CLOCK SYNCHRONIZATION IN CELLULAR/MOBILE NETWORKS PETER CROY SENIOR NETWORK ARCHITECT AVIAT NETWORKS

Transcription

1 CLOCK SYNCHRONIZATION IN CELLULAR/MOBILE NETWORKS PETER CROY SENIOR NETWORK ARCHITECT AVIAT NETWORKS 1

2 Agenda Sync 101: Frequency and phase synchronization basics Legacy sync : GPS and SDH/Sonet overview Q&A part 1: Sync 101 & Legacy sync Summary of packet sync standards Sync in cellular/mobile networks: Overview and challenges Impact of sync on backhaul networks migration Summary Q&A part 2: Clock Sync in cellular/mobile networks 2 AVIAT NETWORKS

3 Classical Music Example Question: What do we need to make a lot of instruments play a classical symphony? 3 AVIAT NETWORKS

4 Synchronizing an Orchestra Answer: A conductor with a baton. 4 AVIAT NETWORKS

5 Synchronization in Telecommunications The network master clock is playing the role of the orchestra conductor, so the independent cellular network users can transmit and receive wireless data at the same rate. Master Clock (PRC) We call this clock the network Primary Reference Clock (PRC) or stand alone sync equipment (SASE). It is usually a highest precision atomic clock oscillator. 5 AVIAT NETWORKS

6 Clock Signal Phase Definitions Phase relations between two clock signals Clocks are in phase, signals are perfectly aligned: Clock frequency and phase are synchronized Clocks have an accumulating phase offset: This is a frequency accuracy problem! Clocks have a changing phase offset: This is a frequency stability problem! 6 AVIAT NETWORKS

7 Clock Signal Quality Definitions Frequency offset: Average phase change accumulated within a 100 seconds observation period example f offset = Δt/t = 1ns / 100s = AVIAT NETWORKS

8 Clock Signal Quality Definitions Frequency accuracy: MTIE (maximum time interval error) is the largest phase change during the observation interval. Note: MTIE only increases and hides periods of small errors 8 AVIAT NETWORKS

9 Clock Signal Quality Definitions Time domain stability uses statistical observations of frequency sources over observation time - Deviations from a nominal value are expressed with statistical methods - TDEV (time deviation, calculated from Allan Deviation ) characterizes the statistical time errors of a frequency source from its nominal frequency Clock source frequency accuracy is defined as percentage and expressed in ppm (parts per million) or ppb (parts per billion): - A typical asynchronous Ethernet switch transmits with ±0.01% accuracy, which is ±100x10 6 or ±100ppm (ppm to ppb: 0.001ppm equals 1pbb) - Example: A local oscillator is specified to deviate from its nominal frequency by a maximum of ± % (50ppb) > A watch with this oscillator would vary only ±4.32 microseconds per day 9 AVIAT NETWORKS

10 Network Synchronization Standards The ITU has released numerous recommendations to define clock accuracy & quality requirements for networks TDM transport networks have used clock synchronization for decades to ensure correct data transmission Requirements SDH/PDH Network Packet Network Functional architecture and network synchronization requirements G.803, G.810, G.823 (E1), G.824 (T1), G.825 (SDH jitter and wander) G.8261 Primary Reference Clock specification G.811 G.811 Equipment clock specification G.812 (Type IV), G.813 (SDH) G.8262 Synchronization layer functions, functional blocks, timing flow, etc. G.783, G.781 G.8264, G AVIAT NETWORKS

11 Network Synchronization using GPS GPS system uses synchronized atomic clocks on the ground and in the satellites to measure time differences of coded signals to determine a position on earth The frequency and phase synchronized GPS system clock can be received and used to sync a local oscillator Precision GPS receivers require a high quality local oscillator to correct clock wander - Use oven controlled oscillators (OXCO) - Frequency accuracy: typ. ± 5ppb - Phase accuracy: typ. ± 0.1µs - Better accuracy achievable (at cost!) 11 AVIAT NETWORKS

12 GPS Receiver Clock Sync: Pro s and Con s Pro s + Very accurate sync of frequency & phase plus time/day (UTC) + Equivalent to distributed PRC clocks + Relatively inexpensive receivers Con s - Additional outdoor antenna installation required - Vulnerable to local interference and malicious radio jamming - GPS selective availability (SA) directly affects accuracy - System controlled by US military (DoD) 12 AVIAT NETWORKS

