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PAPER

Special Section on the Next Generation Ethernet Technologies

Hardware-Based Precise Time Synchronization on Gb/s Ethernet
Enhanced with Preemptive Priority
Yoshiaki YAMADA†a) , Satoru OHTA†b) , and Hitoshi UEMATSU†c) , Members

SUMMARY
Time synchronization is indispensable for wide area distributed systems including sensor networks, automation systems, and measurement/control systems. Another application is clock distribution, whichis indispensable to support continuous information transfer. Because of theincreasing demand for more sophisticated applications, it is essential to establish a time synchronization technique that oﬀers higher accuracy andreliability. Particularly, the accuracy of time synchronization for Ethernet
must be enhanced since Ethernet is becoming more important in telecommunication networks. This paper investigates a precise time synchronization technique that supports Gb/s Ethernet. To obtain accurate time synchronization, delay variation in message transfer and processing must be
minimized. For this purpose, the paper first describes the implementationof preemptive priority queuing, which decreases the message delay variation of Ethernet. Through experiments, it is shown that preemptive priority
queuing eﬀectively achieves very low delay variation. The paper then proposes a method to synchronize the time signal of a slave node to that of themaster node. The proposed time synchronization method is performed in
the lower protocol layer and implemented on FPGA-based hardware. The
method achieves superior time accuracy through the low message transfer/processing delay variation provided by preemptive priority, lower layer
execution, and hardware implementation. The eﬀectiveness of the method
is confirmed through experiments. The experiments show that the time
variation achieved by the method is smaller than 0.1 µsec. This performance is better than those obtained by existing synchronization methods.
key words: Ethernet, synchronization, delay variation, priority, QoS

1.

Introduction

Time synchronization [1] is indispensable for wide area distributed systems including sensor networks [2], automation
systems [3], [4] and measurement/control systems [5], [6].
Another application is clock distribution, which is indispensable to support continuous information transfer. For
clock distribution, the employment of time synchronizationis advantageous because it greatly improves the phase accuracy of the clock signal [7]. Because of the increasing demand for more sophisticated applications, it is essential toestablish a time synchronization technique that oﬀers higher
accuracy and reliability.
Of particular interest is time synchronization for Ethernet. This is because Ethernet is the most popular datalink protocol and covers applications from LANs (Local
Area Networks) to WANs (Wide Area Networks). Namely,
Ethernet time synchronization would enhance various typesof networks. Additionally, because of the popularity ofManuscript received July 11, 2005.
The authors are with the NTT Network Innovation Laboratories, NTT Corporation, Yokosuka-shi, 239-0847 Japan.
a) E-mail: yamada.yoshiaki@lab.ntt.co.jpb) E-mail: ohta.satoru@lab.ntt.co.jpc) E-mail: uematsu.hitoshi@lab.ntt.co.jp
DOI: 10.1093/ietcom/e89–b.3.683†

Ethernet, its components are inexpensive. Thus, Ethernet
time synchronization would also be beneficial from the cost
viewpoint.
Time can be synchronized on Ethernet by employing the NTP (Network Time Protocol [1], [8]) or its subset
SNTP (Simple Network Time Protocol [9]), both of which
run on IP (Internet Protocol). The drawback of NTP and
SNTP is accuracy. The reported time accuracy of NTP is
1–50 msec [8], [12], [13] and this is insuﬃcient for some applications. The accuracy of (S)NTP can be greatly improvedby executing time-stamping in hardware. It was reported
that this approach yields the accuracy of several tens of microseconds [3]. One alternative, IEEE 1588 [5], was createdfor measurement and control on small networks. An implementation of IEEE 1588 achieves the accuracy of several
hundred nanoseconds [6]. Another method is GPS (Ground
Position System). However, GPS is not suited to be installedon many nodes in a network because of its antenna placement limitation.
The time accuracy possible with standard Ethernet
is hampered by the tradition of implementing most functions as software processes. For example, message transfer/processing delay is often varied by the software implementation of packet multiplexing. Reports on precise timesynchronization [3], [6] rely on the standard Ethernet andthe processing is mostly executed in software. Adequate
accuracy can only be achieved by modifying the Ethernet
protocol to decrease message delay variation implementing
more functions on hardware.
This paper investigates a more accurate time synchronization technique for Gb/s Ethernet. Its application area includes clock signal distribution for synchronous networks.
For this purpose, existing methods are not suﬃciently accurate and thus they fail to well handle jitter and wander(See [10] and [11] for jitter and wander requirements). To
obtain higher timing precision, this paper first shows an implementation of preemptive priority queuing, which minimizes the message transfer delay variation. It then proposes a time synchronization method, which is executed in
the lower protocol layer and implemented on FPGA (Field
Programmable Gate Array) hardware. The proposed synchronization method achieves superior accuracy due to thesmaller message delay variation provided by the preemptive priority technique. The lower layer execution and the
hardware-based implementation further suppress the inaccuracy induced by processing delay variation. Its eﬀectiveness is confirmed by an experiment.

c 2006 The Institute of Electronics, Information and Communication Engineers
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2.

