DetNet H. Wang
Internet Draft P. Wang
Interned status: Standards Track H. Yang
Expires: May 19, 2017 Chongqing University of
Posts and Telecommunications
November 15, 2016
Joint Real-Time Scheduling Methods for Deterministic Industrial
Field/Backhaul Networks
draft-wang-detnet-joint-scheduling-00
Abstract
In industrial field/backhaul networks, the joint real-time
scheduling method is important to keep end-to-end data streams
meeting the deadline. This document proposes four joint scheduling
methods, the four methods consider time slotting the industrial
backhaul network, regarding industrial backhaul network as a black
box system, ignoring delay of industrial backhaul and establishing
latency model of an industrial backhaul network.
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Table of Contents
1. Introduction ................................................. 2
2. Deterministic industrial field-backhaul network requirement .. 4
3. Deterministic Industrial field-backhaul network Joint Scheduling
Key Technology .................................................. 5
3.1. End-to-end Network Data Stream .......................... 5
3.2. Network Communication Resource .......................... 5
3.3. Network Time Slot Scheduling ............................ 6
4. Joint real-Time scheduling methods for deterministic industrial
field-backhaul network .......................................... 6
4.1. Time-Slotted Industrial Backhaul Networks ............... 6
4.2. Consider Industrial Backhaul Network as a Black Box .... 10
4.3. Ignore the Delay of Industrial Backhaul Network ........ 11
4.4. Build Delay Model of Industrial Backhaul Network ....... 11
5. Security Considerations ..................................... 11
6. IANA Considerations ......................................... 11
7. References .................................................. 11
7.1. Normative References ................................... 11
7.2. Informative References ................................. 11
Authors' Addresses ............................................. 13
1. Introduction
Industrial field network is a network that can be deployed in
industrial process and monitor industrial field equipment and
systems to achieve the target of control and management. It can
improve production efficiency, reduce human intervention to
industrial production process and decrease the cost of production.
It has significant importance for industrial modernization.
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Industrial field bus and industrial ethernet are two kinds of common
solutions to industrial automation with the development of
industrial field network, however they are both wired network. If
they can combine the technology of wireless sensor network, a new
network, industrial wireless network, can free from being bonded to
wires and cables, and is more easy and flexible to deployment.
Industrial wireless network is a communication network which is
oriented toward building automation, and process automation, and
industrial automation. There are three major international standards
(ISA100[ISA100.11a], WirelessHART[WirelessHART],WIA-PA[WIA-PA]) in
the area of industrial wireless network currently.
Industrial backhaul network is a transition network, which combines
industrial field network with higher level network to achieve the
goal of interconnection. It mainly solves the problem of access of
industrial field network data to higher level network. Industrial
field network is generally limited to a specific region, such as a
plant. By this network, transaction data of industrial field network
can be transferred to internet or other industrial field networks.
Industrial backhaul network is a medium-sized network, which can
cover from a few kilometers to tens of kilometers. The major
technology of industrial wireless backhaul network consists of Wi-Fi,
WiMAX and LET.
In order to adapt the presentation and development of industry 4.0,
which is aimed to elevate the level of manufacturing, industrial
field network should not be confined to a plant network only.
Therefore, it is necessary to introduce the technology of industrial
backhaul network to break the restrictions of interconnection
between different networks, and to form a mixed network of
industrial field network and backhaul network. Figure 1 indicates a
typical network architecture of the mixed network. It is a type of
deterministic network, and had been illustrated about use cases and
architecture in the drafts proposed by DetNet Workgroup of IETF of
draft-bas-usecase-detnet-02 and draft-finn-detnet-architecture-04.
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+-----------------------------------+
| |
| |
| Backhaul network |
| |
| |
+-----------------------------------+
/ \
/ \
+-------------------------------+ +-------------------------------+
| | | |
| | | |
| Field network | | Field network |
| | | |
| | | |
+-------------------------------+ +-------------------------------+
Figure 1. Typical industrial field-backhaul network
In this mixed network architecture of industrial field network and
backhaul network, field network is made up of ISA100, which is
industrial wireless sensor network protocol. In the former network,
a node deployed in a plant can communicate with a node deployed in
another plant through a backhaul network.
2. Deterministic industrial field-backhaul network requirement
The draft of draft-finn-detnet-problem-statement put forward by
DetNet Workgroup of IETF had described the requirement of
deterministic network and deterministic scheduling partially.
