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Reliability of UDP Protocol Data Transmission
in Electrical Power Telecomunication Systems Interworking with the Internet
RELIABILITY OF UDP PROTOCOL DATA TRANSMISSION IN ELECTRICAL POWER
TELECOMUNICATION SYSTEMS INTERWORKING WITH THE INTERNET
Michał Porzeziński / Technical University of Gdańsk
Grzegorz Redlarski / Technical University of Gdańsk
1. INTRODUCTION
Development of modern microprocessor technology and data processing systems witnessed in recent
years is directly connected with the development of electrical telecommunication systems based on, among
others, optical fibre technology. These new solutions find many practical applications in a variety of fields ranging
from land-based and cellular telephony, through data transmission over LAN (Local Area Network) to transmission processes taking place in measuring devices and telemechanics. Thus they play a key role in controlling the
work of electrical power systems, especially in the area of primary and secondary control, dispatch control and
protection automation [3]. In addition, they are readily used in monitoring the condition of numerous systems
and devices which constitute the structure of an electrical power system.
There are two network areas within the structure of an electrical power teletransmission system: backbone SDH/ATM (Synchronous Digital Hierarchy/Asynchronous Transfer Mode) and access PDH (Plesiochronous
Digital Hierarchy). Backbone network devices are installed in the most important locations, such as central company headquarters or power supply districts or, while access network devices are usually installed in electrical
power stations and alerting posts Physical double-ring typology of the B-SHR type is used .within the range of
SDH/ATM networks. In this solution physical connections are carried out through parallel fibre optic lines built
into OPGWs (Optical Ground Wire). Within such physical typology numerous technologies (e.g. STM-1, STM-4,
STM-16) and standards (e.g. V.24, V.35, G.703) are employed allowing for communication of various transmission parameters. Various physical typologies are employed within PDH networks (e.g. bus. star, mesh, ring), as
well as logical typologies (e.g. broadcasting, token passing), which are jointly responsible for continuity and
reliability of communication processes [1, 2, 3].
Teleinformatic infrastructure of an electrical power system and continuous development of modern technologies, as well as the trend to increase reliability of electrical power systems result in a search for increasingly more effective solutions. Research activities conducted in numerous centres concentrate on a number of
important processes which take place in electrical power systems. Without a doubt, one of them is the process
of automatic synchronization which when incorrect may have many serious negative consequences of legal,
social and financial nature. Improving the reliability of this process may be achieved through, among others,
improving the reliability of devices carrying out synchronization functions such as UASP automatic generator
synchronizers.
Despite the fact that modern UASP are high-class microprocessor devices, various types of breakdowns
still occur. Diagnostic processes usually are restricted to the so called periodic maintenance. Therefore, it is
not possible to constantly monitor their current conduction which would be possible after equipping the UASP
Abstract
The paper presents the principle of reliability in
data transmission based on the UDP transmission protocol model without implicit hand-shaking dialogues in electrical power transmission systems interworking with the
Internet. The subject matter applies especially to small
(frequently maintenance free) electrical power objects in
which the monitoring process may be conducted through
the widely available infrastructure of the Internet and a
monitoring centre. The paper points out the most significant sources of errors in the transmission process and indicates possibilities of their elimination. In addition, the
work presents developed diagnostic tools, methodology,
as well as results of conducted research and its most significant conclusions.
77
78
Michał Porzeziński / Gdańsk University of Technology
Grzegorz Redlarski / Gdańsk University of Technology
devices with appropriate diagnostic software and sometimes also interfaces allowing them to work together
with a computer network. This in turn would allow for the information on device condition to be transferred
directly to application located in, for example, the monitoring centre. However, such steps require research and
implementation, among others, in the area of the method of information transfer between examined device .and
applications with access to the Internet. Without a doubt, one of the basic issues which needs to be answered
is the choice of communication protocol and the effectiveness of its function.
In practice, a number of communication protocols may be employed for data transmission. Due to their
popularity, two of them deserve special attention [5, 6]: TCP (Transmission Control Protocol) whose format is
presented in Fig. 1 and UDP (User Datagram Protocol) with format presented in Fig. 2.
Source Port
Destination Port
Sequence Number
Acknowledgement Number
HLEN
Reserved
Window
Checksum
Urgent pointer
Options
Padding
Data...
Control Bits
Fig. 1. TCP protocol segment format
Source Address
Destination Address
UDP Lenght
Checksum
Data...