Related Works

Currently, the most popular time synchronization method is
to employ NTP (or SNTP). Since NTP (or SNTP) works
on the IP protocol stack, it is easily adapted to Ethernet.
However, its accuracy in terms of synchronization is notgood enough. With NTP, the time of the client host is synchronized to that of the server by adjusting the client clock
so as to cancel the time diﬀerence between the client andthe server. The time diﬀerence is measured by exchanging messages between the host and the server. In this process, the measured time diﬀerence is disturbed by variationin the message transfer delay. This disturbance degrades thetime accuracy. Delay variation is unavoidable because timing messages collided with other messages and are queued
in the buﬀers of switches or routers in the network. Moreover, NTP is an application layer protocol, and is usually
implemented as software. Thus, collision among application processes also triggers delay variation and further degrades time accuracy. Thus, it is unrealistic to expect thatNTP can achieve highly accurate synchronization.
It has been reported that the time synchronized by NTP
exhibits deviations of 28.7 msec for a WAN [12] and 1 msec
for LANs [13]. This performance is not suﬃcient for some
applications.
More precise time synchronization methods for Ethernet were reported in [3], [4]. These methods, which basically employ SNTP, successfully improve time accuracy
by the hardware-based timestamp approach. It was reported that they can achieve accuracy ranging from 25 µsec
to 1 µsec. However, since these methods rely on software
processing for the most part, other than timestamp handling,
they can not avoid the accuracy degradation caused by conflicts among software processes. Actually, the measured accuracy achieved varies with the load on the server [3].
One time synchronization scheme designed for measurement and control was standardized as IEEE 1588 [5].
The protocol specified in IEEE 1588 supports precise timesynchronization by measuring the time of message departure on the physical media. It was reported that the meanvalue of time variation achieved by an IEEE 1588 implementation was 49 nsec and the standard deviation was
233 nsec [6]. The drawback of IEEE 1588 is that its accuracy is not guaranteed if the messages pass through routers.
Thus, the method should be used only for synchronization
within a subnet. It is possible to obtain better performance
than IEEE 1588 by implementing time synchronization in
the lower protocol layer.
For precise time distribution, GPS is also an eﬀective
technique. It oﬀers a time accuracy of 10 nsec [14]. Unfortunately, it requires that a antenna sited so as to always havea clear view of one or more of the GPS satellites. Because ofthis limitation on antenna placement, it is diﬃcult to employ
GPS-based time distribution in every node in a network.

3.

Preemptive Priority QueuingThis study employs preemptive priority queuing to minimize the delay and delay variation. Preemptive and nonpreemptive are the two basic approaches to priority queuing[15]. With preemptive priority queuing, if the system receives a high-priority frame while processing a low-priority
frame, it stops processing the latter and begins processing
the former. The system resumes processing the low priority frame after the high-priority frame is processed. In
this scheme, high-priority frames are processed immediately
upon arrival so they experience smaller delay and delay variation.
The basic approach of Ethernet is nonpreemptive queuing. To implement preemptive priority queuing, the receivermust identify and extract high-priority Ethernet frames aswell as reconstructing the low priority frame. Reference[16] reports an implementation of such functions. With themethod of [16], the sender inserts special codes before andafter each high priority frame. The special codes are bit sequences that are unused by the standard Ethernet protocol.
The receiver uses the sequences to identify the position of
the high priority frames and extract them. This process is
illustrated in Fig. 1.
This study used a commercial Gb/s Ethernet multiplexer [17] to implement preemptive priority queuing in amanner similar to that used in [16]. While Ref. [16] placed
the preemptive priority mechanism in the PHY layer, thisstudy placed it between the MAC and PHY layers as shownin Fig. 2. We implemented the priority queuing mechanism
using FPGAs. Note that we did not modify the PHY layer.
Thus, generic and inexpensive PHY layer components (i.e.
ICs) can be employed. This functional arrangement is expected to be more economical than that of Ref. [16]. In addition, this study applies cut-through [18] to high-priority
traﬃc to decrease the delay further.
Our implemented version of preemptive priority queuing was evaluated by an experiment using the configuration shown in Fig. 3. As the figure shows, two multiplexers with preemptive priority queues were connected by Gb/s

Fig. 1 Preemptive priority control executed at (a) the sender and (b) thereceiver, connected by Ethernet.

