the applicability of 1588 time synchronization over packet delay

IEEE 1588 TIME SYNCRONIZATION OVER PDV COMPENSATION SCHEME 1

The applicability of 1588 Time Synchronization over Packet Delay Variance Compensation

Scheme in the Wireless Backhaul Network

Altynbek Malibayev, Sanket Nasre, Ya-Li Kao, Debashish Dash

University of Colorado at Boulder


Table of Contents

Headline ................................................................................................................................... 1

Table of Contents .................................................................................................................... 2

Abstract .................................................................................................................................... 3

I. Introduction ................................................................................................................. 4

i) Research Problem ...................................................................................................... 4

ii) Research Question and Sub-problems ....................................................................... 5

II. Literature review ........................................................................................................ 7

i) Project Extension to the State-of-art ......................................................................... 8

III. Research Methodology ............................................................................................... 9

i) Proposed PDF Algorithm ....................................................................................... 11

IV. Results ........................................................................................................................ 13

V. Discussion................................................................................................................... 15

VI. Conclusion and Area for Further Research ........................................................... 16

VII. References .................................................................................................................. 18

IEEE 1588 TIME SYNCRONIZATION OVER PDV COMPENSATION SCHEME 3 Abstract— Mobile wireless backhaul system demand highly precise clock synchronization within its

network. Precision Time Protocol version 2 is an evolution of clock synchronization standard that meets

the strict Time/Phase & Frequency requirements in different mobile technologies such as LTE and

WiMAX. However, due to the high implementation cost of PTPv2, its deployment or upgrade is a

demotivational factor for mobile operators.

This study examines the effectiveness of the PTP algorithm in software-based implementation intended

to improve the synchronization of distributed network nodes with non-1588v2 switches. This research

study offer a novel algorithm as an alternative during the transition period of deploying PTPv2 network

nodes and implement this C programmed algorithm in a real testing environment to measure the time-

stamped packets between grandmaster and slave nodes in different levels of traffic congestion based on

the Unix-like operating system. The results of performance allow us to understand the effect of C

programmed PTP algorithm on different settings in the backhaul system in terms of Packet Delay

Variance.

Keywords - Packet Delay Variance (PDV), Precision Time Protocol (PTP), congestion, IEEE 1588, C

programming, Linux (Ubuntu-Operating system), backhaul system, mobile operators.


I. Introduction

The mobile networks have different technology generations such as 2G TDM, 4G

IP/Ethernet, 3G ATM/Ethernet delivered simultaneously over the same backhaul network.

All these technology generations have different synchronization requirements in terms of

accurate frequency or time synchronization. The mobile network providers when

implementing network convergence face many challenges one of which is synchronizing the

base stations. The explosive growth of data traffic in wireless industry has become a

milestone for large mobile network operators to gradually migrate from TDM based backhaul

solutions to an IP based Ethernet paradigm. The legacy SONET/SDH and T1/E1 backhaul

technologies were specially designed to meet the strict synchronization requirements of the

mobile base stations. As a result, the implementation of Ethernet in the mobile backhaul

network had been a major challenge for decades until the emergence of 1588v2 (PTP)

Precision Time Protocol [1]. PTP is an industry standard protocol used to synchronize nodes

in the packet switched environment. The underplaying concept of PTPv2 is to synchronize

distributed network nodes by delivering time stamping information between a Grandmaster

clock and slave clocks. The established hierarchy of a master/slave clocks in a network

allows the slave clock to regulate their internal clock source in a continuous manner. Based

on the report made by [2], PTP frequency and time synchronization accuracy heavily depends

on Packet Delay Variance (PDV). As [3] explains, “PDV is the difference between the one-

way delay of the selected packets.” Therefore, PDV is considered to be a key factor

deteriorating the performance of PTP.

