SONET-SDH. Fabio Neri e Marco Mellia Gruppo Reti Dipartimento di Elettronica nome.cognome@polito.it

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1 SONET-SDH Fabio Neri e Marco Mellia Gruppo Reti Dipartimento di Elettronica nome.cognome@polito.it fabio.neri@polito.it - tel marco.mellia@polito.it - tel

2 SONET/SDH Today telephone network is largely based on the evolution of the first digital infrastructure, based on a TDM system, with strict synchronization requirements, or PDH Plesiochronous Digital Hierarchy: SONET Synchronous Optical NETwork (optical signal, based rate of 51.84Mbit/s) SDH Synchronous Digital Hierarchy (European standard equivalent to SONET) STS Synchronous Transport Signal (equivalent standard for electric signals)

3 Plesiochronous Digital Hierarchy Plesiouchonous Digital Hierarchy (PDH) is the original standard for telephone network, now abandoned in favors of SONET/SDH Exploits Time Division Multiplexing Designed to support digital voice channels at 64kb/s No store and forward: imposes strict synchronization between TX and RX. A plesio-synchronous solution is adopted (almost synchronous) Different standard in US/EU/Japan Make it difficult to connect different networks5

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6 Ref: Digital Voice Technical Concepts

7 Plesiochronous Digital Hierarchy Bit rate for one digital voice channel is 64 kbps This is a widely accepted standard and is known as DS0 Signal. Higher-speed streams were designed as multiples of this basic stream. Fiber Optic Communication Systems 3 The transmission is organized into frames. Each frame contains a fixed number of time slots. Each time slot is pre-assigned to a specific input link. If the buffer of an input link has no data, then its associated time slot is transmitted empty. A time slot dedicated to an input link repeats continuously frame after frame, thus forming a channel or a trunk. Fiber Optic Communication Systems 4

8 T/E Carrier The DS signal is carried over a carrier system known as the T carrier. T1 carries the DS1 signal, T2 carries the DS2 signal etc The ITU-T signal is carried over a carrier system known as the E carrier. Fiber Optic Communication Systems 5 Fiber Optic Communication Systems 6

9 DS1 Signal 24 8-bit time slots/frame Each time slot carries 8 bits/ 125 µsec, or the channel carries a 64 Kbps voice. Every 6th successive time slot (i.e, 6th, 12th, 18th, 24th, etc), the 8 bit is robbed and it is used for signaling. F bit: Used for synchronization. It transmits the pattern: Fiber Optic Communication Systems 7 T1 E1 Total transmission rate: 24x8+1 = 193 bits per 125 µ sec, or Mbps 30 voice time slots plus 2 time slots for synchronization and control Total transmission rate: 32x8 = 256 bits per 125 µsec, or Mbps Fiber Optic Communication Systems 8

10 T-,, E-E Hierarchy Level 0 US (T-) Mb/s Europe (E-) Mb/s Japan Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s

11 CH1 CH2.. CH23 CH24 Sample MUX T-1 1 carrier system: US standard 24- voice channels are the results of voice sampling, quantization and coding using the PCM standard and TDM framing Additional signaling channel of 1 bit T1 speed is (24*8+1)*8000=1.544Mb/s frame S CH1 CH2 CH3... CH22 CH23 CH24 x x x x x x x x MSB A sample every 125μsec A frame every 125μsec 193 bits per frame More frames can be multiplexed (TDM) on faster channels. LSB

12 T- and DS- hierarchy S CH1 CH2 CH3... CH22 CH23 CH24 DS1 DS1 DS1 DS k=1.544 Mb/s T1 Frame transmitted over a DS1 It is difficult to have perfect synchronization between ALL nodes. bit stuffing to overcome this. DS2 DS2 DS2 DS2 DS2 DS2 DS2 DS3 DS3 DS3 DS3 DS3 DS3 DS4 4 DS1 = 1 DS = Mb/s 7 DS2 = 1 DS = Mb/s 6 DS3 = 1 DS = Mb/s It is difficult to access a single channel in a highspeed stream: demultiplexing of all tributaries must be performed

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22 PDH Digital transmission system (T-carrier, E-carrier) exploiting TDM to multiplex lower speed streams into higher speed channel Every apparatus has its own clock. No networkwide synchronization is possible Every clock is different, and therefore synchronization errors show up Solution: insert (and remove) stuffing bits in the frame (but stuffing)

23 PDH - Sincronizzazione Frame Source Bit Stuffing 1 2 Node Faster clock 1 2 Dest

24 PDH - Synchronization Positive Stuffing: Data are written in a temporary buffer Data are read from the buffer with a higher rate to transmit data to the (faster) transmission channel Every time the buffer is going to be empty, stuffing bit is transmitted instead of real data Stuffing MUST be signaled to the receiver, so that stuffing bits can be removed. A different frame is used at the data layer and at the transmission layer! This makes mux/demux operation much more complex.

