FAQ's
Q. What
is SDH ?
SDH stands for Synchronous Digital Hierarchy
& is an international Standard for a high
capacity optical telecommunications network.It
is a synchronous digital transport system aimed
at providing a more simple,economical,&
flexible teleccommunication infrastructure.
Q. What led to SDH development
?
Before SDH, the first generations of fibre-optic
systems in the public telephone network used
proprietary architectures, equipment line codes,multiplexing
formats, and maintenance procedures. The users
of this equipment wanted standards so they could
mix and match equipment from different suppliers
The primary reason for the creation of SDH was
to provide a long-term solution for an optical
mid-span meet between operators; that is, to
allow equipment from different vendors to communicate
with each other.This ability is referred to
as multi-vendor interworking and allows one
SDH-compatible network element to communicate
with another, and to replace several network
elements, which may have previously existed
solely for interface purposes.
Traditionally, digital transmission systems
and hierarchies have been based on multiplexing
signals which are plesiochronous (running at
almost the same speed). Also, various parts
of the world use different hierarchies which
lead to problems of international interworking;
for example, between those countries using 1.544
Mbit/s systems (U.S.A. and Japan) and those
using the 2.048 Mbit/s system.
Q. What
are the advantages of SDH over PDH ?
The increased configuration flexibility and
bandwidth availability of SDH provides significant
advantages over the older telecommunications
system.
These advantages include:
 |
A reduction in
the amount of equipment and an increase
in network reliability. |
|
The provision of
overhead and payload bytes - the overhead
bytes permitting management of the payload
bytes on an individual basis and facilitating
centralized Fault sectionalisation.-nearly
5% of signal structure allocated for
this purpose. |
|
The definition
of a synchronous multiplexing format
for carrying lower-level digital signals
(such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s)
which greatly simplifies the interface
to digital switches, digital cross-connects,
and add-drop multiplexers. |
|
The availability
of a set of generic standards, which
enable multi-vendor interoperability. |
|
The definition
of a flexible architecture capable of
accommodating future applications, with
a variety of transmission rates.Existing
& future signals can be accomodated. |
Q. What
are the main limitations of PDH ?
The main limitations of PDH are:
|
Inability to identify
individual channels in a higher-order
bit stream. |
|
Insufficient capacity
for network management |
|
Most PDH network
management is proprietary |
|
There's no standardised
definition of PDH bit rates greater
than 140 Mbit/s |
|
There are different
hierarchies in use around the world.
Specialized interface equipment is required
to interwork the two hierarchies |
Q. What
is meant by "Plesiochronous" ?
If two digital signals are Plesiochronous, their
transitions occur at "almost" the
same rate, with any variation being constrained
within tight limits. These limits are set down
in ITU-T recommendation G.811. For example,
if two networks need to interwork, their clocks
may be derived from two different PRCs. Although
these clocks are extremely accurate, there's
a small frequency difference between one clock
and the other. This is known as a plesiochronous
difference.
Q. What
is meant by "Synchronous" ?
In a set of Synchronous signals, the digital
transitions in the signals occur at exactly
the same rate. There may however be a phase
difference between the transitions of the two
signals, and this would lie within specified
limits. These phase differences may be due to
propagation time delays, or low-frequency wander
introduced in the transmission network. In a
synchronous network, all the clocks are traceable
to one Stratum 1 Primary Reference Clock (PRC).
Q. What
is meant by "Asynchronous" ?
In the case of Asynchronous signals, the transitions
of the signals don't necessarily occur at the
same nominal rate. Asynchronous, in this case,
means that the difference between two clocks
is much greater than a plesiochronous difference.
For example, if two clocks are derived from
free-running quartz oscillators, they could
be described as asynchronous.
Give details of PDH signals.
| Signal |
Digital Bit Rate |
Channels |
| E0 |
64 kbit/s |
One 64 kbit/s |
| E1 |
2.048 Mbit/s |
32 E0 |
| E2 |
8.448 Mbit/s |
128 E0 |
| E3 |
34.368 Mbit/s |
16 E1 |
| E4 |
139.264 Mbit/s |
64 E1 |
|
Give details of SDH signals.
| Bit Rate |
Abbreviated |
SDH |
SDH Capacity |
| 51.84 Mbit/s |
51 Mbit/s |
STM-0 |
21 E1 |
| 155.52 Mbit/s |
155 Mbit/s |
STM-1 |
63 E1 or 1 E4 |
| 622.08 Mbit/s |
622 Mbit/s |
STM-4 |
252 E1 or 4 E4 |
| 2488.32 Mbit/s |
2.4 Gbit/s |
STM-16 |
1008 E1 or 16 E4 |
| 9953.28 Mbit/s |
10 Gbit/s |
STM-64 |
4032 E1 or 64 E4 |
| 39813.12 Mbit/s |
40 Gbit/s |
STM-256 |
16128 E1 or 256 E4 |
| STM = Synchronous
Transport Module |
|
Q. What
are the various steps in multiplexing ?
The multiplexing principles of SDH follow, using
these terms and definitions:
Mapping: A process used when tributaries are
adapted into Virtual Containers (VCs) by adding
justification bits and Path Overhead (POH) information.
Aligning: This process takes place when a pointer
is included in a Tributary Unit (TU) or an Administrative
Unit (AU), to allow the first byte of the Virtual
Container to be located.
Multiplexing: This process is used when multiple
lower-order path layer signals are adapted into
a higher-order path signal, or when the higher-order
path signals are adapted into a Multiplex Section.
Stuffing: As the tributary signals are multiplexed
and aligned, some spare capacity has been designed
into the SDH frame to provide enough space for
all the various tributary rates. Therefore,
at certain points in the multiplexing hierarchy,
this space capacity is filled with "fixed
stuffing" bits that carry no information,
but are required to fill up the particular frame.
Explain 1+1 protection. In
1+1 protection switching, there is a protection
facility (backup line) for each working facility
At the near end the optical signal is bridged
permanently (split into two signals) and sent
over both the working and the protection facilities
simultaneously, producing a working signal and
a protection signal that are identical.At the
Far End of the section, both signalsare monitored
independently for failures. The receiving equipment
selects either the working or the protection
signal. This selection is based on the switch
initiation criteria which are either a signal
fail (hard failure such as the loss of frame
(LOF) within an optical signal), or a signal
degrade (soft failure caused by the error rate
exceeding some pre-defined value).
Explain 1:N protection. In
1:N protection switching, there is one protection
facility for several working facilities (the
range is from 1 to 14). In 1:N protection architecture,
all communication from the Near End to the Far
End is carried out over the APS channel, using
the K1 and K2 bytes. All switching is revertive;
that is, the traffic reverts to the working
facility as soon as the failure has been corrected.
In 1:N protection switching, optical signals
are normally sent only over the working facilities,
with the protection facility being kept free
until a working facility fails.