Synchronisation in Telecom Networks

Transcription

1 Synchronisation in Telecom Networks ITSF / Jean-Loup Ferrant / November 6, 006

2 Network synchronisation history () Page -PSTN and PDH -Switches needed synchronisation in order to comply with slip generation specified in G.8 -Switches used to be synchronised from G.8 clocks (988) -Transport of synchronisation was done via Mbit/s signals transported within the PDH hierarchy, quasi transparently -The quality of these networks is guaranted by the control of wander that allows not to over/underflow buffers. These buffers were specified to allow 8 µs of wander without generation a slip

3 Network synchronisation history () -SDH -With SDH, Mbit/s signals transported via VC were not anymore suitable for network synchronisation due to the phase transients of VC pointer justification. -STM-N was chosen and specified to transport network synchronisation. -G.803 defines the hierarchical architecture of synchronisation network with clocks are defined in G.8, G.8 and G.83. -The respect of these recommendations avoids desynchronisation and allows the control of jitter and wander, prevents pointer justification and consequent wander on PDH tributaries Page 3 SDH networks have proven over last the 0 years their ability to provide excellent synchronisation network

4 Network synchronisation history (3) -GSM, and later UMTS, generated new requirements for the synchronisation network. Rather than Jitter and wander the frequency accuracy on the air ijnterface is the key requirement for synchronisation networks -WDM systems have been introduced Pre OTN point-to point WDM systems with proprietary implementation OTN systems based on G.709 -Packet networks have been introduced in metro and access networks New equipments, MSPP, combine TDM and Ethernet interfaces New standards specify the transport of TDM signals through packet networks New methods and protocols are proposed to transport synchronisation through packet networks Page 4

5 SDH Mapping & PJE due to desynchronisation Page 5 Central Clock Pointer jusification events Desynchronised clock Mapper / Demapper phase 3700 ns for VC /34/40 Mbit/s VC-4, VC-3, VC STM-N SDH - network STM-N VC-4, VC-3, VC Mapper / Demapper /34/40 Mbit/s phase 7400 ns for VC missing pointer 35 pointers

6 SDH Network Synchronisation Synchronisation reference chain This reference chain has been specified in order to maintain jitter and wander within acceptable limits, as specified in G.85 Page 6 Synchronisation direction PRC SSU SSU SSU m m mn n m n+ Maximum numbers according to G.803: - maximum number of 's between SSUs: m, m,... mn+ < 0 - maximum number of SSU's in a chain: n < 0 - maximum number of 's in a chain: 60

7 (SDH Equipment Clock) and SSU Page 7 T3 T4 T T T0 : MHz( Mbit/s) input sync. Signals : MHz ( Mbit/s) output sync. Signals : Mhz derived from STM-N : MHz derived from Mbit/s : MHz station clock T T T3 Sel A Sel B SETS squelch squelch S el C T4 T0 SETS: SDH Equipment Timing Source Using the T-T4 link allows to synchronize the from the SSU without any risk of timing loop SSU T T T3 T4 T0 ~

8 Hierarchical Master-slave solutions Easy and robust architecture, no timing loop May lead to long chains of clocks Page 8 PRC : SSU SSU SSU SSU SSU SSU Mai n synchroni sati on paths (normal operati on) U nder f ai lur e sit uat ions t he dir ect i on indicat ed by t he ar r ow m ay be r ever sed Standby synchronisation paths Pat hs w it hout ar r ow s m ay be used in ei t her di r ect i on, depending on All t he rights f ai lur reserved e sit uat ion 005, Alcatel

9 Distributed architecture Example with use of GPS receivers Short chain of clocks High number of GPS receivers Radio di stributed PRC, e. g. GPSsateli te system Page 9 PRC : RX SSU RX SSU SSU RX RX SSU RX SSU RX SSU

10 Hybrid solutions Each of the architectures, centralised and distributed has its own drawbacks, and most operators are optimising their synchronization network with a mix of both architectures. Page 0 : PRC,,3: Priorities RX SSU SSU SSU RX 3 SSU RX 3 SSU Mai n synchroni sati on paths (normal operati on) SSU

11 SSM and synchronisation protection SSM purpose Provide timing traceability Indicate the Quality Level of the source of synchronization SSM definition A 4 bit code located in S byte of STM-N frame SSM application Generates a DNU code to prevent timing loop In linear chains and rings and combination of them In meshed networks with some restrictions Provide desynchronisation detection Page G.8 source G.8 G.8 G.8 DNU DNU

12 Generalisation of SSM Page External Reference External Reference G.8 0 G.8 G.8 G.8 DNU DNU G.8 G.8 DNU DNU 0 G.8 G.8 G.8 G.8 0 G.8 G.8 G.8 DNU DNU G.8 DNU 0 DNU G.8 G.8 G.8 0 G.8 DNU DNU G.8 G.8 0 DNU G.8

13 Synchronisation of the E layer in SDH When SDH is the sync layer E is floating within the SDH frame through an asynchronous mapping E is inappropriate to transport synchronization due to VC PJE Page 3 Sync ref Digital switch Mbit/s Synchro? SDH network Mbit/s Digital switch Synchro? Solutions Provide a Mhz/ Mbit rom an SSU if the digital switch has a synchronisation port Implement a retiming function with the Mbit/s desynchroniser

