packet synchronization

© Tech Mahindra Limited 2011 © Tech Mahindra Limited 2010

1588 V2 –

A New Paragon for Packet Synchronization

A whitepaper By Ankur Rawat & Sasindran M Prabhu 21st 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 centernetworks & 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…………………….....3 Evolution of 1588 V2………………………………………………………………………..4

Difference between version 1 and version 2 of IEEE 1588……………………......5 Maintaining Synchronization using 1588 V2……………………………………….…7

Application of 1588 V2……………………………………….……………………………10 Conclusion……………………………………………………………………………………12 Tech Mahindra’s Plan………………………………………....................................13 List of tables……………………………………....................................................14 References..................................................................................................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 that network to use a common clock to coordinate all of their activities. Network integrators currently have a number of different time synchronization options available. Each has its own advantages and disadvantages. Table 1 shows the comparison of different synchronization techniques. Inter-range Instrumentation Group (IRIG): The IRIG standard defines a serial time code format for use with serial communications networks. First standardized in 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 was first established in 1985. NTP relies on a hierarchical, layered system to promulgate the current time throughout the network. NTP imposes hierarchical tree architecture on the network to avoid cyclical dependencies. Global Positioning System (GPS): GPS satellites orbiting the earth use highly accurate atomic clocks. Satellite signals carrying timekeeping information can travel at the speed of light to receivers on the ground. These light-speed signals are also corrected according to the principles of general relativity, which gives each receiver on the ground highly accurate time information. IEEE 1588 V1: This standard defines a protocol enabling precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication, local computing and distributed objects. The protocol is applicable to systems communicating by local area networks supporting 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 Precision Time Protocol (PTP) provides a standard method to synchronize devices on a network with sub microsecond precision. The protocol synchronizes slave clocks to a master clock ensuring that events and timestamps in all devices use the same time base. PTP is optimized for user-administered, distributed systems, minimal use of network bandwidth and low processing overhead. PTP was originally defined in the IEEE 1588-2002 standard, officially entitled "Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems". In 2008 a revised standard, IEEE 1588-2008 was released. This new version, also known as PTP Version 2, improves accuracy, precision and robustness but is not backwards compatible with the original 2002 version. Table 2 shows the list of major events during the evolution of 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 for

transparent clocks (including End-to-End and Peer-to-Peer delay options) and industry profiles. Store and forward Ethernet switches exhibit latency times that 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 be much greater than if the transmit queue was empty or transmitting a 500-byte packet. Transparent clock mode can account for the varying latency times; the switch 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 a follow-up message with the correction field in it (two-step mode). Accumulation of switch latency or jitter errors is eliminated with transparent clock mode.

• V2 introduced the delay measurement mechanism. The propagation delay time is 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 slave clock to the master clock that eliminates two likely problems with the previous scheme: In a large network the end-to-end method will traverse many switches, each with varying and unpredictable latency time that leads to timing inaccuracy and jitter, compounded by the possibility of asymmetric data paths, Secondly, all the end-to-end path delay request messages must be answered by the master clock that can cause a traffic and processing bottleneck at the master in a large network. In the peer-to-peer delay mechanism, path symmetry is guaranteed and there will never be processing or traffic overloading due to the one to one relationship.

• V1 packets are larger, making more traffic whereas V2 packets are smaller. V1 is 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 more fault 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 the communication 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 PTP clock devices.

Table 4: Different types of PTP devices

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

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

• General Messages – Messages which do not require timestamps but may contain 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 delay between 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 and Pdelay_Resp_Follow_Up. It is restricted to topologies where each peer-to-peer port 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 establishment

In 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) to establish the Master for the path between two ports. The statistics of the remote end of a path are provided to each state machine by the Announce message. Since the local clocks statistics are already known by the state machine, a comparison can be made as to which is the best Master.

• Phase 2 - Synchronizing Ordinary and Boundary Clocks (using the delay

request-response mechanism or Peer delay mechanism)

Method-1 Clock synchronization phase starts after the Master-Slave hierarchy has been established. This phase consists of the exchange of PTP timing messages 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 using the delay request-response mechanism. 2. Performing the clock offset correction. Once the propagation delay is known the Master can send Sync and optional Follow_Up messages containing its master timestamp. Method-2 After the Master-Slave hierarchy has been established the clock synchronization phase can start. There are two parts to this synchronization method: 1. Peer-to-peer ports maintain a measurement of the link propagation to each peer by using the peer delay mechanism. 2. Performing the clock offset correction. Once the link propagation is known, the master sends Sync and optional Follow_Up messages containing its 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 the synchronization of various wireless telephony and data services, e.g. GSM, UMTS, CDMA, WiMAX etc. These are gradually transitioning from a TDM-based backhaul network to a packet-based network. The problem with eliminating the TDM interface is that this is often used as a source of synchronization for the base station itself. In order to permit correct handover between adjacent base stations in the presence of Doppler shift generated by a moving mobile handset, the RF frequency 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 is replaced by a packet network, the synchronization requirement is fulfilled by 1588 V2.

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 that promises to make power distribution more efficient, sustainable, and cost-effective by applying information technology. The basic concept is simple: upgrade the power grid to accomplish the same amount of work with less electricity by reacting intelligently 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 is achieved by means of front-ending computers connected to remote devices such as switch gears, sensors, power generators, and circuit breakers through intelligent electronic devices (IEDs). Other priority front-end computing tasks in substations include data acquisition, data computing, and protocol conversion between the DNP, IEC, Modbus, and other proprietary protocols used in substation communications. To create a “Smart Grid”, all the network nodes must work together seamlessly. That’s where time synchronization comes into picture. Accurate timekeeping allows the network to coordinate activity more effectively. For example, one embedded computing task is to keep precise data logs of all the substation computers, switches, and IEDs. It’s important to keep accurate timestamps of all the events in these data logs, which are often only milliseconds apart. Accurate timekeeping ensures that these logs can be used to correctly manage and diagnose any problems on the network. Electric utilities have recognized that 1588 V2 offers network-based precision time synchronization that is reliable and accurate enough (i.e. sub-microsecond) for use in electric power applications. IEEE 1588 V2 is currently being considered for inclusion within Edition 2 of the IEC 61850 standard for the design of electrical substation automation. In a network based on IEEE 1588 V2, the grandmaster clock determines the reference time for the entire substation automation system. The Ethernet switch acts as the boundary or transparent clock, and additional devices (such as merging units, IEDs, and protection devices) are designated as ordinary clocks. All of these devices are organized into a master-slave synchronization hierarchy with the grandmaster clock at the top.

• Capital Markets

1588 V2 provides a foundation for accurate performance measurement and transaction logging that is required for next generation electronic trading platforms, exchanges, and other trading venues. It provides accurate system clock and synchronization across server clusters required to measure application performance in an ultra-low-latency environment such as HFT (High Frequency Trading). Accurate measurement is the first step toward gaining advantage in a competitive market where fast trading speed matters.

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Conclusion Synchronization is an important part of today’s IP networks. By using 1588 V2 protocol, carriers can achieve synchronization with accuracy matching that of alternative solutions without the cost or need to build overlay networks required by those solutions. This standard provides an essential technology that allows carriers to efficiently deploy IP networks with accurate synchronization requirements being met. Field trial showcasing excellent interoperability and performance 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 these activities: • 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

packet synchronization

Documents

network time protocol

time base

current time

synchronization needs

accurate time information

network communication

irig standard

standard method