operation, which makes it possible to fit a
variable rate signal into a fixed rate frame.
Suppose the normal rate of a tributary = X bits / sec
The variation of rate = + ∆ bits / sec
To transmit the tributary with in S frame, it is necessary to
allocate highest possible bit rate.
S = X + ∆ bits / sec.
E4 = 139.264 mbps + 15 ppm = 139264 + 2.088 kbps.
1 justification bit is added in a row in Z byte.
IRIS
ET
TCT-5 VER 1.052
IRIS
ET
TCT-5 VER 1.053
The special bytes are called W, X, Y and Z and have the following
roles:
•W is a normal information byte;
•Y is a stuffing byte with undefined structure;
•X is a byte having the structure: C R R R R R O O:
bits O are used as control overhead for the PDH flow;
bits R are fixed stuffing bits;
C is a justification control bit, which indicates if the possible
justification position from the considered row contains information
bit (C=0) or justification (C=1);
IRIS
ET
TCT-5 VER 1.054
•byte X is transmitted 5 times in a row;
are available 5 justification control bits;
identification of the justification operation is realized based on a
majority logic decision applied to the C bits.
IRIS
ET
TCT-5 VER 1.055
•Z byte having the structure: I I I I I I I S R:
bits I are information bits;
bit R is fixed stuffing bit;
bit S is a possible justification bit.
W = IIIIIIII X = CRRRRROO Y = RRRRRRRR
Z = IIIIIISR IRIS
ET
TCT-5 VER 1.056
I = Information bit at 140 mbps
O = Service element bit reserved for future needs
R = Fixed stuffing bit
S = Justification bit (1 per line in Z)
C = 5 Justification indicator bits (by majority detection)
C = 00000 S = Information bit C = 11111
S = stuffing bitIR
ISET
TCT-5 VER 1.057
Total information bits are [(12 x 8 x 20) + (1 X 8) + (1 X 6)] X 9 X
8000 = 139.248 mbps.
This bit rate is the rate of C4 bits, which is less than the E4 -
139.264 mbps.
To add some more information bits, S bit in Z byte is used as
information bit (as justification bit) to the extent necessary,
which gives a max. bit rate of139.320 mbps.IRIS
ET
TCT-5 VER 1.058
Total information bits are
[(12 x 8 x 20) + (1 X 8) + (1 X 6)] X 9 X 8000 = 139.248 mbps.
This bit rate is the rate of C4 bits, which is less than the E4 -
139.264 mbps.
To add some more information bits, S bit in Z byte is used as
information bit (as justification bit) to the extent necessary,
which gives a max. bit rate of139.320 mbps.IRIS
ET
TCT-5 VER 1.059
The POH has the task of monitoring quality and indicating
the type of container.
The format and size of the POH depends on the container
type.
The Path Overhead is assigned to, and transported with
the payload from the time it’s created by path terminating
equipment until the payload is demultiplexed at the
termination point in a piece of path terminating equipment.
The Path Overhead is found in Rows 1 to 9 of the first
column.
Path overhead (POH)
IRIS
ET
TCT-5 VER 1.060
Path overhead (POH) Bytes
IRIS
ET
TCT-5 VER 1.061
VC4 is composed of 261 columns each of 9 rows; the first
column contains the POH and the rest compose the C4
container
IRIS
ET
TCT-5 VER 1.062
This pointer points to the VC-4 in an STM-1 frame.
SDH provides payload pointers to permit differences in
the phase and frequency of the Virtual Containers (VC-N)
with respect to the STM-N frame.
(Lower-order pointers are also provided to permit phase
differences between VC-1/VC-2 and the higher-order VC-
3/VC-4)
AU-4 Pointers
IRIS
ET
TCT-5 VER 1.063
Minimization of multiplexing Delay
• This is the main advantage of pointers. Normally
signals from different originating points differ in
their phases, because of different transmission
length and different clock generation.
• In the usual multiplexing process, to align them,
each signal has to be written into memories and
read out using a new phase of the frame to be
multiplexed.
Functions of a Pointer
IRIS
ET
TCT-5 VER 1.064
• Thus, it is inevitable to cause additional delay of half of
the frame time in average and one frame time at
maximum. Also, it requires large capacity memories.
• To avoid above inconveniences, this pointer method was
introduced into the multiplexing of SDH signal.IRIS
ET
TCT-5 VER 1.065
• A pointer is assigned to each VC to be
multiplexed and it indicates relative phase shift
between the VC and the new frame by using
the address number in the new frame.
• As a matter of course, every VC has different
pointer value.
• The pointer is renewed at every multiplexing
process, so it is not necessary to introduce
undesirable additional delays
IRIS
ET
TCT-5 VER 1.066
When there’s a difference in phase or frequency, the
pointer value is adjusted.
To accomplish this, a process known as byte stuffing is
used. In other words, the VC payload pointer indicates
where in the container capacity a VC starts, and the byte
stuffing process allows dynamic alignment of the VC in
case it slips in time.
On a frame-by-frame basis, the payload pointer indicates
the offset between the VC payload and the STM-N frame
by identifying the location of the first byte of the VC in the
payload.
Payload Pointers
IRIS
ET
TCT-5 VER 1.067
In other words, the VC is allowed to “float” within the
STM-1 frame capacity.
