Sdh basics hand_outs_of_sdh_basics

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SDH BASICS
HAND OUTS OF SDH BASICS
[1] Introduction:
Telecommunication is a process of transmitting or receiving information over a distance by
any electrical or electromagnetic medium. Telecommunication can be possible in 2 ways
1) Analog Communication
2) Digital Communication
In Analog communication, analog signals that vary continuously amplitude and frequency
are used in transmission media In an analog communication, it is difficult to remove noise
and wave distortions during the transmission. For this reason, analog signals cannot perform
high-quality data transmission.
In digital communication, digital signal is an electrical signal, which possesses two distinct
states, on/off or positive/negative. Noise and distortions have little effect, making high-
quality data transmission possible.
In digital communication prior to transmission, analog to digital conversion is needed and
Analog-to-Digital Conversion (ADC) is carried over 3 steps
a. Sampling:
The analog signal is sampled at regular intervals to create pulses, each representing the
amplitude of the signal at an instant in time. This process is called sampling. Sampling
frequency is given by Nyquist Theorem, which says that „ to recover the original analog
signal from the sampled signal, sampling rate should be 2*fmax, where fmax is maximum
frequency of the analog signal.
 Voice frequency range: 300Hz to 3.4kHz rounded off to 0 to 4 kHz
 Sampling occurs at Sampling frequency, 2 * 4 KHz = 8000 Samples/Sec.
 Duration of the samples is 125Sec.
b. Quantization:
Quantization is process of assigning a discrete amplitude value that closely matches the
original sample is now used to present the value of the pulse.
c. Encoding
Encoding is to allot an 8bit binary code to each of the quantized samples and transmit the
binary code.
 Voice channel occupies 8000 samples/sec.*8 bits/sample=64000=64kbps
 Baseband frequency= 64kHz.
When communication is carried using only single Baseband signal on a line, its called as
Baseband communication. But due to high bandwidth requirement Broadband
communication need to be used. Broadband communication is the process of combining a
number of individual baseband signals into a common frequency band or into a common bit
stream for transmission. Broad communication uses Time Division Multiplexing (TDM) to
cater multiple voice frequencies. There are 3 kinds of transmission types
a. Asynchronous transmission
Asynchronous transmission is transmission of data in which time intervals between
transmitted characters may be of unequal length. Transition of signals do not occur at the
same nominal rate generally free running quartz oscillators derive the clock, there is no
timing pulses sent from transmitter to receiver.

SDH BASICS
b. Plesiochronous transmission
Plesiochronous is a Greek word meaning Almost Synchronous, but not fully synchronous.
The digital transitions in the signals occur at almost the same rate. There may be a phase
difference between the transitions of the two signals, and this would lie on specified limits.
Based on this Plesiochronous Digital Hierarchy (PDH) is developed.
c. Synchronous transmission
The digital transitions in the signals occur at exactly the same rate. There will be no phase
difference between the transitions of the two signals, and this would lie on specified limits.
Based on this Synchronous Digital Hierarchy (SDH) is developed.
[2] Plesiochronous Digital Hierarchy (PDH):
Traditionally, digital transmission systems and hierarchies have been based on multiplexing
signals, which are Plesiochronous (running at almost the same speed). Various parts of the
world use different PDH and multiplexing levels Indians followed the European hierarchy
levels.
2.1 Frame format:
The multiple PDH frame formats are starting with baseband signal E0 upto E4 level.
E1 signal level has line rate/data rate of 2.048Mbps consisting of 30 time division
multiplexed (TDM) voice channels, each running at 64Kbps known as E1 and two
additional channels carrying control information. E1 level only used, byte interleaved
multiplexing where 1 byte from each voice signal is combined together within frame
duration of 125µsec. each to form the 2Mbps data.
2.1.1 TDM of E1 level:
Increasing traffic has demanded that more of these basic E1 are to be multiplexed together
to provide increased capacity. In order to move multiple 2 Mbit/s data streams from one
place to another, they are combined together, or multiplexed in groups of four. This is done
by taking 1 bit from stream #1, followed by 1 bit from stream #2, then #3, then #4 which is
Bit interleave TDM. All higher-level frames use bit interleaved TDM. Since each MUX can
use its own clock in PDH, 4 X E1 levels will be having different frequency, multiplexing by
compensating for the frequency difference by adding justification bits wherever possible.
This changes the line rate of E2 level 8.448Mbps instead of 8.192Mbps.This follows for all
higher hierarchy levels.
PDH standards are as depicted in Figure1. The European standard named E0, E1, E2 & E4
hierarchy levels and North American standard named DS1, DS2, DS3 & DS4 levels.
E1 Level: Byte interleaved TDM.
32 * 64 KHz = 2.048 Mb/s, Capacity = 30 Base Channels
E2 Level: Bit interleaved TDM
4 * 2.048 +stuffing bits = 8.448 Mbps, Capacity = 120 Base Channels
4 * 8.448 + stuffing bits = 34.368 Mbps, Capacity = 480 Base Channels
4 * 34.368 +stuffing bits = 139.264 Mbps, Capacity = 1920 Base Channels

SDH BASICS
Figure1
2.2 Limitations of PDH:
Because of Plesiochronous transmission & bit interleave TDM at higher levels, mainly
problem aroused that is
a. Problem of channel segregation:
Access to digital channels from hierarchical signals requires demultiplexing and subsequent
full multiplexing, which leads to the proliferation of equipment at those points where
segregation of channels is needed.
Because of this another problem aroused that‟s voice channels can‟t be Added/Dropped
when at higher hierarchy level and in turn send /receive voice channels. This is the problem
of cross-connection.
b. Homogeneity of equipment:
PDH when deployed with optical fiber, as multiplexing is carried out with electrical signals;
these electrical signals should be converted into optical signal for which Line Terminal
Equipment (LTE) is used. The terminal equipment in a link between exchanges (fiber,
coaxial, etc) must come from the same manufacturer. This limits flexibility when it comes to
reconfiguring and extending the network, since equipment from different manufacturers
cannot be interconnected.
c. Limited functionality:
Supervision and maintenance functions are limited with multiframe structure of E1 level The
PDH don‟t have capability to provide service automatically when there is connection failure.
d. Problem of interoperability:
In the process of successive PDH multiplexing of the 64 Kbps channels, interoperability is
lost between the interfaces used in the European, American and Japanese hierarchies, that is,
the transmission rates are different for the same multiplexing level. This creates a need for
equipment to convert between interfaces in internetworking situations.

SDH BASICS
[3] Synchronous Digital Hierarchy (SDH):
Telecommunication Networking formulated by ITU-T (International Telecommunication
Union for Telecommunication standards) and can have capacity upto 40Gbps (not yet
formalized). SDH solved all the problems, which PDH faced by following ways
1. SDH follows the Master-Slave clock technique with PLL (Phase Locked Loop) to
synchronize the nodes.
2. SDH provides mapping, MUXing (byte interleaved TDM) & framing to mainly carry
PDH & Ethernet traffic to form STM frame.
3. Overhead bytes ensure the management of payload & Pointers allow dynamic allocation
of payload in STM frame with which justification can be done under specified limit.
4. Automatic Protection Switching provide protection to traffic against the fiber cut & errors
with the help of overhead bytes in SDH.
5. Performance Monitoring & Alarms strengthens the SDH by giving sufficient indication
using Overheads bytes against errors like equipment, operator & communication.
SDH does not multiplex in predefined steps; one unit is multiplexing all incoming links, adds
overhead information and creates a synchronous transport module (STM).
3.1 STM frame structure:
The STM frame standards with line rate are as shown in Table1. The zero level of the
synchronous digital hierarchy shall be 51 Mbps. The first level of the synchronous digital
hierarchy shall be 155Mbps. Higher synchronous digital hierarchy bit rates shall be obtained
as integer multiples of the first level bit rate and shall be denoted by the corresponding
multiplication factor of the first level rate.
The STM-1 frame is the basic transmission format for SDH. The frame lasts for 125µSec as
depicted in Figure2.The STM-n frame is arranged in matrix format and STM-1 frame
structure is having 9 rows X 270 columns and hence has 2430 bytes within 125µSec, which
forms the line rate of 155Mbps(2430X64Kbps). Similarly the STM-N frame is formed, will
be having 9 rows only but columns
Figure2
will be multiplied, which is nothing but column multiplexing and is as depicted in Figure3.
The STM-n frames are transmitted in left-right and top-bottom manner, i.e. 1st
row is
transmitted starting from 1x1 byte and continue upto 1x270 byte, then 2nd
row transmitted in
same manner, 3rd
row,4th
row,…….7th
row.

