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SDH Lab Course Manual - Lab Course Communication Networks Experiment

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					                                               L

                                       K             N
                         Lehrstuhl für Kommunikationsnetze
                          Technische Universität München
                                  Prof. Dr. Ing. J. Eberspächer




                                  Lab Course
                     Communication Networks



                                  Experiment

                                          SDH

PDH        SDH                                                                           STM-   622
                                                                                 4*       4     Mbit/s

                                              1*                                      1* STM-
140        C-4                                       VC-4           AU-4   AUG                  155
Mbit/s                                                                                    1     Mbit/s

                                                     3*

34                                   1*      TUG-
           C-3    VC-3   TU-3
Mbit/s                                        3

                                              7*
                                 1* TUG-
                  VC-2   TU-2
                                     2


                                     3*
                  VC-    TU-
2 Mbit/s   C-12
                  12     12




                                Lehrstuhl für Kommunikationsnetze
                                 Technische Universität München
                                            Arcisstr. 21
                                          80333 München
Lehrstuhl für Kommunikationsnetze                                                   Lab Course SDH



Contents

1     Why Lab Course SDH? __________________________________________________ 5
2     General _______________________________________________________________ 6
    2.1 SDH Mapping ______________________________________________________________7
    2.2 Experimental System ________________________________________________________9
      2.2.1   Structure of the Multiplexer _____________________________________________________10
      2.2.2   Management Software UNIGATE ________________________________________________11
      2.2.3   SDH Measuring Tool ANT-20E __________________________________________________12
    2.3 Homework________________________________________________________________13
3     Network Configuration _________________________________________________ 14
4     Clock Synchronization __________________________________________________ 15
    4.1 Background_______________________________________________________________15
      4.1.1   Why Clock Synchronization? ____________________________________________________15
      4.1.2   Clock Signal Distribution _______________________________________________________17
    4.2 Synchronization by External PRC ____________________________________________17
      4.2.1   Hardware Configuration ________________________________________________________17
      4.2.2   Software Adjustment ___________________________________________________________19
    4.3 Synchronization by Internal Clock Source _____________________________________22
    4.4 Without Synchronization____________________________________________________22
    4.5 Homework________________________________________________________________23
5     Connections __________________________________________________________ 24
    5.1 Lower Order Path Cross-Connections_________________________________________24
      5.1.1   Loopback in the same Multiplexer ________________________________________________25
      5.1.2   Mapping of the VC-12 _________________________________________________________25
    5.2 Higher Order Path Cross-Connections ________________________________________27
      5.2.1   Short Way ___________________________________________________________________28
      5.2.2   Long Fiber ___________________________________________________________________28
      5.2.3   Long Way ___________________________________________________________________29
    5.3 Homework________________________________________________________________29
6     Protection Switching Mechanisms ________________________________________ 29
    6.1 Multiplex Section Protection Termination Function (MSPTF) ____________________30
    6.2 Path Protection____________________________________________________________32
      6.2.1   APS caused by Fiber Failure _____________________________________________________33
      6.2.2   APS caused by Connection Failure ________________________________________________33
    6.3 Homework________________________________________________________________34
7     Performance Measurement ______________________________________________ 35
    7.1 Homework________________________________________________________________36
8     Literature ____________________________________________________________ 37
9     Appendix _____________________________________________________________ 38




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Lehrstuhl für Kommunikationsnetze                                                Lab Course SDH




                                    Laser Warning




An invisible laser light is emitted from the OI622 module sender which can
be dangerous, so please take precaution when using it. Please also take
precaution when using the SDH measuring tool ANT-20E optical interface.
Always turn off the laser before disconnecting a fiber connection.
Glass fibers should not be physically damaged or strained! Don't touch the
end face of the connectors!


When turing off the laser on the multiplexer, the following steps should be taken.

• Open the <MSPTF-4/RTF-4> window in the UNIGATE <Function View> of the
  multiplexer in order to choose the optical sender you would like to disconnect.
• When clicking on menu, you may select <RTF work./west #1> or <RTF prot./east #2>,
  depending on wether the fiber is connected on the left or the right side of the OI 622
  module. The OI 622 module should be disconnected by selecting <Configuration>
  <Optical>.
• When the window is opened, select the combo box <Forced Laser Shutdown> and press
  the <Apply> button.


For switching off the laser of the ANT you have to do this:

• Press the <LASER> button in the <Signal Structure> window of the ANT.
• The yellow LASER LED on the ANT, which is next to the sender connector, should be
  extinguished.


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Lehrstuhl für Kommunikationsnetze                                                              Lab Course SDH



Abbreviations
ADM                 Add-Drop Multiplexer
ANT                 Advanced Network Tester
APS                 Automatic Protection Switching
ATM                 Asynchronous Transfer Modus
AU                  Administrative Unit
AUG                 Administrative Unit Group
BBE                 Background Block Error
BSHR                Bi-directional Self Healing Ring
C                   Container
DM                  Degraded minute
DUT                 Device under Test
EB                  Errored Block
EFS                 Error-free second
EI                  Electrical Interface
ES                  Errored Seconds
HOA                 Higher Order Assembler
HOI140M             Higher Order Interface 140 Mbit/s
HPXVC4              Higher Order Path Cross-Connection VC-4
ISDN                Integrated Service Digital Network
ITU-T               Telecommunication Standardization Sector of
                    International Telecommunications Union
LAN                 Local Area Network
Line Signals        SDH signals like STM-1 and STM-4 transported on the SDH network
LOI                 Lower Order Interface
LPXVC               Lower Order Path Cross-Connection VC-
M155                Multiplexer/Demultiplexer 155 Mbit/s
MSP                 Multiplex(er) Section Protection
MSPTF               Multiplexer Section Protection Termination Function
MTS                 Multiplex Timing Source
Mux                 Multiplexer
OC                  Optical Carrier
OI622               Optical Interface 622 Mbit/s
PDH                 Plesiochronous Digital Hierarchy
POH                 Path Overhead
PRC                 Primary Reference Clock
RTF-4               Regenerator Termination Function STM-4
SDH                 Synchronous Digital Hierarchy
SES                 Severely Errored Seconds
SET                  Synchronous Equipment Timing
SN                  Switching Network
SOH                 Section Overhead
SONET               Synchronous Optical Network
STM-N               Synchronous Transport Module, level N=1; 4; 16; 64
STS                 Synchronous Transport Signal
TDM                 Time Division Multiplex
TP                  Termination Point
Tributary Signals   signals extracted from or inserted into the SDH bitstream on the network
TTI                 Trail Trace Identifier
TU                  Tributary Unit
TUG                 Tributary Unit Group
UAS                 Unavailable Second
VC                  Virtual Container




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Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH




1 Why SDH?

What is SDH? SDH is the abbreviation for Synchronous Digital Hierarchy. It is a technique
for broadband transmission which has significant importance in modern commercial
networks, especially in the backbones of wide area networks. SDH can be used with optical
and electrical networks.
SDH is a Time Division Multiplexing (TDM) technology. The multiplexing is done in a
hierarchical way. This allows multiplexing of different channels with different bit rates. This
is very useful because it is always possible to add or drop channels with low expenditure.
Compared with the older PDH system, it is much easier to extract and insert low-bit rate
channels from or into the high-speed bit streams in SDH. It is no longer necessary to
demultiplex and then remultiplex the whole plesiochronous structure, which can be both a
complex and costly procedure.




