SDH Concepts And Principle
Introduction
It is an international standard networking principle and a multiplexing
method. The name of hierarchy has been taken from the multiplexing method which
is synchronous by nature. The evolution of this system will assist in improving
the economy of operability and reliability of a digital network.
1. Historical Overview
In February 1988, an agreement was reached
at CCITT (now ITU-TS) study group XVIII in Seoul, on set of recommendations,
for a synchronous digital hierarchy representing a single world wide standard
for transporting the digital signal. These recommendations G-707, G-708, G-709
cover the functional characteristic of the network node interface, i.e. the bit
rates and format of the signal passing over the Network Node Interface (NNI).
For smooth transformation from existing PDH,
it has to accommodate the three different country standards of PDH developed
over a time period. The different standards of PDH are given in Fig.1.
The first attempt to formulate standards for
Optical Transmission started in U.S.A. as SONET (Synchronous Optical Network).
The aim of these standards was to simplify interconnection between network
operators by allowing inter-connection of equipment from different vendors to
the extent that compatibility could be achieved. It was achieved by SDH in
1990, when the CCITT accepted the recommendations for physical layer network
interface. The SONET hierarchy from 52 Mbit per second rate onwards was
accepted for SDH hierarchy (Fig.1).
2. Merits of SDH
(i)
Simplified
multiplexing/demultiplexing techniques.
(ii)
Direct access to lower
speed tributaries, without need to multiplex/demultiplex the entire high speed
signal.
(iii)
Enhanced operations,
Administration, Maintenance and provisioning capabilities.
(iv)
Easy growth to higher bit
rates in step with evolution of transmission technology.
(v)
Capable of transporting
existing PDH signals.
(vi)
Capable of transporting
future broadband (ATM) channel bit rates.
(vii)
Capable of operating in a
multi-vendor and multi-operator environment.
3. Advantages
(i)
Multi-vendor environment
(mid span meet) : Prior to 1988 international agreement on SDH all vendors used
proprietary non-standard techniques for transporting information on fibre. The
only way to interconnect was to convert to the copper transmission standards
(G702/703/704). The cost and complexity levels were very high.
(ii)
Synchronous networking :
SDH supports multi-point or hub configurations whereas, asynchronous networking
only supports point-to-point configurations.
(iii)
Enhanced OAM&P : The
telecoms need the ability to administer, surveil, provision, and control the network from a
central location.
(iv)
Positioning the network
for transport on new services : LAN to LAN, HDTV, interactive multimedia, video
conferencing.
(v)
HUB : A hub is an
intermediate site from which traffic is distributed to 3 or more spur. It
allows the nodes to communicate as an angle network, thus reducing the
back-to-back multiplexing and demultiplexing.
4. S.D.H. Evolution
S.D.H.
evolution is possible because of the following factors :
(i)
Fibre Optic Bandwidth : The bandwidth in
Optical Fibre can be increased and there is no limit for it. This gives a great
advantage for using SDH.
(ii)
Technical Sophistication : Although, SDH
circuitary is highly complicated, it is possible to have such circuitary
because of VLSI technique which is also very cost effective.
(iii)
Intelligence : The availability of
cheaper memory opens new possibilities.
(iv)
Customer Service Needs : The requirement
of the customer with respect to different bandwidth requirements could be
easily met without much additional equipment. The different services it
supports are :
1.
Low/High speed data.
2.
Voice
3.
Interconnection of LAN
4.
Computer links
5.
Feature services like
H.D.T.V.
6.