To make this possible, within each STM-N frame, there’s
a pointer, known as the VC Payload Pointer that
indicates where the actual payload container starts.
For a VC-4 payload, this pointer is located in columns 1
and 4 of the fourth row of the Section Overhead.IRIS
ET
TCT-5 VER 1.068
H1 H1 H1 H2 H2 H2 H3 H3 H3
AU-4 Pointer bytes
H1 and H2 bytes
These two bytes, the VC payload pointer bytes specify the
location of the VC frame.
H3 byte
•This byte is used for frequency justification.
•Depending on the pointer value, the byte is used to adjust
the fill input buffers.
The byte only carries valid information in the event of negative
justification, otherwise it’s not defined
IRIS
ET
TCT-5 VER 1.069
Frequency Justification
Generally this function is not required in an SDH network
since all network elements are synchronized to a single
clock.
But if the VC’s are transported over different networks, and
if a network element is in an abnormal condition,
justification is necessary to absorb any frequency
differences between payload and the frames.IR
ISET
TCT-5 VER 1.070
There are 2 types of justification in SDH:
Positive justification: If the frame speed of the STM is
higher than the payload arrival speed If the frame rate of
the VC-n is too slow with respect to that of the AUG-N,
then the alignment of the VC-n must periodically slip back
in time This is Positive Pointer Justification (PPJ). PPJ
operation is carried out in 4 consecutive frames.IRIS
ET
TCT-5 VER 1.071
IRIS
ET
TCT-5 VER 1.072
Negative justification: If the frame speed of the STM is
lower than the payload arrival speed. If the frame rate of
the VC-n is too fast with respect to that of the AUG-N,
then the alignment of the VC-n must periodically be
advanced in time It is similar to PPJ till frame (n+1),
Frame (n+2)Pointer value D bits inverted to have 5bit
majority voting at receiver & Buffering is done in H3 bytes
where payload data is loaded (which is extra in VC-n).
Frame (n+3)Pointer value decremented by 1.
IRIS
ET
TCT-5 VER 1.073
IRIS
ET
TCT-5 VER 1.074
The operations performed in this case are the following:
• The plesiochronous tributary signal is assembled into a C4
container;
• VC4 is generated by adding the POH to C4;
• the AU pointer is added to the VC4 and it is obtained the AU4
unit;
• the AU4 administrative unit is converted into an AUG
structure;
• this structure includes the block having 9 rows, 261 columns
and in row 4 an additional number of 9 bytes are used for the
AU pointer;
• AUG is inserted into an STM-1 frame.
IRIS
ET
TCT-5 VER 1.075
Mapping of E-3 into STM-1
IRIS
ET
TCT-5 VER 1.076
The 34,368Mbps (or 44.736Mbps) signal is assembled
into the C3 container.
The VC3 virtual container (composed of 9 Rows and 85
columns) is generated by adding the POH.
The TU3 tributary unit is generated (86 columns and 9
rows) by adding a pointer to the VC3.IRIS
ET
TCT-5 VER 1.077
The TU3 tributary unit generates TUG3 units (TUG3 is
practically identical with TU3) and 3 TUG3 units can be
multiplexed in a C4 container.
The VC4 virtual container is generated by adding the
POH and VC-4 is inserted into an STM-1 frame or an
STM-N frame. IRIS
ET
TCT-5 VER 1.078
The mapping process ensures the following
characteristics of the VC3 SDH flow:
Total bit rate debit (of the payload area)
• VC3 = 48384kbps;
•Useful nominal rate fs = 34368kbps;
•It is obtained by transmitting one information and one
justification bit in the 2 possible justification positions of a
partial frame;
IRIS
ET
TCT-5 VER 1.079
•There are used 3 information and 3 justification bits per
VC3 container.
•Useful rate without the justification positions (the
possible justification positions contain effectively
justification bits) = 34344kbps
•Useful rate with justification positions (the possible
justification positions contain information bits)
139320kbps = 34392kbps
IRIS
ET
TCT-5 VER 1.080
Mapping of E-3 into STM-1
IRIS
ET
TCT-5 VER 1.081
Mapping of E-3 into STM-1
IRIS
ET
TCT-5 VER 1.082
Mapping of E-3 into STM-1
The 34,368Mbps (or 44.736Mbps) signal is assembled
into the C3 container.
The VC3 virtual container (composed of 9 lines and 85
columns) is generated by adding the POH.
The TU3 tributary unit is generated (86 columns and 9
rows) by adding a pointer to the VC3.IRIS
ET
TCT-5 VER 1.083
Mapping of E-3 into STM-1
The TU3 tributary unit generates TUG3 units (TUG3 is
practically identical with TU3) and 3 TUG3 units can be
multiplexed in a C4 container.
The VC4 virtual container is generated by adding the POH
and VC-4 is inserted into an STM-1 frame or an STM-N
frame. IRIS
ET
TCT-5 VER 1.084
Mapping of E-3 into STM-1
The mapping process ensures the following characteristics
of the VC3 SDH flow:
• Total bit rate debit (of the payload area) VC3 =
48384kbps;
• Useful nominal rate fs = 34368kbps;
• It is obtained by transmitting one information and
one justification bit in the 2 possible justification
positions of a partial frame
IRIS
ET
TCT-5 VER 1.085
Mapping of E-3 into STM-1
• There are used 3 information and 3 justification bits
per VC3 container.
