The document summarizes the frame structure of SDH (Synchronous Digital Hierarchy) transmission. It explains that an STM-1 frame comprises 9 rows and 270 columns, with each column representing 1 byte. Transmission occurs row by row at a rate of 125 microseconds per frame. This equates to a payload data rate of 155.52 Mbps. It also lists the various network elements in SDH such as multiplexers, regenerators, and the SDH multiplexing process.

1. Answere1: SDH Frame Structure
A basic STM frame is represented by a matrix of 9 rows and 270 columns; each
column being one byte as shown in Fig. Transmission is row by row, starting
with the byte in upper left corner and ending with the byte in the lower right
corner. The frame repetition rate is 125 s, meaning that a byte in the
payload represents a 64 Kbps channel.
The STM-1 frame is capable of transporting any PDH tributary signal (≤ 140
Mbps). The frame comprises of Section overhead (SOH), pointer and the payload.
How do we arrive at the bit rates? We may proceed through the steps as given
below:
Number of rows in a frame : 9
Number of columns in a frame : 9 + 261 = 270
Number of bytes/frame : 9 x 270
Number of bits /frame : 9 x 270 x 8
Number of bits per second : 9 x 270 x 8 x 8000
= 15552000 or 155.52 Mbps
Answer 2: Network Elements in SDH
Before the evolution of the standards covering synchronous transmission
systems, networks had to be built up from separate multiplex and line terminal
equipment. These are characterized by defined formats and electrical
interfaces at each level of the transmission hierarchy; whereas optical
interfaces were entirely proprietary. This gave rise to large amounts of
multiplex and separate optical line equipment. On the other hand in SDH,
multiplexer performs both multiplexing and line terminating functions.
Synchronous multiplexers can accept a wide range of tributaries and offer a
number of possible output data rates. Though the regeneration of signal at
repeaters is similar to PDH, there is some additional equipment in SDH to
perform function like cross–connection and OA&M functions as explained in
following sections.
Regenerators
Regenerators, as the name implies, have the job of regenerating the
clock and amplitude 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 Kbps
channels (e.g. service channels E1, F1, etc. in RSOH) and also can be
output using these channels

2. SDH Multiplexing
A procedure by which multiple lower order path layer signals are
adapted into a higher order path or the multiple higher order path layer
signals are adapted into a multiplex section.
Answer 3: VC(virtual container): container + POH
TUG(tributary unit group): group of TU
TU(tributary unit): VC + TU pointer
AUG(administrative unit group):group of AU
AU(administrative unit):TUG + AU pointer
POH: pay load container
Answer 5: Synchronization Methods
The Synchronization Equipment Clock (SEC) generates the System Clock (T0)
which is used for synchronization in internal processing and for all out going
SDH signals and resynchronized. E12 (tributary) outputs (as stated earlier
when E12 Resync optional feature is used). The SEC uses one of the four
sources mentioned below as references to generate the T0.
Two STM–1 inputs (This reference is known as T1 Ref.).
Any two 2 Mbps inputs (T2 Ref.)
Two External 2 Mbps Ref. (T3 Ref.)
Internal Oscillator
The system also outputs two reference clocks (T4) for use, if required, for
use in other systems in the same place etc. they can be individually be
Derived from
The two STM-1 inputs
T0
Different Modes of Synchronization
a) Locked Mode:
In this mode, the T0 is kept locked to the selected reference. If the selected
reference is one of the STM–1 inputs, the Synchronization Status Message (SSM)
on this input is transmitted in all the other outputs (in the S1 byte of the
SOH) except in the return direction of this input. Instead, in this direction
a ‘Do Not Use For Sync.’ message is inserted. That is, if the equipment at

3. Station A is taking the STM–1 input coming in from Station B as a reference,
in the S1 byte (bit 5 to 8) of SOH a code which mean ‘Do Not Use Sync.’ is
sent. This is to prevent the station B from using a derivative of its own
clock (after a transmission delay) as reference for its clock. If used this
may lead to the SEC of Stn. B perennially trying to get phase locked to the
reference offered to it (This problem is referred to as a Timing Loop). The
SSM otherwise indicates the quality level of the clock used through standard
codes. When the system is operating in this mode, the difference of T0 and the
internal clock of the system is calculated and stored as the ‘Hold over
Value’ for use in the hold over mode.
b) Hold Over Mode:
When all the selected references for deriving T0 is lost, the system uses the
Hold over Value’ stored in memory to continue to operate retaining the
frequency it had when it was in the locked mode. Usually the system can work
for more than 24 hours in this mode.
c) Free Running Mode:
When both the modes above are not operable, i.e., when no reference source was
ever selected or a reference was lost before a hold over value was stored in
memory etc., the system enters free running mode in which the internal timing
reference is used.