SDH BASICS
Table1 Figure 3
STM-4 will be having 9 rows only but columns will be multiple of 4 (270 x 4) columns,
which simplifies all higher STM-n frame similarly all STM frames are formed.
At the Network Node Interface, node derives timing information from the STM-n frames
for network node synchronization so STM-n frame should have sufficient bit timing content
(long sequence of 1 or 0 must be avoided). This is done by Frame synchronous scrambler,
which generates alternate 1‟s & 0‟s.
Scrambling is performed on 1st
row of RSOH bytes, at transmitter side and will be de-
scrambled at receiver after deriving the timing from it.
The STM-N frame consists of
 Section overhead and Administrative Unit.
 Section overhead – Regenerator section overhead and Multiplex section overhead.
 Administrative unit – Virtual container and Pointer.
 Virtual container - Payload and Path overhead information.
Multiplexing hierarchy forms these entities using the mapping elements, which is explained
in section 3.2.
3.2 SDH multiplexing hierarchy:
SDH multiplexing hierarchy uses mapping elements and perform the mapping, multiplexing
and framing to form STM frame.
3.2.1 Mapping Elements:
Mapping elements basically the packaging units that have got their sizes fixed beforehand
depending upon the traffic path they follow in multiplexing hierarchy. There are 5 mapping
elements
a. Container-n:
Container is basic mapping (packaging) unit for the PDH tributary signals. For every PDH
traffic, container is provided for mapping and unused capacity of container can‟t be filled by
any other traffic. Only one PDH traffic can be mapped into every container and then
remaining bytes will be stuffed.
C11=DS1, C12=E1, C2=DS2, C3=E3&DS3 and C4=E4
Optical Signal Bit Rate Abbreviated as
STM-0 51.84 Mbps 51 Mbps
STM-1 155.52 Mbps 155 Mbps
STM- 4 622.080 Mbps 622 Mbps
STM-16 2488.320 Mbps 2.5 Gbps
STM-64 9953.280 Mbps 10 Gbps
STM-256 39813.12 Mbps 40 Gbps

SDH BASICS
There is separate container for E1&DS1 but single container for E3&DS3 because
maximum 3 E3 & 3 DS3 traffics are possible in STM-1 even if used separate container
where as 63E1 and 84DS1 possible by using separate container.
b. Virtual Container-n
Virtual Container is the information structure used to support path (explained in Section
Hierarchy) connections in the SDH. Adding POH byte(s) to container forms VC. For every
container there is VC.
VC11=C11, VC12=C12, VC2=C2, VC3=C3 & VC4=C4
c. Tributary Unit/Administrative Unit:
Tributary Unit is an information structure, which provides adaptation between two layers
and is formed by adding pointer(s) to VC:
When pointer is added to lower order VC-n it forms TU.
When pointer is added to higher order VC-n it forms AU.
POINTER is an indicator whose value defines the frame offset of a VC with respect to the
frame reference of the payload.
d. Tributary Unit Group (TUG):
TUG combines multiple TU by means of multiplexing.
TUG-2 = 4xTU11 or 3xTU12 or 1xTU2.
TUG-3 = 7xTUG2 or 1xTU3.
e. Administrative Unit Group (AUG):
Similar to the TUG, AUG combines the multiple AU by means of multiplexing.
AUG = 3xAU3 or 1x AU4.
3.2.2 Multiplexing Hierarchy:
The Multiplexing hierarchy caters both European & North American hierarc
Figure 4

SDH BASICS
levels data rates to form STM frame which is as depicted in Figure4. There are 2 types of
mapping AU-4 & AU-3 mapping through which the PDH traffic is mapped into STM frame
and when mapped with AU-3 mapping data rate will be more compared to AU-4 mapping.
Considering E1 traffic to form STM-1 frame through AU-4 mapping which is performed
in following steps:
a. E1 traffic is of 32 bytes with 2.048Mbps data rate.
b. 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. C-12 is mapped into VC-12 by adding 1 byte of VC-12 path overhead byte.
d. 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.
e. TUG-2 is formed by multiplexing three TU-12 containers which is indicated by M=1, 2,
3. The TUG-2 size is 9X12.
f. 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.
g. 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.
h. 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.
i. The STM-1 frame is formed by performing the framing by adding SOH bytes to AUG-1,
with line rate of 155Mbps. Figure5 depicts the byte interleaved TDM which forms TU-12 to
TUG-2 to TUG-3 to VC-4.

SDH BASICS
Figure5
Three TU-12‟s TU-12 #1, #2 and #3 are byte interleaved multiplexed to form TUG-2,
Byte1 #1--1X1, Byte1 #2--1X2, Byte1 #3--1X3, Byte2 #1--1X4………Byte4 #3--1X12.
Every row will be having 4bytes of each TU-12 (every TUG-2 will be having 4 columns of
TU-12 in it). This procedure follows for remaining 8 rows, which forms TUG-2 of 9X12.
Similarly TUG-3 & VC-4 are multiplexed. In STM-1 frame, 63 VC-12‟s (K*L*M) can be
mapped each having 4 columns & these columns will be framed in STM-1 frame (K*L*M)
columns apart.
STM-1 frame is formed through AU-3 mapping from E1 traffic in following steps:
a. TUG2 is formed similar to that of AU4 mapping.
b. Seven TUG2‟s are multiplexed to map into VC3 by adding 1 column path overhead with
2 columns of stuffing, which makes the size of VC-3 of 9X87.
c. VC-3 is mapped into AU-3 by addition of the 3byte pointer, which gives the starting
location of the VC-3 payload.
d. Three AU-3‟s are multiplexed into AUG-1.
e. Then framing is done by adding SOH bytes to AUG-1 to form STM-1 frame.
STM-1 frame when mapped through AU-4 mapping technique will be having 1column VC-4
POH & 8 stuffing columns where as when mapped through AU-3 mapping has got
3columns of VC-3 POH & 6column stuffing. In AU-3 mapping 2columns of data is more
compared to AU-4, which increases the data rate comparatively. So according to data rate
varies according to mapping.
Similar to 63XVC-12 STM-N frame can also cater combination of traffics like 21VC-12 and
2VC-3 can be combined together in STM-1 frame in AU-4 mapping. Similarly there can be
many combinations with respect to K, L, M values and data rate.
STM-4 frame:
The STM-4 frame can be formed through the multiplexing hierarchy of STM-1 till AUG-1
then four AUG-1‟s are multiplexed to form AUG-4 container. Then STM-4 framing is done
by addition of SOH bytes to the AUG-4 to form STM-4 frame with line rate of 622Mbps.In
STM-4 frame there is replication of 4 AUG-1 which indicates presence of 4 VC-4 POH
columns.
Contiguous Concatenation:
STM-4 frame can also be formed by contiguous concatenation as depicted in Figure 3, where
C4-4c is formed by concatenation of 4-C4 containers, VC4-4c is formed by concatenating 4
no. of Higher order POH column then AU4-4c is formed by concatenating 4 no. of AU4
pounters and hence STM-4 frame is formed.STM-4 frame can also be formed by
multiplexing 4 no. of AUG‟s to form AUG-4 hence the STM4 is formed.
Similarly all STM-N frames are formed using the basic frame format of STM-1, with only
multiplexing N times from AUG-1 to form the AUG-N, then directly framing to STM-N.
3.3 SDH network elements:
There are 4 network elements, which form the SDH network, they are
a. Terminal Multiplexer(TM):
The Terminal Multiplexer depicted in Figure6, also called as Path Terminating Element
(PTE) acts as a concentrator of tributary signals, which forms the STM-N frame. It converts
signals from electrical domain to optical domain. It is the end equipment in any link or a line,
which can either multiplex or de-multiplex optical signals/electrical signals.

