/ ٠/ ٣٦ SDH & PDH Fundamental SDH & PDH Fundamental BY Chief Engineer NEAMA A WAD J ASIM Head of North West Communication Dept. Ministry of Electricity / Iraq SDH & PDH Fundamental by Chief Engineer neamah awad jasim Head of communication dept. in north west region the general docto rate of control & opera tion Ministry o f Electrici ty – IRAQ[email protected]please see my whole profile a t the URL : 830 / 56 /a 50 jasim/ - awad - https://www.linkedin.com/pub/neamah Or my page at the facebook: https://www.facebook.com/neamah.jasim
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
The Communication Era before SDHThe development of digital transmission systems started In the early70s , and was based on the Pulse Code Modulation (PCM) method.
In the early 80's digital systems became more and more complex , yetthere was huge demand for some features that were not supportedby the existing systems. The demand was mainly to high ordermultiplexing through a hierarchy of increasing bit rates up to 140Mbps or 565 Mbps in Europe. The problem was the high cost ofbandwidth and digital devices. The solution that wascreated then , was a multiplexing technique , allowed for thecombining of slightly non synchronous rates, referred to as
plesiochronous*, which lead to the term plesiochronous digitalhierarchy (PDH).
*plesiochronous - "almost synchronous , because bits are stuffed intothe frames as padding and the calls location varies slightly - jitters -from frame to frame".
SDH (Synchronous DigitalHierarchy)so SDH is an international standard for high speedtelecommunication over optical/electical networkswhich can transport digital signalsin variable capacities. It is a synchronous systemwhich intend to provide a more flexible , yet simplenetwork infrastructure.
SDH (and its American variant- SONET) emerged from
standard bodies somewhere around 1990.these two standards create a revolution in the
communication networks based on opticalinfibers their cost and performance.
why using SDH ?Although PDH was A breakthrough in the digital transmissionsystems , it has a lot of weaknesses :
- No world standard on digital format (three incompatibleregional standards - European, North American andJapanese).
- No world standard for optical interfaces. Networking isimpossible at the optical level.
- Rigid asynchronous multiplexing structure.
- Limited management capability.
- Because of PDH disadvantages, It was obvious That a newmultiplexing method is needed.The new method was called SDH.
Backward and forward compatibility: Backward compatibilityto existing PDHForward compatibility to future B-ISDN, etc.
Standards of SDH
SDH has been standardized by ITU-T in 1988.
In November 1988 the first SDH standards were approved.
In 1989 , the CCITT (International Consultative Committee on Telephony & Telegraphy)had published in its "Blue book" recommendations G.707 , G.708 & G.709 coveringthe SDH standards.
G.702 - Digital Hierarchy Bit RatesG.703 - Physical/Electrical Characteristics of Hierarchical Digital
Interfaces
G.707 - SDH Bit RatesG.708 - Network Node Interface for the SDHG.709 - Synchronous Multiplexing StructureG.773 - Protocol Suites for Q Interfaces for Management of
Transmission SystemsG.781 - (Formerly G.smux-1) Structure of Recommendations on
Multiplexing Equipment for the SDHG.782 - (Formerly G.smux-2) Types and General Characteristics
of SDH Multiplexing EquipmentG.783 - (Formerly G.smux-3) Characteristics of SDH Multiplexing
This is an even-parity code, used to determine if a transmissionerror has occurred over a path. Its value is calculated over all the
bits of the previous virtual container before scrambling and placed in
the B3 byte of the current frame. (BIP-8 is calculated on the 8 bit
blocks of the nth frame and placed on the (n+1) the frame of VC3/
VC4.)
• C2 - Path signal label byte – This byte specifies whether the virtualcontainer is equipped or not and the mapping type in therespective virtual container.
Application of Pointer – A Pointer is used to address a particular locationwithin an AU or a TU structure. There are mainly 2
types of pointers
1. AU Pointers: used to point at Higher Order VC’s (VC-4,3) in
an STM frame
2. TU Pointers: used to point at Lower Order VC’s (VC-12) in
higher order VC
Each of these pointers carry the offset number (address) at
which the 1st byte of the payload is located, within the frame.The offset numbering of AU4, TU3, TU12, frames are shown
in slide # 66, 68, 71. The offset numbering of TUs/AUs will be
according to the CCITT Rec. G.707.
Functions of a Pointer1. Minimization of multiplexing Delay
– This is the main advantage of pointers. Normally signals from different originatingpoints differ in theirphases, because of different transmission length and
different clock generation. In the usual multiplexing process, to align them, eachsignal has to be written into memories and read out using a
new phase of the frame to be multiplexed. Thus, it is inevitable to cause additionaldelay of half of theframe time in average and one frame time at maximum. Also,
it requires large capacity memories.
