SDH and TDM telecom
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Synchronous Digital Hierarchy and Time Division Multiplexing
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SDH and TDM telecom
- 1. TDM and SDH Basics TDM Time Division Multiplexing SDH Synchronous Digital Hierarchy
- 2. Evolution. A/D 8 kHz 64 kbit/s BUT........
- 3. T1 T2 T3 T4 T5 T6 T7 time Audio Signal Sampler Output time T1 T2 T3 T4 T5 T6 T7 Pulse Amplitude Modulated (PAM) signal Sampling
- 4. 0 0 0 0 X X X X +V 0 0 0 1 X X X X 0 0 1 0 X X X X digital codes 1 1 1 1 X X X X 1 1 1 0 X X X X 1 1 0 1 X X X X 1 1 0 0 X X X X 1 0 1 1 X X X X 1 0 1 0 X X X X 1 0 0 1 X X X X 1 0 0 0 X X X X 112 96 80 64 48 -V In accordance with CCITT’s A-law 1/2V 1/4V 1/8V 1/16V 1/32V 1/64 V Quantization Level 32 Non-Linear Quantization and Encoding
- 5. 8 x = 64kbit/s bits per sample 8000 samples per sec PCM Signal Data Rate
- 6. Time Division Multiplexing (TDM)
- 7. 64 kbit/s Data Signals 139264 kbit/s (+/-15ppm) PDH Multiplex / Demultiplex 2048 kbit/s (+/-50ppm) 1 8448 kbit/s (+/-30ppm) 34 368 kbit/s (+/-20ppm) DSMX 34/140 64 Channel Capacity: 64 x 30 = 1920 DSMX 8/34 DSMX 2/8 1 30 DSMX 64k/2 1 30 PCMX 30 1 PCMX 30 5 1 30 4 1 4 Analog signal
- 8. Plesiochronous Drop & Insert OLTU 34 - 140 8 - 34 2 - 8 OLTU 34 - 140 8 - 34 2 - 8 140 Mbit/s 140 Mbit/s OLTU 34 - 140 8 - 34 2 - 8 OLTU 34 - 140 8 - 34 2 - 8 main stand-by 1,2 ................. 64 1,2 ................. 64 Line Terminating Unit Line Terminating Unit Drop & Insert Station
- 9. PDH vs. SDH — Bit Rates E1 E0 E2 E3 E4 plesiochronous plesiochronous plesiochronous synchronous TM STM-1 TM STM-4 TM STM-16 • E1 (2.048 Mbit/s) • T1 (1.544 Mbit/s) • DS-2 (6.312 Mbit/s) • E3 (34.368 Mbit/s) • DS-3 (44.736 Mbit/s) • E4 (139.264 Mbit/s) • ATM • etc. PDH SDH
- 10. PDH vs. SDH — Add & Drop Function E 3 E 3 E 3 E 2 E 4 E 4 TM TM 4 / 3 4 / 3 E 2 TM TM E 2 E 1 E 1 E 1 3 / 2 3 / 2 2 / 1 2 / 1 PDH SDH STM-4 ADM STM-4 Six TMs needed to drop E1 One ADM is enough to drop E1. TM TM E 1 ( 2 . 0 4 8 k b i t / s )
- 11. Why SDH Why SDH Simpler multiplexing (low SDH level can be directly identified from higher SDH level) Simple D&I of traffic channels (direct access to lower level systems without synchronization) Allows mixing of ANSI and ETSI PDH systems SDH is open for new applications (It can carry PDH, ATM, ETHERNET...) SDH provides TMN (ECCs) (for centralized network control)
- 12. Synchronous Network Structure A B C TM-1 ADM-1 TM-1 42 E1 A <=> C 21 E1 A <=> B 21 E1 A <=> B 21 E1 B <=> C 42 E1 A <=> C 21 E1 B <=> C XC
- 13. 2Mbit/s STM-4/-16 34Mbit/s ADM ADM ATM Switch 34Mbit/s 140Mbit/s STM-1 ADM : Add Drop Multiplexer DXC : Digital Cross Connect TM : Terminal Multiplexer DSC: Digital Switching Center LAN: Local Area Network DWDM: Dense Wavelength Multiplexing 34Mbit/s STM-1 / STS-3c Gateway to SONET DXC STM-1 LAN ADM STM-1, STM-4 2Mbit/s 8Mbit/s 140Mbit/s DSC Synchronous Network Structure DDWWDDMM