i. Research problem

The substantial share of the deployed packet switched networks runs on legacy switches

that are not equipped with IEEE 1588v2 support. Therefore, the effect of PDV on PTP

accuracy can be significant. As [4] explains, the main cause of PDV in networks is related to


the “store and forward” principle where switches and routers perform automatic buffering of

all packets before forwarding. In other words, all packets coming at the input ports are placed

in one queue regardless of their destination. As a result, a switch or router experiences

contention as soon as two packets arrive simultaneously at different input ports with the same

output interface. Since the slave clock synchronization accuracy is considerably dependent on

a symmetric path delay between grandmaster and slave, the exposure of PTP messages to

different delays contributes to the generation of Time Interval Error [3, 4]. Due to the speed

of outgoing interface and number of packets in the queue, PDV can get too large to maintain

the demanded synchronization accuracy in the mobile network.

The implementation of Layer 2 Quality of Service (QoS) appears to be the most sensible

solution as it gives timing packets the highest priority to minimize the queuing delay.

However, L2 QoS is not effective if the switch has started the transmission of a packet that

makes the queuing delay inevitable. For this reason, the introduction of the boundary and

transparent clock concept in PTPv2 has enabled asynchronous network nodes to significantly

reduce PDV value to a negligible figure. The downside of boundary clocking is that only

small share of the industry switches have the capability of synchronizing network nodes with

the relatively new 1588v2 time stamping protocol. As a result, there is an economic challenge

for mobile operators to deploy large scale backhaul networks with 1588v2 supporting

equipment [4].

ii. Research question and sub-problems

The purpose of this study is to improve the synchronization of distributed network nodes

with non-1588v2 intermediate switches. In particular, the research aims at identifying the

potential of the time synchronization algorithm for IEEE 1588 standard to minimize the PDV

to an extent applicable to the synchronization of base stations in the wireless backhaul


network. Table 1.1 shows frequency and time/phase synchronization requirements of

currently deployed mobile technologies.

Table 1.1 – Time/Phase & frequency requirements of some mobile technologies

Mobile technology Frequency Phase/Time

CDMA2000 ±50ppb Goal: <3µs

Must Meet: <10µs for not less than 8

hours, with respect to CDMA System

Time (traceable to UTC)

WCDMA-TDD ±50ppb 2.5µs

TD-SCDMA ±50ppb 3µs

LTE (TDD) ±50ppb 3µs inter-cell phase difference for

small

cells, 10 µs for large cells

LTE MBMS ±50ppb 1 µs inter-cell phase difference

WiMAX (TDD) ±2 ppm 1 – 1.5 µs

It should be noted that the goal of the research is to measure and observe the actual

performance of the proposed algorithm by employing the extended features of IEEE 1588v2

standard. Nevertheless, the research is not intending to obtain results totally fitting the

requirements of Table 1.1 due to time and technical constraints. As a result, the research sub-

problems are organized in the following order:

Design an algorithm of finding symmetric delay packets by employing a feedback

mechanism and increasing PTP packet rate.

Implement the “symmetric packets” programming code with C language on Linux

machines

Build an in-line network topology in the lab environment and identify the actual Time

Interval Error (TIE).

Test the performance of the algorithm under various levels of network congestion.


II. Literature review

The IEEE 1588 standard has become a coveted area of research among scientists since the

last five years. A prime example can be the last year capstone project [5] that focused on

identifying the feasibility of using non-1588 node elements in differential protection system.

However, a majority of recent work done in this area propose and describe different types of

PDF (Packet Delay Filtration) algorithms aimed at enhancing PTP synchronization. Even

though the underlying logic of most PDF algorithms is based on increasing PTP message

rate, the implementation methods are different.

One of the first approaches to lower the PDV effect was proposed by [6]. The algorithm

employs a queuing estimation mechanism in which the probing packets are periodically

exchanged between the master and slave to filter out packets with high delay jitter. The

researchers used the OPNET discrete event simulator to conduct experiments. The main

drawback of the described method is associated with targeting only the minimum delay

experienced packets, because the nature of traffic load can be asymmetric.

