［ Systems・Components ］

High-Precision TimeSynchronization Network Switch
Keywords

Network switch, High-precision time synchronization, Protective relay, PTP

Abstract

26

Yasushi Tateishi,
Ryo Koshiya,
Masaaki Tajima

In power generation and industrial systems, high-precision time synchronization is carried out through a network so that various equipment and units canbe synchronized. If an ordinary network switch is applied to the transmission
path, however, a rear-end collision delay of a packet may be caused and thisdelay can greatly affect time synchronization precision. Such a delay is caused
by a difference in packet transfer time due to the difference in packet size.
Our industrial gigabit Layer 2 network switch, MEISWAY SW600F, has applied the IEEE 1588v2 (PTP※1) technology. Without affecting the variation intransfer time of the network switch, high-precision time synchronization can beassured. In addition, there is a function to stabilize the network management
even in a system where the network data load is relatively high.

1 Preface

2 Necessity for PTP

Protective relays are required to measure current waveforms simultaneously among multiple
substations. When timing control is used on a typical Ethernet line, the timing shifts due to the storeand-forward※2 effect of the switching hub. For thisreason, a conventional protective relay system
adopts dedicated equipment and a dedicated line
by using a cut-through※3 method.
In a protective relay system where a general
Ethernet line is applied, a method of bandwidth control may be adopted in order to relieve the effect ofStore-and-Forward.
In order to eliminate the effect of Store-andForward, it is adequate to adopt the high-precisiontime synchronization conforming to IEEE 1588v2(“Precision Time Protocol (PTP)” hereafter). Since
the waveform data of protective relays is transmitted
in a large volume and in a short period of time, thetransmission environment tends to be rigorouscompared with an ordinary monitor control system.
To resolve these issues, we developed MEISWAY
SW600F (“SW600F” hereafter) as a gigabit Ethernet
network switch for protective relays. This paper
introduces the major features of SW600F applied toprotective relays.

2.1 Network Transmission Delay

MEIDEN REVIEW Series No.181 2021 No.1

When the reference clock※4 delivers the time,a machine on receiving side receives the time slightly
in the past due to a delay. For this reason, it is necessary for the receiving side to adjust the time bycompensating the transmission delay. If the time oftransmission delay is unclear, half of the time forboth-way transmission of a packet is deﬁned as thetransmission delay. Transmit and receive transmission times must be the same for transmission paths
other than network switches. If there is a difference,timing may shift as a result of the wrong compensation and will affect the protective relay operation.
The transmission delay time contains a communication line delay and a transfer processing
delay by a network switch. In the case of an opticalﬁber line, the delay is approximately 70% of the light
velocity for both sending and receiving, and there is
no remarkable difference in the same routing. The
transfer time of a network switch differs according to
the packet size, but there is no difference for bothsending and receiving if the same packet size isused. Fig. 1 shows the comparison of delays dueto the packet size. The larger the packet size, theslower the transfer time.

Networkswitch

Networkswitch

AReceiverunit

Network switch
ASincepacket
size isshort,
it is sentout faster.

BCBCDDFastETime

Waiting time

FTime

E

Networkswitch

Networkswitch

Receiverunit

Transfer time ofnetwork switch

Fig. 2FHTime

Rear-End Collision and Delay by a Small Packet

Packet position on network
1518 byte

Fig. 1

NetworkSince the transfer rate of a large packet is low in the network
switch, a small packet comes in rear-end collision with the large
packet. After, a small packet waits for the completion of large
packet transmission every time, and delay time is extended.

G

Transfer time ofnetwork switch

Slow64 byte

Comparison of Delays Due to Packet Size

A process is shown when a small packet and a large packet passthrough two network switches and are then received. Since thenetwork switch sends out a packet, after it has been received, alarger packet requires a longer transfer time and a delay iscaused.

