Sdh

SDH Concepts & Principle Principle of DWDM, Synchronization DLC

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 interconnection 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, 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.

Administration,

Maintenance

and

3.

Advantages (i) Multi-vendor environment (mid span meet) : Prior to 1988 international agreement on SDH all vendors used proprietary nonstandard 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. 2. 3. 4. 5. 6.

Low/High speed data. Voice Interconnection of LAN Computer links Feature services like H.D.T.V. 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”.

6.

•

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.

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 increased to 9 x 260 bytes. Details as follows :

seconds) is

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 group3. 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)

(x)

Administrative Group AUG consists of a homogenous assembly of AU-3s or an AU-4.

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 C12 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 TUG2 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 interleave.

(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 STM1 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.

Fig. 1 Network Reference Model and Standardization of Digital Hierarchies

One frame

Fig. 2 SDH Interface Frame Representation Method

Fig. 3 STM-N Frame Structure

Fig. 4 SDH Standards – Bit Rates (G.707 Recommendation)

Fig. 5 SDH–based Transport Network Layered Model

Fig. 6(a) SDH Layers

Fig. 6(b) Layer Interaction

Fig. 7(a) Generic Multiplexing Structure

Fig. 7(b) Reduced Multiplexing Structure

Fig. 12 Section and High Order Path Overhead Bytes The purpose of individual bytes is detailed below.

A1,A2

Frame Alignment.

B1,B2

Parity bytes for errors monitoring.

D1…D3

Data communication channel (DCC) network management.

D4…D12

Data communication channel (DCC) network management.

E1,E2

Orderwire channel.

F1

Maintenance

J0

Trace identifier

K1,K2

Automatic protection switching (APS) channel.

M1

Transmission error acknowledgement.

S1

Clock quality indicator.

*

Media dependent bytes.

H.O. POH : PATH OVER HEAD (VC3/VC4) J1

MODE 1 – A 64 BYTE FIXED LENGTH STRING

J – PATH TRACE BYTE (LIKE FAW)

MODE 2 – 15 BYTE STRING & 1 BYTE HEADER

B3

B – BIT INTERLEAVED PARITY CODE (BIP–8) BYTE FOR PATH ERR. MON. CALCULATED OVER ALL BITS OF PREVIOUS VC BEFORE SCRAMBLING.

C2

C – PATH SIG. LABEL BYTE TO INDICATE, SPE EQPD (1) OR NOT (0) ATM – 00010011, MAN – 00010100, FDDI – 00010101, LOCKED TU – 00000011.

G1

G – PATH STATUS BYTE OR REMOTE STN. (BIT 1–4 FEBE, BIT 5 – FERF, BIT 6–8 NOT USED)

F2

F – E.O.W. BETWEEN PATH

H4

H – MULTIFRAME ALIGNMENT BYTE OR DENOTE STARTING POSITION OF ATM CELL

Z (F3)

Z – FUTURE USE

Z

K3 – APS FOR PROTN. SWG. (b1 …. b4) SPARE (b5 …. b8) TO INCREASE N/W CAPABILITY

(K3) Z (N1)

N1 – TANDOM CONN. MON AND PATH DATA BYTE

L.O. P.O.H (FOR VC–11, VC–12, VC–2) V5

BIP–2 FEBE FEBE FERF PT L1 L2 L3

– – – – – –

PT

L1

L2

L3

FERF

FAR END BLOCK ERROR. FAR END RECEIVE FAILURE PATH TRACE MAPPING IS IN ASYNCH. MODE MAPPING IS IN BIT SYNCH. MODE MAPPING IS IN BYTE SYNCH. MODE

J2

PATH TRACE

K–4

PATH APS

N–2

TANDOM CONNECTION

Fig. STM–N Alarm Scheme

Fig. In Service Alarm Events In–Service Maintenance Signals The wide range of alarm signals and parity checks built into the SDH signal structure support effective in–service testing. Major alarm conditions such as Loss of Signal (LOS), Loss of Frame (LOF), and Loss of Pointer (LOP) cause Alarm Indication Signal (AIS) to be transmitted downstream. Different AIS signals are generated depending upon which level of the maintenance hierarchy is affected. In response to the different AIS signals, and detection of major receiver alarm conditions, other alarm signals are sent upstream to warn of trouble downstream. Far End Receive Failure (FERF) is sent upstream in the Multiplexer Section Overhead after Multiplexer Section AIS, or LOS, or LOF has been detected by equipment terminating in a Multiplexer Section span; a Remote Alarm Indication (RAI) for a high order path is sent upstream after Path AIS or LOP has been detected by equipment terminating a Path, and similarly, a Remote Alarm Indication (RAI) for a Low Order Path is sent upstream after Low

Order Path AIS or LOP has been detected by equipment terminating a Low Order Path.