Another approach, an enhanced IEEE 1588 time synchronization method, put forward by

[7] intends to perform time synchronization. The idea is based on an offset-correction

mechanism, where the actual PTP algorithm is complemented with exploratory messages

such as M2S EXPLORER 1-2, S2M EXPLORER 1-2. While the enhanced time

synchronization algorithm provides only 0.3 – 2.8 µs bias error, the research fails to show the

algorithm performance under real traffic profile. Another proposed algorithm, which was

developed at the Heriot-Watt University Edinburg uses a sample-mode PDF method for 1588

synchronization [8]. The algorithm is based on creating more than one pool of sampled

SYNC packets that come from the master. After obtaining W samples of packets from the

master-slave, the delay distribution histogram is generated. By identifying mode bins with


relatively the same number of sampled packets, “good packets” (packets experiencing

minimum delay) are selected in accordance with the following formula:

(1)

Where α stands for the width of a histogram bin, and δmode is the sample mode.

Finally, an application report generated by Texas Instruments Inc. [9] proposes a “lucky

packets” algorithm that determines a minimum Master to Slave and Slave to Master delay to

correct slave clock offset. The basic idea of the algorithm is to search for minimum delay

while increasing PTP sync message rate up to 8 packets/ sec. The lucky packets algorithm

makes offset corrections after ensuring that MSdelay and SMdelay are lower than Min-

MeanPathdelay value. Even though the algorithm meets G.823 requirements, further

improvements are required.

i. Project extension to the state-of-the-art

While our proposed PDF algorithm, called symmetric packets algorithm (SPA), is similar

to the “lucky packets” algorithm, the ultimate goal of the research is not directed at finding

minimum delay packets, but rather packets experiencing relatively equal delay. The packets

experiencing symmetric delay are considered to be “good packets”. The symmetrical line

delay means that the line delay from the master to the slave and from the slave to master

should be of comparable length. First of all, the implementation method uses a feedback

mechanism to adjust the PTP message rate dynamically so that the algorithm puts less

pressure on bandwidth consumption of other applications. Secondly, the algorithm does not

add additional elements to the existing IEEE 1588v2 standard, but rather takes advantage of

its extended features.


III. Research methodology

We approached the research problem quantitatively in order to answer the research sub-

problems. The process of data collection was related to establishing a test environment. The

test platform was based on using the PTP Grandmaster, PTP client, traffic generator, and

non-1588 supporting switches. The whole testing process consisted of two parts: The first

part involved measuring the impact of PDV on TIE between Master and Slave under different

levels of network congestion. The second part was devoted to identifying the effectiveness of

the SPA to deal with background traffic burst. In other words, we did a comparative analysis

by measuring TIE without and with the SPA.

Fig. 3.1 – Test Platform

Fig.3.1 represents a basic test platform where measurements were carried out. Three

Catalyst 2960 switches formed the core of the wireless backhaul network. In addition, a

traffic generator software was used to simulate a unidirectional UDP stream up to 10 Mbps

towards the traffic absorber. For this reason, the switch B was configured with 10 Mbps

interface bandwidth along the path from the Master to Slave. It is also relevant to note that

the packet size for background traffic was set to 1460 bytes. PTP Master and Slave machines

worked under Linux OS in order to exchange timestamps with microsecond precision. The

primary mode for emulating IEEE 1588 standard and obtaining timestamp messages was


achieved with the help of the gettimeofday() and settimeofday() library functions of C

language, which provides information of the local time of the PCs. Besides, the socket

programming part uses UDP in accordance with PTP specification. Finally, there was a

controller PC to measure TIE. The following steps explain the order of the test:

The initial step involved the synchronization of the PTP Master and Slave in

accordance with the basic PTP algorithm experiencing no background traffic (See Fig. 3.2).

Fig. 3.2 – PTP synchronization process

(2)

(3)

(4)

Upon implementing the above calculations, the PTP Slave applied an offset to its clock.