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27 Framing and overhead In addition to locking on to bit-rate we need to recognize the frame structure We identify frames by adding Frame Alignment Signal The FAS is part of the frame overhead (which h also includes "C-bits", " OAM, etc.) Each layer in PDH hierarchy adds its own overhead For example E1 2 overhead bytes per 32 bytes overhead 6.25 % E2 4 E1s = Mbps out of 8.448Mbps so there is an additional Mbps = 3 % altogether 4*30*64 kbps = Mbps out of Mbps or 9.09% overhead What happens next? Y(J)S SONET Slide 17

28 PDH overhead digital data rate voice overhead signal (Mbps) channels percentage T % T % T % T % E % E % E % E % Overhead always increases with data rate! Y(J)S SONET Slide 18

29 PDH limitations Rate limitations Copper interfaces defined Need to mux/demux hierarchy of levels (hard to pull out a single timeslot) t) Overhead percentage increases with rate At least three different systems (Europe, NA, Japan) E 2.048, 8.448, , T 1.544, 3.152, 6.312, , , J 1.544, 3.152, 6.312, , , So a completely l new mechanism was needed d Y(J)S SONET Slide 24

30 From PDH to SONET/SDH SONET: Synchronous Optical Network: American standard SDH: Synchronous Digital Hierarchy: EU and Japan standard Standardization occurred in 80 Telecom Providers drove the standardization process PDH system was not anymore scalable, and did not guarantee to support traffic growth Optical technologies were becoming commonly used, foreseeing bandwidth increase Interoperability among different providers were a nightmare.

31 The original Bellcore proposal: Standardization! hierarchy of signals, all multiple of basic rate (50.688) basic rate about 50 Mbps to carry DS3 payload bit-oriented mux mechanisms to carry DS1, DS2, DS3 Many other proposals were merged into 1987 draft document (rate ) In summer of 1986 CCITT express interest in cooperation needed a rate of about 150 Mbps to carry E4 wanted byte oriented mux Initial compromise attempt byte mux US wanted 13 rows * 180 columns CEPT wanted 9 rows * 270 columns Compromise! US would use basic rate of Mbps, 9 rows * 90 columns CEPT would use three times that rate Mbps, 9 rows * 270 columns Y(J)S SONET Slide 26

32 What is SONET/SDH Set of ITU-T recommendation (first from 1989): Define a structured multiplexing hierarchy Define management and protection mechanisms Define physical layer requirements (optical components) Define multiplexing of different sources and protocols over SONET/SDH

33 SONET/SDH Goals Main goals of SONET/SDH: Fault tolerance of telecom providers requirement (99.999% - five nines - availability) Interoperability among different manufacturers Flexibility of upper layer formats to adapt to different source (not only voice) Complex monitoring capabilities of performance and of traffic (50 ms of recovery time)

34 SONET/SDH hierarchy OC level OC-1 STS level STS-1 SDH level Mbit /s OC-3 STS-3 STM OC-12 STS-12 STM OC-24 STS-24 STM OC-48 STS-48 STM OC-192 STS-192 STM OC-768 STS-768 STM OC-3072 STS-3072 STM

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36 SONET/SDH reference model Path layer (close to OSI layer 3 - Network) Manages end-to-end connection Monitoring and management of user connection Line Layer Multiplexing of several path-layer connection among nodes Protection and Fault Management Section Layer Define regenerator functions SONET s Line and Section layers are almost equivalent to 2 (Data Link) OSI layer Photonic Layer (same as OSI layer 1) Defines all the transmission requirements of signals.

