SynchronizationoverIP Networks

Transcription

1 SynchronizationoverIP Networks Introducing PTP, the Precision Time Protocol Distribuzione Del Segnale di Sincronizzazione per le Reti Digitali Terrestri Operanti in SFN Attraverso Modalità Alternative al GPS Università Tor Vergata 26 Maggio 2011 Heiko Gerstung, Meinberg Funkuhren GmbH & Co. KG

2 Contents 1 IP Based Synchronization The Challenges 2 Precision Time Protocol 3 Example Configurations and Performance 4 Products

3 1 IP BasedSynchronization

4 IP basedsynchronization The Challenges Time and Frequency Synchronization over packet switched networks faces several challenges: Network Path Delay Variation (PDV) Delay variationsin networkchips, OS networkstackandmultitaskingos userspace Embedded solutions only face PDV and Network Chip variations

5 2 Precision Time Protocol

6 Precision Time Protocol Standard published as IEEE (PTP v1) Updated in 2008 andpublishedasieee (PTP v2) V2 allowsusingip Networks (OSI Layer 3, IPv4 andipv6), Ethernet frames (OSI Layer 2, IEEE 802.3) andothers In production in a number of different areas: Finance(Synchronization of Stock Exchange Transaction Processing Systems), Telecommunication Networks (Mobile Phone Backhaul, e.g LTE) and Industrial Applications (Process Automation)

7 Precision Time Protocol (2) Experimental / Research projectsin Power Grid/ SCADA systems(substation Automation, new standard IEEE C37.238) and broadcasting(customer projects)

8 PTP The Approach Master Slave Approach: Oneactivemaster, multiple slaves(additional master clocks in passive mode for redundancy purposes) PTP-optimized network infrastructure(ethernet switches with hardware timestamp support) to reduce/eliminate path delay variation effects Supports a multitude of modes(layer 2/Layer 3, Unicast/Multicast, One- Step/Two-Step, Peer-to-Peer/End-to-End)

9 Time Stamps Both NTP and PTP use timestamps for their delay measurements NTP: Software Timestamps(time ofreceivingorsendingofan NTP packet is determined by NTP software, i.e. on application level) PTP: Hardware Timestamps(network packets are timestamped by network chip) requires special network chip hardware Error in Timestamp= Error in Synchronization= SyncPerformance Degradation

10 Delay Measurements The Holy Grail of Network based Synchronization NTP andptp bothuse4xtimestamp Algorithms(t1, t2, t3, t4) NTP performs delay measurements on every request/response transaction calculating a round-trip delay(client->master->client) PTP uses separate delay measurement mechanisms for both directions (master->slave andslave->master) but attheend ofthedayusesa roundtrip delay, too Asymmetric Delays cannot be determined by NTP and PTP

11 Delay Measurement Sequence PTP Message Sequence: Collecting the 4 timestamps t1,t2,t3,t4 Delay = (t4-t3)+(t2-t1) Roundtrip Delay = Delay/2 is used

12 PTP - PossibleModes Layer 2 / Layer 3: PTP communicationiseitherbasedon rawethernet frames(layer 2) orip Packets(Layer 3) Unicast/ Multicast: In Multicast mode, all PTP related communication is using the reserved PTP multicast address( ). A special procedure called Best Master ClockAlgorithm isusedtodeterminewhichdevicein theptp networkistheactivemaster One-Step/ Two-Step: The hardware timestamping logic either directly modifies the network packets(one-step) or just stores them and they are sent in a second step(two-step)

13 PTP PossibleModes (2) End-to-End / Peer-to-Peer Delay Measurements: In E2E modetheslavesdeterminethedelaybetweenthemandthe Grandmaster over the whole network path(from end to end). In P2P mode each device(grandmaster, Switches, Slaves) only determine thedelaytotheirnearestneighbour(s) andprovidethisvaluebyaddingitto the packets

14 3 ExampleConfigurations

15 PTP An Example Network Topology:

16 PTP -Performance Measurement ofpps over8 hours Yellow: Grandmaster Clock (GPS synchronized) Green: Slave A (after 19 hops) Blue: Slave B (after 10 hops) Purple: Slave C (after 20 hops)

17 PTP Performance (2) SlavesA andb managedtostaywithin200ns ofgrandmaster over8 hours test Slave C showedproblemsdue toa faultyswitchin boundaryclock configuration No temperature controlled environment Low traffic volume, all switches configured to be Transparent Clocks

18 PTP Performance (3) Test Run: 8 hours Frequency Deviation (10MHz) Resolution: Hz Network: Test Lab with 40% Bandwidth usage

19 PTP Performance (3) Test Run: 3 hours Time Deviation (1PPS) Resolution: 10-9 s Network: Test Lab with 60% Bandwidth usage

20 4 PTP Products

21 PTP Products Grandmaster Clocks: LANTIME M600/GPS/PTPv2 GPS Grandmaster Clock (19 1U) LANTIME M900/GPS/PTPv2 GPS Grandmaster Clock (19 3U) Slaves SYNCBOX/PTPv2 (E1/T1 or10mhz FrequencyOut, 1PPS out) PTP270PEX (PCIe Slot Card for servers/pcs) Combined LANTIME M600/MRS/PTPv2 Grandmaster Clock(GPS or1pps, 10MHz, IRIG) orslave Clock(PTPv2 asan input)

22 Grazie Mille! Questions?