• Useful rate without the justification positions (the
possible justification positions contain effectively
justification bits) = 34344kbps
• Useful rate with justification positions (the possible
justification positions contain information bits)
139320kbps = 34392kbps
IRIS
ET
TCT-5 VER 1.086
Mapping of E-1 into STM-1
IRIS
ET
TCT-5 VER 1.087
Mapping of E-1 into STM-1
E1 traffic to form STM-1 frame through AU-4 mapping
which is performed in following steps:
• E1 traffic is of 32 bytes with 2.048Mbps data rate.
• E1 traffic is mapped into C-12 container whose size
is of 34 bytes; the remaining 2 bytes will be stuffed
to complete the mapping (packaging).
• C-12 is mapped into VC-12 by adding 1 byte of path
overhead byte.
IRIS
ET
TCT-5 VER 1.088
Mapping of E-1 into STM-1
• TU-12 is formed by adding 1byte pointer value,
which points to the first byte of VC-12, then TU-12
is framed to form 9X4 information structure.
• TUG-2 is formed by multiplexing three TU-12
containers which is indicated by M=1, 2, 3. The
TUG-2 size is 9X12.
• TUG-2 is mapped into TUG-3 by multiplexing
seven TUG-2‟s which is indicated by L=1, 2,
3…7.TUG-3 size being 9X86, with first 2 columns
of fixed stuffing.
IRIS
ET
TCT-5 VER 1.089
Mapping of E-1 into STM-1
• VC-4 is formed by multiplexing three TUG-3‟s
which is indicated by K=1, 2, 3 and adding VC-4
POH bytes, with 2 columns of stuffing whose size
is 9X261.
• VC-4 is mapped into AU-4 by adding the 1 row
pointer, which gives the offset location of VC-4 in
AU-4; AU-4 is then mapped into AUG-1, which is
of same size.
IRIS
ET
TCT-5 VER 1.090
Mapping of E-1 into STM-1
• The STM-1 frame is formed by performing the
framing by adding SOH bytes to AUG-1, with line
rate of 155Mbps.
IRIS
ET
TCT-5 VER 1.091
CHAPTER - 4
Ethernet over SDH
AGENDA
1. Generic Framing Procedure (GFP
2. Virtual Concatenation (VCAT)
3. Link Capacity Adjustment Scheme (LCAS)
4. Auto negotiation
5. Link Integrity IRIS
ET
TCT-5 VER 1.092
CHAPTER - 4
Ethernet over SDH
Ethernet and SDH are landmark technologies for
computer and telecommunications networking
respectively. However, digital network convergence has
pushed both to work together.
This demand drove the creation of a set of new
technologies to efficiently and flexibly combine both
worlds.
IRIS
ET
TCT-5 VER 1.093
CHAPTER - 4
Ethernet over SDH (EoS) was developed primarily to
provide a simple, flexible and cost-effective solution to
customers offering Ethernet based services.
IRIS
ET
TCT-5 VER 1.094
CHAPTER - 4
An EoS transport solution fundamentally addresses the
following key issues.
1. Generic Framing Procedure (GFP
2. Virtual Concatenation (VCAT)
3. Link Capacity Adjustment Scheme (LCAS)
4. Auto negotiation
5. Link Integrity IRIS
ET
TCT-5 VER 1.095
CHAPTER - 4
ITU-Trecommendation G.7041/Y.1303): provides several
functions to adapt Ethernet traffic to transportation.
Framing protocol to encapsulate Ethernet frames to
generate an SDH payload in SDH networks.
Generic Framing Procedure (GFP)
IRIS
ET
TCT-5 VER 1.096
CHAPTER - 4
Advantages of GFP
• Traffic management and QoS control are
significantly easier
• GFP is more robust than HDLC and less
susceptible to bit errors
• GFP is supported by OTN (Optical Transport
Network)/WDM interfaces in addition to SDH
• GFP permits multiple protocols from different ports
or links to share the same transport path, resulting
in more efficient use of available bandwidth
IRIS
ET
TCT-5 VER 1.097
CHAPTER - 4
Concatenation is the process of summing the bandwidth of X containers
of the same type into a larger container.
There are two concatenation methods:
•Contiguous concatenation, which creates big containers that cannot
split into smaller pieces during transmission. For this, each NE must
have concatenation functionality.
•Virtual concatenation, which transports the individual VCs and
aggregates them at the end point of the transmission path. For this,
concatenation functionality is only needed at the path termination
equipment.
Concatenation
IRIS
ET
TCT-5 VER 1.098
CHAPTER - 4
There are two concatenation methods:
Contiguous concatenation: which creates big
containers that cannot split into smaller pieces during
transmission. For this, each NE must have
concatenation functionality.
IRIS
ET
TCT-5 VER 1.099
CHAPTER - 4
Virtual concatenation: which transports the individual
VCs and aggregates them at the end point of the
transmission path. For this, concatenation functionality is
only needed at the path termination equipment.
IRIS
ET
TCT-5 VER 1.0100
CHAPTER - 4
allows the separation of GFP-adapted traffic into different
paths in an SDH network. Bandwidth mapping of the SDH
payload to SDH channels, which are either high-order or
low-order virtual containers (VCs)
Bandwidth provisioning scheme. Bandwidth mapping of
the SDH payload to SDH channels, which are either high-
order or low-order virtual containers (VCs).