– To avoid above inconveniences, this pointer method was introduced into themultiplexing of SDH signal. A pointer is assigned to each VC to be multiplexed andit indicates relative phase shift between the VC and the new frame by using theaddress number in the new frame. As a matter of course, every VC
has different pointer value. The pointer is renewed at every multiplexing process, soit is not necessary to introduce undesirable additional delays.
2. Frequency Justification – Generally this function is not required in an SDH network since all network
elements are synchronized to a single clock. But if the VC’s are transported overdifferent networks, and if a network element is in an abnormal condition, justification is necessary to absorb any frequency differences between payloadand the frames.
There are 2 types of justification in SDH:a. Positive Justification: If the frame speed of the STM is higher than the payloadarrival speed.
b. Negative justification: If the frame speed of the STM is lower than the payload
STM-1 SignalDue that there are multi rate tributaries like (ATM,GFP(2.114Mb/s)or the rate of E1signal (2.048Mb/s) is not always in regular speed so the concept of multi-frame isintroduced for the convenience of rate adaptation used in multiplexing 2Mb/sinto STM-1signal , i.e. four C-12 basic frames form a multi-frame , each basic C-12frame represent by 9×4-2 matrix (notched matrix )
Since the frame frequency of the C-12 basic frame is 8000-frame per second, theframe frequency of the C-12 multi-frame will be 2000-frame per second.
-if E1 signals have a standard rate of 2.048Mb/s, each basic frame willaccommodate 32-byte (256-bit) which a integer value.
--- While when E1 signals rate of 2.046Mb/s are accommodated into a C-12 basicframe, the average number of bits loaded in each frame is: (2.046 x106b/second)/(8000-frame/second)=255.75 bits. Because this number is not an
integer so if a multi-frame of four basic frames is used, the number of bits can beloaded in the multi-frame is: (2.046 x 106b/second)/(2000-frame/second)=1023
bits. Each of the first three basic frames accommodates 256-bit (32-byte) payloadand the fourth accommodates 255-bit payload.
--A multi-frame can accommodate payloads at the rate ranging from C-12 Multiframe max to C-12 Multi-frame min, as follows:
C-12 Multi-frame max=(1023+1+1)x 2000=2.050Mb/s
C-12 Multi-frame min=(1023+0+0)x 2000=2.046Mb/s while
Mapping 2M to THE SDH streamFirst step is mapping E1 signal into SDH Container (creating
The rest of C-12 Used AS Overhead for E1 AS follows
2-Creation of VC-12 Multi frameTo monitor on a real-time basis the performance of each 2Mb/spath signal during transmission on SDH network, C-12 mustbe further packed ---- adding corresponding path overhead(lower order overhead)---- to form a VC-12 informationstructure. the LP-POH (lower order path overhead) is addedto the notch in the top left corner of each basic frame. Eachmulti-frame has a set of lower order path overhead
composed of total 4 bytes: V5, J2, N2 and K4. Since the VCcan be regarded as an independent entity, dispatching of2Mb/s services later is conducted in unit of VC-12.
A set of path overhead monitors the transmission status of thewhole multi-frame on a network. How many frames of 2Mb/ssignals does a C-12 multi-frame accommodate? One C-12multi-frame accommodates 4 frames of PCM30/32 signals.Therefore a set of LP-POH monitors the transmission status of4 frames of PCM30/32 signals.
6- Creation of Container -4 (C-4)Three TUG-3 can be multiplexed into the C-4 signal structure viabyte interleaved multiplexing method. The arrangement ofthree TUG-3s multiplexed in the VC-4. The TUG-3 is a 9-rowby 86-column structure. The VC-4 consists of one column ofVC-4 POH, two columns of fixed stuff and a 258-columnpayload structure. The three TUG-3s are byte interleaved intothe 9-row by 258-column VC-4 payload structure and have a
fixed phase with respect to the VC-4. The phase of the VC-4with respect to the AU-4 is given by the AU-4 pointer.
Since the TUG-3 is an information structure of 9 rows x 86columns, the information structure composed of three TUG-3via byte interleaved multiplexing is a block frame structure of9 rows x 258 columns. While C-4 is a block frame structure of9 rows x 260 columns. Two columns of stuffed bits are addedto the front of the composite structure of 3 x TUG-3 to form aC-4 information structure. shows the frame structure of oneC-4.
The Add And Drop Multiplexer (ADM)The function of (ADM) is to cross multiplex/de-multiplex the low rate tributary signal
(PDH,STM-M) to the High rate STM-N signal in east or west lines & STM-N signalcross connect west or east line port ( M less than N) . Also it include the cross
connect of the STM-N SIGNAL between the sides of the W/E line. ADM is equivalent
to two TMs, ADM is used in transfer stations of SDH network.
The REGENATOR (REG)is used to regenerate the (high rate) STM-N in case that the
distance between two sites is longer than the transmitter can carry.
REG needs only to handle RSOH in the STM-N frame to reproduce the