Figure 1: Schematic diagram of hybrid communications networks


Figure 1 is a schematic diagram of a SDH ring structure with various tributaries, i.e.
subordinate signals. The mixture of different applications is typical for the data transported by
SDH. Synchronous networks must be able to transmit plesiochronous signals while at the
same time be capable of handling more advanced services such as ATM.
These networks have to be clock-synchronized. Clock synchronization enables data
transmission without unnecessary delays. Its disadvantage, however, is that master clocks with
very high precision are needed. The reference timing signal of these clocks has to be
distributed throughout the whole network and it has to be ensured that "clock islands" do not
appear. Within these "clock islands", timing would then deviate from the timing in the other

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Lehrstuhl für Kommunikationsnetze                                                Lab Course SDH



parts of the network. This can be a problem when bearing in mind that SDH network nodes
can compensate for just relatively small phase differences within the network.
SDH networks provide special mechanisms for protection switching, in case of failures in
cables or nodes, such as switching signals to spare infrastructure. There are different
mechanisms with shared and dedicated protection paths for ring and line networks.
In this lab course, we want to show you what SDH is and how to operate a SDH network. In
the theoretical part, we explain how the SDH multiplexing works. We also want to describe
and explain the configuration of the network, which is already pre-configured in the
beginning. In the lab part, we start with synchronizing the network. After that, we will set up
connections and make quite a few measurements. Protection switching mechanisms will be
introduced. Finally we make some measurements in testing the performance of the network.
The equipment in our experiment consists of three synchronous multiplexers which are
controlled by a PC, which has a commercial network management software installed on it. For
measuring, we use a commercial SDH network tester.
Our thanks goes to Siemens AG, Mr. Sikora and Mr. Wahl for supplying this lab course with
the hardware, software, training and know-how. Hardware has also been supplied by Spinner
GmbH and for Wavetek Wandel Goltermann Eningen GmbH & Co special knowledge of the
lab’s content.

             The pointing hand in the lab course indicates when the student needs to make
             configuarations on the management unit in order to control the hardware.


             The pencil indicates when the student needs to take down measurements.

Further information such as parts of the literature used and some interesting web links for the
SDH lab can be found find on the LKN web page at "Studium" "Lehrveranstaltungen"
"Praktikum Kommunikationsnetze 1 und 2" "SDH".


2 General

As introduced in the ISDN lab course, the analog telephone signal is sampled at a bandwidth
of 3.1 kHz, quantized, encoded and then transmitted at a bit rate of 64 kbit/s. By Time
Division Multiplexing (TDM) 30 such coded channels are collected together into a frame.
Together with two channels reserved for signaling information a transmission rate of 2048
kbit/s results. This transmit rate is called the primary rate and is used throughout the world.
Only the USA, Canada and Japan use a primary rate of 1544 kbit/s, which is formed by
combining 24 channels instead of 30.
The growing demand for more bandwidth meant the need for more adavnced techniques of
multiplexing throughout the world. This led to Plesiochronous Digital Hierarchy (PDH),
which allowed for further multiplexing stages of 8, 34 and 140 Mbit/s. Slight differences in
timing mean that justification or stuffing is necessary when forming the multiplexed signals.


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Lehrstuhl für Kommunikationsnetze                                                      Lab Course SDH



Towards the end of the 1980s, the so-called Synchronous Digital Hierarchy (SDH) was
introduced. This paved the way for a unified network structure on a worldwide scale, resulting
in a means of efficient and economic network management for network providers.


2.1 SDH Mapping

Because of the heterogeneous nature of modern networks, there are many different signals
such as ATM or PDH which need to be transported over SDH networks. The process of
matching the signals to the network is called mapping.
The container is the basic package unit for tributary channels. A special Container (C-n) is
provided for each class of a PDH tributary signal. These containers are always much larger
than the payload to be transported. The remaining capacity is partly used for justification
(stuffing) in order to equalize timing inaccuracies in the PDH signals. When synchronous
tributaries are mapped, fixed fill bytes are inserted instead of justification bytes.
A Virtual Container (VC-n) is made up of a container and a so-called Path OverHead (POH).
The VC is transmitted unchanged over its path through the network.

Tasks of POH are:
• Identification of the VC
• Quality monitoring
• Signaling
A VC-12, the virtual container for PDH signals with a primary rate of 2048 kbit/s, has a bit
rate of 2240 kbit/s.
Because the position of the VC in the Synchronous Transport Module (STM-N) frame is not
fixed, there has to be a pointer which points to the start of the VC. This pointer leads to the
start of the VC. If the pointer value is 0 the first bit of the VC is transmitted immediately after
the last byte of the pointer.
The unit formed by the pointer and the VC is called an Administrative Unit (AU-n) or a
Tributary Unit (TU-n).
Several TUs together form a Tributary Unit Group (TUG-n). These are in turn collected
together into a VC-4. One ore more AUs form an administrative unit group (AUG). These you
call mapping. An idea of what an AUG looks like is displayed below:
                       Pointer

                                 Pointer

                                           Pointer




    Pointer                                          VC-3              VC -3   VC -3


                                                                  VC-4

                                                              STM -1

Figure 2: "Logical" mapping in an AUG

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Lehrstuhl für Kommunikationsnetze                                                                       Lab Course SDH




Lastly, the AUG plus the Section OverHead (SOH) form the STM-N.
The SOH does the following:
• Synchronization of an STM signal
• Identification of an STM-1 frame (especially if there are more than one STM-1 in a STM-
   N)
• Quality monitoring
• Channels for network monitoring
• Channels for voice connection between network nodes
• Signaling for Automatic Protection Switching (APS)
Ways of mapping a PDH signal into a STM-4 frame, which are important for the lab course
are shown in Figure 3. The whole plan for mapping in SDH is shown in the appendix.


  PDH       SDH                                                                                  STM-    622
                                                                                       4*         4      Mbit/s

                                                1*                                          1*   STM-
 140        C-4                                        VC-4         AU-4      AUG                        155
 Mbit/s                                                                                           1      Mbit/s

                                                       3*

 34                                     1*      TUG-
            C-3      VC-3      TU-3
 Mbit/s                                          3                    Path Overhead Processing (“Mapping“)
                                                                      Pointer Processing (“Aligning“)
                                                7*
                                                                      (De-)Multiplexing
                                      1* TUG-                 STM Synchronous Transport Module
                     VC-2      TU-2
                                          2                   AUG Administrative Unit Group
                                                              AU      Administrative Unit
                                        3*                    TUG Tributary Unit Group
                     VC-        TU-                           TU      Tributary Unit
 2 Mbit/s   C-12
                     12         12
                                                              VC      Virtual Container
                                                              C       Container



Figure 3: Mapping in SDH in the lab course


The ITU-T Recommendation G.707 defines a frame with a bit rate of 155.520 Mbit/s as first
level Synchronous Transport Module (STM-1). Figure 4 shows the format of this frame.

                                         2 7 0 b y te s



            P o in t e r                             P a y lo a d
 SOH                                                                                                     9 ro w s
                                         ( tr a n s p o r t c a p a c ity )


            9 b y te s

Figure 4: Schematic diagram of STM-1 frame


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Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH



It is made up of a byte matrix of 9 rows and 270 columns. Transmission is done row by row,
starting with the byte in the upper left corner and ending with the byte in the lower right
corner. The frame repetition rate is 125 µs. Each byte in the payload represents a 64 kbit/s
channel.
The STM-1 frame is capable of transporting any PDH tributary signal (≤140 Mbit/s). A STM-
4 frame, which is used on the optical ring in the lab course, is generated by interleaving 4
STM-1 frames in a byte-by-byte manner.
As already mentioned, in the USA, Canada and Japan, a different system called SONET
(Synchronous Optical NETwork) is used. The base bit rate is 51.84 Mbit/s and is designated
as a STS-1 (synchronous transport signal). If this bit rate is transmitted over an optical cable
system, the signal is designated as a OC-1 (optical container).


2.2 Experiment Setup

The experiment setup consists of a bi-directional ring with a bit rate of 622.08 Mbit/s which is
setup with three synchronous add/drop multiplexers (ADM) which are connected together by
fibers pairs (see Figure 5). The multiplexers can be controlled from a PC because the
multiplexers are connected to a LAN. A measurement tool can be connected to the optical ring
to enable monitoring of the signals transmitted on the ring. It also can even change data in the
header from the packets or insert bit errors into them. Lastly, it can be used for monitoring the
signals on the electrical interfaces of the multiplexers.