SDH BASICS
b. Regenerator:
A regenerator depicted in Figure7 is needed when, due to the long distance between
multiplexers, the signal level in the fiber becomes too low. The regenerator does
regenerating, reshaping and re-timing of received signal and replaces the Regenerator Section
overhead bytes before re-transmitting the signal.
c. Add/Drop Multiplexer (ADM):
One of the major advantages of SDH is its ability to Add & Drop tributaries directly from
higher-order aggregate bit streams and that is performed by Add/Drop multiplexer, which is
as depicted in Figure8.
d. Digital cross Connect (DXC):
Digital cross-connect (DXC)(see Figure9) handles multiple STM-N signals and
simultaneously switches tributaries across multiple STM-N signals. One major difference
between a cross-connect and an add/drop multiplexer is that a cross-connect may be used to
interconnect a much larger number of STM-1s.. There are 2 types of cross-connects
possible; add/drop cross-connect (ADXC) & pass-through cross connect (PTXC). ADXC is
used for grooming (consolidating or segregating tributaries) of STM-1s and PTXC is used
for switching multiple signals at VC-n levels.
Figure6 Figure7
Figure8 Figure9
3.4 Overheads & Pointers:
Several types of overhead have been identified for application in the SDH. They are SOH
and POH byte. SOH & POH bytes perform maintenance operation for their respective
sections like regenerator section, multiplex section and path (higher order & lower order),
which are explained under the heading of Section Hierarchy.
3.4.1 Section Overhead bytes (SOH):
SOH information is added to the information payload to create an STM-N. The SOH
information is further classified as
a. Regenerator section overhead (RSOH):
The Regenerator Section Overhead contains only the information required for the elements
located at both ends of a regenerator section. The section can be between two regenerators
or regenerator & ADM etc. The RSOH is found in the first three rows of STM-N frame as
depicted in Figure10. Byte level description is explained in Table2.
STM-N
E1
E3
T1
Reg
..
STM-N
STM-N
E3
T1 E1
STM-N
STM-N
STM-N STM-N

SDH BASICS
HOPOH
Figure10
b. Multiplex section overhead (MSOH):
The MSOH 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). The Multiplex Section Overhead is found in Rows 5 to
9 of STM-N frame as depicted in Figure10. Byte level description is in explained in Table3.
3.4.2 Virtual Container Path-Overhead (POH):
Virtual container POH provides for integrity of communication between the point of
assembly of a virtual container and its point of disassembly. Two categories of virtual
container POH have been identified:
a. Higher order virtual container POH (VC-4/VC-3 POH):
VC-3 POH is added to either an assembly of TUG-2s or a C-3 to form a VC-3. VC-4 POH
is added to either an assembly of TUG-3s or a C-4 to form a VC-4 as depicted in Figure10.
J1
B3
C2
G1
F2
H4
F3
K3
N1
RSOH Description
A1=F6H
A2=28H
Indicate the beginning of the STM-N frame; The frame alignment word of an STM-
N frame (N=1, 4, 16, 64) is composed of 3XN A1 bytes followed by 3XN A2
bytes.
J0: RS
trace
Regenerator Section (RS) Trace byte is used to transmit repetitively a section access
point identifier so that a section receiver can verify its continued connection to the
intended transmitter. J0 byte transmitted as 16-byte code sent in 16 consecutive
frames.
B1: BIP-8 This is a parity code (even parity), used to check for transmission errors over a RS.
Its value is calculated over all bits of the nth
STM-1 frame and then placed in the B1
byte of (n+1)th
STM-1 frame.
E1 This byte is used as a local orderwire channel for voice communication in RS.
F1 This byte is reserved for user purposes (e.g., to provide temporary data/voice
channel connections for special maintenance purposes).
D1-D3–
DCCR
RS Data Communication Channel bytes form a 192 Kbps message channel
providing a message-based channel for Operations, Administration and
Maintenance (OAM) between pieces of section terminating equipment.
E2
M1
S1
D12
D11
D10
D9
D8
D7
D6
D5
D4
K2
K1
B2
B2
B2
D3
D2
D1
F1
E1
B1
J0
A2
A2
A2
A1
A1
A1
RSOH
MSOH

SDH BASICS
Table2
Table3
VC-3/VC-4 POH bytes are also termed as Higher Order POH (HOPOH) as HOPOH is
added to the higher order VC‟s only. Bytes are explained in Table4.
b. Lower order virtual container POH (VC-2/VC-1 POH):
The bytes V5, J2, N2 and K4 are allocated to the VC-2, VC-12 and VC-11 POH. The V5
byte is the
first byte of the multiframe and its position is indicated by the TU-2, TU-12 or TU-11
pointer. These bytes are explained as in Table5. The LOPOH bytes
MSOH Description
B2: BIP-24 Error monitoring over MS & computed over STM frame excluding RSOH
bytes.3XN B2 bytes in STM-N (N=1,4,16,64).
K1 & K2 Automatic Protection Switching (APS channel) K1 & K2 (b1-b5) bytes are
allocated for APS signalling for the protection of the multiplex section. K2 (b6-
b8) for communicating Alarm Indication Signal (AIS) and Remote Defect
Indication (RDI).
D4 -D12
DCCM
MS Data Communications Channel bytes form a 576 kbps message channel from
a central location for OAM information (control, maintenance, remote
provisioning, monitoring, administration and other communication needs).
S1 Bits 5 to 8 of byte S1 are allocated for synchronization status messages.
M1 The M1 byte is used for a MS layer remote error indication (MS-REI). Bits 2 to 8
of the M1 byte are used to carry the error count of the interleaved bit blocks that
the MS BIP-24xN has detected.
E2 Local orderwire channel for voice communication in MS, E2 is sent in 4
consecutive frames to initiate the communication which includes segment no.(00-
99) & orderwire no.(000-249).