Fig. 11 Synchronous Multiplexers Optional Tributary Interfaces

Fig. 12 Add Drop Multiplexer

10.0

Network Elements in SDH Before the evolution of the standards covering synchronous transmission systems, networks had to be built up from separate multiplex and line terminal equipment. These are characterized by defined formats and electrical interfaces at each level of the transmission hierarchy; whereas optical interfaces were entirely proprietary. This gave rise to large amounts of multiplex and separate optical line equipment. On the other hand in SDH, multiplexers performs both multiplexing and line terminating functions. Synchronous multiplexers can accept a wide range of tributaries and offer a number of possible output data rates. Though the regeneration of signal at repeaters is similar to PDH, there are some additional equipment in SDH to perform function like cross–connection and OA&M functions as explained in following sections. 10.1

Terminal Multiplexers Terminal multiplexers are used to combine plesiochronous and synchronous input signals into higher bit rate STM–N signals as shown in Fig.13 below. On the tributary side, all current plesiochronous bit rates can be accommodated. On the aggregate, or line side we have higher bit rate STM–N signals.

Fig. 13 Terminal Multiplexer 10.2

Add/Drop Multiplexer (ADM) Plesiochronous and lower bit rate synchronous signals can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This feature makes it possible to set up ring structures, which have the advantage that automatic back–up path switching is possible using elements in the ring in the event of a fault.

Fig. 14

Add/Drop Multiplexers

10.3

Digital Cross–Connects (DXC) Cross connection is a synchronous network involves setting up semi– permanent interconnections between different channels enabling routing to be performed down to a VC level. This network element can have widest range of functions such as mapping of PDH tributary signals into virtual containers and switching of various containers up to and including VC–4.

Fig. 15 10.4

Digital Cross–Connects

Regenerators Regenerators, as the name implies, have the job of regenerating the clock and amplitude of the incoming data signals that have been attenuated and distorted by dispersion. They derive their clock signals from the incoming data stream. Messages are received by extracting various 64 kbit/s channels (e.g. service channels E1, F1, etc. in RSOH) and also can be output using these channels.

Synchronisation The role of synchronisation plan is to determine the distribution of synchronisation in a network and to select the level of clocks and facilities to be used to time the network. This involves the selection and location of master clocks for a network, the distribution of primary and secondary timing through out the network and an analysis of the network to ensure that acceptable performance levels are achieved. Improper synchronisation planning or the lack of planning can cause severe performance problems resulting in excessive slips, long periods of network downtime, elusive maintenance problems or high transmission error rates. Hence, a proper synchronisation plan which optimises the performance, is a must for the entire digital network. The status of synchronisation in the BSNL network is as follows : 3 nos. of cesium clocks at VSNL Bombay provide the Master National Reference Clock (MNRC). The back up NRC is available at Delhi. The MNRC feeds the reference signal to the VSNL GDS at Mumbai and from the GDS both the new technology TAXs at Mumbai are synchronised. From these two TAXs at Mumbai, all the other TAXs are to be synchronised. Part of this work has already been done. However, all the Level–I TAXs are yet to be synchronised. A direct synchronisation link is also available between GDS Mumbai and Karol Bagh TAX at Delhi. For synchroisation of the SDH network, it has been decided to use the clock source available through the TAXs at the major stations. The synchronisation plan is based upon provision of Synchronisation Supply Units (SSUs) which will be deployed as an essential component of the synchronisation network which will support synchronised operation of the SDH network. The architecture employed in the SDH requires that the timing of all the network clocks be traceable to Primary Reference Clock (PRC) specified in accordance with ITU Rec.G.811. The classical method of synchronising network element clocks is the hierarchical method (master–slave synchronisation) which is already adopted in the BSNL network for the TAXs. This master–slave synchronisation uses a hierarchy of clocks in which each level of the hierarchy is synchronised with reference to a higher level, the highest level being the PRC. The hierarchical level of clocks are defined by ITU as follows : – P.R.C. – Slave Clock (Transit Node) – Slave Clock (Local Node) – SDH Network Element Clock.