37 SONET Network Elements SONET standard defines several apparatuses to fulfill different functionalities Multiplexer and de-multiplexer Regenerators Add-Drop multiplexers Digital cross-connects All are electronic devices, with no elaboration done in the optical domain except transmission

38 path layer line layer Layering in SONET/SDH standard ITU-T T G.78x Connection line layer path layer line layer section layer section layer section layer section layer physical layer physical layer physical layer physical layer Sonet Terminal Sonet ADM Sonet Terminal Regenerator add/drop mux

39 SONET Network Elements: PTE Multiplexer and demultiplexer: The main function is mux and demux of tributaries Il Path Terminating Element (PTE) Simpler version of multiplexer path-terminating terminal Multiplexes DS 1 channels, and generated the OC-N carrier Two terminal multiplexers connected by a fiber are the simplest SONT topology (section, line, path on the same link) STS-3 STS-3 STS-3c DS-1 DS-1 VT OC-N OC-N DS-3 DS-3 STS-1

40 SONET Network Elements: Regen Regenerator Simplest SONET element. Perform 3R regeneration Allows to overcome distance limit at the physical layer Receives the input stream, and regenerates the section overhead before retransmitting the frame. Does not modify Line and Path overhead (behaves differently from an Ethernet repeater) OC-N T x R x T x R x OC-N

41 SONET Network Elements: ADM Add-Drop multiplexer: multiplexing and routing over ring topologies Multiplexes different tributaries over a single OC N The add/drop operation allows to elaborate, add/drop only signal that must be managed Transit traffic is forwarded without the need of particular operation. It manages alternate routing in case of fault STS-N BUS OC-N OC-N STS-N VT STS-1 OC-N OC-N OC-N DS-1 DS-3 OC-N DS-1 DS-3

42 SONET Network Elements: DCS Digital cross-connect: multiplexing in general meshed topology Different line speed Works at the STS-1 granularity Used to interconnect several STS-1 inputs High-speed cross-connects are used to efficiently mux/demux several channels Transparent Switch Matrix (DS1 Switch Matrix) STS-N (VT1.5) STS-1 (DS1) DS1 (DS1) DS3 (DS1) STS-N (STS-N) STS-1 (DS3) DS1 (DS1) DS3 (DS3) STS-N STS-1 ATM DS1 DS3

43 SONET Network Configurations Point-to-point topology Simplest topology The point-to-point start and end on a PTE, which manages the mux/demux of tributaries No routing, and no demux along the path Regenerators may be used to avoid transmission problems PTE REG REG REG REG PTE

44 SONET Network Configurations Linear add-drop topology Still al linear topology ADM (and regen) along the line ADM allow to add and drop tributaries along the path ADM are designed to work in this kind of topologies, which allows a simpler structure than a general cross-connect (there is no need to mux and remux in transit tributaries) PTE REG ADM REG REG ADM REG PTE

45 SONET Network Configurations Hub network setup Typically on big aggregation point Adopt Digital Cross connect (DCS) working at high rate DCSs are much more complex that ADMs: they have to manage both single tributary and SONET stream REG Mux Mux REG DCS REG Mux Mux REG

46 SONET Network Configurations SONET Rings The most used topology. Can use two or four fibers and an ADM at each node. Bidirectional topology Simple protection funcions ADM ADM SONET Ring Architecture ADM ADM

47 Layers SONET was designed with definite layering concepts Physical layer optical fiber (linear or ring) when exceed fiber reach regenerators regenerators are not mere amplifiers, regenerators use their own overhead fiber between regenerators called section (regenerator section) Line layer link between SONET muxes (Add/Drop Multiplexers) input and output at this level are Virtual Tributaries (VCs) actually 2 layers lower order VC (for low bitrate payloads) higher order VC (for high bitrate payloads) Path layer end-to-end path of client data (tributaries) client data (payload) may be PDH ATM packet data Y(J)S SONET Slide 28

48 SONET architecture Path Termination ADM Line Termination regenerator Section Termination ADM Line Termination Path Termination path line line line section section section section SONET (SDH) has at 3 layers: path end-to-end data connection, muxes tributary signals path section there are STS paths + Virtual Tributary (VT) paths line protected multiplexed SONET payload multiplex section section physical link between adjacent elements regenerator section Each layer has its own overhead to support needed functionality SDH terminology Y(J)S SONET Slide 29