SDH BASICS
Table4
Table5
3.4.3 Pointers:
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. On a frame-by-frame basis, the payload pointer indicates the offset between the VC
HOPOH Description
J1 Path access point identifier sends trace message either as 16byte or 64byte in
consecutive frames.
B3: BIP-8 B3 bytes checks the transmission error over HP.
C2: Signal
label
One byte is allocated to indicate the composition or the maintenance status of the
VC-4Xc/VC-4/VC-3 like 00(H)=Unequipped, 02(H)=TUG structure--- FF(H)= No
valid incoming signal(VC-AIS).
G1: Path
status byte
G1 byte is used to convey the path terminating status and performance back to
the originating PTE. 4bits used as Path –REI that has 0-8 valid BIP violations;
1bit for HP-RDI, which acknowledgement for the AU-AIS.
F2 & F3 These bytes are allocated for user communication purposes between path
elements and are payload dependent.
H4:
Position &
Sequence
Indicator
H4 byte provides a multi frame and sequence indicator for VC-3/4 generalized
position indicator for payloads. In the latter case, the content is payload specific
(e.g., H4 can be used as a multiframe indicator for VC-2/1 payload). H4 byte can
be 00,01,02 or 03 for VC-12 multiframe.
K3 APS signalling is provided in K3 bits 1-4, allocated for protection at the VC-4/3
path levels. K3 bits 5-8 are allocated for future use.
N1:
Network
operator
byte
N1 byte is allocated to provide a Higher-Order Tandem Connection Monitoring
(HO-TCM) function. At each node the B3 data is copied into N1 & sent and at
next node B3 is compared with N1, if they don‟t match the error is sent back to
its previous node as REI, which goes till TC source (terminating node).
LOPOH Description
V5 i) 2 bits allocated for error performance monitoring where BIP-2 scheme is used
which includes POH bytes, but excludes V1, V2, V3, and V4.
ii) 1bit as REI that has 2 valid BIP violations.
iii) 3bits for signal label, 000=Unequipped, 001=equipped-non-specific, all other
values are used by new equipment to indicate specific mapping.
iv) 1bit as LP-RDI as acknowledgement for TU-AIS.
J2 Lower order path trace.
N2 N2 byte is allocated to provide a tandem connection monitoring (TCM) function
that works similar to N1, along with V5.
K4 1bit for as extended signal label which works along with signal label of V5 and
2bits as APS signalling for protection at lower order path level.

SDH BASICS
payload and the STM-N frame by identifying the location of the first byte of the VC in the
payload. In other words, the VC is allowed to “float” within the STM-N frame capacity.
Pointer operation carried out with H1, H2 & H3 bytes in AU & TU-3 pointers and V1, V2
& V3 that works analogous to H1, H2 & H3 respectively in TU-2/TU-1 pointers. Byte
explanation is as depicted in Figure11. The figure shows the byte arrangements for AU-4 &
Figure11
3xAU-3 (due to multiplexing 3xAU-3 to map into AUG-1). 3xH3 bytes (AU-4)/1xH3 byte
each in 3AU-3/3TU-3/1xV3byte(TU-2/TU-1) is used for buffering of payloads in negative
pointer justification & equivalent number of bytes after H3/V3 byte are used for stuffing in
positive justification. There are different pointer values for different pointer that is explained
in Table6.
Pointer
type
Pointer
value
Description
AU-4 0-782 VC-4 has 2349bytes to be allocated pointer values with 10bit(1024)
so 2349/3=783,3byte each 1 pointer value.
AU-3 0-782 VC-3 is783bytes, which is possible with 783 pointer values.
TU-3 0-764 VC-3 (AU-4 map) has 765 bytes so 765pointer values.
TU-n Lower order VC will be structured in multiframe format, in which 4
frames of VC-n are formed by adding 4 POH bytes (V5, J2, N2, K4),
with each of POH byte there will be pointer bytes (V1, V2, V3, V4
respectively) added which indicates location of V5 to form TU-n as
depicted in Figure 12(LO).

SDH BASICS
TU-2
TU-12
TU-11
0-427
0-139
0-103
4X107byte(VC-2)=428.
4X35byte(VC-12)=140.
4X26byte(VC-11)=104.
Table6
Figure 12(LO)
After the pointer value assignment, the hierarchy should be able to process the data even in
case of clock difference between the nodes, which can be because of jitter & wander and in
all cases the hierarchy does the justification. For performing the justification First byte of the
payload should be allocated from 4th
row, immediately after H3 bytes and they can float
anywhere in frame there after. Pointer justification occurs when the sending SDH NE‟s
timing is different than the receiving SDH NE‟s timing. There are 2 types of justification
possible depending upon the clock frequency.
a. Positive Pointer Justification (PPJ):
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 as depicted in Figure12 [a], where
there are 2 nodes (NE1 & NE2) in network, VC-n data rate of NE1 is 782bytes/sec &
AUG-n data rate of NE2 is 783bytes/sec(reference clock recovered) with respective number
of pointers. Because of the difference in clock rate, when NE1 sends only 782bytes, which
will fill the NE2 buffer with leaving 1byte empty (1 pointer no information is present) and
the pointer is pointing to the 1st
position, this is where positive pointer justification is used.
Pointer value is showing the false pointer value where as actual data starts from 2nd
position,
so for that stuffing byte is added at 1st
position (doesn‟t contribute for payload) & pointer
incremented from position 1(dotted arrow) to 2(where actual data starts). This is Positive
Pointer Justification (PPJ). PPJ operation is carried out in 4 consecutive frames as depicted
in Figure12 [b].
Frame-n Start of VC-n, where clock is recovered from incoming bits & set as reference.
Frame (n+1) Pointer initialization, payload slipped back because clock difference.
Frame (n+2) Pointer value I bits inverted to have 5bit majority voting at receiver &
stuffing bytes added after 3H3 bytes.
Frame (n+3) Pointer value incremented by 1.
NE2

SDH BASICS
783
NE1
VC-n AUG-n
Figure12 [a]
Figure12 [b]
b. Negative Pointer Justification (NPJ):
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 as depicted in Figure13 [a],
where VC-n data rate is 784bytes/sec and AUG-n data rate of adjacent node is 783bytes/sec
(reference clock recovered). When NE1 sends the data all 783 positions (50-783 in nth frame
to 49 shown in figure) of NE2 are filled with data sent by NE1, even then 1 byte is
remaining which has to be accommodated in frame of NE2, this is done by vacating the
H3H3H3 bytes, because of which payload is moved in that place and the remaining data is
included within payload by decrementing the pointer from 50 to 49.NPJ operation is carried
out in 4 consecutive frames as depicted in Figure13 [b]. 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.
VC-n(NE1) AUG-n(NE2)
Figure13 [a]
Figure13 [b]
[4] Section Hierarchy:
1
2
3
|
|
|
|
780
781
782
1
1q1
2
3
4
|
|
|
|
781
782
783
1
1q1
2
3
4
|
|
|
|
782
783
784
50
51
52
|
782
783
0
1
|
48
49
49

SDH BASICS
Section Hierarchy is useful for effective traffic planning, where depending upon the NE
capacity and population of traffic cross connect is provisioned.
There are 2 sections in any optical fiber network depending upon the part of the STM frame
processed as depicted in Figure14, they are
a. Regenerator Section (RS): RS is a part (section) of the optical fiber network,
within which RSOH part of STM frame is NOT processed.
Figure14
b. Multiplex Section: MS is a part (section) of the optical fiber network, within which
MSOH part of STM frame is NOT processed.
c. Higher order Path: HP is a part (section) of the optical fiber network, within which
higher order VC part of SDH frame is NOT processed (it may be processed only for
interpreting HOPOH).
d. Lower order Path: LP is a part (section) of the optical fiber network, within which lower
order VC part of SDH frame is NOT processed (it may be processed only for interpreting
LOPOH).
RS & MS are defined in network only on the basis of the type of the equipment used in the
in it, but for defining the HP & LP it depends upon the traffic provisioning (type of the
cross connect) in the NE‟s. While provisioning in NE following points has to be
remembered
i. After receiving the STM frame at NE, while disassembling it; NE follows the sequence of
RSOHMSOHAU-4/AU-3 PointerVC-4/VC-3 POHTU-n pointerVC-n
POH.n-11, 12,2,3.
ii. For Tejas nodes, even if one is making a VC4 level pass-through (an operation with HP
without processing it), he/she is processing MS & therefore terminating the MS.
iii. One can change any HPOH field (e.g., J1 transmitted trace) only when one is processing
HP (e.g., VC12 level pass through cross-connect (PTXC) exists on AU4 mapping), but not
when HP is not disturbed (e.g., VC4 level PTXC on AU4 mapping).
iv. For Tejas nodes, for AU4 mapping, one can make VC4 and VC12/VC11 level and not
VC3 level pass-through for E1/DS1 traffic.
v. If in a STM-1 node, multiple (say, 18) E1/DS1 traffic has to be passed-through with some
other traffic added/dropped from that node, one has to make multiple (18) VC12 level
PTXC.