Broadband ISDN transport
(ATM transport)
5. S.D.H. Standards
The S.D.H. standards exploit one common
characteristic of all PDH networks namely 125 micro seconds duration, i.e.
sampling rate of audio signals (time for 1 byte in 64 k bit per second). This
is the time for one frame of SDH. The frame structure of the SDH is represented
using matrix of rows in byte units as shown in Figs. 2 and 3. As the speed
increases, the number of bits increases and the single line is insufficient to
show the information on Frame structure. Therefore, this representation method
is adopted. How the bits are transmitted on the line is indicated on the top of
Fig.2. The Frame structure contains 9 rows and number of columns depending upon
synchronous transfer mode level (STM). In STM-1, there are 9 rows and 270
columns. The reason for 9 rows arranged in every 125 micro seconds is as
follows :
For 1.544 Mbit PDH signal (North America and
Japan Standard), there are 25 bytes in 125 micro second and for 2.048 Mbit per
second signal, there are 32 bytes in 125 micro second. Taking some additional
bytes for supervisory purposes, 27 bytes can be allotted for holding 1.544 Mbit
per second signal, i.e. 9 rows x 3 columns. Similarly, for 2.048 Mbit per
second signal, 36 bytes are allotted in 125 micro seconds, i.e. 9 rows x 4
columns. Therefore, it could be said 9 rows are matched to both hierarchies.
A typical STM-1 frame is shown in Fig. 3.
Earlier this was the basic rate but at present STM-0 which is just 1/3rd
of STM-1, i.e. 51.840 Mbit per second has been accepted by CCITT. In STM-1 as
in Fig.3 the first 9 rows and 9 columns accommodate Section Overhead (SOH) and
9 rows x 261 columns accommodates the main information called pay load. The
interface speed of the STM-1 can be calculated as follows :
(270 columns x 9 rows x 8 bits x 1/125 s) =
155.52 Mbps.
The STM-0 contains just 1/3rd of
the STM-1, i.e. 9 rows x 90 columns out of that 9 rows x 3 columns consist of
section overhead and 9 rows x 87 columns consist of pay load. The STM-0
structure was accepted so that the radio and satellite can use this bit rate,
i.e. 51.840 Mbit/s across their section.
The different SDH level as per G-707
recommendations is as given in Fig.4.
Principles
of SDH
·
SDH defines a number of
“Containers”, each corresponding to an existing plesiochronous rate.
·
Each container has a
“Path Overhead” added to it
–
POH provides network
management capability.
·
Container plus POH form a
“Virtual Container”.
·
All equipment is
synchronised to a national clock.
·
Delays associated with a
transmission link may vary slightly with time–causing location of VC within the
STM–1 frame to move.
·
Variations accommodated
by use of a Pointer
– points to beginning of VC.
–
pointer may be
incremented or decremented.
·
G.709 defines different
combinations of VCs which can be accommodated in the “payload” of an STM–1
frame.
·
When STM–1 payload is
full, more network management capability is added to form the “Section
Overhead”.
·
SOH remains with payload
for the fibre section between synchronous multiplexers.
·
SOH bytes provide
communication channels to cater for :
– OA&M facilities.
– user channels.
–
protection switching.
–
section performance
–
frame alignment
–
other functions.
6. Basic Definitions
(i) Synchronous Transport Module
This is the information structure used to
support information pay load and over head information field organised in a
block frame structure which repeats every 125 micro seconds.
(ii) Container
The first entry point of the PDH signal is
the container in which the signal is prepared so that it can enter into the
next stage, i.e. virtual container. In container (container-I) the signal speed
is increased from 32 bytes to 34 bytes in the case of 2.048 Mbit/s signal. The
additional bytes added are fixed stuff bytes (R), Justification Control Bytes
(CC and C’), Justification Opportunity bytes (s).
In container-3, 34.368 Mbit/s signal (i.e.,
534 bytes in 125 seconds) is increased to 756 bytes in 125 seconds adding
fixed stuff bits(R). Justification control bits (C-1, C-2) and Justification
opportunity bits (S-1, S-2).