After synchronization, the program forced both parties to simultaneously send a check

message to the PC controller, which in turn calculated the Time Interval Error (TIE).

In the next step, broadcast traffic was sequentially injected into the network with four

levels of link congestion: 20%, 40%, 60%, and 80%, with the help of the IP Traffic – Test &

Measure commercial software. We applied ITU G.8261 [8] recommendation, which defines

the network traffic model for testing purpose of this simulation. As a result, by adjusting the

level of link utilization between Switch A and Switch C, the actual PDV and TIE values were

derived. For the sake of clarity, it is important to mention that the deployment of the PC


controller to measure TIE adds an observational error. The reason is that when a checker

packet issued by both the end parties arrives simultaneously at the Switch B’s controller port,

one of the packets was queued. However, considering the small size of the check packet (64

byte) and the high interface port speed of the Switch B (100Mbps), we quite permissibly

ignored the 5.12 µs transmission error.

The goal of the third experiment was to determine the performance of the proposed

algorithm that relies on increasing synchronization cycles between the PTP Master and Slave.

This allowed the PTP Slave PC to adjust its clock more precisely. Initially, the

synchronization cycle was set to be one “sync” packet per second for reference. In order to

calculate more accurate offset value, we took values for three different synchronization cycle

rates: 16 packets/second, 32 packets/second, and 64 packets/second.

i. Proposed PDF algorithm

The goal of symmetric packets algorithm (SPA) is to select comparatively equal path delay

for MS and SM packets. The algorithm takes the advantage of the extended feature of IEEE

1588v2 protocol that allows the PTP Slave to increase the PTP timing packet rate of the

grandmaster clock depending on the severity of network congestion. The idea behind this

proposal was to ensure that only “good” packets are selected so that no occasional traffic

burst in the network can desynchronize Base Transceiver Stations. The block-diagram of the

SPA is demonstrated below:


Fig. 3.3 – The workflow of the proposed algorithm

According to Fig. 3.3, before the initialization of the SPA algorithm, it is necessary to

perform frequency and time synchronization. Otherwise, the slave clock having an initial

offset value larger than the specified boundary condition would reject even the symmetric

Master-to-Slave (MS) and Slave-to-Master (SM) delay packets. As soon as the slave

calculates the actual MS and SM values, it takes their ratio through the selection criteria,

which has been determined to be in the 0.91< MS/SM < 1.09 range.

The decision to set the boundary condition to such a specific value was based on the

assumption that the Linux system clock on Intel-based PC provides 20 PPM accuracy specs

[10]. Besides, the average delay between the grandmaster master and slave clock was

measured to be 460 µs. Therefore, the slave having a clock drift around ±20 PPM fell into the

pre-calculated boundary condition every second. If the MS and SM packets experienced

relatively the same delay, the computed offset value was applied to the slave clock;

otherwise, the PTPs feedback mechanism came into play.

When the MS/SM ratio was outside the scope of the specified range, the slave employed a

PTPv2’s new feature associated with unicast profile. This meant that the slave was able to


affect the grandmaster packet emission rate by issuing a specific REQUEST UNICAST

TRANSMISSION TLV message [11]. See Fig. 3.4,

Fig. 3.4 – PTP flow diagram with the SPA

According to [11], IEEE 1588 standard supports PTP message rate up to 128 packets per

second. So by accelerating PTP message rate, the slave increased the probability of finding

good packets. In addition, since the PTP message rate was adjusted dynamically, the issue of

large bandwidth consumption was eliminated.

IV. Results

In accordance with the research methodology agenda, experiments were divided into two

parts. The objective of the first experiment was to determine the impact of the increasing

background traffic on the PTP Master – Slave synchronizing process. The following graph

demonstrates the extent of synchronization accuracy between the PTP Master and Slave in

the presence of traffic congestion.