                                                                        Pair of Fibers
                                         w   Mux 1      e               622 Mbit/s



                              LAN

                                         w   Mux 2      e




                                         w   Mux 3      e




 Network Management
 Unit (UNIGATE)

                                                                        SDH Measuring Tool
                                                                        (ANT-20E)

Figure 5: Expirement Setup




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Lehrstuhl für Kommunikationsnetze                                                    Lab Course SDH



2.2.1 Structure of the Multiplexer

In the lab course we use three Siemens SMA4C multiplexers that work as synchronous Add-
Drop Multiplexers (ADM). By means of ADMs, plesiochronous and lower bit rate
synchronous signals can be extracted from or inserted into high speed SDH bit streams. This
feature makes it possible to set up ring structures. The ADMs also provide means for
protection switching in the event of a failure.
On the panel of each multiplexer are connectors for power supply, a connection to the LAN,
an electrical tributary and line signals. Tributary signals are signals, extracted from or inserted
into the SDH bit stream. This SDH bit stream is called a line signal.
The rack on the other side supplies sufficient space for plug-in hardware. The hardware has a
modular design, so that the multiplexer can easily be adapted to the local requirements, e.g.
more interfaces for 2 Mbit/s PDH signals if there are a lot of signals with low bit rates.

                                                                Power Supply




                                                                                  Tributary
                                                                                  Interfaces
       Service and LAN
       Interfaces                                                  Modules




 line west


                                                                      line east


Figure 6: Hardware view of the multiplexer


Mapping of tributary PDH signals into VCs is done in tributary modules. For each class of a
PDH signal there are special tributary modules: the Electrical Interfaces EI2, EI34 and EI140.
The EI155 module can either be used for tributary or line signals. Multiplexing signals and
adding and dropping signal into the line signal is done in a Switching Network module called
SN. One can add an additional SN module for module protection switching in case one SN
module fails. The module M155 consists of a multiplexer and a demultiplexer. The
Multiplexer combines four STM-1 signals into a STM-4 signal and the demultiplexer takes
the STM-4 signal and splits it up back into four STM-1 signals. Lastly, the Optical Interface
OI622 is responsible for transforming the electrical signal into an optical signal and
transmitting and receiving the optical signal. The OI622 module to the left is called the line
west of the multiplexer and the OI622 module to the right represents the line east.
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Lehrstuhl für Kommunikationsnetze                                             Lab Course SDH



The Universal Control Unit module UCU-C and the Local Alarm and Disc module LAD are
responsible for controlling and monitoring the unit. They are connected to a LAN with a PC
which runs the network management software UNIGATE.




    lin e                                                                      lin e
    w es t                                                                    e as t
                S T M -4                                        S T M -4




                                         trib u ta ry


                                                                     LAN



Figure 7: Flow of data through the modules of the SMA4



2.2.2 Management Software UNIGATE

The software for controlling the multiplexers is installed on a PC running under Windows NT.
When logging onto the network management PC as the user “SDH”, UNIGATE will start
automatically. Use the <NCT> mode for the lab course, which will allow you to control all
network elements connected to the LAN. When UNIGATE ,starts, an overview of the
structure of the network is displayed.
When you double-click the icon of a multiplexer, you will get its details. You can choose
either the module or the function view.
The module view is needed when you want to change the hardware setup of the multiplexer. It
is somewhat similar to the hardware view in Figure 6.




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Lehrstuhl für Kommunikationsnetze                                                                               Lab Course SDH


                                                    Number of slot




                                                                     LEDs for signaling Alarms:
                                                                     O Operation Mode             green    Mode Operation
                                                                                                  yellow   Mode Maintenance
                                                                     A   Alarm Suppression        green    No Alarm Suppressions set
                                                                                                  brown    Alarm suppressions set




 working       Ring west            Card         Ring east
                                    protection


Figure 8: UNIGATE Module View


The functional view is a view of the way in which user data is transported and handled in the
multiplexer. This view has similarities to Figure 3. In the functional view window, you can
establish and change connections through the network. This view is much more important in
our lab course than the module view.




Figure 9: UNIGATE Functional View


Red colored or red flashing symbols signal new alarms in the corresponding functional
element. Yellow symbols mean that there are no problems. You can select red or yellow
symbols with the right mouse button to get a sub view of this element or to configure it.
Names of fields, windows and entries in menus as well as entries to be made or selected are
printed in acute brackets<> and in italics in this lab course manual.


2.2.3 SDH Measuring Tool ANT-20E

For measuring tasks, we use a modern commercial SDH measuring tool called the ANT-20E.
It is built on top of the standard Windows 95 graphical user interface. You can use it for
measuring the delay of a signal through the network or measuring the automatic protection
switching time. It can also be used for monitoring and changing the SDH overhead. There is
also the possibility of inserting and monitoring defects. Because of the different possibilities
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Lehrstuhl für Kommunikationsnetze                                                  Lab Course SDH



of using ANT, you first have to edit the signal structure. Often used structures are already
available as files. You can open them in the menu <Application>.
The ANT is also used as a signal source for the connections.
You can find a download for the demo software of the ANT and interesting SDH manuals at
http://www.acterna.com/products/ant/ant.html.

            Anomaly/Defect Analyser           Overhead Generator

                           Pointer Analyser     Overhead Analyser

                                                      Anomaly/Defect Insertion




    Delay Time                Edit Signal Structure         Button for switching
    Measurement                                             On/Off the Laser
              Automatic Protection
              Switching Time
              Measurement

Figure 10: Signal structure window of the ANT



2.3 Homework

1. Calculate the relative loss of transport capacity of a STM-1 due to the SOH and the pointer.
2. Calculate the relative loss of transport capacity of a VC-12.
3. Compare the loss when you only calculate the loss due to the SOH (and pointer) and POH
   (as you calculated in 1. and 2.) with the real loss when the maximum number of VC-12, 63,
   is transported in a STM-1 and explain the difference.
4. How many STM-1 frames are sent per second on a STM-1 link with 155.520 Mbit/s? How
   long does the emission of a STM-1 frame take?
5. Calculate the propagation time on a glass fiber with a length of 2283 m. Remember that the
   refractive index of the glass fiber is about 1.5. The vacuum speed of light is 300000 km/s.
6. How long is a section of a fiber occupied by a STM-1 frame?




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Lehrstuhl für Kommunikationsnetze                                                Lab Course SDH




3 Network Configuration

Before we can start to set up the synchronization and the connections, we have to configure
the network. This however, is already done for you, in order to save time. We want to mention
something before we continue.
Before you can start with the configuration of the network you have to think about the
topology. It can be a line or a ring topology. When there are more than three network elements
,it can also be a mixed structure, e. g. like a star. We have chosen a ring topology with a bi-
directional ring set up by two fibers between each multiplexer. The line west side of a
multiplexer is connected to the line east side of its neighboring network element.
When building up the ring, always be sure that an optical sender and an optical receiver are
connected. When installing the glass fibers, don't go below the minimum radius of 3 cm
because if you do, the fibers will be damaged.
More complicated than the hardware configuration is the software configuration. Each
multiplexer must have its own LAN address. This address has to fulfill some specifications.
The first 13 of 20 bytes and the Area-Codes must be identical in all of the multiplexers
connected by LAN to the same network management PC. When the multiplexer is activated,
plugged-in modules have to be configured, i.e. it needs to be defined that there is a XYZ
module (no automatic recognition!) and if the module is working or protection.
As said in the beginning, this adjustments are already done. This chapter was only for your
further information.




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Lehrstuhl für Kommunikationsnetze                                                  Lab Course SDH




4 Clock Synchronization

4.1 Background

Considering high-bitrate media like optical fiber which offers 2.5 or 10 Gbit/s transport
capacity per optical channel, it is obvious that most of today’s traffic sources do not suffice to
adequately load the links of optical networks.

