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8/8/2019 01423642
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A Switched Ethernet Protocol for Hard Real-Time
Embedded System Applications
Alimujiang Yiming and Toshio Eisaka
Kitami Institute of Technology, Kitami, Hokkaido, [email protected]
Abstract
This paper presents a protocol to support traffic for
hard real-time applications over non real-time LANtechnology, with a switched Ethernet based network
concept without any modifications of existing Ethernet
hardware. Simulation results demonstrate the proposed Ethernet protocol has better real-time performances,higher bandwidth and a larger data frame, and meets the
requirements of reliability for hard real-time systems.
1. Introduction
High bandwidth and determinism are the criticalnecessary conditions for any hard real-time applications.
For the real-time systems that need high-bandwidthcommunications, Ethernet becomes the communications
link most designers think of first today [1]. Unfortunately,Ethernet uses a non-deterministic arbitration mechanism
(CSMA/CD) that makes it unsuitable for real-timecommunications. In spite of this, the advantages of Ethernet
(popularity and high-bandwidth, etc.) still attract the needto make it suitable for real-time applications [2-6].
Protocols which enable hard real-time communicationon Ethernet have also been proposed in [7-9]. However,
these protocols either change the existing Ethernet
hardware or bring to additional time consumption.In this paper, we aim to provide a better way of design
and implementation of a switched Ethernet protocol for
hard real-time applications, without modifications of
existing Ethernet hardware. In the proposed protocol, thereal-time communication supports in end-nodes and
switches according to a real-time dynamic schedulingalgorithm -- EDF (Earliest Deadline First) algorithm, by
software added between the Ethernet protocols and theTCP/IP suite in the OSI reference model [10]. Real-time
traffic from an end-node bypasses the TCP/IP stack andthus considerably reduces the dwell time in the nodes, and
increases the achievable frame rate.Compared to the conventional hard real-time
communication protocols, the proposed Ethernet protocolhas better real-time performances, higher bandwidth and a
larger data frame, and meets the requirements ofreliability for hard real-time systems, without anymodification of the original Ethernet hardware.
2. Real-time communication support
In this section, network configuration, real-time
channel establishment and traffic scheduling are discussedrespectively.
2.1 Network configuration
We applied a full-duplex switched Ethernet for thenetwork configuration, and both switch and end-nodes use
a real-time dynamic scheduling algorithm -- the earliestdeadline first (EDF) algorithm for hard real-time traffic
control. The advantage of using EDF in hard real-time
traffic was shown in [11-12], as it is the optimal and
dynamic-priority scheduling algorithm. MAC function,frame buffering and centralized transmission arbitration isincluded in the switch. In addition, we have used anintelligent switch in our work for which provides better
performance (shorter latency and higher utilization) than
using the ordinary switches, and provide controls toenhance multi-point operations of the network.
2.2 RT channel establishment
Before the real-time traffic is transmitted, the real-timechannel should be established. The establishment of the
real-time channel (RT channel) is including request and
recognition communication, after the source nodes,
destination nodes and switches have agreement withchannel establishment.
We built real-time channels between source nodes and
destination nodes. Each node can connect to more thanone sending and receiving real-time channel.
When receiving RT channel establishment requestframes, the switch calculates the total utilization of all the
Proceedings of the 19th International Conference on Advanced Information Networking and Applications (AINA05)550-445X/05 $20.00 2005 IEEE
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request frames, namely, the feasibility of traffic schedule
between requesting nodes and switch and the destinationnodes (called admission control process). If the schedule
is feasible, the switch responses with the network
schedule parameters to the requesting nodes. If theschedule is not feasible, the switch sends out a set ofrecommended control parameters to the requesting nodes.
These control parameters are suggested based on thestatus of switch queue and the active queue control law.