Fig. 4.1 – Time Interval without the SPA

-1000

-500

0

500

1000

1500

1 2 3 4 5 6 7 8 9 1011121314151617181920

Tim

e in

us

N, samples

no congestion

20%

40%

60%

80%


According to the Fig. 4.1, the actual value of TIE is directly related to the magnitude of the

introduced background traffic. This implies that the MS and SM delay asymmetry have

caused the PTP Slave to calculate erroneous offset. In addition, the Fig.4.1 shows more

positive spikes, which can be attributed to the fact that the MS path fell under congestion.

The second part of the experiment dealt with the evaluation of the performance of the SPA

under different level of background traffic congestion and PTP message rate. Below is the

experiment output results in Table and graphical format. The Table 4.1 represents the

proposed algorithm performance derived from testing the three levels of PTP rate.

Table 4.1 – The SAP with various PTP message rate and congestion Congestion level № of Sent packets № of Accepted packets PTP Rate Accepted/Sent*100%

20% 323 125 16 38.70%

20% 608 271 32 44.57%

20% 1208 68 64 5.63%

40% 320 36 16 11.25%

40% 605 196 32 32.40%

40% 1228 56 64 4.56%

60% 308 29 16 9.42%

60% 622 168 32 27.01%

60% 1106 31 64 2.80%

80% 309 23 16 7.44%

80% 620 112 32 18.00%

80% 1039 29 64 2.79%

Fig. 4.2 – Relationship between PTP message rate and network congestion

0.00%

20.00%

40.00%

60.00%

80.00%

10% 20% 40% 60% 80%

Pac

ket

s ac

cep

ted

/se

nt

rati

o i

n %

Congestion Level in %

Packet Rate 64 Packet Rate 32 Packet Rate 16


From fig. 4.2, we were able to see what particular PTP message rate produced the most

favorable result. Based on the above graph, it was clearly evident that PTP rate 32 packets/

sec appeared to be the most optimal choice regardless of the introduced network congestion

level.

Finally, there is a concluding graph expressing TIE for the case of PTP message rate 32

packets /sec in the form of root mean square (rms). In particular, the graph depicts two plots

helping us to draw a parallel between two experiments in terms of the generated TIE. Based

on Fig.4.3, the proposed algorithm demonstrates the expected ability to cope with large PDV

by filtering out only good packets. As an example, the graph clearly shows that the

application of the SPA algorithm reduces TIE between PTP Master and Slave from 573 to 62

us in the presence of 80% background traffic.

Fig.4.4 – Time Interval Error in the form of rms

V. Discussion

As described in the methodology section, three experiments were conducted to assess the

performance and viability of the proposed PTP time synchronization algorithm. The goal of

the experiments was to identify the extent to which the SPA is capable of minimizing the

0

100

200

300

400

500

600

700

0 20 40 60 80

Tim

e in

us

Network congestion in %

TIE without SPA

TIE with SPA


actual TIE under varying levels of network congestion. As mentioned earlier, the

predominant number of mobile technologies has strict requirements for time-synchronization

of their base stations, which ranges from 1 to 10 us. According to the last graph, it can be

easily seen that the method of increasing PTP message rate in combination with setting the

boundary condition has a potential to significantly reduce TIE. Based on the Fig. 4.2, PTP

message rate of 32 packets /sec provides the highest performance. However, it has been

determined that the increase of PTP message rate does not always results in receiving more

symmetric delay packets, which is seen in the case of PTP message rate of 64 packets /sec.

Referring to the Table 4.1, it has been found that the probability of finding symmetric

packets, p(A), linearly depends on the magnitude of running background traffic. In particular,

p(A) with 20% congestion is determined to be 0.47, while for 80% background traffic p(A) is

estimated to be 0.18. However, no plot presented in the Fig.4.3 satisfies the requirement of

either backhaul synchronization standards. Researchers have identified that the major source

of error is associated with software-based time-stamping. Obtaining time-stamps from the

application layer involves large fluctuation of its values because of multiple running internal

processes. As a result, the study could not identify the ultimate potential of the SPA in the

context of backhaul network synchronization requirements.