13 Legacy Sync: SDH/Sonet & PDH Networks Circuit switched networks are per definition synchronized - SDH/Sonet defines synchronous transmit/receive data clocks for transmission - PDH hierarchy is derived from SDH/Sonet clock timing (plesiochronous clock) - PDH circuits (E1/T1, etc.) use HDB3 coded signal (layer 1) for sync transmission ITU standards specify the sync quality and accuracy - G.813/G.825 for SDH: Specifications for core network links - G.823/G.824 for PDH: Specifications for access/aggregation network links - Includes masks for MTIE, TDEV, wander and jitter tolerance Synchronization is delivered with every transmission circuit - A PRC traceable synced clock is available at every point in the network - Convenient for cellular/mobile backhaul sync delivery to every base station - Most backhaul migration strategies involve retaining PDH circuits for sync

14 Legacy Sync: SDH/Sonet & PDH Networks Pro s +Very accurate sync delivered through physical layer +Ubiquitously available throughout the transmission network +Well established and understood (e.g. trouble shooting) Con s - Frequency only sync; Phase sync separately needed - PDH circuits scalability for data capacity demands 14 AVIAT NETWORKS

15 Q&A SYNC 101 & LEGACY SYNC Questions, comments on synchronization basics? What legacy sync method is your network using? Are your network sync requirements changing?

16 New Standards: Packet Network Synchronization Fixed and mobile network operators are migrating from circuit towards packet based transport for scalability and cost reasons - But: Packet transport is typically based on asynchronous clocks, i.e. independent clocks for transmitting and receiving stations New standards promise to retrofit clock sync functions into packet transport technologies: - Synchronous Ethernet (sync transport over physical OSI layer 1) - IEEE1588v2 & IETF NTP (message based protocols) 16 AVIAT NETWORKS

17 Synchronous Ethernet: A Brief History The first attempt to investigate time synchronization over packet networks dates back to ITU Study Group 15, Question 13 in October 2003 Subsequently became the ITU G.pactiming project Synchronous Ethernet PHY was initially proposed by British Telecom and France Telecom, and incorporated into G.pactiming in 2005 Intention: Enable seamless transport migration from SDH to Sync Ethernet G.pactiming consented in 2006, resulting in recommendation G.8261 The group then continued to enhance G.8261 and began the work on two additional ITU T recommendations, G.paclock and G.pacmod G.pacmod includes clock distribution and SSM usage G.paclock defines clock characteristics (wander, jitter and frequency accuracy) Sync Ethernet clock spec is based on G.813 (SDH equipment slave clocks) G.paclock became G.8262 in 2008; Expected to be ratified in G.pacmod subsequently became G AVIAT NETWORKS

18 Synchronous Ethernet Principle of Ethernet Synchronization Source: ITU G.8261 document Switch A Switch B Switch C MAC MAC MAC PRC clock S M PHY S M PHY S PHY Ethernet connections (electrical/optical) Example: Switch A transports data & clock over the PHY links to switch C > Ethernet ports are Master/Slave daisy chained to transmit the PRC clock 18 AVIAT NETWORKS

19 Synchronous Ethernet: Pro s and Con s Pro s Very accurate sync at physical layer (like SDH/Sonet) Accuracy unaffected by traffic load Con s Frequency only sync; Phase sync separately needed Requires all switches and routers along the transmission path to support Synchronous Ethernet 19 AVIAT NETWORKS

20 Packet Sync IEEE 1588v2: A Brief History IEEE working group was setup in 2001 after prototype work from vendors showed successes Original version 1 released in 2002, but lacked robustness, accuracy; It was multicast only and frames were long and sent infrequently Version 2 was released in 2008 with significant improvements to frame format & message frequency which improved clock quality and robustness Version 2 defined message transport as IP unicast and directly over Ethernet and other technologies IEEE 1588v2 does not specify clock parameters like accuracy, jitter or wander > This is left to ITU (and others) 20 AVIAT NETWORKS

21 IEEE 1588v2 Overview IEEE 1588v2 uses hardware time stamped messages to distribute time information between Master and Slaves The Best Master Clock Algorithm (BMCA) is used to determine the clock sync hierarchy in the 1588v2 domain Standard does not specify the clock recovery algorithm for slave nodes, it is proprietary to each vendor! 21 AVIAT NETWORKS

22 IEEE 1588v2 Clock Timing Calculations 1588v2 is highly sensitive to delay variation of packets and to asymmetries in the transmit/receive path! 22 AVIAT NETWORKS

23 IEEE 1588v2: Pro s and Con s Pro s + Frequency and phase sync along time/day are provided + Relatively simple hardware implementation Con s - Accuracy varies with traffic load across the network path - Requires symmetrical transport and low delay variation - Equipment of different vendors does not interoperate - Requires packet network QoS support with low latency queuing - May add significant priority traffic to oversubscribed links 23 AVIAT NETWORKS