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Fig. 2 Layer diagram for the implementation of preemptive priority
queuing.

Fig. 3ing.

Measurement configuration to evaluate preemptive priority queu-

Ethernet. High priority traﬃc was transmitted from one
multiplexer to the other and the average delay of Ethernet
frames was measured. Simultaneously, low priority frames
were also transferred as background traﬃc. The priority
was indicated by the priority tag specified in IEEE 802.1p.
Namely, high priority traﬃc was tagged QP4, while low priority background traﬃc was tagged QP1. The frame lengthof the measured traﬃc was 1518 bytes, and its inter-arrival
time was constant. Meanwhile, the inter-arrival time of the
background traﬃc was also constant, but its frame length
was random. The bit rate of the background traﬃc was set at
20%, 50%, and 80% of the wire speed, 1 Gb/s. For comparison, the same measurements were executed using commercial switches (Summit 1i, Extreme Networks [19]) in placeof the multiplexers. The switches use non-preemptive priority queuing. SmartBits (SPIRENT [20]) was used for traﬃcgeneration and delay measurements.
Figure 4 shows the results. In Fig. 4, the x-axis is thebit rate of the measured traﬃc, while the y-axis is the average delay. The bit rate is shown as a percentage of the wire
speed. The figure shows that the preemptive priority traffic had delay of less than 10 µsec while the non-preemptive
priority architecture suﬀered delay of more than 30 µsec.
Moreover, this increased to 50–60 µsec when the total bit
rate of the measured and background traﬃc exceeded 100%

Fig. 4 Delay characteristics of preemptive priority queuing and conventional queuing (no preemption).

of the wire speed. By contrast, the delay of the preemptive
priority traﬃc was almost regardless of the traﬃc load. Thatis, the performance of the high priority traﬃc was independent of the background traﬃc. This characteristic suggeststhat preemptive priority queuing prevents the high priority
frames from contending with the low priority frames fortransmission resources. Since the delay variation is broughtby frame contention, it is expected that preemptive queuingyields very low delay variation for high priority frames. Byutilizing this characteristic, the accuracy of time synchronization can be greatly improved.
Figure 4 also suggests that non-preemptive priority
produces substantial time oﬀsets if the traﬃc is strongly
asymmetric. Suppose that heavy background traﬃc runs
in one direction. The delay in that direction can become
20 µsec larger than that in the reverse direction. This is a serious problem since popular synchronization methods (NTPor IEEE 1588) assume that the delay is the same in both directions. These methods allow the delay diﬀerence to createtime oﬀset. Preemptive priority, by contrast, holds the delay
the same in both direction because delay does not depend on
the background load. This characteristic eﬀectively avoids
the time oﬀsets brought by delay diﬀerence between the directions.
4.

Time Synchronization

This section proposes a highly accurate time synchronization method. The proposed method employs relatively simple logic that is easily implemented on hardware; it avoids
the processing delay variation and performance bottleneck
inherent in the software implementations used by the conventional methods. Another feature is that it works on alower protocol layer. This feature also eliminates the delay
variation caused by the upper layer protocol processes andleads to better accuracy.
The proposed method was realized on two nodes connected by Gb/s Ethernet with preemptive priority queues as

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Fig. 5 Basic configuration for the proposed time synchronization
method.

and the master node. Note that d does not depend on the
direction because media length is the same in both directions
and the queuing delay is basically independent of direction.
The master and slave nodes transmit synchronization
frames when their local clocks indicate t0 , t1 , t2 , . . .. If theslave node sends a synchronization frame at time ti on theslave clock, its departure time is ti +∆ on the master clock. If
this frame arrives the master node at time ti,r,M on the masterclock, then,ti,r,M = ti + d + ∆.