VI. Conclusion and Area for Further Research

The Precision Time Protocol has established itself as a de-facto standard in the deployment

of next-generation wireless backhaul networks. While the IEEE 1588 standard is

characterized to provide sub-microsecond/nanosecond precision with the master-slave

architecture, the actual accuracy is heavily dependent on a Packet Delay Variation value

sourced from the queuing delay. The research paper proposed a new PDV filtration

algorithm, which is based on the utilization of extended features of the IEEE 1588v2

standard. The experiments conducted in the research aimed at collecting Time Interval Error


in the presence of varying levels of network congestion. A careful analysis of gathered results

has proved the viability of the proposed algorithm to cope with occasional heavy traffic

bursts. In particular, the proposed algorithm performance achieves the most desirable results

with PTP message rate 32 packets /sec. Finally, it has been found that the true performance of

the algorithm heavily relies on timestamp uncertainties generated by software-based time-

stamping. Therefore, further work is needed to answer the research question by testing the

proposed algorithm with PTP Master and Slave devices capable of hardware time-stamping.


VII. References

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Available: http://www.silabs.com/Support%20Documents/TechnicalDocs/AN420.pdf

[3] Chimento, P., and C. Demichelis, "IP Packet Delay Variation Metric for IP

Performance Metrics (IPPM)," RFC3396, November, 2002.

[4] “Synchronization and precise timing in packet networks,” Transpacket, June 2011.

Available: http://www.transpacket.com/wcontent/uploads/2012/06/white_paper_Synch

_100612.pdf

[5] “A Formula for Deploying IEEE 1588v2 and Synchronous Ethernet: Investigate – Test

– Deploy,” The Metro Ethernet Forum, January 2012. Available:http://metroethernet

forum.org/Assets/White_Papers/Packet_Synchronization_over_Carrier_Ethernet_Netw

orks_for_MBH_2012021.pdf

[6] J. Hoyos, J. Khanin, S. Pujani, S. Hinck, “Effects of Network Asymmetry and Traffic

on IEEE std. 1588 Precision Time Protocol (PTP) Transmitted over Wide Area

Networks for Power System Differential Protection Applications,” Capstone Paper.

University of Colorado at Boulder, CO, 2013

[7] T. Murakami, Y. Horiuchi, “Improvement of synchronization accuracy in IEEE 1588

using a queuing estimation method,” in Proc. International IEEE Symposium on

Precision Clock Synchronization for Measurement, Control and Communication,

Brescia: Italy, Oct. 2009

[8] S. Lv, Y. Lu, and Y. Ji, “An Enhanced IEEE 1588 Time Synchronization for

Asymmetric Communication Link in Packet Transport Network,” IEEE Comm. Letters,

Vol. 14, No.8, pp. 764-766, August, 2010.

[9] M. Anyaegbu, C. Wang, “A Sample-Mode Packet Delay Variation Filter for IEEE 1588

Synchronization,” International Conference on ITS Telecommunications, Nov. 2012

[10] “IEEE 1588 Synchronization over Standard Networks Using the DP83640,” Texas

Instruments Inc., April 2013. Available: http://www.ti.com/lit/an/snla116a/snla116a.pdf

[11] J. Rothweiler, “Linux system clock and hardware clock accuracy?,” April, 2013.

[Online]. Available: http://blog.sensicomm.com/2013/04/linux-system-clock-and-

hardware-clock.html

[12] Institute of Electrics and Electronics Engineers, “IEEE 1588 Version 2,” Institute of

Electrics and Electronics Engineers, IEEE 802, 2008. [Online]. Available:

http://www.ieee 802.org/1/files/public/docs2008/as-garner-1588v2-summary-0908.pdf.

[Accessed: April. 10, 2014]