SDH, SONET and PDH - 高雄應用科技大學

1 By Othmar KyasAn Agilent Technologies PublicationNetworkTroubleshootingAgilent TechnologiesSDH, SONET and PDH14 SDH, SONET and PDH14 Most problems are either unimportant or impossible to solve. VICTOR SDH, SONET and PDH:Specification and ImplementationBack in the late 70s, Bellcore (now Telecordia) saw the need to replace thePlesiochronous (near synchronous) Digital Hierarchy (PDH) in the North Ameri-can Bell System (as it was then known) with a new synchronous started work on what we now know as SONET , the Synchronous OpticalNETwork. PDH networks had evolved in a rather ad hoc manner and it was timeto improve on this. A transmission standard was needed that allowed higher ratetransmission, properly planned network management facilities and, most im-portantly, a means to time lock the digital channels being carried so thatindividual lower rate channels could be accessed directly without the need tobreak down the PDH signal by hierarchy level, taking into account the justifica-tion (stuffing) that had occurred at each level during signal construction.

2 SONET would be able to provide all , SONET was focused on handling PDH rates used in North America only,for example, T1 ( Mbit/s) and T3 (45 Mbit/s), and was thus based on a framestructure of nine subframes of 60 octets (bytes). It turned out, this precluded themore international rates of E1 (2 Mbit/s), E3 (34 Mbit/s), etc. The ITU-T (thencalled the CCITT) also saw the need for a new synchronous network standardand worked with Bellcore to modify the SONET system to allow a more generalstandard, based on a frame structure of nine subframes of 90 octets (usuallyrepresented diagrammatically as a two dimensional drawing of nine rows by90 columns) that would be compatible with North American and internationalPDH rates after all, a new standard had to interwork with what was already inexistence. The Synchronous Digital Hierarchy (SDH) was thus defined by theITU-T in 1988 as an international recommendation (standard) for wide-areadata communications and is almost identical to SONET .

3 The main differencesare as follows: first, the basic rate of SONET is Mbit/s whereas SDH has aSDH, SONET AND PDH48614 SECTION IIITROUBLESHOOTING WIDE-AREA NETWORKS basic rate of Mbit/s (three times Mbit/s). Second, SONET definesthe optical layer while SDH defines the signal protocol structure above theoptical layer, other ITU-T recommendations focus on the optical layer. And third,different terminology is used with each standard, a source of constant confusionand irritation. There are also some minor interoperability issues that will bementioned frames are universal transport containers for all types of digitizeddata, including data streams, such as ATM, IP ( Packet over SONET ), FrameRelay, and leased lines, as well as the entire range of digital and analog tele-phony. Even in telecommunication systems that supply subscribers with analogservice, voice signals have long been transmitted in digitized form over wide-area backbones and re-converted to analog signals at the destination SDH or SONET is used by all major telecom service providers to imple-ment high-speed backbones in wide-area ATM was chosen as the transfer mechanism for the ITU-T s BroadbandISDN project, SDH/ SONET frames became the transmission vehicles of choicefor ATM cell streams.

4 This coupling of ATM and SDH/ SONET was still wide-spread when, some years later, ATM began to be used in local-area is how SDH/ SONET , originally developed for wide-area networks, also cameto be used in LANs as already mentioned, the main advantage of SDH/ SONET over the older PDHstructures lies in its use of a transparent multiplexing method that allowsindividual channels to be accessed directly. This means that a 64 Kbit/s channel,for example, can be directly read out of, or inserted into, the highest SDH/ SONET multiplex level (currently Gbit/s). This capability is also calledsingle-stage multiplexing. This is not possible in PDH networks, where allhierarchical layers must be demultiplexed in succession, taking stuffing intoaccount, in order to make a single channel accessible, and then multiplexedagain in order to be forwarded further. A given 64 Kbit/s channel that ismultiplexed through two or three hierarchical levels, to the 140 Mbit/s level forexample, cannot be directly located in the PDH data stream.

5 SDH/ SONET istherefore less expensive to use than PDH because it does not require a largenumber of expensive multiplexing/demultiplexing systems, and allows fargreater flexibility in network advantage of SDH/ SONET is its overhead structure, which is designedto support modern, highly automatic switching and network management sys-tems. When communication errors occur, the problem domain can be quicklyidentified by evaluating overhead bytes. This is why the conversion of datatransmission structures to SONET or SDH has been increasing steadily over theSDH, SONET AND PDH48714 SECTION IIITROUBLESHOOTING WIDE-AREA NETWORKS past few years. All PDH multiplex hierarchies can also be transmitted over theSDH/ SONET network, so that the transition from PDH to SDH/ SONET is Plesiochronous Digital Hierarchy (PDH)The Plesiochronous Digital Hierarchy (PDH), specified in 1972 by the ITU-T forNorth America, Europe and Japan, based on earlier national standards, is also ahierarchy of data structures at different bit rates (see Figure ).