SDH BASICS
Traffic planning is explained with the example as depicted in Figure15.As depicted there are 3
kinds of traffics needs to have communication between the NEs with respective number as
shown. There are 7 ADM & 1 Regenerator, with this knowledge, there will are 7RS & 6MS
defined as depicted irrespective of traffics going to be provisioned in them.
Figure15
Amongst the NEs, A, B, F and C (port 1) is of STM1 capacity and C (port 2&3), G, E and F
is of STM4 capacity all are following AU4 mapping technique.
For any traffic terminating NEs must be provisioned with ADXC of respective VC, for
example: E1VC12, E3VC3.
Now the PTXC provision in intermediate NEs is part of effective traffic planning so that
maximum traffic can be catered with existing capacity.
At B:
There are 3 traffic 21E1 from port 1 to 3,21E1 & 1E3 from port 2 to 3.STM1 capacity NE
means each port of the NE can receive or deliver traffic of STM1 capacity.
a. For 21E1 from 1-3, according to point iv both VC4/VC12 PTXC is possible but
according to point v only VC-12 PTXC has to be provisioned as port3 capacity is STM1 and
if VC4 PTXC is provided then no other traffic can be either delivered or received.
b. Similarly for traffic from 2-3, 21E1 & 1E3; VC12 & VC3 PTXC is provisioned
respectively and all traffic must be in STM#1 as NE is of STM1 capacity.
At C:
From port 1-3 there 2 traffic of 42E1 capacity & 1-2 1E3 traffic, here port1-STM1 & port
2&3-STM4 capacity.
a. If VC4 PTXC is provisioned from 1-3 then all traffic including E3 will go to port3, which
is not expected so VC12 PTXC from 1-3 carried within STM#2 for 42E1 traffic and VC3
PTXC from 1-2 within STM#4.
b. Port 2&3 can cater traffic in any of the STM number (1-4), in port 3 42E1 traffic is
catered within STM#2 in which can cater still 21E1 traffic.
Similarly at E VC12 PTXC is provisioned from 1-2 within STM#2.After all traffic are
provisioned in all NEs, HP & LP can be indicated for each traffic as depicted in Figure15.

SDH BASICS
[5] Automatic Protection Switching (APS):
Protection is required for the traffic against 3 conditions Signal Degrade (SD), Signal Failure
(SF) and fiber cut. When first 2 conditions occur the traffic quality will be lost and in the
later case traffic itself will be lost. So there was service level agreement done between service
provider & customer, according to which 99.999% of the time connection should be
available and this is possible only by Automatic Protection Switching. APS can be classified
in general as
i. 1+1 protection scheme/Dedicated protection scheme
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). Protection line is redundant line
dedicated for the working line. 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. The switch over is triggered by a defect such as fiber cut, SF or SD.
When working line is on the toss, traffic will be selected from the protection line. The main
disadvantage of the 1+1 is high bandwidth redundancy but 1+1 is faster than any of the
protection-switching scheme.
ii. 1:1 protection scheme/Shared protection scheme
1:1 in general can be called as 1:N; in 1:1 protection for each of the working line (which can
be either port or path) there will be a corresponding protection line. There will be 2 types of
traffics identified for catering it, high priority traffic & low priority traffic (Not supported in
Tejas products). High priority traffic will be catered in the working line and low priority
traffic will be taken through protection line. When the working line goes on the toss, traffic
has to be catered through protection line where low priority traffic is present, which will be
pre-empted from protection line for serving the high priority traffic.
In 1:N there will be N working line, which are getting protected by 1 protection line through
which low priority traffic is catered and the switching occurs depending upon the priority
given to the working line and low priority traffic will be given least priority.
Above said 2 protection schemes can be discussed in detail based on network topology,
which are
5.1. Linear protection scheme:
1+1 & 1:N protection switching scheme when deployed in linear network will be termed as
linear protection switching scheme.
a.1+1 MSP:
1+1 protection scheme in linear network are termed as 1+1 MSP as each section in our
Tejas optical network is MS. Figure16 depicts the normal operation of the 1+1 MSP, where
communication needs to establish between 2 NE‟s A & B.
Figure16
1+1 MSP is port level protection scheme where traffic catered in one port called as Working
port (3-1 or 4-1) as depicted will be protected by Protecting port (3-2 or 4-2). Each port will
be having Tx. & Rx fiber which will be connected to its adjacent port to Rx. & Tx.
respectively, which will be either Working Path (WP) if its for working port & Protecting

SDH BASICS
Path(PP) if its for protecting port. Both WP & PP will be carrying the signal simultaneously
and at the far end the traffic will be selected by switch depending upon healthiness of the
traffic, in normal case it will be from working path and switching to protecting path occurs
when SD, SF & fiber cut is observed on working path which are traffic affecting. For
example if single fiber cut occurs in WP as depicted in Figure17, then selector switch in A will
be switched from WP to PP (ie. Only affected direction of traffic will be switched over to
PP) and traffic will be undisturbed. This type of switching is called as Unidirectional
Switching.
Figure17
On single fiber cut if both direction of traffic is switched over from WP to PP then it will be
called as Bi-directional switching. User can configure both switching types.
1+1 MSP can also be configured by user as Revertive or Non-Revertive protection, when
provisioned as Revertive protection, after WP is repaired back, switching takes place PP to
WP and traffic will be selected from WP but after Wait-To-Restore (WTR variable from 5-
12min which is user configurable) time is elapsed which ensures proper splicing in WP.
If Non-Revertive protection is provisioned then traffic will not be switched back from PP to
WP even if the WP is repaired completely, until user forcefully switches the traffic using
external commands.
5.2Ring protection scheme:
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. There are 2 types of protection switching schemes.
a. Sub Network Connection Protection (SNCP):
Sub Network Connection Protection (SNCP) or Unidirectional Path Switched Ring
(UPSR)(in SONET terminology) is dedicated protection scheme, which gives path level
protection for each traffic and traffic will be flowing in both WP & PP simultaneously. The
SNCP has to be provisioned at terminating nodes, for each traffic individually Destination
Work (DW), Destination Protect (DP) and Source Work (SW), Source Protect (SP) has to be
specified as depicted in Figure18.Consider communication is needed between A & D and the
HP or LP will be as shown from A to D in 2 ways, one is selected as Working HP/LP (WP)
(for this traffic) and another one selected as Protecting HP/LP (PP) (for this traffic).
Whenever the WP is on the toss its PP will protect the traffic and corresponding switching
occurs at far end. PP is dedicated to WP for individual traffic, like this 63 E1 can be
protected in STM-1 node, each having different WP and PP.