49 Main Features The frame is presented in matrix form and it is transmitted row by row. Each cell in the matrix corresponds to a byte The first three columns contain overheads The remaining 87 columns carry the synchronous payload envelope (SPE), which consists of user data, and additional overheads referred to as the Path overhead (POH) Fiber Optic Communication Systems 17 STS-1 payload The payload consists of user data and the path overhead User data: Virtual tributaries: sub-rate synchronous data streams, such as DS-0, DS-1, E1, and entire DS-3 frames ATM cells and IP packets Fiber Optic Communication Systems 18

50 SONET Physical Layer SONET physical layer is strongly optically-centric More important recommendations are: ITU-T G.957: Optical interfaces for equipments and systems relating to the synchronous digital hierarchy Single span, single channel link without optical amplifiers ITU-T G.691: Optical interfaces for single-channel STM-64, STM-256 and other SDH systems with optical amplifiers Single channel, single or multi span, optically amplified links at 622 Mbit/s, 2.5 Gbit/s, 10 Gbit/s ITU-T G.692: Optical interfaces for multichannel systems with optical amplifiers Multi channel, single or multi span, optically amplified Definition of the ITU frequency grid Large variety of possibilities, from very short-haul interoffice links up to a ultra-long haul, WDM backbone links All physical parameter of all interfaces are defined

51 SONET Framing SONET/SDH TX send a continuous, synchronous streams of bit a given rate Multiplexing of different tributaries is performed by the means of TDM Apparently complex, but the TDM scheme has been designed to simplify the VLSI implementation A SONET frame is a very organized stream of bits At a given multiplexing level, every tributary becomes a Synchronous Payload Envelope (SPE) Some bits, the Path Overhead, are added to the SPE, to implement monitoring, management, control functions SPE + Path Overhead are a PDU, called Virtual Tributary (VT)

52 STS-1 1 framing 1 frame = 810 Byte in 125μs Row/colum representation STS-1 OC-1 9 rows 0 µs (1st bit) 3 rows 6 rows 3 Bytes 87 Bytes SOH LOH SPE Path Overhead: Is removed only at the path layer Transport Overhead Payload 125 µs (last bit)

53 SONET: Framing STS-1 0 μs 90 byte 3 byte 87 byte A1 Framing A2 B1 E1 D1 D2 H1Puntatori H2 B2 K1 D4 D5 D7 D8 D10 D11 Z1 Z2 C1 F1 D3 H3 K2 D6 D9 D12 E2 Section overhead Payload J1 B3 C2 G1 F2 H4 Z3 Z4 Z5 Line overhead Payload Path overhead time 125μs

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56 Higher layer multiplexing STS-1 #1 STS-1 #2 STS-1 #3 9 MUX Byte interleave 3x3 3x87 9

57 Virtual Tributary (VT) VTs are identified by pointers along the frame. Pointers are stored in the line overhead A pointer states where a VT begins in the frame A recursive approach is allowed: a VT may multiplex other smaller VTs Pointer Pointer VT VT VT This allows to multiplex contributing tributaries running at very different speed in an efficient manner.

58 SONET hierarchy A sample of SONET hierarchy SONET has been designed to support a very large set of technologies: IP, ATM, PDH, Lower speed tributaries are multiplex by PDH DS1 (1.544 Mb/s) E1 (2.048 Mb/s) DSIC (3.152 Mb/s) DS2 ( Mb/s) DS3 ( Mb/s) ATM ( Mb/s) E4 ( Mb/s) ATM ( Mb/s) VT1.5 VT2 VT3 VT6 x4 x3 x2 x1 VT group x7 SPE STS-1 SPE STS-3c STS-1 STS-3c xn xn/3 STS-N Byte interleaved multiplexing

59 Scrambling Scrambling is performed during insertion into the SONET STS SPE/SDH Higher Order VC to provide adequate transparency and protect against potential security threats Ref.