Therefore, the capacity of optical network links is usually shared by quite a number of
different traffic streams, which undergo several (de)multiplexing processes on the
transmission path in general.

When multiplexing several traffic streams, the resulting bitstream needs to be augmented by
additional (control) information which enables subsequent demultiplexing. In packet-switched
networks, for example, each individual packet contains its own control information, whereas
in circuit-switched systems, like SDH, a continuous scheme of back-to-back frames
encapsulate the payload data. SDH frames are of deterministic size/duration, which causes a
constant frequency of incoming frames (8000Hz @ 155Mbit/s).

This is especially advantageous in high-bitrate systems. The SDH bitrate range starts from 155
Mbit/s and goes up to 10 Gbit/s due to the fact that subsequent frame starts can be predicted
by a prior knowledge (previous frame starts & given frame size), which avoids the need of
tracking the control and payload information.


4.1.1 Why Clock Synchronization?

The need for synchronization1 isn‘t really obvious until the interaction of different traffic
streams on the high-bitrate links of an optical network is considered.

Consider three traffic streams as depicted in Figure 11. Given an initial state where these
streams share a common bitrate on the link in an ideal situation (Figure 11a), consider when
one of them sends out its frames at a slightly lower frequency than the others such as 7999
frames/s, rather than 8000 frames/s.

The frames of the lower-frequency source would gradually be delayed in relation to frames of
other streams, and at some point in time, the probability that collisions would occur would be
higher, as shown in Figure 11b.


1
  The dispute over the definition of bit synchronization (i.e. recovery of the ideal bit sampling
time) has been considered in this lab manual. When using the term synchronization, we refer
to clock synchronization throughout this manual.

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Lehrstuhl für Kommunikationsnetze                                                Lab Course SDH




Thus, proper measures must be taken to prevent the occurence of this situation. The options
are:
(a) Removing affected frames when a collision occurs
(b) Provide a mechanism which controls and adjusts the timing of the frames with respect to
     each other at their sources
(c) Provide a mechanism which controls and adjusts the timing of the frames with respect to
     each other on the transmission links

While option (a) is undesirable in general, option (b) is exactly what clock synchronization in
synchronous systems (including SDH) is about. Once the frames are synchronised properly on
the shared link, the sources share exactly the same timing when considering the subsequent
transmission of frames. See Figure 11c.

Note that option (c), known in SDH as „pointer mechanism“, has been described above.




                                                    (a)




                                                    (b)




                                                    (c)




                              Figure 11: Contending signals on a shared link




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4.1.2 Clock Signal Distribution

In order to provide the same (in)accuracy in timing to all network nodes, the nodes must be
organised in a master-slave relationship with clocks of the higher level nodes supplying the
timing signals for clocks of the lower-level nodes. All nodes can be traced to a primary
reference clock (PRC) with extremely high stability and accuracy. The ITU-T
recommendation G. 811 specifies a timing accuracy ∆f/f ≤ 10-11.

This reference clock signal must be distributed throughout the entire network, which can
either be done via an external synchronization network, or by deriving it immediately from
one of the incoming SDH signals. In other words, the frequency f of frame arrivals is the clock
signal itself and its measured deviation ∆f represents the clock accuracy ∆f/f.

For the reason that it would take a long time to verify clock accuracies as low as 10-11 just by
measurements, the accuracy of the timing source is coded in a special byte (“timing marker”)
of the SOH and the receiver trusts that it is correct.

If the clock supply fails, the affected network element switches over to a clock source with the
same or lower quality, or if this is not possible, it switches into a hold-over mode. In this
situation, the clock signal is kept relatively accurate by applying the stored frequency
correction values from the previous hours and taking the temperature of the oscillator into
account.
To avoid the creation of clock "islands", the state of the clock supply is transmitted to the
neighboring network elements by the marker. Clock islands cause increased clock drift and
would eventually result in a synchronization disaster. Thus, when a network element uses a
timing signal transmitted by another element, it in turn sends back the marker <Don't use for
sync>, so that a network element doesn't use its own clock.
Special problems arise in gateways between networks with independent clock supplies. SDH
network elements can compensate for clock offsets within certain limits by means of pointer
adjustment. Pointer activity, called pointer jitter, is thus a reliable indicator if problems or
failures have arisen with the clock supply.


4.2 Synchronization by External PRC

In our experiment, we want to first use the ANT-20E as a primary reference clock (PRC).
Although it doesn't fulfil the ITU-T recommendation G.811, its timing accuracy of ± 2ppm is
better than that of the internal clock of the multiplexers with an accuracy of ± 4,6 ppm.


4.2.1 Hardware Configuration



                                             A-17
Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH



To synchronise the timing of the ANT with the PRC, one needs to configure the hardware as
described below. We have to supply one multiplexer with the ANT timing signal. We also
want to see possible pointer actions. For this we have to watch the signals on the optical ring.
Because we can't disconnect the ring during the measurement, we use the ANT optical power
splitter, so that the power of the signal on the ring reduced to 90%. 10% of the optical power
is transmitted to the optical input of the ANT, so that the signal can be monitored. We want to
insert the ANT into the optical link from Mux 2 to Mux 1. This configuration is shown in
Figure 11.

            w                 e                     w               e

                M ux 1                                     M ux 3



                                      M ux 2

                                  e            w



                                         G .8 1 1

                                      PRC

                ANT

Figure 12: Configuration of the ring with external PRC


             • Connect the electrical output ANT-20E 1,5...156 Mbit/s with the coax cable
               with the140 Mbit/s Mux 2 input .
             • Switch off the laser in Mux 2 line east. Instructions for doing this can be
               found in the beginning of this lab course manual or is again restated below:
                  − Double click the field <MSPTF-4/RTF-4> in the functional view of Mux
                      2 with the left mouse button.
                  − In the window opened, select the field <RTF prot./east #2>, which
                      represents the optical line east module OI622 of Mux 2, with the right
                      mouse button.
                  − In the menu opened, choose <Configuration> <Optical>.
                  − Select the combo box <Forced laser shutdown> and press the <Apply>
                      button.
                  − The field <RTF #2> will turn red: This signals an alarm. When you
                      choose <Fault> in the menu of field <RTF #2>, you can see where the
                      failure or problem is. This symbol represents an optical Loss Of Signal
                      (LOS).
             • Disconnect the optical connection between the Mux 2 east sender and the Mux
               1 west receiver. When you point with the open end of the fiber connected with
               the sender of Mux 2 east on the active field of the infrared indicator card, you
               should see nothing. But be careful nevertheless, because the system tries to
               restart the laser and therefore checks the optical connection.
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Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH



             • Plug the open end of the fiber connection with the sender of Mux 2 east into
               the input of the optical splitter of the ANT.
             • Connect the 90 % output of the optical splitter of the ANT with the Mux 1 west
               receiver.
             • Connect the 10% output of the optical splitter of the ANT to the optical input
               of the ANT.
             • Start the signal structure <140 - STM4 VC4 140 - Pointer Jitter> in the ANT
               menu <Application> <Open...>.
             • Switch off the forced laser shutdown in Mux 2 line east.

4.2.2 Software Adjustment

After having set up the hardware connection, we can start making software adjustments. Now,
we select which timing signals the multiplexers are allowed to use for synchronization.
It would be favorable if Mux 1 and Mux 3 use the signal they get from line east. They should
not use it if it has the ´status <Unknown> or <Don't use for sync>. Mux 2 shall use the signal
received at the 140 Mbit/s input <HOI140M> only. Otherwise, ´clock islands` can occur. We
set the marker for the quality of this signal to <G.811> although it doesn't fit the ITU-T
recommendation.

