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