Techmahindra - PacketSynchronization

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© Tech Mahindra Limited 2011 © Tech Mahindra Limited 2010

1588 V2 –

A New Paragon for

Packet Synchronization

A whitepaperBy

Ankur Rawat

&

Sasindran M Prabhu

21 st Mar, 2011

Abstract:

Ethernet continues to gain traction as a cost-effective way to achieve higher bandwidth.

With the emergence of 'all Ethernet'-basednetworks, packet-based timingsynchronization is now of fundamentalimportance to ensure maximum networkperformance as more demanding technologiesand applications are deployed. The transitionto Ethernet from traditional PlesiochronousDigital Hierarchy (PDH) and SynchronousOptical Network (SONET)-based networksrequires efficient timing synchronizationtechniques in backhaul networks tosynchronize base stations and avoid droppedcalls as the call is handed off from one basestation to the next. Similarly Data cente rnetworks & Electrical Sub Stations alsorequire tighter synchronization to ensure theaccuracy and performance.

To address the synchronization needs, 1588V2 came into existence. IEEE 1588 V2 is aprotocol designed to synchronize real-timeclocks in the nodes of a distributed systemthat communicate using a network.



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© Tech Mahindra Limited 2011

Table of Contents

Overview of Historical Time Synchronization Technologies…………………….....3Evolution of 1588 V2………………………………………………………………………..4Difference between version 1 and version 2 of IEEE 1588……………………......5Maintaining Synchronization using 1588 V2……………………………………….…7Application of 1588 V2 ……………………………………….…………………………… 10Conclusion……………………………………………………………………………………12

Tech Mahindra’s Plan………………………………………....................................13List of tables……………………………………....................................................14References..................................................................................................15



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Acronyms & Abbreviations

BMC Best Master Clock

BMCA Best Master Clock Algorithm

CDMA Code division multiple access

GSM Global System for Mobile Communications

GPRS General packet radio service

HFT High Frequency Trading

IP Internet Protocol

LTE Long Term Evolution

PTP Precision Time Protocol

TEM Telecom Equipment Manufacturer

TSP Telecom Service Provider

WiMAX Worldwide Interoperability for Microwave Access



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Overview of Historical Time Synchronization Technologies

In a data network, time synchronization allows all of the different devices on thatnetwork to use a common clock to coordinate all of their activities. Network

integrators currently have a number of different time synchronization optionsavailable. Each has its own advantages and disadvantages. Table 1 shows thecomparison of different synchronization techniques.

Inter-range Instrumentation Group (IRIG): The IRIG standard defines a serialtime code format for use with serial communications networks. First standardizedin 1956, IRIG signals are a legacy technology used with older serial systems.IRIGB 205-87 is the latest update of this standard.

Network Time Protocol (NTP): NTP is a time protocol for data networks; it wasfirst established in 1985. NTP relies on a hierarchical, layered system topromulgate the current time throughout the network. NTP imposes hierarchicaltree architecture on the network to avoid cyclical dependencies.

Global Positioning System (GPS): GPS satellites orbiting the earth use highlyaccurate atomic clocks. Satellite signals carrying timekeeping information cantravel at the speed of light to receivers on the ground. These light-speed signalsare also corrected according to the principles of general relativity, which giveseach receiver on the ground highly accurate time information.

IEEE 1588 V1: This standard defines a protocol enabling precise synchronizationof clocks in measurement and control systems implemented with technologiessuch as network communication, local computing and distributed objects. The

protocol is applicable to systems communicating by local area networkssupporting multicast messaging including but not limited to Ethernet.

Table 1: Comparison of different time synchronization techniques



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Evolution of 1588 V2

The P recision T ime P rotocol (PTP) provides a standard method to synchronize

devices on a network with sub microsecond precision. The protocol synchronizesslave clocks to a master clock ensuring that events and timestamps in all devicesuse the same time base. PTP is optimized for user-administered, distributedsystems, minimal use of network bandwidth and low processing overhead.

PTP was originally defined in the IEEE 1588-2002 standard, officiallyentitled "Standard for a Precision Clock Synchronization Protocol for NetworkedMeasurement and Control Systems". In 2008 a revised standard, IEEE 1588-2008 was released. This new version, also known as PTP Version 2, improvesaccuracy, precision and robustness but is not backwards compatible with theoriginal 2002 version. Table 2 shows the list of major events during the evolutionof 1588 V2.