             • Select the <SET> field in the UNIGATE functional window of Mux 2 with the
               right mouse button and chose <Configuration> in the menu opened.
             • In the window <SET Configuration> you can select which timing source
               should be used. Mux 2 shall use the timing signal received at the 140 Mbit/s
               input <HOI140M>. This is adjusted in the <SET Configuration T2> window.
               To open this window double click the <T2> field with the right mouse button.
               The <Class of PDH Port> is <HOI140M>, the <PDH Port> is <1> and the
               <Quality of Clock Source> has to be defined as <G.811>. Use either T2(1) or
               T2(2), the choice is arbitrary.
             • Set the <Priorities of Clock Source for T0> in the <SET Configuration>
               window for the used clock to <Prio1 High>.
             • The signal should not be used if it has the status of <Unknown> or <Don't use
               for sync>. Therefore, select in the field <Quality of Clock Source to Reject for
               Timing Reference> in the window <SET Configuration T1>.
             • Mux 1 and Mux 3 should use the timing signal they get from line east, i.e. the
               <Class of SDH Port> <RTF-4>, <SDH Port> <2>. They should not use it if it
               has the status of <Unknown> or <Don't use for sync>. These adjustments are
               made in the window <SET Configuration T1>. The usage of T1(1) and T1(2) is
               arbitrary again. The priority of a clock source for the port to be used is <Prio1
               High> again.

You can see the correct adjustments in Figure 12 and Figure 13.



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Lehrstuhl für Kommunikationsnetze                                             Lab Course SDH



To be able to observe the pointer jitter on the ANT we have to establish a bi-directional
connection between line west and line east of the Mux 2. This is done in the <HPXVC4
Configuration> window of Mux 2:

                • Double-click on the function group <HPXVC4> with the left mouse button to
                  open the <HPXVC4 Configuration> window.
                • Select the card <Create> and choose the <Direction> <Bidirectional> and
                  <Unprotected>.
                • The <TP A Port class and No.> is <MSPTF4 #001> and the <TP Index> is
                  <01/01w>.
                • The <TP B Port class and No.> is <MSPTF4 #001> and the <TP Index> is
                  <03/01e>.
                • Choose the <Action> <Create, Connect> and press the <Apply> button.

 Interfaces for
 receiving the clock
 source

              Line

         Tributarty


          Auxilliary




Figure 13: SET Configuration in Mux 1 and Mux 3 for external PRC




                                               A-20
Lehrstuhl für Kommunikationsnetze                                                Lab Course SDH




Figure 14: SET configuration in Mux 2 for external PRC


Look at the markers of the transmitted and the received timing signals in the window
<MSPTF4 Configuration MSTTP Working> for the line west and in the window <MSPTF4
Configuration MSTTP Protection> for the line east. Write down the clock qualities. To open
these windows select <Configuration> in the menu of the <MSPTF> field in the <MSPTF-
4/RTF-4> subview.
There is also the possibility to watch the marker of the timing signal by the overhead analyzer
of the ANT. The quality of the timing signal is coded in byte S1. To decode the bytes, use the
Interpreter.
To watch pointer activities use the Pointer Analyzer of the ANT. In the Pointer Analyzer
window there is a button <CSR>. When you press it you will get a numerical view of pointer
activities.

             1. Which markers do the the transmitted timing signals have? Write them down
                in Figure 11.
             2. Are there any pointer activities?


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Lehrstuhl für Kommunikationsnetze                                                  Lab Course SDH



4.3 Synchronization by Internal Clock Source

Now we examine what happens if the external PRC fails. One should disconnect the electrical
connection between the ANT and Mux 2. Mux 2 gets no external signal for timing anymore.
So, it switches to the internal clock source. You can see this in the <SET Configuration>
window because the arrow pointing at the used clock source disappears.
In case the timing signal of the PRC appears again, the multiplexer shall switch back to this
clock source. This is the <Revertive> Mode. To avoid permanent jumping between two timing
sources in case of an unstable signal, the multiplexer has to wait a certain time before
switching back. The <Wait to Restore Time> and <Revertive> Mode are adjusted in the <SET
Configuration Mode> window.




Figure 15: SET Configuration window


The internal clock is situated on the module SN. The SN module in Mux 2 is doubled, so that
we gain better timing accuracy and higher reliability in case of a failure in the SN module.
When the internal clock is used for timing, the transmitted timing marker is <MTS>. The
other stations now use this timing signal.
             1. Look at the transmitted and received timing markers in the <MSPTF4
                Configuration MSTTP Working> and the <MSPTF4 Configuration MSTTP
                Protection> window. Use the <Update> button to get the actual view. Again,
                write down the changes.
             2. Do the changes have an influence on the pointer activities?
             3. How could a clock island emerge in this situation?


4.4 Without Synchronization

Finally we try to see what happens if the transmission of the timing signal fails.
To do this, we set the priority of each clock source in each multiplexer in the <SET
Configuration> window to <Don't use for sync>. The result is that the multiplexers think
they do not get any timing signal. In turn, they all use their internal clock sources. Because the
accuracy of the clocks in the multiplexers slightly differ from each other, we will see pointer
activity, which is an indicator for problems with the clock supply.



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Lehrstuhl für Kommunikationsnetze                                                Lab Course SDH



             1. Which markers do the transmitted and the received timing signals have now?
             2. Try to see what happens when Mux 2 uses the <MTS> timing signal of Mux 1
                for synchronization and Mux 3 still uses its own clock source. Can you see
                differences in the step width of the pointer jitter in comparison with the
                situation when each multiplexer uses its own clock source?

Reset the software adjustments in UNIGATE for synchronization by external PRC.
Remember that the ANT as PRC can be connected to the 140 Mbit/s input <HOI140M> <1>
or to the 2 Mbit/s input <LOI2M>, <PDH Port> <15> of Mux 2. The 140 Mbit/s input shall
have the higher priority. A timing signal received at the 140 Mbit/s input shall get the marker
<G.811> and a signal received at the 2 Mbit/s input <G.812 Transit>. Turn off the laser for
reestablishing the direct optical connection of Mux 1 and Mux 2 without going through the
optical splitter of the ANT. Delete the connection between line west and line east in Mux 2.


4.5 Homework

1. Sketch what happens when the station getting the timing signal from the external PRC
   fails.




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Lehrstuhl für Kommunikationsnetze                                              Lab Course SDH




5 Connections

Once the SDH network is synchronized, one can set up connections between the network
elements. If a PDH signal is to be sent over a SDH network it first has to be mapped into a
STM-N frame, as mentioned in chapter 2. In this chapter, a step by step procedure will be
represented, which shows mapping of a STM-N frame. In section 5.1., an unprotected
connection will be established and then later on in chapter 6, different types of proetection
mechanisms will be considered.


5.1 Lower Order Path Cross-Connections

We start off with a 2 Mbit/s PDH signal. Connect the electrical 1,5...156 Mbit/s interface of
the ANT with the 2Mbit/s interface #015 of the Mux 2.
Before continuing, we have to adjust the ANT signal; e. g. the input and output used. This
time one´s aim should be to edit it on one´s own:

             • In the <Signal Structure> window click on <Edit> in the menu bar and then
                click <Signal Structure>.
             • In the <Edit Signal Structure - TX> window press the <Clear> button to delete
        the existing structure.
             • We want to have a <2M> <PDH> signal. The <PDH Mode> should be
                <Framed>.
             • Press the <TX=>RX> button to make the receiver structure identical to that of
                the transmitter structure.




                                            A-24
Lehrstuhl für Kommunikationsnetze                                              Lab Course SDH




  Transmitter
   Receiver


    ITU-T
    Standart

  Make Receiver
  equal to
  Transmitter

 Clear existing
 structure



Figure 16: Edit signal structure window of the ANT


5.1.1 Loopback in the same Multiplexer

At first we only want to make an internal loopback in Mux 2, i.e. a channel from the incoming
signal is sent in the other direction through the ring back to the sender.