Architecture for Primary Rate Networks

SDH Equipment Clock

Each node is associated with a particular hierarchical level of clock prescribed above and is referred to as a nodal clock. The SSU is an important component of this hierarchical master–slave synchronisation network scheme and of a slave clock belonging to the transit node level or the local node level as defined in ITU Rec. G.812. 4.4 The BSNL, therefore, has decided to go in for 10–20 nos. of SSUs to provide a clean reference primary source for other stations. These SSUs are basically high stability filter clocks which eliminate phase transients, jitter and wander and provide the exact sync. signal needed for every network element.

DWDM 1.

Evolution of Transmission Capacity In the 80’s, it was possible to transmit 140 Mbit/s with optical PDH – systems. SDH technology in the 90’s has improved this capacity. SDH can transmit the capacity of 16 times 140 Mbit/s or 155 Mbit/s (16 X STM 1 = STM 16, 2.5 Gbit/s) or up to 64 times 140 Mbit/s or 155 Mbit/s (64 X STM 1 = STM 64, 10 Gbit/s). Currently, it is possible with WDM wavelength division multiplex systems to transmit between 32 and 96 times 10 Gbit/s (320 Gbit/s) over very large distances. Soon we will have 160 times 10 Gbit/s, and in the laboratory it is possible to transmit in the terabit range (10 X 1012). In the case of optical systems the available bandwidth can exceed several Terahertz (1012Hz). TDM could not be used to take advantage of this tremendous bandwidth due to limitations on electrical technology. Electrical circuits simply cannot work on these frequencies. The solution was to use frequency multiplexing at the optical level or Wavelength Division Multiplexing. The basic idea is to use different optical carriers or colours to transmit different signals in the same fibre. Consider a highway analogy where one fibre can be thought of as a multi-lane highway. Traditional TDM systems use a single lane of this highway and increase capacity by moving faster on this single lane. In optical networking utilizing DWDM is analogues to accessing the unused lanes on the highway (increasing the number

of wavelengths on the

embedded fibre base) to gain access to an incredible amount of untapped capacity in the fibre. An additional benefit of optical networking is that the highway is blind to the type of traffic that travels on it. Consequently the vehicles on the highway can carry ATM packets, SDH and IP. A distinction is made between WDM and DWDM (Dense Wavelength Division Multiplexing). With channels can be relatively large.

WDM

the

spacing

between

In Dense multiplexing the frequency spacing between channels can be as small as 50 GHz or less, increasing the overall spectral density of the transmitted signal.

TDM

FDM

WDM 3 Fig. 1 Comparison between TDM, FDM and 2.

Transmission Windows Today, usually the second transmission window (around 1300 nm) and the third and fourth transmission windows from 1530 to 1565 nm (also called conventional band) and from 1565 to 1620 nm (also called Long Band) are used.

Technological reasons limit

DWDM applications at the moment to the third and fourth window. The losses caused by the physical effects on the signal due by the type of materials used to produce fibres limit the usable wavelengths to between 1280 nm and 1650 nm. Within this usable

range the techniques used to produce the fibres can cause particular wavelengths to have more loss so we avoid the use of these wavelengths as well.

0.4 nm 50 GHz

1510.0 nm 1528.77nm 198.6THz 196.10THz 1480.0 nm 202.6THz

3.

1560.61 nm 192.1 THz

Fig.2. Wavelength Plan for 50 GHz Grid

Application Advantages Optical

networks

are

opening

up

new

horizons

for

telecommunication operators. Technologies such as wavelength division multiplexing (WDM) and optical amplification are giving them a multitude of ways to satisfy the exploding demand for capacity. New architectures will increase network reliability and decrease the cost of bit rates and distance, therefore, creating economic benefits for network operators and users alike.

Based on

existing fibre optic backbone networks, the idea of an all optical network (AON) is revolutionizing the structures of our communication networks. In short, optical networks are the future of the information superhighway.

The biggest advantages of such an optical network would be :

Properties

Applications

Multiple use of fibres

Ideal in cases of fibre shortage

Extremely high transport

Multiple

capacity at low cost

yielding

use

decreased

of

optical

amplifiers

investments

and

maintenance costs. Format

and

bitrate

transparency

4.