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2.2 Effect of Store-and-Forward
The reason the transfer time of a network
switch is changed by the packet size results fromthe effect of Store-and-Forward. When the network
switch begins to receive a packet, this packet isstored in the inner memory and the transmission isstarted after all of the packet is received. Accordingly,a larger packet size requires a longer time for retention in the inner memory, thus extending the transfertime.
When a large-sized packet is received, the network switch begins to transfer the packet fromanother port. When another small-sized packet isreceived thereafter, packet transfer is suspended
because the larger packet is in the middle of transmission. Since a smaller packet needs a shorter
processing time, there can be a phenomenon ofrear-end collision while a larger packet is processed.
Fig. 2 shows rear-end collision and delay by a smallpacket.
Such a time difference caused by rear-end collision is substantial. For example, when a 1518
byte※5 packet is sent at a transmission rate of
1 Gbps and another packet of 64 byte is transmitted
in succession, a packet-to-packet distance becomesshorter by approximately 12 μs each time a packet
passes through a network switch. There is a delay9TimeFig. 3

Delay Caused by Transmission Load andNumber of Network Switches

If there is a transfer load, a packet with a small time synchronization comes in rear-end collision with a large one, and an error isgenerated in the time synchronization. In the case of transmission timing in Fig. 3, a rear-end collision occurs at the 9th packet
and a delay is caused at the 10th packet.

of 12 μs after the occurrence of rear-end collisionaccordingly.
Fig. 3 shows a delay caused by a transmission
load and the number of network switches. In Fig. 3,a rear-end collision and delay is shown caused
when a packet of 64 byte is transmitted under thecondition of a 10% load at 1518 byte and 1 Gbps
respectively. The distance between packets is longand there is ample allowance in the transmission
load. Under such a condition, however, there can bea rear-end collision and delay even though transmission is forwarded at a timing of 64 byte packet.
In this case, the delay time amounts to 6 to 121μs
and the time difference is half that at 3 to 61μs.
In order to reduce such an effect of Store-andForward, it is necessary to regulate the packet size,transmission load, and number of network switches.
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3 Basic Speciﬁcations and Featuresof SW600F
Table 1 shows the basic speciﬁcations ofSW600F. Table 2 shows the environmental conditions. Table 3 shows the board conﬁguration.
Table 1 Basic Specifications of SW600F
Time synchronization conforms to IEEE 1588v2, and functions ofbandwidth control and access list are provided.
Item

When the PTP is applied, the transfer time in anetwork switch is measured and adjusted. Becauseof this adjustment, time synchronization is neverinﬂuenced even though a transmission delay should
be caused by a packet rear-end collision.
For time synchronization, there are two methods of adjustment: a Point-to-Point (PtoP) adjustment only for two neighboring sections, and an Endto-End (EtoE) adjustment performed by an apparatus installed in the center of the reference clock
and terminal unit. In SW600F, a method of EtoE

Switching mode

Store-and-Forward

Switch capacity

13.7 Gbps
2.66 Gbps (For full duplex/all-port 100 Mbps)

Transmission system

Full duplex/half duplex

Table 2 Environmental ConditionsPortconstruction(18 portsMax.)16 ports (4 ports are used for
combo-board with optical ports)

The environmental conditions conform to the Standard of PowerElectric Utilities (on Relay) B-402.

10/100/1000BASE-T

Optical 1000BASE- Connector used: LCports
LX
Applicable to Single Mode
(4 ports
Fiber (SMF )Max.)
Applicable to 10 km and long
distance (40 km)

Error output terminal

1 contact point

Media Access Control (MAC)
address capacity

8000 pcs.