60 Scrambling Scramblingis is the process of encoding digital 1s and 0s onto a line in such a way that provides an adequate number for a 1s density requirement. The ANSI standard for T1 transmission requires an average density of 1s of 12.5 percent (a single 1 in 8 bits meets this requirement) with no more than 14 consecutive 0s for unframed signals and no more than 15 consecutive 0s for framed signals. The primary reason for enforcing a 1s density requirement is for timingrecovery or network synchronization. However, other factors such as automatic line build out (ALBO), equalization, and power usage are affected by 1s density. Ref: Packet over SONET

61 SONET Overheads Path Terminating Element Section Line Regenerator Section Path Digital Crossconnect o Add/Drop Multiplexer Section Line Regenerator Section Path Terminating Element Section overhead Line overhead Transport Overhead Path overhead Different overhead Section: used and managed by two section equipment Line: to allow signaling among STS multiplexers Path: end-to-end, added to the SPE to transform it in a VT Each exploit a different function Multiplexing Management Allocation of resources

62 SDH Paths and Selections Figure 1 displays how Regenerator Section Overheads (RSOHs) terminate at each end of the RS, and how Multiplex Section Overheads (MSOHs) terminate at each end of the MS. Path OHs (POHs) terminate at the end of the path, and will be Higher Order (HO) or Lower Order (LO). Ref: 016c3dc.shtml, Troubleshooting Guide for Synchronous Digital Hierarchy

63 SONET overhead Section Overhead: Generated and removed by Section Terminal Equipments (STE) Monitoring of the section performance Operation, administration and maintenance (OAM) voice channel Framing

64 SONET overhead - SOH A1 A2 framing bytes States the beginning of the STS-1 frame. J0 section trace (J0)/section growth (Z0) section trace byte or section growth byte B1 section bit-interleaved parity code (BIP 8) byte Parity code (even parity), used to detect transmission errors on this section. It is evaluated on the previous frame after the scrambling operation. E1 section orderwire byte 64Kbit/s digital channel to transport a voice signal between operators at the section endpoints F1 section user channel byte not defined D1, D2, D3 section data communications channel (DCC) bytes 192Kbit/s channel used for OAM&P.

65 Error Monitoring in the SDH Network a B1 byte is used to check for errors in the RS. a B2 byte is used to check for errors in the MS. a B3 byte is used to check for errors in the VC 4 path. a V5 byte is used to check for errors in the VC 12 path.

66 SONET Overheads Line Overhead: Generated and removed by the line terminating equipment (LTE) VT identification in a frame Multiplexing/routing Performance monitoring Protection switching Line management STS Path Overhead: Generated and removed by Path Terminal Equipment (PTE) end-to-end monitoring of VT/SPE Connection management

67 SONET overhead - LOH H1 H2 STS payload pointer (H1 and H2) stores the pointer. It is the offset between the first payload byte and the actual VT first byte. H3 pointer action byte (H3) used when a negative stuffing is performed. It stores the additional byte of the last frame. B2 line bit-interleaved parity code (BIP 8) byte parity check code, used to detect errors at the line layer K1 K2 D4 D12 S1 Z1 M0 M1 Z2 E2 automatic protection switching (APS channel) bytes signaling to face fault management line data communications channel (DCC) bytes 9 bytes for a 576Kbit/s management channel to carry over O&M operations synchronization status (S1) Used to carry global network-wide synchronization. growth (Z1) not defined STS 1 REI L (M0) to carry signaling in case of remote error indication STS N REI L (M1) to perform restoration operation growth (Z2) not used orderwire byte 64Kbit/s digital channel to transport a voice signal between operators at the section endpoints

68 SONET pointers How to manage de-synchronization among apparatus clocks? Use pointer to absorb frequency and phase shifting They allow to dynamically follow the phase shifting in a simple manner And avoid the need of buffering Section overhead Separate clocks with almost same timing (plesiochronous) Line overhead H1 H2 SPE SPE Bit Stuffing was used in PDH. Byte stuffing is used in SONET When the SPE speed is smaller than STS-1 speed, an extra byte is inserted When the SPE speed is larger than STS-1 speed, an extra byte is removed and transmitted in the overhead 125 µs

69 Positive stuffing Positive stuffing SPE slower than STS-1 Periodically, when the SPE has a delay of 1 byte, odd bits of pointers are negated, to signal a positive stuffing operation An additional byte is added in the VT, allowing it to be delayed by 1 byte The additional byte is always put close to the H3 header field The pointer is then incremented by 1 in the next frame, e following frames will hold the new value.

70 Negative stuffing SPE faster then the STS-1 Periodically, when the SPE has an additional byte, pointer even bits are negated, to signal a negative stuffing operation In next frame, the VT starts 1 byte earlier The additional byte of the previous VT is put into the H3 header field The pointer is then decremented by 1 in the next frame, e following frames will hold the new value.