Detail follows : 756 bytes are in 9 x 84
bytes/125 seconds frame. They are further subdivided into 3 sub frames 3 x 84
(252 bytes or 2016 bits). Out of this
1431 information bits (I),
10 bits (two sets) (C-1, C-2)
2 Justification opportunity bits (S-1, S-2)
573 (fixed bits)
In container-4, 139.264 Mbit/s signal (2176
bytes in 125 seconds) is increased to 9 x 260 bytes. Details as follows :
9 x 260 bytes are partitioned into 20 blocks
consisting of 13 bytes each. In each row one justification opportunity bit(s)
and five justification control bit(s) are provided.
The first byte of each block consists of
either
eight information bit (I)
or
eight fixed stuff bits (R)
or
One
justification control bit (C) plus five fixed stuff bits (R) plus two overhead
bits (o).
or
Six
information bits (I) plus one justification opportunity bit (s) plus one fixed
stuff bit (R).
The last 12 bytes of one block consists of
information bits (I).
(iii) Virtual Container
In Virtual container the path over head
(POH) fields are organised in a block frame structure either 125 seconds or
500 seconds. The POH information consists of only 1 byte in VC-1 for 125
seconds frame. In VC-3, POH is 1 column of 9 bytes. In VC-4 also POH 1 column
of 9 bytes. The types of virtual
container identified are lower orders VCs VC-1 and VC-2 and higher order VC-3
and VC-4.
(iv) Tributary Unit
A tributary unit is a information structure
which provides adaptation between the lower order path layer and the higher
order path layer. It consists of a information pay load (lower order virtual
container) and a tributary unit pointer which indicates the offset of the pay
load frame start relating to the higher order VC frame start. Tributary unit 1
for VC-1 and Tributary unit 2 is for VC-2 and Tributary unit 3 is for VC-3,
when it is mapped for VC-4 through tributary group-3. TU-3 pointer consists of
3 bytes out of 9 bytes. Three bytes are H1, H2, H3 and remaining bytes are
fixed bytes. TU-1 pointers are one byte interleaved in the TUG-2.
(v) Tributary Unit Group
One or more tributaries are contained in
tributary unit group. A TUG-2 consist of homogenous assembly of identical TU-1s
or TU-2. TUG-3 consists of a homogenous assembly of TUG-2s or TU-3. TUG-2
consists of 3 TU-12s (For 2.048 Mbit/sec). TUG-3 consists of either 7 TUG-2 or
one TU-3.
(vi) Network Node Interface (NNI)
The interface at a network node which is
used to interconnect with another network node.
(vii) Pointer
An indicator whose value defines frame
offset of a VC with respect to the frame reference of transport entity, on
which it is supported.
(viii) Administrative Unit
It is the information structure which
provides adaptation between the higher order path layer and the multiplex
section layer. It consists of information pay load and a A.U. pointer which
indicates the offset of the pay load frame start relating to the multiplex
section frame start. Two AUs are defined (i) AU-4 consisting VC-4 plus an A.U.
pointer indicating phase alignment of VC-4 with respect to STM-N frame, (ii)
AU-3 consisting of VC-3 plus A.U. pointer indicating phase alignment of VC-3
with respect to STM-N frame. A.U. location is fixed with respect to STM-N
frame.
(ix) Administrative Group
AUG consists of a homogenous assembly of
AU-3s or an AU-4.
(x)
Concatenation
The procedure with which the multiple
virtual container are associated with one another, with the result their
combined capacity could be used as a single container across which bit sequence
integrity is maintained.
7. S.D.H. Layer Structure
The S.D.H. can be based on layered concept
as shown in Fig.5. The Fig.6 shows the layer interconnection in detail.
8. Multiplexing Principles
The basic
multiplexing principles and processing stage by stage, the information signal
is shown in Fig.7. In C-11, 1.544 Mbit per sec is mapped. In C-12 container,
the entry is 2.048 Mbit/sec. In C-2 container the entry, i.e. 6.312 Mbit/sec
which is of American standard. These three containers passes through their
respective virtual containers and tributary unit pointers. At TUG-2 it can be
either 4VC-11 with TU-11 or 3VC-12 with TU-12 or 1 VC-2 with TU-2. The C-3 container
takes the input 34 Mb/s or 44.7 Mb/s of the American Standard. These through
VC-3 container and with tributary unit-3 goes to Tributary Unit Group–3. 3 Nos.