SDH BASICS
Figure18
SNCP has to be provisioned only in terminating nodes and PTXC has to be provisioned in
intermediate nodes. SNCP is necessarily follows Unidirectional switching only as the
protection directly provided for the traffic separately and if traffic flowing from 3-1 to 4-1 is
affected then traffic will be received from 5-2 in PP. Traffic is still received in A from 3-1
through WP only. If both direction traffic is affected in WP then both direction traffic will
switch over to PP. SNCP also can have both Revertive & Non-Revertive protection.
Tandem SNCP is another application of SNCP where single fiber cut in N rings can be
protected simultaneously for which can be simulated using 4-way XC. SNCP protection
can‟t be provisioned in the presence of MSP or MSSP protection.
b. Multiplex Section Shared Protection (MS-SP):
MS-SP ring is also called as Bi-directional Line Switched Ring (BLSR) in SONET terms,
which is ring network application of 1:1 protection switching scheme There are types of
rings
i. MS-SP 2F ring:
MS-SP-2Fiber ring will be having 2 fibers with bandwidth divided for the working traffic &
protecting traffic, for example in STM-N ring having 6 nodes as depicted in Figure19,
communication needs to be carried out between NE A & D. In MS-SP ring within every
port each fiber is divided as working traffic & protecting traffic of N/2 capacity each (i.e. in
each fiber AU4 1-8 will be working traffic & AU4 9-16 will be protecting traffic). Working
traffic of one direction say from A to B in AU4 1-8 is protected by protecting traffic in
opposite direction from B to A in AU4 9-16.
The protection process is explained with example as depicted in Figure 20, if Multiplex
Section between B & C is affected then all other Multiplex Sections in ring will share their
bandwidth to protect the traffic because of which it‟s named as MS-Shared Protection.
When the traffic between B & C is affected the traffic will reach from A to B in
AU4#1(working traffic) then at B, RING SWITCHING takes place where traffic will be
switched from AU4#1 to AU4#9(protecting traffic) and will go back from B to A. The
traffic will reach the node C in AU4#9 only then traffic will be switched back to AU4#1 as
the MS between B & C was on the toss so traffic got protected by other MS. After traffic
reached the C another ring switch takes place where traffic will be switch back to
AU4#1(working traffic) then will reach the destination D in AU4#1only.Similarly the
opposite direction traffic also will be protected in Ring switching.
As name suggests this follows only Bi-directional switching and MS-SP 2F can‟t be Non-
Revertive at all, as main advantage of Non-Revertive protection is to avoid the fiber ageing
when it is heavily loaded, keeping another path idle. Only disadvantage of the MS-SP 2F is
that maximum bandwidth utilization is STM-N/2 capacity, which was overcome by MS-SP
4F. Tejas products do not support MS-SP 4F for now.

SDH BASICS
Figure 19 Figure 20
There can be maximum of 16 NE‟s (0-15) in a ring because every NE is given Node ID for
which 4 bits reserved in K1& K2 byte. Provisioning needs to be done at every NE giving
East/West chassis-slot-port, ring ID, node ID and ring map.
ii. MS-SP 4F ring:
MS-SP 4F ring require four fibers for each span of the ring. As illustrated in Figure21,
working and protection traffic are carried over different fibers: two NE‟s transmitting in
opposite directions carry the working traffic while two NE‟s, also transmitting in opposite
directions, carry the protection traffic. This permits the bi-directional transport of working
traffic.
Figure 21
MS-SP 4F rings support RING SWITCHING as a protection switch, as well as SPAN
SWITCHING, though not concurrently. Multiple span switches can coexist on the ring
since only the protection channels along one span are used for each span switch. Certain
multiple failures can be fully protected using span switching. MS-SP 4F can support only Bi-
directional switching and it supports both Revertive & Non-Revertive protection.
5.3 External Commands:

SDH BASICS
External commands enable the user to have control over routing of the traffic whenever
application arises. There are 4 external commands whose priority is given by 4bits of K1
byte.
a. Clear/Release
This external command clears all external commands applied on any port, which is having
the highest priority.
b. Lock Out of Protection (LOP):
Lock out of protection is external command given by user, when maintenance operation has
to be carried on the PP, in which case if WP goes on the toss traffic continuously tries to
acquire the PP and selector switch keeps on switching between WP & PP. To avoid this
loop LOP is given so that PP is locked & no traffic will be allowed to go through PP.
c. Forced Switch: Work or Protect
Forced Switch is the explicit command, which forcefully switches traffic to intended path; it
can be Forced Switch to Work or Forced Switch to Protect. The switching takes place
regardless of condition of the path to which it is switching to because SF/SD are of lower
priority compared to this command.
d. Manual Switch: work or Protect
Manual switch is similar to the Forced switch but main difference is while switching the
traffic, condition of the path is checked as SF/SD are of higher priority. Hence whenever
Manual switch to work is given and WP is having SF/SD then switching doesn‟t take place.
Following table illustrates the different types of protection switching schemes.
Protection
Schemes
Topology
Uni-Directional
/ Bi-Directional
Revertive /
Non-Revertive
Shared /
Dedicated
Switching
Time
1+1 MSP
(Port protection)
Linear Uni / Bi-
Directional
Revertive /
Non-Revertive
Dedicated Low
1+1 SNCP
(Path protection)
Ring Uni-Directional
(UPSR)
Revertive /
Non-Revertive
Dedicated Low
MSSP2F
(Line protection)
Ring Bi-Directional
(BLSR)
Revertive Shared Switching
time is more
MSSP4F
(Line protection)
Ring Bi-Directional
(BLSR)
Revertive /
Non-Revertive
Shared Switching
time is more
1:N
(Card
protection)
Linear NA Revertive/Non
Revertive
Shared Switching
time is more
[6] Synchronization:
Synchronization is required to enable service providers to transport bits of information
within and across network without losing any bits, which can be due to mis-timing (phase

SDH BASICS
variation) inside transmission equipment when data is regenerated. When mistiming
becomes large, errors are produced and the system can become unusable. Even at low values
of mis-timing (phase variation), sensitivity to amplitude and phase variations is increased and
performance suffers. There are 2 types of phase variation Jitter & Wander. Jitter is a short-
term variation of the digital signal from its ideal position in time and its frequency is >10Hz.
Figure 22
Wander is long-term variation of the digital signals from its ideal position and its frequency
is <10Hz.Jitter & wander is as depicted in Figure 22.
Synchronization is attained in the SDH using Master-Slave clock technique along with Phase
Locked Loop (PLL), which is performed by Synchronous Status Message (SSM) state
machine.
6.1 SSM state machine:
SSM state machine present in every NE will synchronize all NE in network, which is as
depicted in Figure 23.
.
Figure 23
SSM state machine nominates the clocks through 3 interfaces
i. Optical interface (STM ports),
ii. Electrical interface (E1/DS1 interface)
iii. BITS (clock/data) (external reference signal)

SDH BASICS
Every NE will be having multiple of interfaces representing different clocks SSM will lock to
the particular clock depending upon the provisioning done by the user in synchronization
configuration in NES.
A Nominated clock signal can be either in “Signal Fail” or “No Signal Fail State”. If
Signal Fail is present on nominated clock, it will not be considered for timing reference
clock selection process until and unless it is overridden by an external command. The
following Alarms are considered for declaring/clearing a signal fail: LOS, LOF and AIS.
User can configure following parameters, which is applied to the SSM for synchronization.
a. Priority:
Priority of the nominated clock can have range of 0-8. A “0” Priority, which is also the initial
default, indicates that this clock should not be considered for selecting the timing reference.
Valid priorities can range from 1-8. “1” is the highest priority and “8” is the lowest. More
than one clock can have the same priority.
b. QL mode:
The following quality modes are available to the user. The quality of a clock is
automatically detected in case of STM clocks from their S1 byte. The S1 byte is as shown
in Table7.
The user can configure the QL label of the BITS/PDH clocks manually. When a
PDH/BITS clock is nominated, it comes up with an “INVALID” Quality Label. There are
mainly 3 quality levels of clock.
i. Primary Reference Clock (PRC): G.811
a. Accuracy of clock is 10 –11
(10pico second
deviation/sec.).
b. Stratum 1 clock derived from GPS satellite, Cesium
clock.
ii. Synchronization Supply Unit (SSU): G.812
a. Accuracy of the clock is 1.6X10-8
(0.016Sec
deviation/sec).
b. Stratum 2 clock derived from Quartz crystal oscillator Table 7
iii. SDH Equipment Clock (SEC): G.813
a. Accuracy 4.6X10-6
b. Stratum 3 clock derived from Quartz crystal oscillator.
When all quality of clocks are mixed together maximum number of clocks to be used is
given by G.803 series; according to which
Maximum number of SEC's between 2 SSUs: < 20
Maximum number of SSU's in a chain: < 10
Maximum number of SEC's in a chain: 60.
i. QL Disable:
Nominated clock input QL is not considered for clock selection. In this mode, clock with
high priority will be selected. The next highest priority clock gets selected as the secondary
reference.
ii. QL Enable:
Nominated clock input QL is considered for clock selection. In this mode clock with better
QL is selected. In case of two or more having the QL label the user configured priorities are
used to break the deadlock.
c. Output QL mode:
Depending on NE configuration (SONET/SDH) and Output QL mode.
S1: 1234 Description
0000 Quality Unknown
0010 PRC
0100 SSU-A
1000 SSU-B
1011 SEC
1111 DNU