Fig. 2. UDP protocol segment format
Definitions of segment fields in Fig. 1 and Fig. 2 [1,5,6]:
• Source Port – specifies the calling port, which may be connection initiating point
• Destination Port – specifies the destination port number
• Sequence Number– specifies the sequence order within data chain transmitted to receiver
• Acknowledgement Number– specifies the number of the next expected octet of data
• HLEN – specifies the number of 32-bite words appearing in data segment
• Reserved – field with entered ‘0’ value
• Control Bits – field of control functions (e.g. relevant to the way of configuring and ending given session)
• Window – field determining the size of sliding window (in bites) which may be accepted by device
• UDP Length – specifies the length of data field in case of UDP protocol
• Checksum – determines the correctness of data transmission, based on a defined working algorithm and
the information contained in the header and data fields
• Urgent Pointer – specifies the completion point of important data transfer
• Options – allows for defining the maximum size of TCP segment
• Data – field of the actual data sent with TCP or UDP protocols.
TCP protocol is characterised by the fact that prior to initiating the transmission process, a connection
with the target device is established as part of the process known as the three-way-handshake [1]. In addition, it contains mechanisms ensuring reliability of data transmission, such as sequence numbers, confirmation
numbers and sliding windows. In contrast, the UDP protocol does not establish connections with target device
prior to beginning the transmission process and is not equipped with mechanism ensuring reliability. Therefore,
without implicit handshaking dialogues in the UDP protocol, it is necessary to ensure sufficient transmission reliability level at the application layer level. However, in order to make sure that the mechanism is effective and
efficient it is first necessary to assess the probability of transmission errors and their character.
In practice, one can frequently encounter situations where the Internet is employed in monitoring the
condition of devices in distributed objects of low power-rating. In addition, due to the high cost of establishing
and maintaining a separate network it seems justifiable to use the Internet for communication in certain situations (e.g. where the reliability of data transmission in real time is not critically important). Therefore, if one
takes into account the efficient, ‘enclosed’ and so highly reliable structure of electrical power teletransmission
Reliability of UDP Protocol Data Transmission
in Electrical Power Telecomunication Systems Interworking with the Internet
network, as well as the necessity of data transmission between that network and the monitoring centre through
the widely accessible connections of the Internet (Fig. 3), it is very important to conduct the Internet network
reliability research, especially if it is not a familiar transmission path or if diverse interference causing factors
may appear.
Monitoring Center
Wide Area Network
in Power Electric System
Internet
Fig. 3. General schematic presenting interworking between teleinformatic WAN network of electrical power system and the Internet
SOURCES OF UDP TRANSMISSION ERRORS AND CAUSES OF THEIR FORMATION
Potential errors causing UDP protocol data loss are mostly connected with random faults which usually
occur at the physical surface level of computer network and with errors of devices serving higher protocol layers.
On the level of physical network layer especially important role is played by the EMI and RFI type interferences,
incorrect functioning of devices within a given network which are often the result of badly designed physical network typology, mechanical damage of transmission pathways, as well as factors causing power supply failures.
There are many possible causes of incorrect design of physical typology. Considered to be among the
most important ones are the following: exceeding allowed parameters of employed transmission media (e.g. cable length), inaccurate completion of cable connections (e.g. excessive value of transfer impedance), insufficient
width of transmission band, or employing network devices unfit for requirements. These factors may constitute
the causes of, among others, increased amount of collisions, possibly leading to not only decreased network efficiency but also to increase in the amount of lost data transmitted through the UDP protocol [1, 2].
Defective network devices which cause loss of transmission are frequently connected with interface and
power supply malfunctions. In such cases, restoring network convergence requires a defined period of time
necessary for, among others, finding another (functional) transmission path. In addition, loss of IP packets in
network devices, such as switches and routers, may be the result of excessive loads caused by large amount of
data transmitted in a short period of time and its related overfilling of internal buffers.
79
Michał Porzeziński / Gdańsk University of Technology
Grzegorz Redlarski / Gdańsk University of Technology
80
1. NETWORK MONITORING TOOLS
Researching the reliability of UPD protocol based data transmission requires development of software for
recording selected transmission parameters based on which a given transmission channel can be evaluated. Fig.
4 presents a window view from TestUdp - an example of software developed for such a purpose.