VC-3 with AU-3 can directly go to AUG and enter STM-frame. Similarly, 7 TUG-2
can be mapped into one VC-3. Otherwise one VC-3 with TU-3 or 7 TUG-2 can go to
TUG-3 and 3 TUG-3 are mapped into one VC-4. A 139.264 Mbit/sec signal can be
mapped into one VC-4 through C-4. VC-4 with AU-4 goes to AUG and then to
STM-frame. The different possibilities are shown in Fig.7.
The details of
processing and adding pointers from the base level to VC–4 container and then
to AUG and then to STM–N is given in Fig.8, where the entry 2M bit/sec is
shown. In the Fig.8, it can be noted that pointers gives the phase alignment
between the shaded and unshaded areas, i.e. the pointer locates the position of
the virtual container which are floating in the STM–frames. Figure 9 shows the
processing of 34 M/bit signal through VC–3 container and going to
Administrative group unit and then to STM frame.
In Fig.10, it is
shown that 140 M/bit signal is mapped into VC–4 container and then enter into
STM frame through AUG. Figure 11, gives the details of processing 2.048 M/bit
signal into VC–3 container and then directly through AUG entering into STM
frame. This method is also posssible.
9. Section Overhead Brief Description
The section overhead portion of the STM-1
frame with their relevant bytes are indicated in Fig. 12. From the figure, it
is seen that 4th row 9 bytes are reserved for AU pointers and this
will be discussed separately. The top 3 rows x 9 columns of STM-1 frame
reserved for Regenerator Section Overhead (R SOH). From the 5th row
to 9th row with 9 columns are reserved for Multiplex Section
Overhead (M SOH). A brief idea of the different bytes in regenerator section
overhead and multiplex overhead are
given below :
A-1, A-2 are framing bytes. Their values are :
A1 : 11110110
A2 : 00101000
(i)
These two types of bytes
form 16 bit Frame Alignment Word (FAW). FAW formed by the last A-1 byte and the
adjacent A-2 byte, in the transmitter sequence defines the frame reference for
each of signal rates. There are 3 A-1 bytes in STM-1 and 3 A-2 bytes in STM-1. In higher order STM their number increases
with the STM order, i.e. in STM-4, there will be 12 A-1 bytes and 12 A-2 bytes.
(ii)
STM Identifier with C-1
Byte : In STM-1 there is a single C-1 byte which is used to identify each of
inter-leaved STM’s and in an STM-N signal. It takes binary equivalent to the
position in the inter-leave.
(iii)
D-1 or D-12 : These bytes
are for data communication channel. Inthis D-1, D-2 and D-3 are for regenerator
section. It can support 192 kilo bit per section. D-4 to D-12 are for multiplex
section. They can support 576 kilo bit per second.
(iv)
E-1, E-2 for order wire
purposes.
E-1
is for regenerator section order wire.
E-2
is for multiplex section order wire.
(v)
F-1 is used for fault
control purposes.
(vi)
B-1 byte are called bit
inter-leave parity-8. This is used for error monitoring in the regenerator
section. There is only 1 byte in STM-1 or STM-4 or STM-16. On line monitoring
can be done in this case.
(vii)
B-2 bytes. These are used
for error monitoring in the multiplex section. There are 3 bytes for STM-1,
STM-4 and 16 will have more number of B-2 bytes as per their order.
(viii)
K-1, K-2 bytes. There are
2 bytes for STM-1, 4 or 16. These are used for co-ordinating the protection
switching across a set of multiplex section organised as protection group, they
are used for automatic protection switching.
(ix)
Z-1, Z-2 : These bytes
are located for functions and yet defined, as per CCITT recommendations.
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