71 SDH Framing In SDH a different naming scheme is used, but the design is similar to SONET s Base tributary is STM-1, with a period of 125 μs Frame has bits, giving a total speed of Mbit/s Information is organized in bytes, using 9 rows of 270 bytes each Virtual container (VC) carried the payload (261 x 9 = 2349 bytes) Administrative unit (AU) is the VC plus headers (like the VT)

72 STM-1 frame in SDH 0 μs 3x3 byte Framing 3x90 byte administrative unit 3x87 byte time time Pointers overhead virtual container 125 μs

73 SONET OC3 Versus OC3c Frame Format OC 3c ("c"" stands for "concatenated") concatenates three STS 1 (OC 1) frames into a single OC 3 look alike stream. Ref: IRIS ATM Configuration Guide

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75 Payloads and Mappings Y(J)S SONET Slide 50

76 STS-1 HOP SPE structure We saw that the pointer the line overhead points to the STS path overhead POH (after re-arranging) POH is one column of 9 rows (9 bytes = 576 kbps) Y(J)S SONET Slide 51

77 STS-1 HOP column of SPE is POH 2 more ( fixed stuffing ) columns are reserved We are left with 84 columns = 756 bytes = Mbps for payload This is enough for a E3 (34.368M) or a T3 (44.736M) Y(J)S SONET Slide 52

78 STS-1 Path overhead J1 B3 C2 G1 F2 H4 F3 K3 N1 POH 1 column of overhead for path (576 Kbps) POH is responsible for path type identification path performance monitoring status (including of mapped payloads) virtual concatenation path protection trace Y(J)S SONET Slide 53

79 J1,,B3,,C2 (path overhead) J1 path trace enables receiver to be sure that the path connection is still OK B3 BIP-8 even bit parity of bytes (without scrambling) of previous payload C2 path signal label identifies the payload type (examples in table) C2 Payload type (hex) 00 unequipped 01 nonspecific 02 LOP (TUG) 04 E3/T3 12 E4 13 ATM 16 PoS RFC LAPS X.85 1A 10G Ethernet 1B GFP CF PoS - RFC1619 Y(J)S SONET Slide 54

80 G1,,F2,,H4,,F3,,K3,,N1 (path overhead) G1 path status conveys status and performance back to originator 4 MSBs are path FEBE, 1 bit RDI, 3 unused F2 and F3 user specific communications H4 used for LOP multiframe sync and VCAT (see later) K3 (4 MSBs) path APS N1 Tandem Connection Monitoring Messaging channel for tandem connections Y(J)S SONET Slide 55

81 LOP VTGs To carry lower rate payloads, divide the 84 available columns into 7 * 12 interleaved columns, i.e. 7 Virtual Tributary (VT) Groups VT group is 12 columns of 9 rows, i.e. 108 bytes or Mbps VT group is composed of VT(s) there are different types of VT in order to carry different types of payload all VTs in VT group must be of the same type (no mixing) but different VT groups in same SPE can have different VT types A VT can have 3, 4, 6 or 12 columns Y(J)S SONET Slide 56

82 SONET/SDH : VT/VC types VT/STS VC column payload rate VT 1.5 VC DS1 (1.544) 4 per group LOP HOP VT 2 VC E1 (2.048) VT DS1C (3.152) VT 6 VC DS2 (6.312) STS-1 VC E3 (34.368) STS-1 VC DS3 (44.736) STS-3c VC E4 ( ) 3 per group 2 per group 1 per group standard PDH rates map efficiently into SONET/SDH! Y(J)S SONET Slide 57

83 LO Path overhead LOP OH is responsible for timing, PM, REI, LO Path APS signaling is 4 MSBs of byte K4 H4=XXXXXX00 H4=XXXXXX01 V1 pointer V2 pointer 125 sec V5 J2 VC11 25B VC12 34B 500 sec H4=XXXXXX10 V3 pointer N2 H4=XXXXXX11 V4 pointer VC11 27B VC12 36B K4 Y(J)S APS Slide 58

84 Payload capacity VT1.5/VC-11 has 3 columns = 27 bytes = Mbps but 2 bytes are used for overhead (V1/V2/V3/V4 and V5/J2/N2/K4) so actually only 25 bytes = 1.6 Mbps are available Similarly VT2/VC-12 has 4 columns = 36 bytes = Mbps but 2 bytes are used for overhead So actually only 34 bytes = Mbps are available Y(J)S SONET Slide 59