Major Events during evolution of 1588 V2

Version 1 published as IEEE Std. 1588 ‐ 2002 – on November 8, 2002

Version 1 approved as IEC standard IEC 61588 on May 21, 2004

V1 products and installations began appearing in late 2003

Conferences on IEEE 1588 held, 2003 – 2007

Version 2 PAR approved March 20, 2005

Version 2 technical work completed February 9, 2007

Version 2 sponsor ballot opened July, 2007 and closed August 8, 2007

Version 2 sponsor ballot comment resolution and coordination with IEEE Registration Authority

Committee (RAC) occurred during August – December, 2007

Version 2 recirculation ballot occurred January 14 – 24, 2008

P1588 committee voted on January 31, 2008 to send version to IEEE RevCom

Version 2 approved by RevCom on March 26, 2008 and by IEEE Standards Board on March 27, 2008

Version 2 published as IEEE Std 1588TM – 2008 on July 24, 2008 (reference 1)

Table 2: Evolution of 1588 V2



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Difference between version 1 and version 2 of IEEE 1588

• V1 does not support transparent clocks or profiles whereas V2 allows fortransparent clocks (including End-to-End and Peer-to-Peer delay options) andindustry profiles. Store and forward Ethernet switches exhibit latency timesthat can vary depending on the data that the switch is currently processing.For instance if the device is transmitting a 1500-byte packet, the latency will bemuch greater than if the transmit queue was empty or transmitting a 500-bytepacket. Transparent clock mode can account for the varying latency times; theswitch timestamps the time packet as it enters, measures the residence time,and corrects the time packet either as it leaves (one-step mode), or with afollow-up message with the correction field in it (two-step mode). Accumulationof switch latency or jitter errors is eliminated with transparent clock mode.

• V2 introduced the delay measurement mechanism. The propagation delay timeis measured only between the switch and its upstream peer. This is an

alternate method to measuring the total end-to-end path delay from the slaveclock to the master clock that eliminates two likely problems with the previousscheme: In a large network the end-to-end method will traverse many switches,each with varying and unpredictable latency time that leads to timinginaccuracy and jitter, compounded by the possibility of asymmetric data paths,Secondly, all the end-to-end path delay request messages must be answered bythe master clock that can cause a traffic and processing bottleneck at themaster in a large network. In the peer-to-peer delay mechanism, pathsymmetry is guaranteed and there will never be processing or trafficoverloading due to the one to one relationship.

• V1 packets are larger, making more traffic whereas V2 packets are smaller. V1is now completely redundant and is obsolete.

• V2 introduced announce messages which improved the operation of the BMC(Best Master Clock) algorithm which made reconfiguration faster, so V2 is morefault tolerant



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Table 3 shows the major features of version 2 which were missing in version1.

Table 3: Differences between 1588 V1 & V2



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Maintaining Synchronization using 1588 V2

In a packet transport system, clocks communicate with each other over thecommunication network using PTP. All clocks, whether master or slave, lead back

to – and ultimately derive their time from – the ‘Grandmaster’ clock. There are 4 types of PTP clock devices. Table 3 lists all the 5 major type of PTPclock devices.

Table 4: Different types of PTP devices

Master and slave are kept in sync by exchange timestamps, which are sent withinPTP messages. There are two types of message in the PTP protocol –

• Event Messages – Timed messages whereby an accurate timestamp isgenerated both at transmission and receipt of the message.

• General Messages – Messages which do not require timestamps but maycontain timestamps for their associated event message.



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Table 5 shows the different type of PTP messages.

Table 5: PTP message types

There are two mechanisms used in PTP to measure the propagation delaybetween PTP ports:

1. The Delay Request-Response Mechanism This mechanism uses the messages Sync , Delay_Req , Delay_Resp and Follow_Up .

2. The Peer Delay Mechanism This mechanism uses the messages Pdelay_Req , Pdelay_Resp andPdelay_Resp_Follow_Up . It is restricted to topologies where each peer-to-peerport communicates PTP messages with, at most, one other such port.

There are two phases in the normal execution of the protocol:

• Phase 1 - Master-Slave hierarchy establishmentIn each port of any Ordinary or Boundary clock there is a PTP state machine.

These state machines use the ‘Best Master Clock Algorithm’ (or BMCA) toestablish the Master for the path between two ports. The statistics of theremote end of a path are provided to each state machine by the Announcemessage. Since the local clocks statistics are already known by the statemachine, a comparison can be made as to which is the best Master.

• Phase 2 - Synchronizing Ordinary and Boundary Clocks (using the delayrequest-response mechanism or Peer delay mechanism)

Method-1Clock synchronization phase starts after the Master-Slave hierarchy hasbeen established. This phase consists of the exchange of PTP timingmessages on the communications path between the two clocks.



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There are two parts to this synchronization method:

1. Measurement of the propagation delay between Master and Slave usingthe delay request-response mechanism.

2. Performing the clock offset correction. Once the propagation delay isknown the Master can send Sync and optional Follow_Up messagescontaining its master timestamp.

Method-2After the Master-Slave hierarchy has been established the clocksynchronization phase can start.

There are two parts to this synchronization method:

1. Peer-to-peer ports maintain a measurement of the link propagation toeach peer by using the peer delay mechanism.

2. Performing the clock offset correction. Once the link propagation isknown, the master sends Sync and optional Follow_Up messages containingits master timestamp.