Our purpose in this work is to create a protocol for
switched Ethernet by scheduling hard real-time trafficusing EDF algorithm. Between different nodes, we can
get information about the other real-time data frames bysending the absolute deadline messages of the next traffic.According to this deadline information, we sort real-time
data frame on each node with EDF algorithm, andestablish a deadline table which including the absolute
deadlines of each node. The determination of sends
real-time traffic will be obtained on the table which gives
a reference of the traffic deadline of other nodes.According to this dynamic table, real-time traffic from theend-node must have lowest deadline than other traffic inthe same node and in the other nodes, and bypasses the
TCP/IP stack and put in a deadline-sorted queue in the RT
layer. The function of bypassing the TCP/IP stack of eachnode is implemented by software added between the
Ethernet protocols and the TCP/IP suite. The processes ofreal-time traffic transmission are describing as follows:
1. If a real-time traffic data frame sent from an
end-node has the lower deadline comparing with deadline
in the table, the data frame skip to use the services of theTCP/IP protocol suite and the EDF traffic-scheduling
algorithm is applied to directly put the data frame as adeadline-sorted queue in the RT layer.
2. Update the table by the deadline according to thereceived data frame. The real-time data frame to be sent
should include the deadline of the next data frame in localqueue if there are some following data frames. If not, then
send a deadline with maximum value.Figure 1 describes the establishment of an RT channel.
When an end-node wants to setup an RT channel, it will
directly deal with the RT layer. The RT layer then sendsan RT channel establishment request to the RT channeltraffic management software in the switch. Real-time
traffic from the end-node bypasses the TCP/IP stack andthus considerably reduces the dwell time in the nodes, and
increases the achievable frame rate by evasion of thenon-deterministic behavior inherent in the TCP and IP
stack. This is important for the proposed Ethernetprotocol has better real-time performances, and meets therequirements of reliability for hard real-time systems. Nonreal-time traffic from the end-node uses TCP/IP stack and
put in an FCFS-sorted (First Come First Serve) queue inthe RT layer.
The switch has two MAC addresses: one is for control
traffic (e.g., Request Frames); and another is for real-time
traffic over RT channels. Therefore the switch will beable to recognize the different kinds of frames: control
frames, real-time data frames and non-real-time.
Intelligent
Node1 Switch Node N
Figure 1. Real-Time channel establishment
2.3 Scheduling of real-time frames
We assume that Node 1 (source node) wants to send
real-time traffic to Node 2 (destination node). Thereal-time guarantee is supported by RT channel, and the
scheduling of real-time frames in the switch and end-node
is made according to EDF theory. According to basicEDF theory, the utilization factor of real-time traffic is
defined as:
,
ii
p d i
CU T (1)
where Tpd, i is the RT channel period duration, Ci is theamount of data per period. In order to satisfy all deadlines,
the utilization of real-time traffic link has to be less thanor equal to a certain level:
,
1ii pd i
CU
T (2)
For all RT channels, the maximum transmission latencyis characterized by:
_ , 1, 2,m lat i n i n i l T T T T (3)
where Tl is the worst-case latency which is defined as:
_ _l n s s d ct T T T T (4)
In the equation, Tn_s is the latency from source nodes to
switch; Ts_d is the latency from switch to destinationnodes, and Tct is the corner-turn latency inside the
intelligent switch.The source node latency is: Tn_s = QTf where Q is the
Best
effort
Real
Time
Real
Time
Best
effort
TCP
UDP
TCP
UDP
IP
Traffic managing
&
Frame Reorganizing
IP
RT layer R T R T RT layer
M AC M A C M A C M AC
PH. L. PH . L .
.
.
. P H . L . PH . L.
Proceedings of the 19th International Conference on Advanced Information Networking and Applications (AINA05)550-445X/05 $20.00 2005 IEEE
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0
200
400
600
800
1000
1200
1400
1600
1 10 100 500 1000 1498 1538
Data frame size (bytes)
Transm
issionlatency(s)
1553BRTCCProposed
number of the frames which is storable on NIC (Network
Interface Card). The switch latency is described as:
_ ,s d f f T MAX T QT (5)
The first term in the MAX expression is the wait timeof maximal sized frames Tf. The second term shows thesame thing as for the source node latency.