SDH BASICS
The following is the list of options available to the user for configuring the output Quality
mode:
i. Auto: SSM will send quality level/label of the selected clock as NE Quality to all interfaces
connected to NE.
ii. Manual: SSM will send user (operator) specified value as NE Quality to all interfaces
connected to NE
Note: Irrespective of the Output QL Mode, a DNU/DUS is sent on the selected clock
interface and its MSP mate port, if any.
d. External Commands:
The following external commands are supported in the SSM timing subsystem as described
with a decreasing order of priority
i. Lockout
This is an explicit request given by the user to the timing subsystem indicating that the clock
that is put in the lockout state should not be considered as a reference clock. Indicating a
Priority of “0” also is equivalent to putting a clock into lockout. A user can also manually
bring a clock out of lockout. Lockout has the highest priority among all external commands
and clears the last external command if any on the clock.
ii. Forced Switch
SSM timing subsystem chooses the Primary (and the secondary) reference clock based on
various parameters. However, the user can explicitly force the SSM module to latch onto a
lower quality/priority clock by giving a forced switch to a particular clock.
Network conditions like Signal Fail, DNU and signal PPM going above the -/+ 17 (1byte
slip =4.6ppm) mark do NOT take precedence over the Forced Switch. In the above
conditions the SSM module goes into holdover waiting for the forced clock to come back to
a usable state. Forced switch has a priority over a Manual Switch command
Doing the following and only the following can explicitly clear forced switch:
 Issuing a Clear Command.
 Locking out the clock on which Forced Switch was given.
 Reducing the Priority of the forced clock to zero.
iii. Manual Switch
A Manual Switch command is a user-configured command forcing the SSM module to latch
on to a particular reference clock. The following conditions manual switch fails:
 Existence of Lockout of the requested clock.
 Existence of a Forced Switch on the same/any other clock.
 If the clock has it‟s Priority Disabled.
 If Quality is disabled and if the clock is a SF state.
 If Quality is enabled and the clock has a DNU or a quality lower than the existing
reference clock.
The manual switch command can be cleared due to the following conditions:
 An explicit clear command.
 A higher priority lockout or forced switch.
 SF/DNU on the clock for priority disabled/enabled respectively.
 If the quality of another source becomes higher than the clock on which manual switch
is active.
6.2 SSM status advertising:
SSM After selection of the clock, PLL will synthesize the frequency as depicted in Figure 24.
Selected clock will be applied as Frequency Reference Input and with the help of Voltage

SDH BASICS
VCO
Offset
Regist
er
Output
frequency (f0)
Nominated clock (fR)
Controlled Oscillator (VCO) Output Frequency will be synthesized. SSM will keep on
surveying clock & will be indicating to user with the help of SETG status and resulting
alarms.
Figure 24
a. SETG (Synchronous Equipment Timing Generation) status:
The SETG status is the indicator of what state the SSM module is in. It has the following
states:
i. Free Running: When the offset register value is completely ignored, PLL becomes an
open loop system. The output frequency cannot be tracked to the nominated clock or no
external clock is nominated or in holdover mode for more than 24 hours.
ii. Holdover: The offset register value is fixed to the last value when both inputs were
available or the Timing reference on all nominated clocks failed or PPM offset on
selected clock is high (> +/- 17 PPM).
iii. Lock: The offset register value in PLL changes dynamically to track the output
frequency to the input frequency and thus, derives timing from the nominated clock
source. The node also shows which timing source it is deriving its reference from by
indicating a “*” against the Timing reference which is current reference clock.
b. Alarms:
SM provides following Alarms to indicates the current status of SSM
i. Timing reference failed:
This alarm is raised whenever nominated clock source has LOS, LOF or AIS or if the
Primary or the Secondary clock PPM offset is higher than +/- 17 PPM.
ii. System clock in holdover mode:
This alarm is raised in the following scenarios.
a. Current selected clock received signal fail and no other nominated clock is available for
timing reference.
b. Current selected clock PPM offset crosses +/-17PPM
iii. Timing generation entry free run:
This alarm will be generated when
a. No clock is nominated for timing reference.
b. Internal clock is selected as reference clock.
[7] Performance Monitoring (PM) Parameters:
Even though Digital Communication is immune to noise addition, noise set to occur in
transmission and performance is inversely proportional to the noise, hence performance of
the hierarchy is going to change accordingly. In STM frame while carrying the voice “data” is

SDH BASICS
of not much importance so even if noise introduction is monitored then respective measures
can be taken and in SDH PM will be monitoring the every section using Bit Interleaved
Parity (BIP) monitoring.
7.1 BIP monitoring:
BIP monitoring will be used to monitor error, which may be getting introduced in the
transmission path and every transmission path in SDH is divided as RS, MS, HP & LP.
Every section in any optical network will be necessarily a RS, which is as depicted in Figure14
in Section Hierarchy. So if RS is monitored then all transmission paths will be monitored but
in SDH PM BIP monitoring is done on all sections. Because parity check only gives
information about probability of error occurring only but not about the exact number of
errors & mainly in which part of the frame the error occurred so to inform to the SDH user
probable place of the error, all section are monitored which will be explained under BIP
calculation.
In STM frame for every section there are bytes reserved which are used in BIP monitoring
there are 4 BIP monitoring bytes
B1 byte --- RS.
B2 (3bytes) --- MS.
B3 byte --- HP.
V5 (BIP-2) --- LP.
7.2 BIP calculation:
BIP calculation is done on the STM frames and part of the STM frame over which BIP bits
are calculated is called as BLOCK. BIP monitoring bytes will be calculated over different
part of the STM frame.
a. BLOCK: The Block over which BIP monitoring bytes are calculated is as given below.
B1 byte --- STM frame --- Monitoring RS.
B2 (3 bytes) ---- STM frame-RSOH --- Monitoring MS.
B3 byte --- VC4/VC3 --- Monitoring HP.
V5 byte --- LO VC --- Monitoring LP.
So if only RS was monitored then we will get information that error occurred but its over
complete STM frame (2430 bytes), which makes it very difficult to find probable byte of
error occurrence. So if MS, HP & LP are also monitored then it will be easy to find the part
of STM frame in which error occurred. To explain this consider the example when B1
calculation shows error occurred and B2 byte shows no error occurred then obviously the
probable error occurred in RSOH bytes only.
Example: B1 byte calculation over STM frame as depicted in Figure 25.