Fig. 4. Window of a network monitoring software programme
The software programme presented above was prepared in two versions. The first version allows for sending of consecutive segments with a 10 second time interval. In the second version three segments containing
the same information are sent after the same pause (with user defined pause duration). The software has to be
run in the diagnostic points between which the data transmission process is taking place. One of the applications
(Station A) plays the sending (local) host1 role while the other (Station B) plays the role of receiving host (remote) which sends the received data back to the sender. Configuring the software requires providing IP address
of the remote host and defining numbers of the ports to be used in the transmission process. The quantity of
sent and received UDP segments is visualised in a continuous way in the fields Send and Receive or , RcvCnt and
RetCnt (depending on software version). Detailed information on the obtained research results, i.e. transmission time, number of data segments, transmission delay, as well as information on successful or unsuccessful
transmission attempt are collected in a text file acting as event log.
1. RELIABILITY ANALYSIS OF UDP TRANSMISSION IN THE INTERNET
Investigation was limited to a few nodal points between which the process of UDP protocol data exchange
was established for various time periods. The research range was divided into two stages. In the first transmission reliability was examined based on the first software version. Single data transmission after which the sent
segment reached its destination was regarded as the correct condition. The study results obtained for the connections indicated in Fig. 5 with the symbols P-P 1 to P-P 7 are presented in Table 1.
1
etc.
In the terminology of computer networks, the term host refers to devices with a MAC address, i.e. personal computers, terminals, servers
Reliability of UDP Protocol Data Transmission
in Electrical Power Telecomunication Systems Interworking with the Internet
Fig. 5. Map of connections carried out during the study
Table 1. Results of reliability analysis of UDP protocol based data transmission
Connection
No. (Fig. 5)
Connection duraNumber
Amount of
of errors
tion
data
[hh:mm:ss]
(1/2/3/4/5)*
Number
of errors
[%]
Average delay
[ms]
Maximum
delay [ms]
Minimum
delay[ms]
Node information
P-P 1
23:32:10
8474
10
(5/0/0/0/1)
0.12
0.056
47
10
Academic network
– Academic network
P-P 2
10:46:31
3817
38
(38/0/0/0/0)
0.99
2.003
4641
0
Academic network
– Cable TV network
P-P 3
07:08:40
2597
11
(3/0/1/0/1)
0.42
186. 49
1829
31
Neostrada TP
– Neostrada TP
P-P 4
06:38:16
1969
148
(118/12/2/
0/0)
7.52
200.67
1 482
15
WiFi network
– Neostrada TP
P-P 5
02:07:31
766
1
(1/0/0/0/0)
0.13
38.56
250
31
Academic network
– Neostrada TP
P-P 6
50:17:31
18106
149
1.16
(142/2/1/0/0)
0.0612
78
0
Academic network
– Neostrada TP
P-P 7
26:03:23
9374
54
(54/0/0/0/0)
52.84
3650
15
WiFi network
– Neostrada TP
0.58
* Number of errors occurring in succession: single/double/triple…
Based on the analysis of study findings presented in Table 1, it may be concluded that they are of a random
character depending on the degree of transmission delay and the number of lost UDP segments. In addition, in
all completed transmission processes there appears a variable amount of lost UDP data, which without a doubt
depends on the current condition of transmission links. . In various transmissions links there also appear different delay values in transmission processes.
Additional research which took the above factors into account was done to examine the relation between
the degree of delay and the number of faulty data segments, the impact on transmission quality of additionally
81
Michał Porzeziński / Gdańsk University of Technology
Grzegorz Redlarski / Gdańsk University of Technology
82
delay [ms]
placed transmission sessions containing identical data segments, as well as the effect of redundancy based on
multiplying the number of sent signals contain the same information on the quality of transmission.
The first of the discussed aspect was addressed to the transmission process marked in Fig. 5 with the
symbol P-P 8. The obtained results are presented in Fig. 6.
Fig. 6. 24-hour characteristic of transmission
delay times and lost UDP
data in a selected link in
the Internet
Time of the day
Based on information from Fig.6 it can be concluded that in the hours of heightened connection activity (working hours and evening) there appear the largest delays in transmission processes. In addition, during the working
hours (7:12–16:48) there is a significantly lower degree of data loss (15 segments) than outside those hours (37
segments). Analysis of the results also points out that the majority of the errors were single ones which were most
probably connected with accidental data loss. However, one should also bear in mind the possibility of longer breaks
in the functioning of transmission channels ranging from several seconds to a minute or even minutes.
Researching the impact of extra channels on transmission quality (constituting of establishing additional
sessions for sending the same data segments) was limited to the connection sample P-P 8. The study findings for N
3Ni
= 1, 2, 3, 4 redundancy paths are presented in Table 2 ( C N2Ni and CN represent the biggest number of errors in
series: two out of four established transmission paths and three out of four established transmission paths).