Besides utilization and worst-case latency, anotherimportant performance evaluation standard of a real-time
data frame is a runtime overheadRi. It is defined as:
,
,
8 /
100
pd i i
i
pd i
T L BR
T
(6)
where Li is the length of data in a message frame, for
example, the data length a frame can be able to transmit;Li 8 is the number of bits in the frame, and B represents
the Ethernet bandwidth.According to the utilization, transmission latency and
runtime overhead that we discussed above, an EDF traffic
scheduling is proposed, which can guarantee all the aboverequirements:
1. If it is a hard real-time traffic data frame, the EDF
traffic-scheduling algorithm is used in order to guarantee thedynamic absolute deadline in earliest priority of the traffic.
2. Transmission of hard real-time traffic is alwaysguaranteed first in the EDF algorithm. Then, transmit
other traffics (for example, soft real-time traffic).
3. A non real-time traffic which come from other best-effort protocols (like HTTP, SNMP, FTP etc.), ituses the services of the TCP/IP protocol suite and put in
an FCFS-sorted queue in the RT layer, then transmit the
traffics at the idle time of the schedule.The real-time traffic schedule is performed according
to EDF algorithm with dynamic-priority scheduling,therefore it is flexible and efficient, and can be adopted inhard real-time traffic applications.
3. Performance evaluation
We made a local area network (LAN) with a
full-duplex switched Ethernet and end-nodes, by usingdesktop computer and several embedded Ethernet
development boards which produced by YDKTechnologies Inc. that provides a hardware platform to
immediately start developing embedded systems based on
Altera ACEXTM
devices (see Figure 2).We use a 100 Mbit/s Ethernet switch with Ethernet
frames that has the data field maximized (1538 bytes inIEEE 802.3 standard). Real-time traffic from the
end-node bypasses the TCP/IP stack and put in adeadline-sorted queue in the RT layer, the intelligent
switch scheduler applied with the EDF algorithm alwaystransmits the message frame according to the closeness oftheir absolute deadline. In the proposed work, there are no
modifications in the Ethernet hardware on the NIC. This
allows connecting the proposed switched Ethernet LANto the existing Internet network.
Desktop
Figure 2. LAN with full-duplex switched Ethernet
Data frame transmission latency and the frame runtime
overhead can be obtained by implementing the proposedprotocol to hard real-time communications.
Figure 3a shows the comparison of data frame
transmission latency of three kinds of hard real-time
communication protocols: MIL-STD-1553B protocol, realtime communication control (RTCC) protocol and the
proposed Ethernet protocol. Runtime overhead of data
frame of the three protocols is demonstrated in Figure 3b.
Table 1 listed some main performance parameters of these
hard real-time communication protocols. Figures andtable shows that the proposed protocol provides betterreal-time performance, higher bandwidth, and a largerdata frame than the other hard real-time communication
protocols.
Figure 3a. Transmission latency of data frame
I n t e l li g e n t S w i tc h
E n d n o d e s
1 0 / 1 0 0 B a s e - T X
B e s t
e f f o r t
R e a l
T i m e
T C P / U D P
I P
R T l a y er
M A CSRAM/
FROM
P h L
Proceedings of the 19th International Conference on Advanced Information Networking and Applications (AINA05)550-445X/05 $20.00 2005 IEEE
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0
0.2
0.4
0.6
0.8
1
1 10 100 500 1000 1498 1538Data frame size (bytes)
Frameruntimeov
erhead%
1553BRTCCProposed
Figure 3b. Runtime overhead of data frame
Table 1. Parameters of hard real-time networks
MIL-STD
-1553BProtocol
Real Time
Ethernet(RTCC)
Proposed
EthernetProtocol
Speed (bps) 1M 10M 100M
Determinism Best Good Good
Length (km) < 0.09 < 2.5 < 2.4
Max nodes 32 1024 1280
Frame 1-32(16bits)
1-1498(8bits)
1-1538(8bits)
4. Conclusion
In this paper, we have presented a protocol to support
traffic for hard real-time applications over non real-timeLAN technology, with a switched Ethernet based networkconcept. For the network configuration, both switch andend-nodes use the earliest deadline first (EDF) algorithm
for hard real-time traffic control; both switch and
end-nodes have an RT layer added to support hardreal-time traffic. Real-time traffic from the end-node
bypasses the TCP/IP stack and put in a deadline-sortedqueue in the RT layer, and the switch always transmits the
data frame according to the closeness of their absolutedeadline that has stored in the switch when establishing
their RT channel. Non real-time traffic from the end-nodeuses TCP/IP stack and put in an FCFS-sorted queue in the
RT layer, and transmit the traffics at the idle time of the
schedule.Simulation results comparing with some conventional
hard real-time protocols has demonstrate that the proposed Ethernet protocol has better real-timeperformances, higher bandwidth and a larger data frame,
and meets the requirements of reliability for hardreal-time systems.