Data, video and voice over a common transport network

Transponder Applications A Transponder Terminal can be used to transmit a wide variety of signal types, like SDH, ATM or PDH signals. The Transponder adapts to the arbitrary bit rate of the incoming optical signal, and maps its wavelength to the chosen WDM channel. Its main function is OEO. It converts wavelength (say 1550 nm) coming from user equipment to electrical signal and electrical signal is converted into optical signal of a specific wavelength, which forms an optical channel for particular user. Optical transparency yields a multitude of new application options and enables network operators to utilize existing network resources in a far more flexible manner. It provides major advantages such as : •

Greatly enhanced transmission capacity.

•

New services offered.

•

Transmission of restructured signals.

•

Use of devices and interfaces from other vendors.

The semitransparent transponder keeps one of the major advantages of the DWDM i.e. Protocols are transmitted transparently, providing a very high flexibility.

SDH NE

SDH NE

Regenerators

Fig.3. Situation without WDM

SDH NE

SDH NE

Optical Terminal MUX

Optical Amplifier

Optical Terminal MUX

Fig. 4. Situation with WDM

Fig.4 Situation without WDM

IP

IP

Tr

T ra n sp o n er

SD H

DW DM M UX

Tr DW DM M UX

T ra n sp o n er

SD H

ATM

ATM

PDH

PDH

SD H M U X

SD H M U X

F ig .5 . T ra n sp o n er A p p lic a tio n Fig.5. Transponder

5.

Optical NE Types We have already met following NEs : (a) Optical Multiplexer/Demultiplexer Multiplexing and Demultiplexing of different wavelength signals. (b) Optical Amplifiers Pure optical 1R regeneration (just amplification) of all transmitted signals. (c)

Transponders Wavelength “change” and 2R regeneration (reshaping and amplification) or 3 R regeneration (reshaping retiming and amplification).

(d)

Regenerators Real 3 R regeneration (reshaping, retiming and amplification) of the signal. Therefore, the signals have to be demultiplexed, electrically regenerated and multiplexed again. They are necessary if the length to be bridged is too long to be covered only by optical amplifiers, as these only perform reshaping and retiming.

(e)

Optical Add/Drop Multiplexer

Adding and Dropping only specific wavelengths from the joint optical signal. This may use complete de-multiplexing or other techniques. (f)

Optical cross-connects

To cater for the huge amount of data expected in an optical network even the cross-connects have to work on a purely optical level. 6. (a)

Future Trends Use of Optical Amplifier – The best developed optical amplifiers are

Erbium doped fibre amplifier (EDFA) which operate at 1550 nm and praseodymium doped fibre amplifiers operating at 1300 nm. (b)

Use of non-zero dispersion shifted fibre (NZ - DSF).

(c)

Use of passive optical components (PON).

(d) 7.

Wave Division Multiplexing of Optical Signal (WDM). Description of Optical Multiplexer and Demultiplexer :

An optical demultiplexer can be built as an association of optical filters or as a single stand device. The purpose is to extract the original channels from a DWDM signal. The requested properties of this device are the same as for the optical filter : isolation and signal distortion.

However channel number and

spacing must be considered now because demultiplexers can

impose

limitations on the number of channels or the total available bandwidth. Most demultiplexers are symmetrical devices and can also be used as multiplexers. (a)

By using Prism

The easiest and best-known optical demultiplexer is the prism. Using the effect of dispersion (different speed of light for different wavelengths), light is split into its spectral components. (b)

By using Diffraction Grating The function of a diffraction is very similar to that of a prism, only here interference is the important factor. A mixture of light is also split into its contributing wavelengths. With such a grating sometimes also called a bulk grating channel spacings of done to 50 GHz can be achieved.

Red White

Blue Effect of a prism

Effect of a grating

8.