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Application of 1588 V2

• GSM and UMTS Base station Synchronization

One of the most common applications currently being cited for 1588 V2 is for thesynchronization of various wireless telephony and data services, e.g. GSM, UMTS,CDMA, WiMAX etc. These are gradually transitioning from a TDM-based backhaulnetwork to a packet-based network. The problem with eliminating the TDMinterface is that this is often used as a source of synchronization for the basestation itself. In order to permit correct handover between adjacent base stationsin the presence of Doppler shift generated by a moving mobile handset, the RFfrequency at a GSM or UMTS base station must be accurate to within 50ppb(parts per billion) of the nominal frequency at all times When the TDM backhaul isreplaced by a packet network, the synchronization requirement is fulfilled by 1588V2.

Figure 1: 1588 V2 in Wireless Network



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• Smart and Synchronized Electrical Substation Automation

The latest buzzword in the power industry is “Smart Grid,” a revolution thatpromises to make power distribution more efficient, sustainable, and cost-effectiveby applying information technology. The basic concept is simple: upgrade thepower grid to accomplish the same amount of work with less electricity by reactingintelligently to changes in power supply and demand with a more responsive,adaptable, and decentralized power distribution network.

“Smart Grid” requires the electrical infrastructure to be smart & intelligent. This isachieved by means of front-ending computers connected to remote devices suchas switch gears, sensors, power generators, and circuit breakers throughintelligent electronic devices (IEDs). Other priority front-end computing tasks insubstations include data acquisition, data computing, and protocol conversionbetween the DNP, IEC, Modbus, and other proprietary protocols used insubstation communications.

To create a “Smart Grid”, all the network nodes must work together seamlessly. That’s where time synchronization comes into picture. Accurate timekeepingallows the network to coordinate activity more effectively. For example, oneembedded computing task is to keep precise data logs of all the substationcomputers, switches, and IEDs. It’s important to keep accurate timestamps of allthe events in these data logs, which are often only milliseconds apart. Accuratetimekeeping ensures that these logs can be used to correctly manage anddiagnose any problems on the network.

Electric utilities have recognized that 1588 V2 offers network-based precision timesynchronization that is reliable and accurate enough (i.e. sub-microsecond) foruse in electric power applications. IEEE 1588 V2 is currently being considered forinclusion within Edition 2 of the IEC 61850 standard for the design of electricalsubstation automation.

In a network based on IEEE 1588 V2, the grandmaster clock determines thereference time for the entire substation automation system. The Ethernet switchacts as the boundary or transparent clock, and additional devices (such asmerging units, IEDs, and protection devices) are designated as ordinary clocks. Allof these devices are organized into a master-slave synchronization hierarchy withthe grandmaster clock at the top.

• Capital Markets

1588 V2 provides a foundation for accurate performance measurement andtransaction logging that is required for next generation electronic tradingplatforms, exchanges, and other trading venues. It provides accurate system clockand synchronization across server clusters required to measure applicationperformance in an ultra-low-latency environment such as HFT (High Frequency

Trading). Accurate measurement is the first step toward gaining advantage in acompetitive market where fast trading speed matters.



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Conclusion

Synchronization is an important part of today’s IP networks. By using 1588 V2protocol, carriers can achieve synchronization with accuracy matching that of

alternative solutions without the cost or need to build overlay networks requiredby those solutions. This standard provides an essential technology that allowscarriers to efficiently deploy IP networks with accurate synchronizationrequirements being met. Field trial showcasing excellent interoperability andperformance results, 1588 V2 is a proven solution for IP synchronization.



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Tech Mahindra’s Plan

Tech Mahindra has vast experience in Optical, Ethernet & Wireless technologies.Leveraging these skills, Tech Mahindra will be able to contribute in the following

1588 V2 areas.• 1588 V2 protocol stack development• System testing• Performance testing• Interoperability testing• EMS/NMS module

Based on the opportunity from the vendors, we will be able to select among theseactivities:

• Requirement Analysis• Product Design & Development• Testing & Validation• Interoperability Testing• Network Design, Deployment & Maintenance for Telecom Service Providers.



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List of Figures and Tables

Figures

• Figure 1: 1588 V2 in Wireless Network

Tables:

• Table 1: Comparison of different time synchronization techniques• Table 2: Evolution of 1588 V2• Table 3: Differences between 1588 V1 & V2• Table 4: Different types of PTP devices• Table 5: PTP message types



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References

• Precision Time Protocol webpage on www.wikipedia.org -

www.en.wikipedia.org/wiki/Precision_Time_Protocol•

www.metroethernetforum.org• Ixia solution for 1588 V2 testing –

www.ixiacom.com/downloads/library/application_notes/ixnetwork/ieee1588_application_note.pdf

• IEEE 802 LAN/MAN Standards Committee www.ieee802.org/1/files/public/docs2008/as-garner-1588 V2-summary-0908.pdf

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Techmahindra - PacketSynchronization