(1)

Similarly, if the master node sends a synchronization
frame at ti , its departure time is ti − ∆ on the slave clock.
If this frame arrives the slave node at time ti,r,S on the slave
clock, then,ti,r,S = ti + d − ∆.
Fig. 6Structure of time synchronization frame.

shown in Fig. 5. In the following, the system shown in Fig. 5
is referred to the “basic configuration.” In the basic configuration, one node acts as the “master” while the other nodeacts as the “slave.”
Since the proposed method performs in the lower protocol layer (below the MAC layer), it does not work between
two nodes connected via routers. Nevertheless, the method
can be applied to a network with more than two nodes.
Namely, in such a network, we can discover a tree, whoseroot is the time source. The basic configuration is then applied to each edge of the tree such that the node closer to theroot is the master. The node clocks are then synchronized tothe clock of the root following the paths from the root to theleaves.
The proposed method synchronizes the slave clock to
the master clock by periodically exchanging synchronization frames. As shown in Fig. 6, the synchronization frame
has three fields. The first field is the identifier. This field indicates that the frame type is “synchronization frame.” Thesecond field carries a timestamp that shows its departure
time. The third field is the time diﬀerence between frame
departure and frame receipt. Each synchronization frame istransmitted as high priority traﬃc using preemptive priority queuing and thus suﬀers very slight delay variation. By
exchanging synchronization frames, the slave node can findthe time oﬀset between its local (slave) clock and the master clock. Canceling the time oﬀset synchronizes the local
clock to the master clock.
4.1 Time Oﬀset Evaluation
In the basic configuration, the slave node needs to find thetime oﬀset between its local clock and the master clock. The
time oﬀset is computed as follows.
Let ∆ denote the time oﬀset. If the time indicated on
the slave clock is t, the time indicated on the master clock is
t + ∆; d denotes the one-way delay between the slave node(2)

Each node computes the time diﬀerence between thedeparture and receipt of the just received synchronization
frame. For the frames sent at time ti , let ui,M denote thetime diﬀerence computed at the master node, and let ui,Sdenote that computed at the slave node. Then, the following
equations are obtained from Eqs. (1) and (2).
ui,M = d + ∆.
ui,S = d − ∆.

(3)
(4)

Time diﬀerences ui,M is written to the time diﬀerence field (Fig. 6) of the next synchronization frame, whichleaves the master node at time t j ( j > i). By receiving thisframe, the slave node can recognize values ui,M and ui,S . ∆and d are then computed at the slave node using the following equations.
∆ = ui,M − ui,S /2.
(5)d = ui,M + ui,S /2.
(6)Time is synchronized by advancing or retarding theslave clock to cancel ∆ derived by Eq. (5). Namely, theclock should be advanced for ∆ > 0 or retarded for ∆ < 0.
In the synchronization frame, the frame transmission time ti
should be indicated in the timestamp field (Fig. 6) to compute ∆ correctly even if ∆ is larger than the transmissioninterval ti+1 − ti . Figure 7 illustrates the above procedureof computing time oﬀset ∆. Although one-way delay d obtained by Eq. (6) is not used for synchronization, it will be
useful for network management.
The proposed method of evaluating time oﬀset uses
fewer parameters than NTP. This feature is obtained bytransmitting synchronization frames at set times, t0 , t1 , . . ..
The decrease in parameters is desirable because it simplifies
hardware implementation. Additionally, since fewer parameters means shorter message length, it is also advantageouswith regard to bandwidth consumption.
4.2 Time Signal and Its Control
The time synchronization mechanism requires an appropriate time signal that is controllable by time oﬀset. Such a

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Fig. 8 Time signal generated by the transmission of synchronization
frames.

Fig. 7 Principle of measuring time diﬀerence between the master andslave nodes.

signal is obtained in the following way.
The proposed method sets the interval between two
successive synchronization frames to a constant value, denoted by T . If time oﬀset is 0, the master and the slave
have identical synchronization frame departure times. Thismeans that the synchronization frame departure time is usable as the synchronized time signal.
The time signal defined above is easily controlled tocancel time oﬀset by employing two types of synchronization frames. The diﬀerence between these types is the bit
length. One frame type has a shorter bit length, denoted by
y1 bits, while the length of the other frame is denoted by y2(y1 < y2 ) bits. After a node sends a synchronization frame,
it waits for the time specified for sending x bits. It then sends
the next synchronization frame. During this process, if theshort synchronization frame is used, the frame interval decreases and thus the time signal is advanced. Conversely, if
the long frame is used, the time signal is retarded. Therefore, the time signal can be adjusted through frame type selection depending on the time oﬀset.
If the time oﬀset is 0 or if the node is the master, the twoframe types are transmitted alternately. The period length of
the time signal remains synchronized even if the bit rates
diﬀer between the slave and the master. If α is the ratio of
the short frames among the synchronization frames, the average interval between synchronization frames is expressed
as follows.
T = {x + αy1 + (1 − α)y2 } /r,(7)where r is the bit rate on the transmission media. The proposed control automatically sets value α such that the masterand the slave have the same T value.
In the experiment described in the next section, the parameters are x = 124800 bits, y1 = 192 bits, y2 = 208 bits
and r = 1 Gb/s. This setting yields T = 125 µsec. Figure 8
explains the control process of the time signal.