Optical Amplifiers

(a)

Introduction Fiber loss and dispersion limit the transmission distance of any fibre-optic

communication system. For long-haul WDM systems this limitation is overcome by periodic regeneration of the optical signal at repeaters, where the optical signal is converted into electric domain by using a receiver and then regenerated by using a transmitter. Such regenerators become quite complex and expensive for multichannel lightwave systems. Although regeneration of the optical signal is necessary

for

dispersion-limited

systems,

loss

limited

systems

benefit

considerably if electronic repeaters were replaced by much simpler and potentially less expensive, optical amplifiers which amplify the optical signal directly. Several kinds of optical amplifiers were studied and developed during the 1980 s. The technology has matured enought that the use of optical amplifiers in fiber-optic communication systems has now become widespread. (b) Optical Amplifier Applications (i)

In-line amplifiers

(ii)

Booster amplifiers

(iii)

Pre-amplifiers

In-line amplifiers are used to directly replace optical regenerators.

Booster

amplifiers are used immediately after the transmitter or multiplexer to increase the output power. Pre-amplifiers are used before the receiver or demultiplexer to increase the received power and extend distance.

The

use

of

each

configuration as advantages and disadvantages that must be considered by the systems designer.

The problems come when considering non-linear effects in

the transmission fiber and also generated by the amplifiers. Some of the requirements for optical amplifiers for DWDM purpose are : •

high gain

•

low noise

•

flat amplification profile

ODM X

OMX O /E /O O /E /O O /E /O

O p tical A m p lifier

F ig.8. P assage from optical/electrical regen erators to op tical am plifiers

B ooster

Rx

Tx P ream p lifier

Rx

Tx Fi

In -lin e am plifier

Rx

Tx F ig.9. A pp lications for optical am p lifiers

DIGITAL LOOP CARRIER SYSTEM The digital loop carrier (DLC) system is a small to medium size pair - gain system (Pair-saving system) which consists of a central office terminal (COT) Remote Terminal (RT) and digital transmission system. In order to accommodate rural subscribers into the existing public telephone net work the RT collects subscriber lines around the target area and transfers the collected telephone signals to the existing local switching equipment (central office) through digital transmission line. At the central office side the COT receives the signals from the RT and after demultiplexed the receipt signals the COT transfers them to the local switching equipment through its subscriber line terminals. In this way the DLC system can expand subscriber line up to rural area without any degradation of signal quality. Both terminals COT and RT are connected by PCM metallic transmission line or optical fibre cable transmission line preferable transmission network type to DLC system is in line type and tree type. The DLC system collects plenty of traffic from remote subscribers and carries them to the local switching office of the public telecommunication network. It can provide various modern telecommunication services to remote subscribers with high quality. Today, the fibre optic transmission system becomes popular in technical and its material and equipment cost becomes reasonable then the combination of the DLC system and fiber optic transmission system can provide feasible rural telephone system. This section describes the Digital Loop Carrier system (DLC) as the optimum rural telephone subscriber accommodation method. Features of the digital Loop carrier (DLC) system are summarized below: A typical DLC system consists of a Central Office Terminal (COT) installed in an existing local switching office a Remote Terminal (RT) installed at the subscriber lines collection Point, and digital multichannel transmission link. individual subscriber is connected to the RT with usual metallic line (drop wire). • Various types of interface condition for subscriber and signaling are acceptable. Mixed use of them are permitted in a group (a channel bank) • An elementary group of the DLC transmission is the digital primary group produced by PCM-30, and it can be transmitted over an existing digital transmission line. • The fibre optic transmission line can extend subscriber lines up to about 50 Km without a repeater • The DLC system has remote testing functions for convenience of maintenance the connection from a subscriber to RT and the multi-

channel transmission link between RT to COT can be tested at the COT. The connection from COT to the subscriber line link frame of telephone switching equipment will be tested from the test board of the switching system, • The RT equipment can be installed in a special outdoor cabinet and it saves the construction fee and period • The configuration of digital transmission line can be a in line type or tree type. The DLC system can be applied to discrete type or homogeneous type of subscriber distribution. • The multi-channel transmission line equipment of the DLC system has the same technical specifications as the digital toll connection line equipment. TYPICAL SYSTEM CONFIGURATION Fig.1 typical DLC system configuration . This figure is illustrated under following imaginary conditions : • The urban area is provided good telephone service and LS-1 has enough extra subscriber accommodation capacity. • The trunk capacity and transmission capacities of the junction line between LS are enough for newly added traffic from sub-urban and rural areas. • The sub-urban area is newly developed business center and it has emergent demand of the 200-300 subscriber lines this means that the demand is emergent but too small to introduce a new local switching center. • The urban area and suburban area is connected by well designed highway that has good roadside space for cable duct construction. • Each rural area has telephone demand of several tens subscriber lines but they are 5-10 kilometers away from the main road . The main road has enough space for buried cable laying. • The subscriber distribution radius of each rural area is smaller than 2-3 kilometers. • Existing wooden poles of open wire transmission line are available for overhead optical fiber cable installation between main road and rural areas. •

At each rural area, commercial power supply is available for RT.