Because industrial field network directly faces the monitoring of
industrial process, it is a difference between industrial field
network data and general network data. Industrial field network has
high demands about the deterministic delay bounds. It will affect
the productivity, and even generate industrial accidents, when there
are high packet loss and latency in a field network. For instance,
real-time monitoring of level measurement and control are required
to avoid overfilling of oil tanks that may lead to serious economic
loss and environmental threats.
So, it is needed that a deterministic joint scheduling method can
guarantee the determination of network data in such a new network
architecture.
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3. Deterministic Industrial field-backhaul network Joint Scheduling Key
Technology
3.1. End-to-end Network Data Stream
In an industrial field network, end-to-end network data stream
indicates a complete transmission path that a source device node
transfers to a destination device node (common node or gateway).
While in an industrial field-backhaul network, it indicates a
complete transmission path that a field network source device node
transfers through an industrial backhaul to another field network
destination device node.
Industrial field-backhaul network data stream have following
features:
o Period. Every data stream in network generates period data.
o Deterministic. Every data stream in network has a deadline,
network scheduling should ensure every data stream arrive at
destination node before its deadline.
o Sequential. A path of an end-to-end network data stream are made
up of every two sequential node transmission link. In the process
of scheduling, it must be scheduled by the order of sequence of
links in the path.
3.2. Network Communication Resource
In the deterministic industrial field networks with backhaul network
architecture, schedulable network communication resources are time
slot, channel and link. If the backhaul network is SDN architecture,
then the SDN controller could schedule the bandwidth and cache of
switch. Therefore, bandwidth and cache resources can be included in
schedulable network communication resources.
o Time slot. Time slot is the basic unit in the TDMA based network
communications. The length of time slots is settled and is the
same in the entire network. Only one packet or ACK can be
transmitted in one time slot.
o Channel. In order to increase network throughput, industrial
field network standards provide a number of channels of different
frequencies. If the links do not interfere with each other, then
we can use different channels to transmit simultaneously.
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o Link. Link refers to a direct communication link between one node
and another and no intermediate switching nodes. The network data
stream is composed of a lot of links. The devices in the
industrial field network devices are half-duplex, so the links in
the industrial field network are unidirectional.
3.3. Network Time Slot Scheduling
In TDMA-based industrial field network, time is divided into time
slots of the same length. One transmission can be conducted in each
time slot and the links using different channels to transmit if they
do not interfere each other.
In the time slot scheduling process, it will cause link collision
when a node arranged to transmit and receive simultaneously, and it
will cause channel collision when the same channel is used within a
certain range. AS shown in figure 2, the network time slot
scheduling process should avoid such collisions.
+---+ +---+ +---+ +---+ +---+ +---+ +---+
| A |-->| B |-->| C | | A |-->| B | | C |-->| D |
+---+ +---+ +---+ +---+ +---+ +---+ +---+
+---------+------------+ +---------+------------+
|Time slot| Time slot 0| |Time slot| Time slot 0|
+---------+------------+ +---------+------------+
|Channel 0| A->B | |Channel 0| A->B |
+---------+------------+ | | C->D |
|Channel 1| B->C | +---------+------------+
+---------+------------+
Figure 2. Link Collision & Channel Collision
4. Joint real-Time scheduling methods for deterministic industrial
field-backhaul network
Joint real-time scheduling methods for deterministic industrial
field/backhaul networks, which cross networks, are intend to solve
the deterministic problem of industrial field /backhaul networks.
Since the current network infrastructure imports backhaul network,
the deterministic scheduling algorithm need to collaborate with
backhaul network to conduct joint scheduling to ensure data
certainty. The proposal put forward the following solutions.
4.1. Time-Slotted Industrial Backhaul Networks
In order to ensure determinism, industrial field networks utilize
TDMA to make the network time-slotted. If the industrial backhaul
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network can also be time-slotted, then the deterministic scheduling
algorithm can jointly schedule with minor alterations. Industrial
backhaul network can be built with a variety of network standards
such as Wi-Fi, WiMAX, LTE and so on. But in consideration of the
high cost and poor feasibility of time-slotted WiMAX and LTE, we
assume that the IEEE802.11 can be time-slotted. Wi-Fi network has
various networking modes, such as peer to peer networking mode,
point to multi-point networking mode and the relay network mode.