- 14. STM-1 Frame Structure AU Pointer Payload RSOH: Regenerator section overhead MSOH: Multiplex section overhead Payload: Area for information transport Transport capacity of one Byte: 64 kbit/s Frame capacity: 270 x 9 x 8 x 8000 = 155.520 Mbit/s Frame repetition time: 125 μs 1 3 4 5 9 270 270 Columns (Bytes) 1 9 transmit row by row RSOH MSOH (transport capacity) 2430 bytes/frame × 8 bit/byte × 8000 frame/s = 155.52 Mbit/s transmitted from top to bottom and left to right
- 15. 1 3 5 4 9 270 STM-1 Frame Structure 270 Columns (Bytes) 1 9 RRSSOOHH MMSSOOHH AAUU--44 AAUU PPooiinntteerr VVCC--44 VVCC--44 PPOOHH CC--44
- 16. Basic Elements of STM-1
- 17. Embedded Overhead Bytes J1 B3 C2 G1 F2 H4 F3 K3 N1 V5 J2 N2 K4 AU - PTR VC-3/4 POH VC-11/12/ 2 POH STM-1 SOH Media dependent bytes X Reserved for national use SOH: Section overhead POH: Path overhead The overheads (SOH, POH) are used for maintenance and supervision of the SDH transmission network. RSOH P O H Pointer MSOH Payload A1 A1 A1 A2 A2 A2 J0 X X D 1 D2 D3 B2 B2 B2 K1 K2 D4 D5 D6 D7 D8 D9 D10 D11 D12 S1 M 1 E2 X X B1 E1 F1 X X H1 Y Y H2 1 1 H3 H3 H3
- 18. Functions of Regenerator Section Overhead AU - Pointer Parity check Frame Alignment (B1 calculated by regenerator and multiplexers) Data communication channels (D1...D3, F1 between regenerators) Voice communication channels (E1 between regenerators) (A1, A2) Section Trace (J0 Identfication of regenerator source) A1 A1 A1 A2 A2 A2 J0 B1 E1 F1 D1 D2 D3 B2 B2 B2 K1 K2 D4 D5 D6 D7 D8 D9 D10 D11 D12 S1 M1 E2
- 19. Functions of Multiplexer Section Overhead Parity check (B2) Alarm information (K2) Remote error indication (M1,K2) A1 A1 A1 A2 A2 A2 J0 B1 E1 F1 D1 D2 D3 AU - Pointer B2 B2 B2 K1 K2 D4 D5 D6 D7 D8 D9 D10 D11 D12 S1 M1 E2 Automatic protection switching (K1, K2 Bytes) Data communication channels (D4 to D12 between multiplexers) Clock source information (S1) Voice communications channels (E2 between multiplexers)
- 20. A1, A2 Frame synchronisation B1, B2 Parity bytes for transmission error monitoring J0 Regenerator section trace D1... D3 Regenerator section DCC D4.. D12 Multiplex section DCC E1, E2 Orderwire for voice communication F1 User channel for maintenance purposes (data, voice) K1, K2 Automatic protection switching (APS) S1 Synchronisation status message M1 MS-REI (remote error idication) J1 Higher order path trace B3 Path parity byte for error monitoring C2 Signal Label (composition of payload) G1 Path status and performance F2, F3 Path user channels H4 Payload specific byte K3 Automatic protection switching (APS) N1 Network operator byte (Tandem Connection Monit.) V5 Error check, path status, signal label J2 Lower order path trace N2 Network operator byte (Tandem Connection Monit.) K4 Automatic protection switching (APS) SOH VC-3/4 POH VC-1/2 POH Overhead Byte Functionality
- 21. SDH Network Elements TM Terminal Multiplexer STM-N or PDH STM-M • Terminates RSOH and MSOH. • May terminate HOP and LOP overhead. • Multiplexes/maps tributary signals. • Includes timing and management functions.