Fig.1 Typical Configuration of DLC System

Generally, the radius of a service area belonged to a local switching office will be limited within 4-5 kilometers but introduction of DLC system expands the service area radius up to 50-60 kilometers. This will require some modifications of the charging system and maintenance system. A simplified block diagram of the DLC system is shown in fig-2 Both terminal equipment, COT and RT consist of channel bank (CH), line terminal multiplexer (LTM) and fiber optic transmission line .DC power supply is necessary for the Remote Terminal Equipment .The Channel bank consist of four PCM-30 multiplexer with signalling function. The line terminal multiplexer (LTM) consists of higher order multiplexer and optical line terminal which converts the electronic signal to / from the optical signal.

Fig. 2 Simplified Block Diagram of DLC System CH - Channel Bank LTM - Line Terminal & Multiplexer SV - Supervisory facility • 1. Subscriber end interface • 2. Office end interface • 3. Analog (VF) interface (Digital or Analog Switching Equipment) Further descriptions are given by using of the details Fujitsu Digital loop Carrier Equipment, Model DLC-120 and Line Terminal Multiplexer Equipment Model LTM-8. LTM- The digital hierarchy bit rates of the DLC120 conforms to the CCITT recommendations G 702.

The model DLC -120 is major equipment of COT and RT of a DLC system. It is deigned as subscriber line interface unit which has 120 lines capacity for larger capacity system, the unit system will be stacked up to the capacity and higher order multiplexer and optical line terminal are combined. The model LTM- 8 is used for 8Mb/s (equivalent to 120 telephone channels) digital data transmission line and LTM-34 is used for 34Mb/s (equivalent to 480 telephone channels) digital data transmission line. More larger capacity system can be established by combining the 140Mb/s higher order multiplexer equipment 140 Mb/s optical line terminal equipment and DLC-120S. This combination can provide subscriber accommodation capacity up to 1920 lines No larger capacity than 1920 subscriber lines might be required in rural area application. 1. Model DLC -120 equipment

2. Typical terminal configuration (COT,RT)

Fig. 3 Typical configuration of DLC 120 & Terminal of DLC System As shown in Fig 3 the DLC -120 consists of four channel banks and together with a Multiplexer and optical line terminal it composes a major part of COT or RT of the DLC system The channel bank is a PCM -30 equipment with respective subscriber interface The 2Mb/s digital group transmission interface of the channel bank conforms to the CCITT recommendation G.703 Central Office Terminal and Remote Terminal have similar equipment’s configuration except for the interface condition of their channel cards. In the Figs. 4, 5, 6, three application examples of DLC–120 are shown. In each example 2Mb/s standard interface condition is applied between DLC–120 and LTE (LTM)

Fig. 4 2Mb/s System Over Metallic Pair Cable

Fig. 5 8 Mb/s System Over Optical Fibre Cable

Fig. 6 34Mb/s System over Optical Fibre Cable

APPLICATION AREA ESTIMATION The DLC system application area is estimated which is made under bold manner such as • Existing local switching equipment has enough extra capacity and no additional cost is needed for new accommodation of the rural subscriber. •

Subscriber distribution radius of each rural is less than 1 Km.

• There are no difficulties for the construction work such as cable laying, antenna and tower construction. •

No need of access road construction for radio station is estimated.

•

AC commercial power is available at every rural area.

•

All cables and wires are installed in overhead type.

In this figure, those application area of each system is estimated by comparison of the estimated initial investment cost referring to the experience. At the less demand area, there will be a big discrepancy between actual cost estimation and an expectation by this figure, because a little difference from above assumptions will cause a big difference of the cost estimation results In actual feasibility study, detail cost estimation should be done under the particular conditions.

From this figure, it is clearly shown that the DLC system has very broad application area if there is an existing local switching center near the target rural community, the number of subscriber are more than 50 and their distribution area is limited with in 1 Km radius .The upper border of the DLC will be determined by the application area of an RSU (Remote Switch Unit) and the border number of subscriber will be about 2000.