Concatenation is the process of summing the bandwidth of
X containers of the same type into a larger container.
Virtual Concatenation (VCAT)
IRIS
ET
TCT-5 VER 1.0101
CHAPTER - 4
• VCAT is an inverse multiplexing technique that
allows granular increments of bandwidth in single
VC-n units.
• At the source node VCAT creates a continuous
payload equivalent to X times the VC-n
• The set of X containers is known as a Virtual
Container Group (VCG), and each individual VC is
a member of the VCG.
IRIS
ET
TCT-5 VER 1.0102
CHAPTER - 4
• Lower-Order Virtual Concatenation (LO-VCAT)
uses X times VC11, VC12, or VC2 containers
(VC11/12/2-Xv, X = 1... 64).
• Higher-Order Virtual Concatenation (HO-VCAT)
uses X times VC3 or VC4 containers (VC3/4-Xv,
X = 1... 256), providing a payload capacity of X
times 48 384 or 149 760 kbit/s.IRIS
ET
TCT-5 VER 1.0103
CHAPTER - 4
Provisioning of Ethernet data into STM-1IR
ISET
TCT-5 VER 1.0104
CHAPTER - 4
Scalability: Allows bandwidth to be tuned in small
increments on demand to match desired data rate and
avoid wastage. Traditional contiguous concatenation
comes in coarse increments.
Efficiency: More easily routed through a network and
aids to eliminate stranded bandwidth. Allows for more
efficient usage of an existing network’s available
bandwidth.
Advantages of VCAT
IRIS
ET
TCT-5 VER 1.0105
CHAPTER - 4
Compatibility: Requires only end nodes of the network
to be aware of the containers being virtually
concatenated. Transparent to core network elements.
Resiliency: Individual members of a virtually
concatenated group can be routed as diversely as
possible across a network. So if one member is lost, the
others are likely to be operational albeit with a reduced
bandwidth.
IRIS
ET
TCT-5 VER 1.0106
CHAPTER - 4
Link Capacity Adjustment Scheme (LCAS) dynamically
adjusts the capacities of SDH paths according to source
and/or destination needs.
In fact, this new network vision was named Next
Generation SDH (NGSDH).
ITU-T as G.7042, designed to manage the bandwidth
allocation of a VCAT path.
LCAS can add and remove members of a VCG that
control a VCAT channel. LCAS cannot adapt the size of
the VCAT channel according to the traffic pattern.
Link Capacity Adjustment Scheme (LCAS)
IRIS
ET
TCT-5 VER 1.0107
CHAPTER - 4
LCAS applications
VCAT bandwidth allocation, LCAS enables the resizing
of the VCAT pipe in use when it receives an order from the
NMS to increase or decrease the size.
Network Resilience, In the case of a partial failure of one
path, LCAS reconfigures the connection using the
members still up and able to continue carrying traffic.IR
ISET
TCT-5 VER 1.0108
CHAPTER - 4
Asymmetric Configurations, LCAS is a unidirectional
protocol allowing the provision of asymmetric bandwidth
between two MSSP nodes to configure asymmetric links
Cross-Domain Operation, because LCAS resides only
at edge nodes it is not necessary to coordinate more than
one configuration centreIRIS
ET
TCT-5 VER 1.0109
CHAPTER - 4
Auto negotiation: Auto negotiation is a feature to detect
the link partner capabilities. It allows the devices at both
ends of a link segment to advertise abilities,
acknowledge receipt and understand the common
mode(s) of operation that both devices share.
IRIS
ET
TCT-5 VER 1.0110
CHAPTER - 4
Auto negotiationIRIS
ET
TCT-5 VER 1.0111
CHAPTER - 4
Allows detecting faults along the end-to-end Ethernet
transport connection. For any traffic affecting VCG side
problem, no point in keeping the ETH port UP (or pumping
ETH traffic).Link Integrity Enabled forces the ETH port to
go down.
Following faults can be detected
•Near-end Ethernet Link Failure
•SDH Link Failures
•CSF/Far-end Ethernet Link Failure
Link Integrity
IRIS
ET
TCT-5 VER 1.0112
CHAPTER - 4
Link IntegrityIRIS
ET
TCT-5 VER 1.0113
CHAPTER - 5
Network Topology and automatic
protection switching (APS) in SDHAgenda
• Linear networks
• Mesh Architecture
• Ring Architecture
• Automatic Protection Switching (APS)
• Recommendation G.841
• Classification of APS
• Linear protection scheme
• Multiplex section protection (MSP)
• Ring protection scheme
• Uni directional path protection
• Unidirectional Operation
• Path Switching
• Line Switching
• Sub network connection protection
IRIS
ET
TCT-5 VER 1.0114
CHAPTER - 5
Network Topology and automatic
protection switching (APS) in SDH
A network element is ready to carry live traffic.
Different network topologies in which the proposed optical
fiber cable faults management system can work.
IRIS
ET
TCT-5 VER 1.0115
CHAPTER - 5
There are 4 types of network topologies:
Linear topology – used when the appropriate network
topology is linear (ex. Access networks in a high speed
network) and when is not necessary a high protection to
faults.
Mesh topology – each node is connected with a number
of other nodes – high management flexibility, high
protection to faults but also high redundancy of the
physical channels between nodes.