Fig. 9 Experimental network for the evaluation of the proposed time synchronization scheme.

5.

Experiment

The proposed synchronization method (Sect. 4) was implemented with preemptive priority queuing (Sect. 3) on an
FPGA. The synchronization method was placed also placed
between the MAC and PHY layers for preemptive priority queuing. This hardware implementation was used in
an experiment to confirm the eﬀectiveness of the proposed
method. The network used in the experiment consists of six
serially connected nodes as shown in Fig. 9. In this configuration, node #0 works as the time source. By applying
the basic master/slave configuration to the two directly connected nodes, the clocks of nodes #1, #2, . . ., #5 are synchronized to that of node #0. Time synchronization was examined by comparing the wave forms of the time signals onan oscilloscope.
Figure 10 shows the results gathered. The upper side of
the figure compares the time signals of nodes #0, #3 and #5.
The time signal period is 125 µsec. As shown in the figure,the signals of nodes #3 and #5 are completely synchronized
to the signal of node #0.
The lower side of Fig. 10 shows enlargements of the
wave forms on the time axis. From the figure, the time signals of nodes #3 and #5 vary slightly. The time variation,
however, is smaller than 0.1 µsec. Previous implementations of precise time synchronization on Ethernet allowed
the time variation to range from several tens of milliseconds[3] to several hundreds of microseconds [6]. Therefore, theproposed method achieves much better accuracy than the existing methods. This is due to the use of preemptive priority
and hardware processing, both of which greatly reduce message/processing delay variation.

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Fig. 10

Measurement results of the synchronized timing signal.

In the lower side of Fig. 10, no significant diﬀerence isseen between the time variations of nodes #3 and #5. Thisimplies that the proposed method achieves highly accurate
synchronization even if the basic configuration is concatenated many times in a large network. This characteristic issuitable to the clock signal distribution application, which
requires accuracy as well as scalability for a large sized network.
6.

Conclusion

This paper presented a precise time synchronization method
for Gb/s Ethernet. First, the paper described preemptive priority queuing as implemented on Ethernet. This queuing
scheme eﬀectively decreases the frame transfer delay andits variation, and so enhances time accuracy. Next, the paper proposed a time synchronization method that works on alower layer of Ethernet and is well suited to hardware implementation. The method utilizes preemptive priority queuing to obtain better time accuracy. The accuracy is further
improved by our hardware-based implementation. An experiment showed that the proposed synchronization method
achieves superior time accuracy over existing methods. In
addition, time was accurately synchronized even if the basic
master-slave configuration was repeatedly applied for several serially connected nodes. These results imply that the
proposed mechanism can provide precise clock signal overa large network successfully.
Acknowledgments
The authors would like to thank Dr. Masahiro Morikura forhis support and encouragement.
References[1] D. Mills, “Internet time synchronization: The network time protocol,” IEEE Trans. Commun., vol.39, no.10, pp.1482–1493, Oct.
1991.