6 These ratesare defined in ITU-T Recommendation , and the physical and electricalproperties of the interfaces are specified in The bit rates in the varioushierarchical levels are calculated as follows:Ti+1 = mi (Ti + xi)Hierarchical level01234 North AmericaEuropeTransatlantic642048804834,3 68139,264 Japan 641544631232,06497,728 642048631244,736139,264641544631244,7361 39,264 Figure Bit rates in the Plesiochronous Digital Hierarchywhere mi and xi are specified for each hierarchical level individually. ITU-TRecommendation defines a time-multiplex structure based on 64 Kbit/schannels for the basic bit rates of Mbit/s in E1 and Mbit/s in T1. The64 Kbit/s specification dates back to the early days of digital voice signaltransmission, when the conversion of voice signals into digital code was alwaysperformed at a sampling rate of 8 kHz. The analog signal is sampled at intervalsof 125 s, which according to Nyquist is sufficient to digitize all the informationcontained in a 4 kHz voice channel.

7 Because every measured value is coded in8 bits, the voice channel is transmitted at 64 T1 Interface (Carrying DS1 Signals)The North American standard defines a primary rate of Mbit/s called provides for the transmission of 24 channels at 64 Kbit/s per channel or forpayloads like ATM. Note that T1 (Transmission level 1) describes the electricalsignal, independent of the frame structure. DS1 (Digital Signal level 1) definesthe frame structure carried within T1. In practice, the terms tend to be usedSDH, SONET AND PDH48814 SECTION IIITROUBLESHOOTING WIDE-AREA NETWORKS interchangeably, although strictly speaking the physical interface should becalled T1 . DS1 signals from T1 interfaces can be multiplexed to higher ratesignals (DS2, DS3, etc.), whereas it would be wrong, strictly speaking, to talkabout DS3 as being a multiplex of T1 DS1 frame is 192 bits long (24 x 8 bits). The addition of 1 bit for framealignment yields a total of Mbit/s (193 bits x 8 kHz).

8 The pattern for framealignment consists of 6 bits (101010), which are spread out over six framesbecause each frame carries only one alignment bit. The alignment bit is also usedto identify the frames containing signaling bits, by means of another 6-bitpattern (001110). The alignment bit changes between framing and signal fram-ing, so that each of the two patterns is completed once in every 12 frames. Amultiframe sequence of 2,316 bits (12 frames of 193 bits) containing bothcomplete alignment patterns is also referred to as a 1SS 1FA 2SS 2FA 6SS 6192 bits (24 channels 8 bits) / 125 s1 bitSuperframeFrame 1 Frame 2 Frame 12 Transmitted row by row, beginning with Frame 1FA (1-6) .. Frame alignment bits (101010)SS (1-6) .. Signaling alignment bits (001110)Figure DS1 superframeSignaling in DS1 is comparable to the function of Timeslot 16 in the E1 interface,and is transported in the least significant bit (LSB) of every sixth sampling valuefor each channel.

9 This method is also called robbed bit signaling. The decreasein transmission quality due to this misuse of the LSB in every sixth byte perchannel is negligible. For data transmission in North America, the least signifi-cant bit in a 64 Kbit/s channel is avoided because it is easier to do this thanidentify which of the one bit in six has been robbed from the full 64 Kbit/ssignal; this results in a net throughput of 56 Kbit/s (7 bits x 8 kHz).SDH, SONET AND PDH48914 SECTION IIITROUBLESHOOTING WIDE-AREA NETWORKSB ecause networks have grown increasingly complex over the years, it has be-come necessary to include more monitoring information in data transmissionframes. This has led to a new definition of Channel 0 in the EuropeanE1 interface (see the following), and to the introduction of the 24-frame Ex-tended Superframe (ESF) in the North American DS1. The alignment pattern inthe ESF consists of six frame alignment bits alternating with six CRC bitsforming a CRC-6 checksum of the preceding ESF, and 12 signaling and monitor-ing bits.

10 The transportation of 12 management bits per 24 frames yields a 4 Kbit/s channel for signaling and error management (Figure ).M 12FA 6192 bit (24 channels 8 bit) / 125 s1 bitFrame 1 Frame 2 Frame 24M 1C 1M 2FA 1M 3C 2M 4FA 2C 6M (1-12) .. M anagement bits (12 per 24 frames @ 4 Kbit/s)C (1-6) .. CRC-6 of the previous ESFFA (1-6) .. Frame alignment bits (101010)Transmitted row by row, beginning with Frame 1 Figure DS1 extended superframe (ESF)At a data rate of Mbit/s, the payload bandwidth in DS1 frames Mbit/s, corresponding to a capacity use of percent. T1 bit streamsare AMI or B8ZS-encoded. The specified transport medium is 100 , SONET AND PDH49014 SECTION IIITROUBLESHOOTING WIDE-AREA NETWORKSP ulse ns50 Pulse mask for the DS1 E1 InterfaceThe E1 system is based on a frame structure of 32 x 8 bit timeslots (that is, atotal of 256 bits); the timeslots are numbered 0 to 31. Like the DS1 frame, the E1frame repeats every 125 s; this creates a signal of Mbit/s (256 bits x8 kHz).