IRIS
ET
TCT-5 VER 1.0116
CHAPTER - 5
Ring topology – used most often; ensures a high
management flexibility and good protection to faults.
Star topology – used to connect distant and less
important nodes; ensures low protection to faults.
IRIS
ET
TCT-5 VER 1.0117
CHAPTER - 5
In linear networks, SDH ADM nodes are connected in a
linear fashion where two terminal multiplexers exist at both
ends.
This topology provides drop and insert capability to all
network elements.
There may be unprotected linear networks, establishing
two fiber connections between two ADMs or protected with
four fiber connections where two of them are working and
other two serving as a backup or protection pair.
Linear networks
IRIS
ET
TCT-5 VER 1.0118
CHAPTER - 5
Linear SDH network
IRIS
ET
TCT-5 VER 1.0119
CHAPTER - 5
A point-to-multipoint (linear add/drop) architecture
includes adding and dropping circuits along the way.
The SDH ADM (add/drop multiplexer) 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 re-multiplexing.
The ADM typically is placed in an SDH link to facilitate
adding and dropping tributary channels at intermediate
points in the network
IRIS
ET
TCT-5 VER 1.0120
CHAPTER - 5
The SDH building block for a ring architecture is the
ADM. Multiple ADMs can be put into a ring configuration
for either Bi-directional or Unidirectional traffic.
The main advantage of the ring topology is its
survivability; if a fibre 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.
Ring Architecture
IRIS
ET
TCT-5 VER 1.0121
CHAPTER - 5
The demand for survivable services, diverse routing
of fibre facilities, flexibility to rearrange services to
alternate serving nodes, as well as automatic
restoration within seconds, have made rings a popular
SDH topology. IRIS
ET
TCT-5 VER 1.0122
CHAPTER - 5
IRIS
ET
TCT-5 VER 1.0123
CHAPTER - 5
Rings are very important because they give greater
flexibility in the allocation of bandwidth to the different
users and they allow rerouting of traffic should a link
fail under normal operation, a 2 Mb/s tributary is sent
round the ring in both the directions.
The ADM assigned to drop the 2 Mb/s tributary
monitors the two SDH signals for errors and delivers
the one with better performance.
IRIS
ET
TCT-5 VER 1.0124
CHAPTER - 5
This is known as path switching.
When a catastrophic failure occurs, for example, when
the fiber is cut by a road digger, the nodes either side of
the failure loop the clockwise ring to the anticlockwise
ring, allowing traffic to avoid the failed ring segment.
This forms an extended ring which carries all the traffic to
each node in the ring, allowing service to continue.IRIS
ET
TCT-5 VER 1.0125
CHAPTER - 5
is a functionality of carrier-grade transport networks, is
often called resilience.
Since it enables service to quickly recover from failures, it
is required to ensure high reliability and availability.
Automatic Protection Switching (APS)
IRIS
ET
TCT-5 VER 1.0126
CHAPTER - 5
APS is also responsible to :
• detection of failures (signal fail or signal degrade)
on a working channel
• switching traffic transmission to a protection
channel
• selecting traffic reception from the protection
channel
• (optionally) reverting back to the working channel
once failure is repaired
IRIS
ET
TCT-5 VER 1.0127
CHAPTER - 5
Protection schemes are classified as:
a) SDH trail protection (at the section or path layer);
b) SDH sub network connection protection (with
inherent monitoring, non-intrusive monitoring, and
sub-layer monitoring).
IRIS
ET
TCT-5 VER 1.0128
CHAPTER - 5
Protection is required for the traffic against 3 conditions
• Signal Degrade (SD),
• Signal Failure (SF)
• And fiber cut.
IRIS
ET
TCT-5 VER 1.0129
CHAPTER - 5
Classification of APS
1. 1+1 protection scheme or Dedicated protection scheme
W - working
P - protection
The simplest of all the forms is the 1+1 type of protection. Each
working line (port or path) is protected by one dedicated
protection line (port or path).
Traffic is taken through both working line & protection line
simultaneously and at the far end traffic will be selected by
switch depending upon the healthiness of the traffic.
IRIS
ET
TCT-5 VER 1.0130
CHAPTER - 5
2. 1:1 protection scheme or Shared protection scheme
W - working
P - protection
in 1:1 protection for each of the working line (which can be
either port or path) there will be a corresponding protection
line. IRIS
ET
TCT-5 VER 1.0131
CHAPTER - 5
1+1, 1:1 and 1:N Configurations can be used in linear
protection switching scheme.Linear topology leads to different configuration of protection
fibers depending on the type of equipments and their
capabilities.The ‘1+1’ configuration means that a supplementary fiber
pair (called the ‘protection channel’) is dedicated to protect
the primary fiber pair (referred to as ‘working channel 1)
Linear protection scheme
IRIS
ET
TCT-5 VER 1.0132
CHAPTER - 5
Depending of the operation (unidirectional or bidirectional)
and switching configuration (revertive or non-revertive), the
data/voice traffic is redirected to the protection fiber pair in
case of problems detected on the primary fiber pair.
IRIS
ET
TCT-5 VER 1.0133
CHAPTER - 5
The protection switching mode can be configured either as
revertive & non-revertive mode.
Non-revertive protection allows the traffic remains on the
protect path even after the working path is repaired.