85 LOP overhead V5 consists of BIP (2b) REI (1b) RFI (1b) Signal label l (3b) (uneq, async, bit-sync, byte-sync, test, t AIS) RDI (1b) J2 is path trace N2 is the network operator byte may be used for LOP tandem connection monitoring (LO-TCM) K4 is for LO VCAT and LO APS Y(J)S SONET Slide 60

86 SDH Containers Tributary payloads are not placed directly into SDH Payloads are placed (adapted) into containers The containers are made into virtual containers (by adding POH) Next, the pointer is used the pointer + VC is a TU or AU Tributary Unit adapts a lower order VC to high order VC Administrative Unit adapts higher order VC to SDH TUs and AUs are grouped together until they are big enough We finally get an Administrative i i t ti Unit Group To the AUG we add SOH to make the STM frame Y(J)S SONET Slide 61

87 Formally C-n n = 11, 12, 2, 3, 4 VC-n = POH + C-n TU-n = pointer + VC-n (n=11, 12, 2, 3) AU-n = pointer + VC-n (n=3,4) TUG = N * TU-n AUG = N * AU-n STM-N = SOH + AUG Y(J)S SONET Slide 62

88 Multiplexing An AUG may contain a VC-4 with an E4 or it may contain 3 AU-3s each with a VC-3s withane3 In the latter case, the AU pointer points to the AUG and inside the AUG are 3 pointers to the AU-3s J1 B3 C2 G1 F2 H4 F3 K3 N1 H1 H1 H1 H2 H2 H2 H3 H3 H3 Y(J)S SONET Slide 63

89 More multiplexing Similarly, we can hierarchically build complex structures Lower rate STMs can be combined into higher rate STMs AUGs can be combined into STMs AUs can be combined into AUGs TUGs can be combined into high order VCs Lower rate TUs can be combined into TUGs etc. But only certain combinations are allowed by standards Y(J)S SONET Slide 64

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91 AUG All SDH mappings STM-N AUG AU-4 VC-4 C-4 AUG * 3 TUG-3 TU-3 VC-3 * 3 E M ATM M STM-0 E M T M ATM M T M TU-2 VC-2 C2 ATM 6.874M AU-3 VC-3 C3 *7 *7 TUG-2 * 3 * 4 TU-12 VC-12 C12 E M ATM M TU-11 VC-11 C11 T M ATM 1.6 M Y(J)S SONET Slide 65

92 All SONET mappings STS-N STS-3c STS-3 SPE E M ATM M *N STS-1 STS-1 SPE E M T M ATM M *7 VTG T M VT6 VT6 SPE ATM 6.874M * 3 VT-2 VT2 SPE E M ATM M pointer processing * 4 VT1.5 VT1.5 SPE T M ATM 1.6 M Y(J)S SONET Slide 66

93 Tributary mapping types When mapping tributaries into VCs, PDH-like bit-stuffing is used For E1 and T1 there are several options Asynchronous mapping (framing-agnostic) Bit synchronous mapping Byte synchronous mapping (time-slot aligned) E4 into VC-4 4, E3/T3 into VC-3 are always asynchronous T1 into VC-11 may be any of the 3 (in byte synchronous the framing bit is placed in the VC overhead) E1 into VC-12 may be asynchronous or byte synchronous Y(J)S SONET Slide 67

94 Survivability in SONET

95 Network Survivability/Fault Management Survivability: the network uses additional capacity to keep carrying all the traffic when a fault occurs It s a must on backbone networks Protection: Immediate and automatic answer of the network to recover from fault Survivability Protection Restoration Self-healing Restoration: typical of complex topologies in which find the network can reconfigure itself slowly (or manually) Reconfiguration Mesh Network Architectures Protection Switching Linear Architectures Ring Architectures

96 Survivability in SONET SONET adopts several technique for Survivability, Protection and Restoration Typically, they are based on ring topologies that offer two alternating paths ADM ADM ADM Fiber fault Nodes close to the fiber fault create a loopback to reconfigure the ring The topology after the reconfiguration is a monodirectional ring