14 Synchronisation of the E layer: SDH NE retiming The retiming function is basically a buffer in which a Mbit/s signal is entered with its own clock and which is extracted with the SDH clock of the SDH NE. Note that retiming is also implemented in some SSUs. This allows to deliver a network synchronization quality to the Mbit/s and get rid of phase jumps caused by VC PJE Page 4 This must be used only on synchronized Mbit/s, otherwise bits will be periodically lost in the buffer Output clock (locked to SDH clock) VC clock Mbit clock Low pass filter VC data buffer Mbit data Mbit/s desynchroniser retiming (functional representation)

15 Optical networks Page 5 WDM system have been specified to be transparent to client timing SDH synchronisation network are not jeopardized by WDM, OTN Synchronisation Layer - Physical Synchronisation STM-n STM-n STM-n STM-n FE/FX WDM Layer Physical SDH WDM

16 Synchronisation choices for OTN OTN is plesiochronous ITU has stated that there is no need for OTN to carry synchronisation, since there is already one network layer that does it, SDH. OTN is transparent to CBR client timing, jitter and wander are specified in G.85 Each OTN NE has its own free-running clock within ±0 ppm OTN is a plesiochronous network G.709 specifies justification scheme to adapt client and G.709 frame rate All client signal can be within ±0 ppm, even with multiplex function When OTN does not transport SDH client, it couldnot transport timing, but this might change using new synchronisation methods transported on packet networks Page 6

17 microseconds Mobile requirements In mobile applications, the most important requirement is that the frequency accuracy on the air interface remains within 50 ppb (red line) in order to provide handover when a mobile moves from one cell to another one. Mbits interfaces vs 50 ppb Page traffic synchro 50ppb 0, 0,0 0, seconds Requires low clock bandwidth implementation in BTS/ nodeb

18 Mobile Backhauling: Typical TDM Architecture Page 8 50ppb G.83 interface Synchronisatio n interface Synchronou s network PRC BTS/ nodeb TDM BSC/ RNC TDM MSC BTS/nodeB locked to a PRC: TDM generated in a MSC that is locked to a PRC via a synchronisation interface (E, MHz, STM-N) BTS/nodeB synchronized on TDM BSC synchronized on MSC by the TDM traffic signal

19 Mobile Backhauling, example with CES Page 9 50ppb BTS/ nodeb G.83 interface TDM I W F Packet network CES I W F Synchronisatio n interface TDM BSC/ RNC TDM Synchronou s network MSC PRC BTS/nodeB locked to a PRC: TDM generated in a MSC that is locked to a PRC via a synchronisation interface (E, MHz, STM-N) BSC synchronized on MSC by the TDM traffic signal BTS/nodeB synchronized on TDM recovered from CES packets

20 New technologies in transport networks Page 0 Vc4nv Layer Metro Ethernet GbE VCn GbE/FX VC4nv - Packet Ring Layer - Physical VCn FE/TX STM-n STM-n STM-n STM-n FE/FX FE/FX WDM Layer Physical SDH WDM FE/FX

21 Packet networks and synchronisation Page st phase: pseudowire CES: for transport of TDM Adaptive Method Differential Method nd phase: packet networks Time Protocols Precision Time Protocol (IEEE588) Network Time Protocol (NTP) Real Time Protocol (RTP) Synchronous Ethernet

22 Multi-service provisioning platform (MSPP) Customer Access/Service Technology E, E3, E4 STM-, STM-4, STM-6 ATM Ethernet, GE IP/MPLS ESCON, FICON Fibre Channel FDDI, DVB Client Signals MSPP NG-SDH/xWDM Traffic Switching Layer Layer Network Transmission Metro Network xwdm/sdh/ge.5g/0g MSPP clock can be synchronised by STM-N, Sync Eth or external synchronisation ports The clock can be used to synchronise STM-N, sync Eth ports and CES when a clock is needed. Page

23 TDM-PSN connexion Page 3 Sync port STM-N VC VC STM-N TDM GFP TDM OTN STM-N STM-64 STM-N CES VC VC VC VC matrix VC PDH VC VC CES CES PDH Eth Eth CES CES 4 5 CES Packet networ k Eth 0GBE-WAN Eth Eth GFP Ethernet SWITCH Eth Eth Eth Packet networ k CES 3

24 Candidate techniques for PSN Pro Con Page 4 CES Pseudowire Adaptive - No specific requirement on intermediate equipments Medium quality as PDV sensitive CES Pseudowire Differential Synchronous Ethernet -No specific requirement on intermediate equipments -Good performance -Excellent quality, similar to SDH -No influence of payload - Need network ref clock at both end points - all switches of the link need to process the sync Eth feature IEEE588 TM V Applicable to Telecom (Expected approval early 007) NTP/RTP - good performance - Possibility to bypass some switches not processing 588 ( - suits several packet network applications such as VOIP -full performance achieved only if all switches are IEEE588 -Current accuracy too low for TDM applications

25 Conclusion Introduction of packet networks creates a similar situation as that one that occured when SDH was introduced in PDH networks, corruption of the existing synchronisation network by a new layer. VC pointer were the SDH problem and PDV is the packet network problem. There has been one solution to solve the issue with SDH, the transport of timing STM-N signals. Many solutions are currently presented to solve the issue with packet networks. Page 5 Many presentations will describe these solutions during the next days.

26 Page 6