Revertive systems restore working traffic on the original
path after the Wait To Restore time (WTR).IRIS
ET
TCT-5 VER 1.0134
CHAPTER - 5
is a port level protection supported on STM-1 interfaces.
The term MSP differentiates it from ring systems.The ‘1+1’ configuration uses a complete duplication of
services and implies that two identical lines are active at
the same time.
Multiplex section protection (MSP)
IRIS
ET
TCT-5 VER 1.0135
CHAPTER - 5
Multiplex section protection is based on failure detection
at the multiplex section level, by both ADMs located on
both sides of the failure.
If a failure occurs in one section, the STM-N signal is
completely rerouted by switching over to the protection
fibre, even if the failure is due to only one of the
containers in the frameIRIS
ET
TCT-5 VER 1.0136
CHAPTER - 5
Ring network is made up of ADM and any traffic added
can reach to its destination in 2 ways, which can be useful
in APS.
Ring protection scheme
Ring networks are classified into
1. Uni directional path protection
2. Bi-directional path protectionIRIS
ET
TCT-5 VER 1.0137
CHAPTER - 5
Two fiber unidirectional path switched ring
Let us assume that there is an interruption in the circuit
between the network elements A and B.
Direction y is unaffected by this fault. An alternative path
must, however, be found for direction x.
The connection is therefore switched to the alternative path
in network elements A and B.
IRIS
ET
TCT-5 VER 1.0138
CHAPTER - 5
The other network elements (C and D) switch through the
back-up path.
This switching process is referred to as line switched. A
simpler method is to use the so-called path switched ring.
Traffic is transmitted simultaneously over both the working
line and the protection line.
If there is an interruption, the receiver (in this case A)
switches to the protection line and immediately takes up
the connection
IRIS
ET
TCT-5 VER 1.0139
CHAPTER - 5
Path Switching
Today, path switching is only used on unidirectional rings
– hence the name Unidirectional Path Switched Rings
(UPSR)
What does Path Switching mean?
− At the exit node, both fibers are monitored and the
path traffic extracted.
− Based on several factors, especially error rate, the
“best” traffic is selected to be handed off to the
customer.
IRIS
ET
TCT-5 VER 1.0140
CHAPTER - 5
Path Switching
IRIS
ET
TCT-5 VER 1.0141
CHAPTER - 5
Line Switching is done on bi-directional rings, either two
fiber or four fiber.
Thus the name Bi-directional Line Switched Ring
(BLSR).
The total capacity of the fibers must be twice the carried
traffic.
For four fiber systems, this means two fibers are dedicated
for protection.
Line Switching
IRIS
ET
TCT-5 VER 1.0142
CHAPTER - 5
What does Line Switching mean?
Two adjacent nodes monitor the traffic between them.
If one detects “failure” on a fiber, it signals the other.
The two nodes coordinate switching to the protection
fiber(s).
IRIS
ET
TCT-5 VER 1.0143
CHAPTER - 5
Bi-directional Ring Backup for a Bi-directional Ring
IRIS
ET
TCT-5 VER 1.0144
CHAPTER - 5
The ring protection types are as follows
a.In Europe, 2f-MS-SPRing, 4f-MS-SPRing or 2f-SNCP
b.In USA, 2f-BLSR, 4f-BLSR or 2f-UPSR
BLSR --- Bi-Directional Line-Switched Rings,
UPSR --- Uni-Directional Path-Switched Rings and
MS-SPRing --- the Multiplex Section-Shared Protected Rings,
SNCP --- Sub-Network Connection ProtectionIR
ISET
TCT-5 VER 1.0145
CHAPTER - 5
Sub network connection protection
Sub network connection protection is a dedicated
protection mechanism that can be used on any physical
structure (i.e. meshed, rings, or mixed).
It may be applied at any path layer in a layered network.
IRIS
ET
TCT-5 VER 1.0146
CHAPTER - 5
It can be used to protect a portion of a path (e.g. that
portion where two separate paths segments are available)
between two Connection Points (CPs) or between a CP
and a Termination Connection Point (TCP), or the full end-
to-end path between two TCPs.
It switches on server failures (using inherent monitoring) or
it switches using client layer information (using non-
intrusive monitoring).
IRIS
ET
TCT-5 VER 1.0147
CHAPTER - 5
SNC protection is a linear protection scheme which can be
applied on an individual basis to VC-n signals.
It need not be used on all VCs within a multiplex section.
It need not be used on all LO VCs within a HO VC.
SNC protection operates in a unidirectional protection
switching manner IRIS
ET
TCT-5 VER 1.0148
CHAPTER - 5
Linear multiplex section protection switching can be a
dedicated or shared protection mechanism.
It protects the multiplex section layer, and applies to point-
to-point physical networks.
One protection multiplex section can be used to protect the
normal traffic from a number (N) of working multiplex
sections.
It cannot protect against node failures. It can operate in a
unidirectional or bidirectional manner, and it can carry extra
traffic on the protection multiplex section in bidirectional
operation.
Linear multiplex section protection switching
IRIS
ET
TCT-5 VER 1.0149
CHAPTER - 5
Switch time – In a ring with no extra traffic, all nodes in the
idle state (no detected failures, no active automatic or
external commands, and receiving only Idle K-bytes), and
with less than 1200 km of fibre, the switch (ring and span)
completion time for a failure on a single span shall be less
than 50 ms.