Flow control

IEEE 802.3x (full duplex)・
Back pressure (half duplex)

Spanning Support protocol
treeprotocolNetwork
configuration

Improved spanning tree
protocol (RTP)

Processing time
for spanning tree
protocolLoop configuration by opticalport
RTP: within 0.5 s for 32 nodes)

VLAN ( Virtual Local AreaNetwork)

Tag and port base VLAN
conforming to IEEE 802.1Q

Time synchronization

IEEE 1588v2 conformance(EtoE transparency※6)
External pulse output, signal
output in correct synchronization

Priority control, bandwidthcontrol, and access list

Error packet filtering function

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Specifications

3.1 Time Synchronization by SW600F
3.1.1 PTPClassification of packets
according to IP and port
numbers
Priority transmission of specified packets, bandwidthcontrol, and destruction areenabled.
Short packet, long packet,
Frame Check Sequence (FCS),
error packet, symbol errorpacket

Network control

SNMPv1 (conforming toRFC1157)・MIBⅡ

Network management

telnet・http・ICMP (InternetControl Message Protocol)・IP

Storm control

Broadcast, multicast, andaddress unlearned packet
discarding are enabled.

Serial console

By round connector conforming to EIA/TIA-232-E

Setup save, writing, firmware
updating

Via Web (http) or serial port

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ItemSpecifications

TypeUT227/＊＊＊A (＊defined by type)

Power source connector3P terminal board (DC)Power source voltage range DC80 V〜143 V
Power consumption(in operation)

25 W or below

Operating temperature
range (in operation)−20〜55℃

Withstandvoltage

Power source
primary – FGor SG

AC2000 V for 1 min

Insulation
resistancePower source
primary – FGor SG

DC500 V 5 MΩ or above

Power source noise durability

Rectangular impulse noise 2 kV,
50 ns/1 μs

Mass

Approx. 4 kgExternal dimensions

W255 × H88 × D250 mm(excluding embossed parts)

Applicable standard

Standard of Power Electric Utilities(on Relay) B-402 (Except for inrush
currents)

Table 3 Board Configuration
16 ports of 10/100/1000BASE-T and 2 or 4 optical ports (1000BASELX) are installed.
Port configuration classified by typeType

10/100/1000BASE-T

Optical port (1000BASE-LX )TYPE 1016

2 (single mode 10 km) LC connectors,
1 GbpsTYPE 1116※1

4 (single mode 10 km) LC connectors,
1 GbpsTYPE 1216

2 (single mode 40 km) LC connectors,
1 GbpsTYPE 1316※1

4 (single mode 40 km) LC connectors,
1 GbpsNote. ※1. Since 2 ports out of 16 ports are used as a combo-board with
optical ports, they cannot be used at the same time.

adjustment is adopted. Assuming that a 1518 byte
packet is a 100% transmission load, the resultant
time difference is not more than 1μs when passage
is attempted through ten SW600F units.

3.1.2 External Output of Synchronization
Timing
SW600F has a function of outputting a synchronized clock signal at a frequency set at 1 Hz,50 Hz, 60 Hz, or more. Although SW600F is designedon the assumption that it will be applied to protective relay systems, it can also be synchronized
remotely if its external clock signals are connected to any other measuring apparatus or variousdevices.

3.1.3 Reference Clock
Each remote unit (SW600F) can make timesynchronization in relation to the reference clock.
This reference clock is an expensive machine generally operated on absolute time of the Coordinated
Universal Time (CTU) or Japan Standard Time
(JST). In the case of the protective relay systems, it
is unnecessary to be synchronized with such astandard time, provided that synchronization of unit
to unit timing is secured. The SW600F itself canwork as a simpliﬁed reference clock. Even when thereference clock is lost because of a failure, other
SW600F units can work as a reference clock to succeed in the function.