                • Open the <LPX VC12 Configuration> window in the functional view of Mux
                  2 and select the <Create> card.
                • In the field <Direction> select <Loopback>.
                • The Termination Point <TP A> is <LOI2M #015 TP01>.
                • Select <Create, Connect> in the <Action> field.
                • Press the <Apply> button.
                • The red <LOS> indication of the ANT should vanish. This signifies that the
                  connection is built up successfully.

                1. Press the <Delay> button in the ANT <Signal Structure> window for
                   measuring the delay on the connection. Write the result down.


5.1.2 Mapping of a VC-12

Once the task in Section 5.1.1 is done, one is ready to map the 2 Mbit/s signal into a VC-4.
The VC-4 can then be mapped into a STM-4 that can be transmitted over the SDH network.
To do this we have to make some adjustments to the VC-4 before continuing.


                                                 A-25
Lehrstuhl für Kommunikationsnetze                                                     Lab Course SDH



We want to set up a connection to the next Mux 1 and make a loopback there. When doing
this, bear in mind that our multiplexer provides up to four VC4 connections on the ring; two
in every direction. Normally, there is room for four VC4 in a STM-4 frame, but in our
configuration, there are two in each direction which are reserved for protection means.
Therefore only two are left to be connected.

             • Create a bi-directional unprotected connection in the <HPX VC4
               Configuration> of Mux 2 between <MSPTF4 #001 TP 03/01e> or <MSPTF4
               #001 TP 04/02e> representing the optical unit and the Higher Order Assembler
               <HOA #001 TP01> which is the functional unit where the mapping is done.
             • Open the <HOA #1> window of the <HOA> subview.
             • Once the <HOA #1> is opened, one has to adjust the size of the VC which one
               wants to map into the VC-4. A VC-4 can carry 3 VC-3s. Our aim is to map a
               VC-12, which is the smallest possible VC. So, select the combo box <7
               TUG2> under the <VC3 (1)> field. Then one can see that one could map a 7
               VC-2 into a VC-3, but only a VC-12 is available. Therefore, mark the combo
               box to the left of the VC2 (1) field. In the VC-2 field 3 new fields are opened.
               Each resembles a VC-12. Note that there is room for 3 * 7 * 3 = 63 VC-12 in a
               VC-4. When seeing the HOA window, one will see similarities to Figure 16,
               which shows the multiplexing heirarchy in SDH.


                                                                                      1*
                                                                                            VC-4

                                                                                            3*
                                                                                1*    TUG
                                                           VC-3     TU-3
                                                                                       -3

                                                                                      7*
                                                                           1*   TUG
                                                           VC-2     TU-2
                                                                                 -2


                                                                                3*
                                                            VC-     TU-
                                                            12      12



Figure 17: UNIGATE window HOA #1


             • Create the bi-directional connection between <LOI2M #15 TP 01> and <HOA
               #001 TP01> in the <LPX VC12 Configuration> window in Mux 2.
             • When adjusting the <HPX VC4 Configuration> in Mux 1 remember that the
               line east of Mux 2 is connected with the line west of Mux 1! So the TP for the
               loopback is the <MSPTF4 #001 TP 01/01w> when using the <MSPTF4 #001
               TP 03/01e> in the Mux 2 or the <MSPTF4 #001 TP 02/02w> when using
               <MSPTF4 #001 TP 04/02e>.




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Lehrstuhl für Kommunikationsnetze                                                          Lab Course SDH



When building up a connection start with the high order path in the <HPXVC4
Configuration>. When deleting a connection start with the low order path in the <LPXVC
Configuration>.


                1. When the connection works correct redo the delay measurement.




5.2 Higher Order Path Cross-Connections

Now you know how the mapping in the multiplexer works. In the last chapter, it was
mentioned that one needs a VC-4 for transmission over the SDH network. Now, the aim is to
fill the VC-4 with a C-4. So, therefore connect the Mux 2, 140 Mbit/s with the electrical
1,5...156 Mbit/s ANT interface. The ANT signal has to be changed into a 140 Mbit/s signal on
the sender and the receiver side. Try to edit the <Signal Structure> window of the ANT by
yourself without using the saved structure.
A C-4 signal can be easily sent without specific adjustments to mapping. Delete the
connections of chapter 5.1 first. Then you can go directly to the <HPX VC4 Configuration> in
order to make the new connection in the Mux 2. We want to send our C-4 signal received at
the <HOI140M #001 TP 01> of Mux 2 to Mux 1 and make an external loopback at the Mux 1
140 Mbit/s interface by a loopback plug.

            1
                       O I 4 0M
                     H L O I1 2M    TP1               TP 3
                                                      TP 4                  e   R T F -4
                                          HPX VC4            M S P T F -4                   M ux 1
                                                      TP 1
                                                      TP 2                  w   R T F -4




                 1                  TP1               TP 3
                     H O I 1 40 M
                                                      TP 4                  e   R T F -4
                                          HPX VC4
                                                                                            M ux 2
                                                             M S P T F -4
                                                      TP 1
  ANT                                                 TP 2                  w   R T F -4




                                                      TP 3
                                                      TP 4                  e   R T F -4
                                                                                            M ux 3
                                          HPX VC4            M S P T F -4
                                                      TP 1
                                                      TP 2                  w   R T F -4


Figure 18: Connections between multiplexers


As can be seen in Figure 17, one can take one of two ways when connecting Mux 1 and Mux
2. One can either go the direct way from Mux 2 line east to Mux 1 line west or go a longer
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Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH



way from Mux 2 line west to Mux 3 line east and than on from Mux 3 line west to Mux 1 line
east.


5.2.1 Short Path

Let us first take the shorter path over Mux 2 line east. Here you can choose again between
<MSPTF4 #001 TP 03/01e> and <MSPTF4 #001 TP 04/02e>, because we have two
connections in every direction.
When adjusting the <HPX VC4 Configuration> in Mux 1 remember that the east side of Mux
2 is connected with the west side of Mux 1! Connect the <MSPTF4 #001 TP 01/01w> or
<MSPTF4 #001 TP 02/02w> respectively with the <HOI140M #001 TP 01> of Mux 1. When
you have used <MSPTF4 #001 TP 03/01e> in Mux 2, the corresponding port in Mux 1 is
<MSPTF4 #001 TP 01/01w>.


             1. This time we will use the delay measurement for calculating the delay time of a
                signal in a multiplexer and for verifying the estimated time for propagating
                along a fiber of a certain length. Make the delay measurement and write down
                the result.


5.2.2 Long Fiber

We want to check our calculation for the delay in a fiber of a certain length. To do this, first
turn off the laser in Mux 2 east. Now you can disconnect the optical connection and insert
the long fiber.

             1. When you have turned off the <Forced Laser Shutdown> and the system works
                properly again, start the delay measurement again and compare the result with
                the calculated delay for a fiber with a length of 2283 m.
             2. We then want to measure the dimming of the light in the fiber. To do this, we
                have a measurement tool for measuring the optical power. So, turn off the
                laser in Mux 2 east again and disconnect the connection between the long
                fiber and the receiver in Mux 1. Connect the long fiber with the measurement
                tool and make a <Manual Laser restart> for <90 seconds>. Write down the
                optical power. The optical power can be displayed in dBm or mW.
             3. Turn off the laser in Mux 2 east and remove the long fiber. Connect the
                sender of Mux 2 east by the short fiber with the measurement tool and redo the
                power measuring, in order that you can calculate the loss of optical power per
                kilometer and compare it with the value given on the fiber.
             4. What is a reason for possible differences between the given dimming and the
                result of the measurement?


                                             A-28
Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH



             5. Turn off the laser in Mux 2 east for reestablishing the connection between
                Mux 2 and Mux 1 without using the long fiber.