On rings under all other conditions, the switch completion
time can exceed 50 ms (the specific interval is under
study) to allow time to remove extra traffic, or to negotiate
and accommodate coexisting APS requests.
Regarding multiplex section shared protection rings, G.841 states
IRIS
ET
TCT-5 VER 1.0150
CHAPTER - 5
For linear VC trail protection, it says :
Switch time – The APS algorithm for LO/HO VC trail
protection shall operate as fast as possible.
A value of 50 ms has been proposed as a target time.
Concerns have been expressed over this proposed target
time when many VCs are involved.
Protection switch completion time excludes the detection
time necessary to initiate the protection switch, and the
hold-off time.
IRIS
ET
TCT-5 VER 1.0151
CHAPTER - 5
Typical SNCP in Rly usage
IRIS
ET
TCT-5 VER 1.0152
CHAPTER - 5
Where to use – Rule of thumb
1. Point-to-point
1. MSP
2. SNCP
2. Ring
1. SNCP
2. MS-SPRing
3. Leased line
1. SNCP (individual VC-Xs)
2. MSP (whole MS)
4. Mixed traffic
1. SNCP (individual VC-Xs)
Protection Mechanisms
IRIS
ET
TCT-5 VER 1.0153
CHAPTER - 5
Comparison of protection schemes
IRIS
ET
TCT-5 VER 1.0154
CHAPTER - 5
The Ring Protection Architectures
IRIS
ET
TCT-5 VER 1.0155
CHAPTER - 5
Sub-Network Connection protection based on Inherent
monitoring (SNC/I) and Non-intrusive monitoring (SNC/N)
is implemented.
The traffic in the ring can be planned allocating both a
working path and a protection path.
If for example a fiber cable is damaged the connection is
preserved by automatic switch-over to the protection path.
Traffic Protection
IRIS
ET
TCT-5 VER 1.0156
CHAPTER - 5
The protection based on inherent switching is controlled by
receipt of AIS (alarm indication signal) or LOP (loss of
pointer).
Errors leading to AIS and LOP and therefore protection
switching are: LOS (loss of signal), LOF (loss of frame) or
excessive errors.
The SNC/N protection will initiate a switch, if an SF (Signal
Failure) or SD (Signal Degrade) condition is detected..
IRIS
ET
TCT-5 VER 1.0157
CHAPTER - 6
Synchronization in SDH networks
Agenda
• Synchronization
• Synchronization network
• Poor synchronization
• Elements of a Synchronization Network
• Synchronization Clock types
• Primary Reference Clock (PRC)
• SSUs (Synchronization Supply Units
• SECs (SDH Equipment Clocks)
• Modes Of Synchronization
• Free running mode:
• Holdover mode
• Locked mode
• Synchronization Status Message (SSM)
IRIS
ET
TCT-5 VER 1.0158
CHAPTER - 6
Synchronization in SDH networks
Synchronization: Synchronous is the first word in the
term SDH for a very good reason.
If synchronization is not guaranteed, considerable
degradation in network function, and even total failure of
the network can be the result.
To avoid this worst case scenario, all network elements
are synchronized to a central clockIR
ISET
TCT-5 VER 1.0159
CHAPTER - 6
Synchronization network is a network that shall be able
to provide all types of telecommunication traffic networks
with reference timing signals of required quality.
The objective for the traffic networks, for example
switching, transport, signaling, mobile, is to not lose
information.
Loss of information is often caused by poor
synchronization.
IRIS
ET
TCT-5 VER 1.0160
CHAPTER - 6
This can be avoided by properly connecting the traffic
network to an adequate synchronization network (how to
connect to a synchronization network is normally called
network synchronization) synchronization network is set up
to ensure that all of the elements in the communications
network are synchronous.IRIS
ET
TCT-5 VER 1.0161
CHAPTER - 6
A network where transmission system payloads are
synchronised to a master (network) clock and traceable
to a reference clock.
A network where all clocks have the same long term
accuracy under normal operating conditions.
IRIS
ET
TCT-5 VER 1.0162
CHAPTER - 6
Poor synchronization causes loss of information in
varying degrees. Examples of results of poor
synchronization are:
• degraded traffic throughput;
• inhibition of set-up of calls (#7 signaling) due to re-
transmission;
• re-sending of files;
• corrupt fax messages;
IRIS
ET
TCT-5 VER 1.0163
CHAPTER - 6
• degraded speech quality;
• freeze-frames on video;
• disconnection of calls during hand-over in mobile
networks;
• Partial or complete traffic stoppage.IRIS
ET
TCT-5 VER 1.0164
CHAPTER - 6
Synchronization Clock types
•G.811 Primary Reference Clock (PRC)
•G.812 Synchronization Supply Unit (SSU)
•G.813 SDH Equipment Clock (SEC)
IRIS
ET
TCT-5 VER 1.0165
CHAPTER - 6
IRIS
ET
TCT-5 VER 1.0166
CHAPTER - 6
The central clock is generated by a high-precision primary
reference clock (PRC) unit conforming to ITU-T
Recommendation G.811.
This clock signal must be distributed throughout the entire
network.
A hierarchical structure is used for this; the signal is
passed on by the subordinate synchronization supply units
(SSU) and synchronous equipment clocks (SEC).