- 22. SDH Network Elements Add/drop Multiplexer STM-M STM-M ADM STM-N or PDH • Terminates RSOH and MSOH. • May terminate HOP and LOP overhead. • Includes connection function between the two aggregates. • Multiplexes/maps tributary signals. • Includes a connection function for allocation of TUs within the STM frame. • Includes timing and management functions.
- 23. SDH Network Elements Cross-connect STM-M DXC STM-M STM-N or PDH • Terminates RSOH and MSOH. • Includes connection function between the aggregates. • Can have tributary ports, for which it may terminate HOP and LOP overhead. • Includes timing and management functions.
- 24. SDH Network Elements Regenerator STM-N STM-N • Regenerates the optical signal. • Terminates RSOH. REG
- 25. Termination of Sections Regenerator Sections TM TM Multiplexing Sections Multiplexing Sections Path
Hinweis der Redaktion
- Sampling is the periodical measurement of the value of the analogue signal. A sampled signal contains all the information if the sampling frequency is at least twice the highest frequency of the signal to be sampled. As the analogue signals in telephony are band-limited from 300 to 3400Hz, a sampling frequency of 8000Hz - every 125usec - is sufficient.
- The amplitude of a typical telephone speech signal can vary enormously, both from one speaker to another and over the normal speaking range of a single individual. In fact, the range of variation can be as great as 50 - 60dB. With a human voice the low level signals are more important than the high levels, so using quantization levels which are closer together at lower amplitudes and get wider as the amplitude increases is more efficient. This is known as non-linear encoding and has two CCITT recommendations A-law &gt;&gt; European &gt;&gt;E1 system u-Law (mu-law) &gt;&gt; USA &gt;&gt; T1 system 8-bit/s used in non linear coding would require an equivalent 12-bit/s in linear coding.
- Each telephone channel has a PCM signal which is the analogue signal sampled at 8kHz and then is non-linearly encoded with 8 bits giving 64kbit/s data rate.
- As you saw earlier an encoded telephone speech signal is transmitted at rate of 64kbit/s (8 bits /sample; 8kHz sampling frequency). However, long distance telephone trunks are designed to handle data at a much greater rate then this. It therefore makes sense for a number of channels to share the same transmission link, using the technique of Time Division Multiplexing (TDM). In the diagram a separate encoder and decoder is shown for a 4-channel multiplexed system. The complete signal is divided into repeated sequences of four successive time slots. When transmission begins, time slot 1 is used to transmit the 8 bit code for the first sample of channel 1; time slot 2 is then used to send the first sample of channel 2...... ....after four slots have elapsed the process begins again, with time slot 1 containing the second sample of channel 1 and so on.
- What are Plesiochronous Tributaries? ITU-T define plesiochronous digital hierarchy tributaries with a certain permitted deviation of bitrate. Each multiplexer has its own clock source (oscillator), making the accuracy of the output frequency vary from system to system. Tolerance ranges have been standardized for the bit rate accuricy. These systems are described as &quot;free running&quot; and networks based on such systems are &quot;plesiochronous&quot;. (&quot;plesio-&quot; comes from the Greek word for &quot;near&quot; since these systems are nearly synchronous.)
- Another problem is that insertion of 2 Mbit/s channels into e.g. a 140Mbit/s local line requires a minimum investment of 4 systems (3 multiplex systems and 1 terminal equipment. Mechanical switching equipment is used for the most part. Although electronic routers have been developed for 64kbit/s and 2 Mbit/s channels. The development cost associated with plesiochronous technology are too high for the upper hierarchy levels. To access a single channel (e.g.64 or 2048 kbit/s) of a multiplex signal it is necessary to go twice through the whole multiplex chain (redundacy of hardware).
- These are only some advantages where we will go into more detail later on. First let&apos;s have a look to the synchronous network structure.