4 Priority Control, Bandwidth Control,and Access List
For a network of a general monitoring control
system, it is necessary to assuredly transmit all
information to the destination. Therefore, the application transmits slowly to prevent the packets frombeing discarded due to lack of internal memory inthe hub because of instantaneous transmission
load. Alternatively, a software delivery conﬁrmation
and retransmission protocol needs to be used.
Waveform data, however, of protective relays
are transmitted in a short period and in a large volume and the transmission load is, therefore, highwhen compared with general monitoring control
systems. Transmission check and resending bysoftware are difﬁcult to accomplish in terms of time.
Consequently, there is a substantial hindrance inprotective functions if a packet entered exceeds thecapacity of memory processing in the network
switch. Depletion of inner memory will be caused in
the network switch for the following reasons:( 1) Difference in I/O transmission rate
For a transfer from a 1 Gbps port to a100 Mbps port, the inner memory is used for 10 times
the capability.
( 2 ) No. of I/O ports
When inputs are entered simultaneously frommultiple ports and they are transferred convergently
to a single port, the inner memory retention time
becomes a total of the respective packet sizes.
( 3 ) Transmission queues due to half duplex andﬂow control
For the setting of transmission conditions,half-duplex communication or ﬂow control is carriedout. When this setting is made, transmission queues
are caused. During this period, a received packet isretained in the inner memory.
For protective relays, problems can arise dueto the number of ports. Since the inner memory has
a small capacity compared with the transmissionrate, a depletion of the inner memory may be caused
by a momentary high burden. The transfer load isexpressed by the amount of data transmitted per
second, but the depletion of the inner memory must
be addressed for a very short time. Since the waveform data of a protective relay is transmitted periodically, the transfer load is kept constant. In time synchronization, however, transmission timing may beconcentrated. For other types of packets, it is also
necessary to consider preventing a momentary
concentration of loads.
SW600F is provided with three functions
described below. By virtue of these functions, thesystem can be stabilized without disturbing thefunction of each protective relay.

4.1 Priority Control
The packets to be transmitted are classiﬁed
into a maximum of eight types. This function isintended to transmit a package that has the highest
priority. When time-synchronized packets, waveform data, or such signiﬁcant packets are preset tohave a higher priority, loss and delay of importantdata can be prevented.

4.2 Bandwidth Control
This function is used to make bandwidth allocation for transmission lines according to the type ofa packet. Any packet exceeding the speciﬁed bandwidth will be destroyed. Even normal packets likewaveform data are also destroyed if they are transmitted in a large volume due to any malfunction.
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Since transmission in conjunction with Address
Resolution Protocol (ARP) is also limited, the ARPload can be reduced for other equipment anddevices.

4.3 Access List
This function is used to transfer only the packets that meet the conditions set in the accesslist. Unexpected or false communication can berestricted.

5 Postscript
According to the PTP, SW600F can make highprecision time synchronization by removing theeffect of delay due to Store-and Forward. By virtueof the functions of priority control, bandwidth control, and access list, the network stability can be
maintained without disturbing the functions of protective relays.
For ordinary monitoring and control systems,
the functions of priority control, bandwidth control,
and access list are also available when sending alarge volume of data relating to videos, audios,30

MEIDEN REVIEW Series No.181 2021 No.1

Internet of Things (IoT), and Artiﬁcial Intelligence(AI). In addition, the PTP can be extended to othersystems where timing control is carried out.
In the future, we will improve its performancesof redundancy, stability, and maintainability to better
develop our products.
・Ethernet is the registered trademark of Fuji Xerox Co., Ltd.
・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.
Notes※1. Precision Time Protocol (PTP): A communication protocol forhigh precision time synchronization.
※2. Store-and-Forward: A method of communication after a packet
has been captured in an inner memory. The delay time is variable
and a broken packet can be discarded.
※3. Cur-through: A method of transmission in a “bit” unit. The delay
time is fixed (several bits) and a broken packet cannot be discarded.
※4. Reference clock: A clock to distribute a correct time. InIEEE1588, it is called Grandmaster clock.
※5. 1518 byte: Maximum packet size for the Ethernet (Generally, it
is 1518 byte, but it can be modified by various types of setting.)※6. EtoE transparency: A function to make time synchronization
between the last devices. (PtoP transparency is a function to maketime synchronization between the neighboring devices.)