5.2.3 Long Way

Now we want to take the longer path via Mux 3. For this you have to change the connections
in the <HPX VC4 Configuration> of Mux 1 from west to east and vice versa in Mux 2. The
path now passes Mux 3. So we have to make some adjustments in this network element as
well, so that signals can pass.
In the <HPX VC4 Configuration> of Mux 3 we have to build up a bi-directional connection of
<MSPTF4 #001 TP 03/01e> <MSPTF4 #001 TP 04/02e> with <MSPTF4 #001 TP 01/01w>
respectively <MSPTF4 #001 TP 02/02w>.

             1. When the connection works properly start the delay measurement again.
             2. While the length of the fibers between the multiplexers is comparable short, we
                can say that the difference in the delay times is originated by the delay in the
                Mux 3. When calculating the delay of the signal in the multiplexer, remember
                that the signal passes Mux 3 twice.


Delete all the connections in the HPX's of the multiplexers.

5.3 Homework

1. The given dimming per kilometer for a signal with a wavelength of 1300 nm is 0.5 dB.
   When a signal with a power of 1mW (=0 dBm) run through a fiber with a length of 1 km,
   what is its optical power once it reaches the end of the fiber?




6 Protection Switching Mechanisms

In modern high speed networks it is very important to have fast automatic protection
switching (APS) mechanisms in case of the failure of a fiber connection or a network element.
When the protection switching is fast enough, the user doesn't realize that parts of the network
have failed.
In our lab course, we only want to take a look at protection mechanisms in case of a failure in
a fiber. Our multiplexers provide two different protection switching mechanisms for our
network configuration. They have protection mechanisms in case of the failure of a module as
well, but we won't study them here.




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Lehrstuhl für Kommunikationsnetze                                                  Lab Course SDH



6.1 Multiplex Section Protection Termination Function (MSPTF)

For a Multiplex Section Protection Termination Function for STM-4 (MSPTF-4), we have the
bi-directional self healing ring with 2 fibers (BSHR-2). MSPTF means that all the multiplex
sections, which are connections between neighbored multiplexers , are protected.
The BSHR-2 is built up out of a minimum of 3 network elements, each with two OI622
modules, with their line interfaces connected to a ring. Each line interface is connected to a
coming in and gong out fiber. From these bi-directional fibers, we get virtually two rings, one
clockwise and one anti clockwise ("2-Fiber-Ring").
The BSHR-2 is a 1:1 protection switching mechanism, which means that there is no reserved
protection path. In case of a failure, traffic with lower priority would have to be removed from
the protection path.
In the following figure there are four multiplexers instead of three that we previously had.
We don´t have three this time because with three you won't see the difference to path
protection as clear as with four.
In the upper part of the figure there is a logical view of the BSHR-2 with a bi-directional
connection and a protection ring. These rings are not comparable with the optical rings formed
by the fibers which are unidirectional. This physical layer is shown in the lower part of
Figure 18. The two directions of the working and the protection ring are divided up and
transported alternately on the different fibers as you can see there.
As you can see in the figures, in case of protection the connection doesn't take the shortest
path any longer. Instead, the network elements adjacent to the failure redirect the signal so that
it takes a detour around the whole ring in order to reach the next network element which is on
the other side of the failed ring segment. This network element then redirects the signal to the
receiver.




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Lehrstuhl für Kommunikationsnetze                                                          Lab Course SDH



 Logical view       Mux 1                           Mux 2           Mux 1                         Mux 2




                    Mux 4                           Mux 3           Mux 4                         Mux 3


 Physical view       Mux 1                         Mux 2            Mux 1                        Mux 2




                    Mux 4                           Mux 3           Mux 4                        Mux 3


                         Working ring           Protection ring             Optical link

Figure 19: BSHR-2 in working situation (left) and in protection situation (right)


In this chapter of our lab course we want to build up this protection mechanism and do some
measurements on it. We want also to take a look at the signals responsive for the APS.
The MSPTF has to be activated in each multiplexer. This is done by the network
management:

             • In the subview of the function group <MSPTF-4/RTF-4> in the Unigate
               functional view select the function unit <MSPTF>.
             • Open the window at <Configuration> <MSPTF...>.
             • Select the combo box <Enable MSPTF> and press the <Apply> button.

Now you can build up the 140 Mbit/s connection between Mux 1 and Mux 2 on the short way
again. The connections had to be deleted, because the adjustments for this APS mechanism
can only be made when there are no connections.

             1. When the connection works properly again you can start the measuring of the
                APS time on the ANT. For this press the button <APS> in the ANT signal
                structure window. Now turn off the laser in Mux 1 line west. Write down the
                APS time.
             2. Look at the alarms occurred in the UNIGATE alarm history window.




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Lehrstuhl für Kommunikationsnetze                                                          Lab Course SDH



Now go back to the working path. For this first restart the laser in Mux 1 line west. In the
<MSPCo Configuration> window of the multiplexers you can see which line is used. We
want to go back to the working line. So select <Lockout of Protection> as <Protection Switch
Request> in the <MSPCo Configuration> window of Mux 2 and press the <Apply> button.
When the line works properly the <Active Line> should be <Working Line / Normal State>
again. Finally we have to choose <Clear> as <Protection Switch Request> for enabling
MSPTF again.

                 3. Do a APS measurement during switching back to the working path.


6.2 Path Protection

Path protection is a 1+1 protection mechanism, i. e. for each path to be protected there is a
reserved protection path. But path protection has to be established severally for each
connection to be protected. This means much more work when arranging the connections for
the whole network and you loose transport capacity. But when reminding the picture with the
BSHR-2 you see that protection paths can use shorter paths than they would do with MSPTF.
In Figure 20 is only displayed path protection for a unidirectional connection. This is for
easier understanding of the picture.
We want to use two methods for releasing APS. One is by shutting down the laser which
simulates a break of a fiber. The other method is by means of trail trace identifiers (TTI)
which are given to a connection for avoiding false connections. We will change this TTI with
the ANT to simulate a misconnection.

                              M ux 1                                   M ux 2
 L o g ica l v ie w




                              M ux 4                                  M ux 3

                              M ux 1                                  M ux 2
 P h ysica l vie w




                              M ux 4                                  M ux 3

          W o rk in g rin g            P ro te c tio n rin g           O p tic a l lin k


Figure 20: Path protection for a unidirectional connection

                                                               A-32
Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH



6.2.1 APS caused by Fiber Failure

For installing path protection switch off the MSPTF. This is done in the <MSPTF
Configuration> window. Otherwise you would get a mixture of protection switching
mechanisms.
The protection paths are adjusted in the <HPXVC4 Configuration> window for the 140
Mbit/s connection by the <Action> <Add Protection Path>. Select also the combo box
<Switch Report> so that we can see which path is taken actually. As protection path we want
to use the longer path between Mux 1 and Mux 2 via Mux 3. In Mux 1 or 2 you have to make
a forced switch to the protection path so that we get the correct time for APS. If both
multiplexers had to switch, the APS time would be much to large, because it takes a lot of
time to make an automatic laser shut down for the disturbed connection in the receiving
multiplexer, so that both ends of the connection notice the disturbance. When the protection
path is built up correctly - don't forget the connection in Mux 3 between line west and line
east - start the APS measurement on the ANT and switch off the laser in Mux 2 line east.

            1. What can you see in comparison with the MSPTF?
            2. How would you describe this?

Restart the laser in Mux 2 line east.