SDH BASICS
Figure 25
STM frame 2430 bytes are arranged in vertical manner as shown and EVEN PARITY is
followed to calculate the B1 byte, according to which in every column it will see that even
number of 1‟s are present. B1 byte is calculated over nth
STM frame & that B1 byte will be
placed in (n+1)th
STM frame because if placed in nth
frame itself then it is difficult to debug
the error which may occur in B1 byte of nth
frame only. Then nth
frame will be transmitted
which when received will be again subjected to Even parity check over STM frame and it
will be stored as B1‟.Then (n+1)th
frame is received and B1 byte is retrieved from that which
will be compared with B1‟ by B1(XOR)B1‟.If they match then probably no errors have
occurred(its probable because even parity can find only odd number of errors occurred in
case of even number of errors it will not trace the error) and if 1 bit doesn‟t match then in
that column 1 or more errors occurred. In B1 if 8bits don‟t match then it can be concluded
that 8BIP bits are in error. Similarly all BIP monitoring bytes are calculated over their
respective blocks.
7.3 Performance Monitoring Parameters:
BIP monitoring bytes only will give information regarding errors occurred in the section and
will report the alarms like SD (1 bit error in 105
– 109
bits) & SF (1 bit error in 103
– 105
bits),
but PM parameters will give the conclusion drawn over period of 15min or 24Htrs.
a. Errored Block (EB):
Errored Block is block in which one or more BIP bits are in error.
b. Errored Second (ES):
Errored Second is one-second period in which one/more Errored Blocks occurred or defect
second occurred. A second is said to be defect second if traffic affecting alarm is reported.
Traffic affecting alarms like LOS, LOF, AIS, RDI.
1 Block 125Sec.
1 Sec.  8000 blocks (For B1, B2 & B3).
1 Sec.  2000 blocks (For V5 bytes because of multiframe).
ES only gives the account of error occurred in the second but not is it very severe condition
to be considered so that traffic is affected, so for that another PM parameter will be counted.
c. Severely Errored Second (SES):
Severely Errored Second (SES) is one-second period, which contains >=30% Errored
blocks or the defect second has occurred.
1s  8000 blocks.
30% of 8000 blocks  2400 blocks.
30% of 2000 blocks 600 blocks (Only for V5).
SES will be counted only when 2400(600) blocks are in error but if user can control over
the situation performance degradation from ES to SES then it adds error performance
monitoring.

SDH BASICS
d. Background Block Error (BBE):
When performance degrades gradually from ES to SES, EB‟s are counted as BBE to indicate
gradual degradation. BBE doesn‟t increment when SES is counted as depicted in Figure 26.
Till 5 sec. BBE goes on incrementing but at 6th
sec. EB >30% EB in 1 Sec. BBE retains
value which is indication of performance degradation from ES to SES. Errored Block Count
(EBC) is equal to BBE but BBE is preferred because
i. EBC is cumulative count, which keeps on incrementing (Doesn‟t contribute to any
performance monitoring), hence is not included in PM parameter in UI of NES..
ii. BBE is also cumulative count but doesn‟t count when part of SES.
Figure 26
e. Unavailable Second (UAS):
UAS is PM parameters which indicates more severe condition compared to SES, where
there is burst of SES events. UAS count start incrementing by 10 its previous count, at the
occurrence of contiguous 10th
SES event and period of unavailable time begins, where
these 10 consecutive SES started their burst of 10 event as depicted in Figure 27.
Figure 27
There are 33 seconds under consideration, in which 10 SES event started at 6th
sec, previous
to that SES count was 1 & ES count was 3,so when UAS set to 10(at 15th
sec), the count was
ES=13 & SES=11,which will be decremented by 10,as UAS started counting and period of
unavailable time started at the beginning of 10 SES event from 6th
Sec.
Resetting Logic of UAS:
UAS count decrements by 10 at the onset of 10th consecutive non-SES and period of
available time begins where 10 non-SES event started.
In Period of Unavailable time ES, SES, UAS counts will counted as shown in Table 8.

SDH BASICS
Time interval(sec) 0 1 2 3 4 5 6 7 8 9 10 11 12 13
SES count 1 2 3 4 5 6 7 8 9 9
Table 8
NOTE:
# UAS increments because, its period of unavailable time and if its 10th
error free
second(non-SES) then it will be part of available time where UAS will decrement by
10.
* ES count increments because that second can be the 10th
ES (non-SES) event which
will make it part of available time. UAS count increments but if ES is 10th
ES
(non-SES) then it will be part of Available time depicted in Figure 26.
@ES & SES retains value as its SES and no way it can become part of available time
in past 10 sec, and also UAS increments as it part of Unavailable time.
i. Max. count possible in 900 sec of
ES: 900
SES: 810(9SES & 1non-SES(SEP)
UAS: 900.
f. Severely Errored Period:
If 10 consecutive SES event was counted as Unavailable period but if there is burst of SES
(as in NOTE i.) but less than 10 counts then it can also affect the performance of the
network in that case it will be declared as Severely Errored Period.
A SEP begins at the onset of 5 to 9 consecutive SES events. For Tejas Nodes, after 7
consecutive SES, SEP is set as depicted in Figure 28.
Figure 28
g. Errored Second Ratio (ESR):
The ratio of ES to total seconds in available time during a fixed measurement interval.
Say interval is of 13 sec then,
ESR: (13 sec/ 13 sec)…. Refer Figure 28.
h. Severely Errored Second Ratio (SESR): The ratio of SES to total seconds in available
time during a fixed measurement interval.
SESR: (9 sec/ 13 sec)… Refer Figure 28.
i. Background Block Error Ratio (BBER): The ratio of Background Block Errors (BBE)
to total blocks in available time during a fixed measurement interval. The count of total
blocks excludes all blocks during SES.
BBER: (3000)/48000) …Refer Figure 26.
Condition of Sec. ES count SES count UAS count
#Error free sec. Retains value Retains value Increments (if 10th
DEC. by 10)
*
ES Increments Retains value Increments (if 10th
DEC. by 10)
@
SES Retains value Retains Value Increments.

SDH BASICS
j. Severely Errored Period Intensity (SEPI): The number of SEP events in the available
time, divided by total available time in seconds.
SEPI: (1/ 13 sec)…Refer Figure 28.
7.4 Performance Monitoring Event:
Performance Monitoring Processing is done with help of PM primitives (DS & EBC)
In every NE in 2 directions Near End & Far End. Consider example in which NE A,B & C
are connected as depicted in Figure 29.
Figure 29
At NE B: a-b & c-b trunk will be Near End (NE) and b-c & b-a trunk will be Far End (FE).
Similarly in all NE there will be 2 trunks and for each trunk PM will be done as shown
below.
For Near End Performance Monitoring Events refer Figure 30(a).
NES = (NDS) + (NEBC)
NSES = (NDS) +(NEBC)
NBBE = !(NSES) + (NEBC)
Figure 30(a)
For Far End Performance Monitoring Events refer Figure 30(b).
FES = !(NDS) + (FEBC) + (FDS)
FSES = !(NDS) +(FEBC) +(FDS)
FBBE = !(NSES) + (FEBC)
Figure 30(b)
For calculating FES, FSES & FBBE Near end Defect Second (NDS) should be absent
because if NDS is present then it will give the information for PM of NE and which will be
propagated to FE trunk also. So PM processing on the FE trunk NDS must be absent.
References:

SDH BASICS
i. ITU-T Recommendation G.707/Y.1322
ii. ITU-T Recommendation G.784
iii. Training Manual (TejTM) Version 1.0
iv. ITU-T Recommendation G.841
v. SSM: Timing Sub-system Version 0.2
vi. Synchronizing SDH/SONET Application Note 1264-2
viii. Training Team Tejas Networks slides.

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