[2] F. Sivrikaya and B. Yener, “Time synchronization in sensor networks: A survey,” IEEE Netw., vol.18, no.4, pp.45–50, July/Aug.
2004.
[3] T. Skeie, S. Johannessen, and Ø. Holmeide, “Highly accurate timesynchronization over switched Ethernet,” Proc. 8th IEEE Conference on Emerging Technologies and Factory Automation (EFTA’01), pp.195–204, Antibes, France, Oct. 2001.
[4] OnTime Networks, http://www.ontimenet.com/[5] IEEE Standard 1588, IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,
Nov. 2002.
[6] J. Eidson and K. Lee, “Sharing a common sense of time,” IEEE
Instrum. Meas. Mag., vol.6, no.1, pp.26–32, March 2003.
[7] A. Imaoka and M. Kihara, “Time signal distribution in communication networks based on Synchronous Digital Hierarchy,” IEEE
Trans. Commun., vol.45, no.2, pp.247–253, Feb. 1997.
[8] D. Mills, Network Time Protocol (Version 3) specification, implementation and analysis, IETF RFC1305, March 1992.
[9] D. Mills, Simple Network Time Protocol (SNTP) version 4 for IPv4,
IPv6 and OSI, IETF RFC2030, Oct. 1996.
[10] ITU-T Recommendation G.812, Timing Requirements of Slave
Clocks Suitable for Use as Node Clocks in Synchronization Networks, June 1998.
[11] ITU-T Recommendation G.824, The Control of Jitter and Wander
within Digital Networks Which Are Based on the 1544 kbit/s Hierarchy, March 2000.
[12] D. Mills, A. Thyagarjan, and B. Huﬀman, “Internet timekeeping
around the globe,” Proc. Precision Time and Time Interval (PTTI)
Applications and Planning Meeting, pp.365–371, Long Beach, CA,
Dec. 1997.
[13] D. Mills, “Precision synchronization of computer network clocks,”
ACM SIGCOM Comput. Commun. Rev., vol.24, no.2, pp.28–43,
April 1994.
[14] W. Lewandowski, G. Petit, and C. Thomas, “Precision and accuracyof GPS time transfer,” IEEE Trans. Instrum. Meas., vol.42, no.2,
pp.474–479, April 1993.
[15] L. Kleinrock, Queuing Systems—Vol.II: Computer Applications,John Wiley & Sons, 1976.
[16] S. Yanagimachi, T. Yoshikawa, T. Baba, and K. Fukuchi, “An architecture of CIAO (Cut-In-Any Occasion) which makes a virtualprivate channel in a PHY layer of the Ethernet,” IEICE Technical
Report, CS2003-105, Nov. 2003.
[17] NTT Electronics, http://www.nel.co.jp/product/optical/gigabee2.
html[18] P. Kermani and L. Kleinrock, “Virtual cut-through: A new computer communication switching technique,” Comput. Netw., vol.3,
pp.267–286, 1979.
[19] Extreme Networks, http://www.extremenetworks.com/[20] SPIRENT Communications, http://www.spirentcom.com/

Yoshiaki Yamada
received the B.E. and
M.E. degrees from the University of ElectroCommunications, Tokyo, Japan, in 1990 and
1992, respectively. Since 1992, he has beenwith NTT Corporation, working on the researchand development of optical switching and optical burst communications. He is currently asenior research engineer at NTT Network Innovation Laboratories, NTT Corporation. His
research interest includes the time synchronization technique and high-speed layer 2 protocols.
He is a member of the IEEE. He received the Young Researchers’ Award
from the IEICE in 1998.

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Satoru Ohta
received the B.E., M.E., andDr. Eng. degrees from the Tokyo Institute ofTechnology, Tokyo, Japan, in 1981, 1983, and1996, respectively. Since 1983, he has beenwith NTT, where he worked on the research anddevelopment of cross-connect systems, broadband ISDN, network management, and telecommunication network planning. He is currently
involved in research on network protocols andnetwork performance evaluation at NTT Network Innovation Laboratories, NTT Corporation. Dr. Ohta is a member of the IEEE. He received the Excellent Paper
Award from the IEICE in 1991.

Hitoshi Uematsu
was born in Hyogo, Japan,
on July 1, 1960. He received the B.S., M.S.
and Ph.D. degrees in communication engineering from Osaka University in 1983, 1985, and1993, respectively. He joined the NTT Laboratories, Yokosuka, Kanagawa, Japan, in 1985.
From 1985 to 1995, he was engaged in researchon ATM (asynchronous transfer mode) crossconnect switches. He also researched circuit
signal transfer systems and their clock transfer
techniques by the SRTS (synchronous residual
time stamp) method in AAL (ATM adaptation layer) type 1. From 1996 to
1998, he was engaged in the development of virtual path based ATM leased
line systems, named ”ATM Mega-Link service” and ATM-PON (passive
optical network) access systems. In 1999, he joined NTT Network Innovation Laboratories and he engaged in research on optical ADM transportsystems based on wavelength multiplexing techniques. Since 2002, he has
been engaged in Ethernet voice transfer techniques with priority control
and a digital RoF (radio on fiber) system. Dr. Uematsu is a member of theIEEE.