97 What is protection? SONET/SDH need to be highly reliable (five nines) Down-time should be minimal i (less than 50 msec) So systems must repair themselves (no time for manual intervention) Upon detection of a failure (dlos, dlof, high BER) the network must reroute traffic (protection switching) from working channel to protection channel The Network Element that detects the failure (tail-end NE) initiates the protection switching The head-end NE must change forwarding or to send duplicate traffic Protection switching is unidirectional Protection switching may be revertive (automatically revert to working channel) working channel head-end NE protection channel tail-end NE Y(J)S SONET Slide 70

98 How does it work? Head-end and tail-end NEs have bridges (muxes) Head-end end and tail-end NEs maintain bidirectional signaling channel Signaling is contained in K1 and K2 bytes of protection channel K1 tail-end status and requests K2 head-end status head-end bridge working channel tail-end bridge protection channel signaling channel Y(J)S SONET Slide 71

99 Linear protection Simplest form of protection Can be at OC-n level (different physical fibers) or at STM/VC level (called SubNetwork Connection Protection) or end-to-end path (called trail protection) Head-end bridge always sends data on both channels Tail-end chooses channel to use based on BER, dlos, etc. No need for signaling If non-revertive there is no distinction between working and protection channels BW utilization is 50% channel A channel B Y(J)S SONET Slide 72

100 Linear 1:1 protection Head-end bridge usually sends data on working channel When tail-end detects failure it signals (using K1) to head-end Head-end then starts sending data over protection channel When not in use protection channel can be used for (discounted) extra traffic (pre-emptible emptible unprotected traffic) May be at any layer (only OC-n level protects against fiber cuts) working channel extra traffic protection channel Y(J)S SONET Slide 73

101 Linear 1:N protection In order to save BW we allocate 1 protection channel for every N working channels N limited to 14 4 bits in K1 byte from tail-end to head-end 0 protection channel 1-14 working channels 15 extra traffic channel working channels protection channel Y(J)S SONET Slide 74

102 Two fiber vs. Four-fiber rings Ring based protection is popular in North America (100K+ rings) Full protection against physical fiber cuts Simpler and less expensive than mesh topologies Protection at line (multiplexed section) or path layer Four-fiber rings fully redundant at OC level can support bidirectional routing at line layer Two-fiber rings support unidirectional routing at line layer 2 fibers in opposite directions Y(J)S SONET Slide 75

103 Unidirectional vs. bidirectional Unidirectional routing working channel B-A same direction (e.g. clockwise) as A-B management simplicity: it A-B and B-A can occupy same timeslots t Inefficient: waste in ring BW and excessive delay in one direction Bidirectional routing A-B and B-1 are opposite in direction both using shortest route spatial reuse: timeslots can be reused in other sections A-B B A-B B B-A B-C A A B-A C C-B Y(J)S SONET Slide 76

104 UPSR vs. BLSR (MS (MS-SPRing) SPRing) UPSR Unidirectional Path switching Two-fiber BLSR Bidirectional Line switching Four-fiber Of all the possible combinations, only a few are in use Unidirectional Path Switched Rings protects tributaries extension of to ring topology Bidirectional Line Switched Rings (two-fiber and four-fiber versions) called Multiplex p Section Shared Protection Ring g in SDH simultaneously protects all tributaries in STM extension of 1:1 to ring topology Y(J)S SONET Slide 77

105 UPSR Working channel is in one direction protection channel in the opposite direction All traffic is added in both directions decision as to which to use at drop point (no signaling) Normally non-revertive, so effective two diversity paths Good match for access networks 1 access resilient ring less expensive than fiber pair per customer Inefficient for core networks no spatial reuse every signal in every span in both directions node needs to continuously monitor every tributary to be dropped Y(J)S SONET Slide 78

106 BLSR Switch at line level l less monitoring i When failure detected tail-end NE signals head-end NE Works for unidirectional/bidirectional l fiber cuts, and NE failures Two-fiber version halfofocn OC-N capacity devoted to protection only half capacity available for traffic Four-fiber version full redundant OC-N devoted to protection twice as many NEs as compared to two-fiber Example recovery from unidirectional fiber cut Y(J)S SONET Slide 79

107 Protection and Restoration Fault recovery is very quick: Less than 50ms Restoration time In PDH was in the order of minutes In IP networks is takes several minutes In Ethernet LANs it takes tens of seconds (up to 60 seconds to reconfigure the spanning tree)