The synchronization signal paths can be the same as
those used for SDH communications
IRIS
ET
TCT-5 VER 1.0167
CHAPTER - 6
The clock signal is regenerated in the SSUs and SECs
with the aid of phase-locked loops.
If the clock supply fails, the affected network element
switches over to a clock source with the same or lower
quality, or if this is not possible, it switches to hold-over
mode.
In this situation, the clock signal is kept relatively accurate
by controlling the oscillator by applying the stored
frequency correction values for the previous hours and
taking the temperature of the oscillator into account.
IRIS
ET
TCT-5 VER 1.0168
CHAPTER - 6
As pointed out earlier, the switches in digital
communication networks in which time division
multiplexing is applied, need a common timing reference.
The requirements on the accuracy and stability of the
reference result from the connection quality objectives
(measured in terms of the slip rate) of a digital connection,
specified in ITU-T Recommendation G.822 .
Primary Reference Clock (PRC)
IRIS
ET
TCT-5 VER 1.0169
CHAPTER - 6
Currently those requirements can only be met with atomic
(Caesium-beam) clocks used as the network primary
reference clock (PRC: Primary Reference Clock) or with
use of GPS receivers; but by deploying different strategies
on clock holdover, repair time and network planning, these
objectives can also be met under failure condition for a
limited period of time.IRIS
ET
TCT-5 VER 1.0170
CHAPTER - 6
The task of network synchronization is to distribute the
reference signal from the PRC to all network elements
requiring synchronization. The PRC determines the long-
term stability of the reference frequency.
IRIS
ET
TCT-5 VER 1.0171
CHAPTER - 6
regenerate the clock signal after it has passed a chain of
SECs and serve as temporary references for parts of the
network when the connection to the PRC is lost in failure
situations.
Usually SSUs are located in network nodes where they
distribute a timing signal to all network elements in the
node.
SSUs (Synchronization Supply Units)
IRIS
ET
TCT-5 VER 1.0172
CHAPTER - 6
SECs (SDH Equipment Clocks) are the clocks
incorporated in SDH network elements.
SECs offer great flexibility in the selection of references
and support automatic reconfiguration mechanisms in
rings or chains of SDH network elements.
IRIS
ET
TCT-5 VER 1.0173
CHAPTER - 6
Elements of a Synchronization Network
up to N=20 SECs cascaded between any two SSUs
up to K=10 SSUs in one chain
total number of SECs in one chain limited to 60IRIS
ET
TCT-5 VER 1.0174
CHAPTER - 6
SEC (SDH Equipment Clock)
T0 : 2 MHz station clock (delivers timing to all functions
of the equipment)
T1 : 2 Mhz derived from STM-N
T2 : 2 MHz derived from 2 Mbit/s
T3 : 2MHz input sync. Signals (may be derived from 2
Mbit/s without traffic)
T4 : 2MHz output sync. Signals (may be derived from 2
Mbit/s without traffic)
IRIS
ET
TCT-5 VER 1.0175
CHAPTER - 6
Modes Of Synchronization
1. Locked mode:
An operating condition of a slave clock in which the
output signal is controlled by an external input reference
such that the clock’s output signal has the same long-
term average frequency as the input reference, and the
time error function between output and input is bounded.
Locked mode is the expected mode of operation of a
slave clock.
IRIS
ET
TCT-5 VER 1.0176
CHAPTER - 6
An operating condition of a clock which has lost its
controlling reference input and is using stored data,
acquired while in locked operation, to control its output.
The stored data are used to control phase and frequency
variations, allowing the locked condition to be reproduced
within specifications.
Holdover begins when the clock output no longer reflects
the influence of a connected external reference, or
transition from it. Holdover terminates when the output of
the clock reverts to locked mode condition.
2. Holdover mode:
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An operating condition of a clock, the output signal of which
is strongly influenced by the oscillating element and not
controlled by phase-locking techniques.
In this mode the clock has never had a network reference
input, or the clock has lost external reference and has no
access to stored data, that could be acquired from a
previously connected external reference.
3.Free running mode
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Free-run begins when the clock output no longer reflects
the influence of a connected external reference, or
transition from it.
Free-run terminates when the clock output has achieved
lock to an external reference.
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This is a communication channel in the SDH protocol
designated for setting-up of protected synchronization of an
SDH network.
The status message contains information on the quality
level of the timing source.
The quality message, together with a list of synchronization
sources in each network element, allows controlled
switching of the synchronization source if there is a fault.
Synchronization Status Message (SSM)
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•Has a configurable list of priorities for synchronization
sources (a maximum of four STM-N sources and two 2
MHz sources) with specified switch criteria (i.e. expected
quality level).
•Monitors SSM on incoming STM-N (with SSM change,
another source is selected)
•Inserts SSM on outgoing STM-N, in accordance with
current reference source
•Monitor the 2 MHz source (out-of-range conditions)
•Squelches 2 MHz output signals if reference is below the
specified quality level.
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Note: An SDH network will operate completely
satisfactorily even if it is not synchronized to very precise
external references.
The important requirement is the selection of one source
and to use that to distribute synchronization.
In the initial phase, the internal clock of one TM/ADM can
be used to synchronize the others.
If one of the external interfaces is synchronized, it can be
used as a reference.
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