Table 2. Impact of channel redundancy on the quality of UDP transmission process
Maximum number of transmission errors
Amount of data
18106
N1 ∨ N2 ∨ N3 ∨ N4
149
N1 ∧ N2 ∧ N3 ∧ N4
20
C N2Ni
3Ni
6 CN
4
Max. transmission
delay [ms]
94
Due to the observed random character of transmission errors (see Table 2), it can be concluded that establishing redundant transmission channels, as well as repeating the information at appropriately selected time
intervals is an effective method of improving the reli ability of UDP transmission. The strongest impact occurs
after employing just a single additional transmission channel. In such a case, the number of transmission errors
decreases nearly by an order of magnitude (out of the observed 149 for a single channel to 20 for the scenario
with one additional channel).
Examining the impact of redundancy based on multiplying the sent amount of data containing the same
information was limited to the connection sample P-P 8. The software version based on sending three data segments in each transmission cycle was employed in this case. The three segments contain the same information
and the assumption is that correct transmission takes place if at least one of the segments reaches its destination. The pause between successive sent packets is defined by the operator in the field entitled ‘Pause’, while
the time period between successive packets containing the information is set in the field „Sending” (see Fig.4).
Results of the conducted study are presented in Table 3.
Reliability of UDP Protocol Data Transmission
in Electrical Power Telecomunication Systems Interworking with the Internet
83
Table 3. Impact of redundancy of sent segments on the quality of UDP transmission process
Software settings and study results
Sending / Pause
10 s / 10 ms
10 s / 3 s
60 s / 10 ms
60 s / 3 s
Amount of data
8655
8644
1438
1436
Number of errors
182
82
19
13
Number of transmission pauses
88
1
6
1
Covering coefficient
Full
After analyzing the data from Table 3, it can be concluded that separating redundant data segments in
time has a positive impact on the reliability of the transmission process. In both of the examined transmissions,
i.e. for the sending times of 10 and 60 seconds and the pauses of 10 ms and 3 s respectively, the number of lost
data segments is smaller when the pauses are longer and amount to 3s. This is caused by the fact that interferences in the transmission channel are as a rule of short duration, which means that if the channel remains nonactive for a sufficiently long period of time, then despite the sending of three segments containing the same data
at 10 ms time intervals, they are not going to reach their destination. Furthermore, a more detailed analysis of
the data collected in programme logs, allows us to observe that longer pauses of transmission channel inactivity
appear in the same way in all the programme logs and increasing the pause between redundant data signals can
not change anything. The degree to which such a situation may appear is indicated by the parameter ‘Covering
coefficient” (see Table 3). In the discussed case, long duration of channel inactivity was recorded in all the programme logs and lasted about 13 minutes.
CONCLUSION
The dynamically developing infrastructure of the Internet working together with the WAN teletransmission network functioning in electrical power system may constitute the foundation for constructing inexpensive
systems for monitoring objects distributed over a large geographic area [4]. The research work carried out by
the authors indicates that:
• quality of data transmission based on the standard UDP protocol in the examined networks is sufficient for
executing monitoring and diagnostic tasks in which the required reaction time is measured in the range
of minutes or even hours
• due to the possibility of the occurrence of random transmission errors it is appropriate to employ additional methods which effectively improve the reliability of transmission processes, for example, establishing redundant transmission channels and sending extra data segments containing the same set of
information at appropriately selected time intervals
• protecting data and system against sabotage by other users of the network is a separate issue which requiring further research.
LITERATURE
1. Cisco Networking Academy program, CCNA 1 and 2 Companion Guide, 3rd Edition. Cisco Systems Inc., 2004.
2. Cisco Networking Academy program CCNA 3 and 4 Companion Guide, 3rd Edition. Cisco Systems Inc., 2004.
3. Poradnik inżyniera elektryka, v. III, chapter 7, Editor in chief Prof. Zbigniew Szczerba. Wydawnictwa Naukowo-Techniczne, Warszawa, 2005.
4. Porzeziński M., Mazur L., Remote monitoring and control of technical systems using internet network technology,
Proceedings of the IEEE International Conference on Technologies for Homeland Security and Safety TEHOSS, Gdańsk
2005.
5. http://www.ietf.org/rfc/rfc0768.txt (March 2009).
6. http://www.ietf.org/rfc/rfc0793.txt (March 2009).