6.2.2 APS caused by Connection Failure

To avoid falls connections in case of MSPTF in big networks or because of wrong
adjustments it is possible to give each connection a so-called Trail Trace Identifier (TTI). This
TTI is at the tail end of a connection compared with the expected. When there is a mismatch,
protection switching is done.
We want to use this for our next experiment. We want to cause TTI mismatch by false
adjustment in the network management software or by changing the TTI with the Overhead
Generator of the ANT.
The TTI string is adjusted in the <HOI140M Configuration> window of Mux 1 and Mux 2
for the connection between those network elements. You have to double-click the yellow
<TTI> field for changing the TTI strings. The sent string can be chosen arbitrary, but use
different strings for the sent TTI strings in Mux 1 and Mux 2. The received and expected
strings doesn't matter.
The comparison and the triggering of protection switching is done in the termination point for
the optical connection, i.e. the AU4CTP #1/1 west or AU4CTP #2/2 west for connections
received from line west and the AU4CTP #3/1 east or AU4CTP #4/2 east for connections
received from line east. The configuration windows for this functional elements are found in
the menu <Configuration...> of the <MSPTF #1> in the subview of the <MSPTF-4/RTF-4>.
To active the controlling of the TTI select <Enable TTIP> for <Trail Trace Identifier
processing> in the related AU4CTP. Change the expected TTI strings so that they match with
the received. You have to do this in Mux 1 and Mux 2.

                                              A-33
Lehrstuhl für Kommunikationsnetze                                               Lab Course SDH



When there are no more alarms displayed - check this in the alarm list of the NE-UNIGATE
window - go back to the working paths and make sure that the Protection State is Clear.

             1. For this time we change the expected TTI string in one of the used AU4CTPs
                to simulate a connection failure. Before doing this start the APS measurement
                on the ANT and write down the switching time.
             2. Watch also the alarms occurred.

Now we want to use the overhead generator of the ANT for changing the TTI string. For using
the overhead generator we have to switch the ANT directly into the ring. So switch of the
laser in Mux 2 line east and connect the optical sender of Mux 2 line east with the optical
input of the ANT. The optical receiver of Mux 1 line west has to be connected with the optical
output of the ANT. Edit the signal structure of the ANT so that you can send and receive a
VC-4 with 140 Mbit/s in an optical STM-4 stream and restart the laser in Mux 2 line east.
To edit the TTI string which is coded in the J1 byte in the overhead generator of the ANT you
have the button <TI> on the overhead generator. The TTI string has a length of 16 bytes, i.e.
the bytes not used are blanks. Consider this when adjusting the expected string in the
AU4CTP #1/1 west in Mux 1! When all alarms have disappeared switch back to the working
path and to the Protection State Clear.
Try different TTI strings in the overhead generator and watch the received ones on with
UNIGATE. Use the <Update> button to get the current view.
Switch of the laser in Mux 2 line east and in the ANT and remove the ANT from the ring.
Connect the ANT with the electrical 140 Mbit/s interface of Mux 2 and restart the laser in
Mux 2 line east. Suit the signal structure of the ANT to the current used.


6.3 Homework

1. How long can the APS last maximum if only one byte per ISDN connection is allowed to
   be lost?
2. What do you think, is the APS for MSPTF or for path protection faster? Try to explain
   your supposition.




                                             A-34
Lehrstuhl für Kommunikationsnetze                                                 Lab Course SDH




7 Performance Measurement

Finally in our experiment we want to know something about the quality management in SDH.
Although our network management software UNIGATE provides a very comfortable tool for
quality monitoring we will use the performance analyzer of the ANT. By means of this tool
we can see the number of seconds with bit-errors occurred. The results are divided in errored
seconds (ES), severely errored seconds (SES) and unavailable second (UAS). They are
defined in the ITU-T recommendation G. 821:
• Errored second (ES): A one-second time interval in which one or more bit errors occurs.
• Severely errored second (SES): A one-second time interval in which the bit error ratio
   exceeds 10 -3.
• Unavailable second (UAS): A circuit is considered to be unavailable from the first of at
   least ten consecutive SES. The circuit is available from the first of at least ten consecutive
   seconds which are not SES.
• Error-free second (EFS): A one second time interval in which no bit errors occur.
• Verdict shows if the availability on the path is good enough during the measurement.
In this chapter we want to try what happens when we insert defects and anomalies in the
bitstream with the ANT. We can also see how great the influence of APS is on the quality of
service.




Figure 21: Performance Analyzer Window of the ANT


We want to use the anomaly/defect insertion on the ANT for adding a frame alignment signal
(FAS-140) on the 140 Mbit/s PDH signal. We can adjust the rate of this signal from 10-10 to
10-3.




                                              A-35
Lehrstuhl für Kommunikationsnetze                                              Lab Course SDH



             1. Start the measurement and try when the performance analyzer remarks an ES
                or a SES.
             2. Check also the influence of some other anomalies and defects.
             3. Switch the anomaly/defect insertion off and try what the performance analyzer
                registers in case of MSPTF APS. Redo this a few times. Why can occur
                different results?


7.1 Homework

1. What influence do you think has an APS, which lasts 25 ms, on the performance?




                                             A-36
Lehrstuhl für Kommunikationsnetze                                       Lab Course SDH




8 Literature

[1] Eberspächer, J.: Skript zur Vorlesung "Kommunikationsnetze 2"
Lehrstuhl für Kommunikationsnetze, TU München 1999

[2] Schultz, S.: Pocket Guide to Synchronous Communications Systems
Wavetek Wandel Goltermann Eningen GmbH& Co., Eningen u. A., without year.

[3] Sexton M., Reid A.: Broadband Networking: ATM, SDH, and SONET
Artech House, Norwood, 1997

[4] without name: Information SMA1-2.3 und SMA4-2.3 TED
Siemens AG, München, 1998

[5] without name: Bedienung SMA1-2.3 und SMA4-2.3 OMN:LCT
Siemens AG, München, 1998

[6] without name: Inbetriebnahme SMA1-2.3 und SMA4-2.3 ITMN
Siemens AG, München, 1998

[7] without name: Wartung SMA1-2.3 und SMA4-2.3 MMN
Siemens AG, München, 1998




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Lehrstuhl für Kommunikationsnetze                                                                        Lab Course SDH




9 Appendix

                                                                                                        STM-   2488
                                                                                                 16*     16    Mbit/s


                                                                                                        STM-   622
   PDH        SDH                                                                                        4     Mbit/s
                                                                                                   4*

                                                         1*                       1*              1* STM-      155
 140 Mbit/s   C-4                                              VC-4        AU-4        AUG
                                                                                                      1        Mbit/s

                                                               3*                           3*
                                                  1* TUG-
                       VC-3         TU-3
                                                      3
                                                    7*
                                                         1*                                       1* STM-      52
 34 Mbit/s    C-3                                              VC-3        AU-3
                                                                                                      0        Mbit/s

                                                              7*
                                           1* TUG-
 6 Mbit/s     C-2      VC-2         TU-2
                                               2
                                                                           Path Overhead Processing (“Mapping“)
                                           3*                              Pointer Processing (“Aligning“)
                        VC-         TU-                                    (De-)Multiplexing
 2 Mbit/s     C-12
                        12          12
                                                                      STM Synchronous Transport Module
                                             4*
                                                                      AUG Administrative Unit Group
                        VC-         TU-                               AU   Administrative Unit
 1,5 Mbit/s   C-11
                        11          11
                                                                      TUG Tributary Unit Group
                                                                      TU   Tributary Unit
                                                                      VC   Virtual Container
                                                                      C    Container


Figure 22: Complete mapping structure in SDH




                                                     A-38

				
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Description: SDH (Synchronous Digital Hierarchy) is a way of multiplexing, line transmission and switching functions integrated, unified network management system operated by the integrated messaging network, Bell Communications Research Institute in the United States proposed synchronous optical network (SONET ). Advisory Committee on International Telephone and Telegraph (CCITT) (now ITU-T) in 1988 accepted the concept and renamed SONET SDH, making it applicable not only to fiber also apply to microwave and satellite transmission of the common technical system. It enables the effective management of the network, real-time business control, dynamic network maintenance, interoperability between different vendors equipment and many other features, can greatly increase the utilization of network resources, reduce management and maintenance costs, flexible, reliable and efficient network operation and maintenance Therefore, the field is the world's information transmission technology in the development and application of the hot spots, attracting widespread attention.