Here we consider the hierarchical network constructed in point to
multi-point networking mode, as shown in Figure 3.
+----------------------------------------+
| |
| +--------+ |
| +-------| Head AP|-------+ |
| | +--------+ | |
| | | |
| +--------+ +--------+ |
+---+---| AP1 | | AP2 |---+---+
| | +--------+ +--------+ | |
| +----------------------------------------+ |
| |
+------------------------------+ +-------------------------------+
|ISA100 field wireless network | | ISA100 field wireless network |
+------------------------------+ +-------------------------------+
Figure 3. Industrial Backhaul Network consisting of WIFI
Although IEEE802.11 also supports 13 channels, but the AP was not
free to switch channels, which means that the AP cannot use a
channel in the current time slot and use another channel the next
time slot. However, we assume that the network architecture, the
following points AP under head AP, which are AP1 and AP2 in FIG 1,
can transmit packets simultaneously as long as their transmission
task do not contain the same AP, i.e. head AP. For example, when a
data stream of field network is transmitting packets to AP1 in a
time slot, AP2 is able to receive packets from head AP, or send
packets to field network in the same time slot. Therefore, the
backhaul network constructed with wireless APs can be considered as
a single-channel linear network, which is shown in Figure 4.
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+---------+ +--------+ +--------+ +--------+ +---------+
| Gateway |--> | AP |--> | AP |--> | AP |--> | Gateway |
+---------+ +--------+ +--------+ +--------+ +---------+
Figure 4. A single-channel linear network
Therefore, the data stream in industrial field/ backhaul network can
be deemed to be equivalent to the data stream in field network, only
that every piece of data streams need to go through the linear
network consisting of wireless APs. So the scheduling process is
proposed as follows:
1. Abstract end to end data stream in the entire network, and
initialize a different priority for each stream.
2. Establish the delay model of network data stream. If collisions
happened between different priority data stream, the low-priority
data stream will be delayed by high-priority data stream, so a
model can be built under the worst circumstances that the low-
priority data streams impacted by higher priority data streams.
3. Estimate the network schedulability. A data stream is schedulable
if the minimum time for the data stream to complete transmission,
plus the worst delay time caused by higher priority data streams,
is less than or equal to deadline, In the current priority
allocation scheme, if each data stream is schedulable, the
network can be considered as schedulable. If the data stream
cannot be scheduled, then change the priority allocation scheme
and estimate again until a corresponding scheme is found.
4. Allocate time slot and channel for every data stream. Traverse
data streams according to their priority, and each data stream
should allocate the next link that is about to be released in
each time slot to the greatest extent. According to the rule that
low-priority data streams should give way to high-priority data
streams, the spare channels can be utilized if there is no
collision. However, if collisions happened between data streams
of different priority, then the lower-priority data stream should
be placed in the next time slot until there are no unallocated
higher priority data streams. Follow these rules until the whole
network scheduling is completed.
The scheduling process is shown in Figure 5:
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+----------+
| Begin |
+----------+
|
|
+---------------------------+
| Initial the priority of |
| each data stream |
+---------------------------+
|<--------------------------------------+
| |
+--------------------+ +------------------------------+
/ Traverse every data \ no | If the data stream cannot be |
/ stream and estimate the\--------->| scheduled, then change the |
\ schedulablity according/ | priority allocation scheme |
\ to delay model / | and estimate again |
+--------------------+ +------------------------------+
|
|yes
+-----------------------------------+
| Traverse data streams according to|
| their priority, each data stream |
| should allocate the next link that|
| is about to be released in each |
| time slot to the greatest extent |
+-----------------------------------+
|
|
+-----------------------------------+
| The spare channels can be utilized|
| if there is no collision. If |
| collisions happened, then the |
| lower-priority data stream should |
| be placed in the next time slot |
+-----------------------------------+
|
|
+-------+
| End |
+-------+
Figure 5. Scheduling of times-slotted industrial backhaul network
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4.2. Consider Industrial Backhaul Network as a Black Box
In order to solve the deterministic problem of industrial backhaul
network, industrial backhaul can be deemed as white box to conduct
fine controls through inner mechanism. While it can also be regarded
as a black box so that we can only consider its delay impacts and
ignore its internal details.
When the packet goes through the industrial backhaul network, we can
give it a timestamp at the application layer and read it after the
transmission completed. Then delay caused by the backhaul network
can be figured out and a fitting curve of delay can be worked out by
collecting large amount of data. It has been verified experimentally
that the delay is concentrated in a numerical range despite its
randomness. Therefore, we can get the approximate delay of packets
caused by the industrial backhaul network.