- Here we must consider the telecommunications network as a whole. In the area of the subscriber network nodes the users are connected to the exchanges (DSC) via the user network interface (UNI). Instead of this central switching points local cross connects (DXC) should be used. In a PDH network a fixed network is performed by point to point links. The channels are switched via these links. Signals from other networks use this transmission technology via flexible multiplexers up to 2 Mbit/s. The growth in data traffic is much higher than in voice communication. The greatest demand lies in the area of high bit rate access from the subscriber area. Such transmission capacity should be available at a reasonable cost and on short notice. The Terminal multiplexer (TM) with diverse interfaces feed this traffic into the SDH network directly or via Add and Drop Multiplexers (ADM) which are configured in a ring network (Back Bone). This ring is formed by two fibre optical cables with variouse back-up switching possibilities. The Network Management (TMN) sets up the necessary connections.
- Here we must consider the telecommunications network as a whole. In the area of the subscriber network nodes the users are connected to the exchanges (DSC) via the user network interface (UNI). Instead of this central switching points local cross connects (DXC) should be used. In a PDH network a fixed network is performed by point to point links. The channels are switched via these links. Signals from other networks use this transmission technology via flexible multiplexers up to 2 Mbit/s. The growth in data traffic is much higher than in voice communication. The greatest demand lies in the area of high bit rate access from the subscriber area. Such transmission capacity should be available at a reasonable cost and on short notice. The Terminal multiplexer (TM) with diverse interfaces feed this traffic into the SDH network directly or via Add and Drop Multiplexers (ADM) which are configured in a ring network (Back Bone). This ring is formed by two fibre optical cables with variouse back-up switching possibilities. The Network Management (TMN) sets up the necessary connections.
- The European Telecommunications Standard Institute (ETSI) did not accept the elements unneeded in Europe (AU-3, VC-3, C-2 and TU-11). 1.5 Mbit/s signals are transported in Europe within the VC-12.
- A STM-1 signal has a byte-oriented structure with 9 rows and 270 columns. A distinction is made between three areas: the payload area, which uses 261 columns the pointer area the section overhead, which is splittet up into two parts the Regenerator- and the Multiplex-Section Overhead. Each byte corresponds to a 64kbit/s channel. The overall bit rate of the STM-1 frame corresponds to 155.520 Mbit/s. The frame repetition time is 125µs.
- A STM-1 frame is built-up in the following way. A basic unit known as a container (C) is formed from plesiochronous signals. Stuffing is used to give the plesiocronous signals a fixed bit rate. The clock frequency of the signal is adapted using positive or positive - zero - negative (bit) stuffing. The container bit rate itself is formed through an addidional fixed stuffing process. The container is nominally synchronized to the STM-N frame. Insertion of the path overhead (POH) produces a virtual container (VC). The transmission paths through the SDH network are formed by these VC’s which are the smallest transport units in SDH. This means that a VC has to be terminated at the end of a path at the SDH/PDH transition point. The VC’s are coupled to the STM-1 frame by pointers (PTR) These pointers are used along with stuffing techniques (byte-stuffing) to compensate for unavoidable phase fluctations and other interferences which occurs in synchronous operating. The pointer and the VC forms the Administrative Unit (AU). Finally the Administrative Unit Group (AUG) or STM-1 is formed by adding the SOH.
- A number of functions are defined in the overhead channels to ensure proper transport of the payload. The Section Overhead (SOH) The overall capacity of the SOH is 4.608 Mbit/s (9x8x64kbit/s), of which 30 bytes (1.920 Mbit/s) have fixed definitions. The remaining 64kbit/s channels are not specified. Six are reserved for national use. Although six bytes are reserved for medium dependent functions (e.g. radio link systems). The columns 1,4 and 7 corresponds also to the STS-1 frame. Functions of the SOH: Contains maintenance, monitoring and operational functions Each byte refers to a 64kbit/s channel Splitted into RSOH and MSOH Protect the connection from point of STM-1 assembly to point of disassembly. The Path Overhead (POH) The POH of VC-4/VC-3 consists of 9 bytes and the POH of the VC-11/VC-12 and VC-2 consists of 4 bytes.
- The RSOH is reformed (terminated) by each regenerator. Each regenerator section passes the MSOH transparently.
- The MSOH is reformed (terminated) by each multiplexer and cross connect .