24 IETF NTP: A brief history Network Time Protocol (NTP) is in use since 1985 and is currently in version 4 (NTPv4) It computes UTC time from a hierarchy of clock sources, Stratum 1 being the most accurate (atomic clocks), Stratum 2 and Stratum 3 derived and synchronized to Stratum 1 master NTPv4 supports hardware time stamped messages and computes clock timing offsets similarly to 1588v2 It is widely used to synchronize Internet server clocks for use in databases, logs and other applications Proprietary extensions to NTPv4 claim to achieve clock sync required for cellular/mobile radio base station use 24 AVIAT NETWORKS

25 IETF NTP: Pro s and Con s Pro s + Well established algorithm for timing offset calculations Con s - Accuracy varies with traffic load across the network path - Requires very accurate (expensive) local oscillators - Originally designed only for computer clock synchronization 25 AVIAT NETWORKS

26 Cellular/Mobile Base Station Sync BTS 2 drifts outside 50ppb window F 1 +f 2 +/ 50ppb BTS 2 Mobile cannot lock to BTS 2 and call is dropped F 1 +/ 50ppb BTS 1 T 1 T 2 Handset moves from BTS1 coverage area to BTS2 Traffic to/from the core network needs the exact same timing, otherwise the handset can not keep the session during the transition time T1/T2 Synchronous radio signals also improve radio coverage through minimizing neighbor cell interference Time 26 AVIAT NETWORKS

27 Cellular/Mobile Radio Sync Requirements Frequency Synchronization A T A =1/f A t Mobile Network Architecture Frequency Sync Time/Phase Sync B f A =f B T B =1/f B t CDMA2000 GSM Phase Synchronization A B f A =f B T A =1/f A T B =1/f B t t UMTS-FDD LTE-FDD UMTS-TDD LTE-FDD with MBMS- Single Freq. Network Time Synchronization A B f A =f B 01:00:00 T A =1/f A T B =1/f B 01:00:10 t t 01:00:00 01:00:10 LTE-TDD Mobile WiMAX TD-SCDMA 27 AVIAT NETWORKS

28 Clock Frequency Accuracy Requirements In cellular/mobile applications, the frequency accuracy on the air interface must remain within 50 ppb for seamless handover of mobile devices between radio base stations - 50pbb applies at the radio/antenna interface Transport nodes in a synchronization chain require better clock accuracy than 50pbb - A tighter accuracy value at the network interface of a cellular/mobile base station is required - 16pbb is mentioned as a potential target in G.8261 for Sync Ethernet - Discussion about recommendations are still ongoing in the community Very precise local clock oscillators in the radio base station equipment are needed to maintain frequency accuracy in case of sync loss - Clock holdover time to maintain radio performance until sync fault is fixed 28 AVIAT NETWORKS

29 Synchronization Transport Chain for 2G/3G/LTE PRC Core Transport Network BTS1 BTS2 BTS3 RNC S M S M S M S x pbb y pbb z pbb Transport chain accuracy x+y+z<50pbb against PRC M S Daisy chained synchronization accuracy decreases with the number of transport network hops Tighter clock accuracy per hop ensures overall accuracy stays within the radio air interface specification target 29 AVIAT NETWORKS

30 Complexity of Transport Network Migration BTS config in 2010: 2G: E1/T1 3G Voice: E1/T1 HSPA: IP/Ethernet (no sync) BTS config in 2012: 2G: E1/T1 (converted to IP) 3G/HSPA: IP/Ethernet (no sync) LTE: IP/Ethernet (sync+phase) Network Migration Progress Keeping clock sync from SDH/PDH circuits may help to de risk network migration to packet based backhaul transport Co existence of multiple transport requirements likely to affect the choice of synchronization architecture Cost pressures favour outsourced backhaul models Synchronization might be a thorny SLA topic with the partner! 30 AVIAT NETWORKS

31 Summary of Synchronization Precise and synchronized frequency oscillators are essential for modern telecommunication systems Traditional TDM transmission systems inherently provides hierarchical clock synchronization Cost pressures and capacity limits of TDM networks accelerate move to asynchronous, packet based transport Clock synchronization in packet networks can not be ignored for cellular/mobile applications Multiple efforts of standards bodies are promising to solve the synchronization challenge over packet networks Synchronization solutions are evolving, so watch this space! 31 AVIAT NETWORKS

32 THANK YOU! TIME FOR YOUR QUESTIONS