After that, a few of scheduling paths of different priority can be
implemented in the industrial field network. A main scheduling path
can be configured according to the average delay of the backhaul
network. And some redundant paths should be pre-configured in case
the delay of the main path is too high.
The scheduling process of industrial field/backhaul network can be
divided into three periods, as shown in Figure 6:
+--------------------+ +-----------------+ +------------------------+
| Scheduling of | | Delay of | | Scheduling of |
|source field subnet |->| backhaul network|->|destination field subnet|
| (deterministic) | |(indeterministic)| |( deterministic dynamic)|
+--------------------+ +-----------------+ +------------------------+
Period 1 Period 2 Period 3
Figure 6. Three periods of scheduling
In source field subnet we can apply the deterministic scheduling
algorithm of field network to conduct deterministic polymerization
and get the time spent by each data stream to go through the source
subnet. Then the data stream goes through the backhaul network,
which is a black box and it will cause indeterministic delay which
is in a numerical range. When the data stream comes out the backhaul
network, the timestamp should be parsed. If the deadline is missed,
it indicates that the packet has gone through poor network and need
to be retransmitted. If there is time left, scheduling path can be
dynamically selected at downward gateway to get the schedulability
of the end to end data stream.
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4.3. Ignore the Delay of Industrial Backhaul Network
Since the field network is slow-speed (256 KB/s), while industrial
backhaul network is a high-speed, if the industrial backhaul
networks adopt IEEE802.11, gigabit wireless routers supporting
IEEE802.11 ac can make the delay of industrial backhaul network
quite small. As a result, the joint deterministic scheduling of the
entire network only needs to cover the field network that is located
at the ends of the backhaul network.
4.4. Build Delay Model of Industrial Backhaul Network
If industrial backhaul network is built with IEEE802.11, the network
access delay test model under IEEE802.11 DCF mode can be established
by using Markov chain or queuing theory. At the same time, the model
under IEEE802.11 PCF mode can be established based on queuing theory.
Therefore, the field network only need to build the delay model of
backhaul network that follows one delay model, then the total
transmission scheduling delay will follow certain regularity. The
total transmission delay will meet delay requirements with specified
probability by scheduling, in other words, the unsuccessfulness of
scheduling is acceptable, but the scheduling success rate should be
in a range of 90% ~ 95%.
5. Security Considerations
This memo includes no request to IANA.
6. IANA Considerations
This memo includes no request to IANA.
7. References
7.1. Normative References
7.2. Informative References
[ISA100.11a]
ISA/IEC, "ISA100.11a, Wireless Systems for Automation,
also IEC 62734", 2011, <
http://www.isa100wci.org/enUS/Documents/PDF/3405-ISA100-
WirelessSystems-Future-brochWEB-ETSI.aspx>.
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[WirelessHART]
www.hartcomm.org, "Industrial Communication Networks -
Wireless Communication Network and Communication Profiles
- WirelessHART - IEC 62591", 2010.
[WIA-PA]
CN-GB. GB/T 26790.1-2011. Industrial wireless networks WIA
specification.Part 1: WIA System architecture and
communication specification for process automation (WIA-
PA)[S]. China: CN-GB, 2011.
[I-D.finn-detnet-problem-statement]
Finn, N. and P. Thubert, "Deterministic Networking Problem
Statement", draft-finn-detnet-problem-statement-04 (work in
progress), October 2015.
[I-D.finn-detnet-architecture]
Finn, N., Thubert, P., and M. Teener, "Deterministic
Networking Architecture", draft-finn-detnetarchitecture-03
(work in progress), March 2016.
[I-D.bas-usecase-detnet]
Kaneko, Y., Toshiba and Das, S, "Building Automation Use
Cases and Requirements for Deterministic Networking", draft-
bas-usecase-detnet-00 (work in progress), April 2016.
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Authors' Addresses
Heng Wang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6248-7845
Email: wangheng@cqupt.edu.cn
Ping Wang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6246-1061
Email: wangping@cqupt.edu.cn
Hang Yang
Chongqing University of Posts and Telecommunications
2 Chongwen Road
Chongqing, 400065
China
Phone: (86)-23-6246-1061
Email: 18716322620@163.com
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