Furthermore, we have used an intelligent switch in our
work, for which provides better performance (shorterlatency and higher utilization) than using the ordinary
switches. Also, the intelligent Ethernet switches providecontrols to enhance multi-point operations of the network.
This is useful for many applications with high demandson multi-point communication performance. Example for
this should be the application in industry distributed hardreal-time communications, robotics, and in parallel signal
processing applications such as radar signal processing.
References
[1] Jan Axelson, Embedded Ethernet and Internet Complete,
Lakeview Research, September 2003. ISBN: 1-931448-00-0.
[2] C. Venkatramani and T. S. Chiueh, Supporting real-timetraffic on Ethernet, Proc. IEEE RTSS94, pp. 282-286, Dec.
1994.
[3] D. W. Pritty, J. R. Malone, S. K. Banerjee and N. L. Lawrie,
A real-time upgrade for Ethernet based factory networking,Proc. IECON, pp. 1631-1637, 1995.
[4] Malcolm, N. and Zhao, W., The timed token protocol for
real-time communication, Proc. IEEE Computers, 27(1): 35 41,
1994.[5] S. Varadarajan and T. Chiueh, "EtheReal: A host-transparent
real-time Fast Ethernet switch", Proc. ICNP, Oct. 1998.
[6] H. Hoang and M. Jonsson, "Switched Real-Time Ethernet
with Earliest Deadline First Scheduling - Protocols and Traffic
Handling", Proc. WPDRTS2002, Fort Lauderdale, Florida,
USA.[7] Z.P. Wang, G.Z. Xiong, J. Luo, M.Z. Lai and W. Zhou, A
hard real-time communication control protocol based on the
Ethernet, Proc. PART2000) pages 161170, Sydney, Australia,Nov. 2000. Springer-Verlag. ISBN 962-430-134-4.
[8] J. Loeser and H. Haertig, Low-latency hard real-time
communication over switched Ethernet Proc. 16th Euromicro
Conference on Real-Time Systems, Catania, Sicily, July 2004.[9] S. Ouni and F. Kamoun, Hard and soft real time messagescheduling on Ethernet networks, Proc. 2002 IEEE SMC, vol. 6,
October 6-9, 2002, Hammamet, Tunisia.
[10] C. Bergenhem and M. Jonsson, Fiber-ribbon ring network
with inherent support for earliest deadline first messagescheduling, Proc. WPDRTS'2002 in conjunction with
IPDPS'02, Fort Lauderdale, FL, USA, April 15-16, 2002.
[11] C. L. Liu and J. W. Layland, "Scheduling algorithms for
multi programming in hard real-time traffic environment",Journal of the Association for Computing Machinery, vol. 20, no.1, Jan. 1973.
[12] Phillip A. Laptane, Real-Time System Design and Analysis,
IEEE Press, third edition 2004. ISBN: 0-471-22855-9.
Proceedings of the 19th International Conference on Advanced Information Networking and Applications (AINA05)550-445X/05 $20.00 2005 IEEE