EVALUATION OF PARAMETERS FOR IMPROVING HANDOFF PERFORMANCE IN MOBILE WIMAX NETWORKS

International Journal of Distributed and Parallel Systems (IJDPS) Vol.3, No.5, September 2012
DOI : 10.5121/ijdps.2012.3508 85
EVALUATION OF PARAMETERS FOR IMPROVING
HANDOFF PERFORMANCE IN MOBILE WIMAX
NETWORKS
Sharmistha Khan1
, Ji Hyun Lee2
, Golam Rosul Khan1
1
Department of Electrical and Computer Engineering, Prairie View A&M University,
Prairie View, TX
skhan2@student.pvamu.edu; gkhan@student.pvamu.edu
2
Department of Electrical Engineering, Tuskegee University, Tuskegee Institute, AL
jhlee@mytu.tuskegee.edu
ABSTRACT
Mobile WiMAX is the latest technology that promises broadband wireless access over long distance. In
order to support mobility and continuous network connectivity in mobile WiMAX, it is necessary to
provide handoff. Handoff is an essential process in wireless networks to guarantee continuous and
effective services. This paper focuses on the analysis of the performance of the handoff process in Mobile
WiMAX. This paper aims to find out the factors/parameters of the WiMAX module that affect handoff
performance the most, such as handoff duration of less than 50 ms and mobility speed up to 120km/hour.
Simulation results show that some of the parameters of WiMAX do not have any influence on handoff
latency, while others have great impact towards achieving shorter handoff duration time. The results also
show that handoff times could vary for different speeds of the Mobile Station (MS).
KEYWORDS
Handoff , Mobile WiMAX.
1. INTRODUCTION
In early 2001, the WiMAX Forum developed the most modern wireless technology named
WiMAX, which is a telecommunications protocol that provides fixed and fully mobile Internet
access. There are many positive aspects of this technology; one of the most important is the
support of a large coverage area. WiMAX provides the support of wireless connectivity with a
minimum range of 30 miles. WiMAX technology also offers high speed broadband access to
mobile internet which transfers data, voice, and video. In WiMAX, when a user uses a 20 MHz
data rate the bandwidth for this data rate can be up to 75 Mbps. Whereas WiFi offers a short
range of data transfer with a maximum bandwidth of 54 Mbps [1] [2].
The IEEE 802.16 standard forms the basis of WiMAX technology. The WiMAX Forum
gradually improves the functionality and approves different generations for this standard.
Usually these standards differ in two different forms generally known as “802.16d” or “Fixed
WiMAX” and “802.16e” or “Mobile WiMAX”. The 802.16d-2004 standard has no support for
mobility. Mobility support is when a user is in motion or in a vehicle and can easily access the
wireless network. To solve the problem of mobility the IEEE 802.16e-2005 standard was

86
published and had full support for mobility. Since this standard introduced the support for
mobility it is known as “Mobile WiMAX” [2].
In Mobile WiMAX networks when a Mobile Node (MN) changes its location the MN moves the
point of attachment to the network. In this situation it is essential to provide continuous network
connectivity to satisfy high levels of mobile service quality. Here the major issue concerning
implementation of Mobile WiMAX is providing effective handoff. Handoff is the process of
changing a Mobile Station’s (MS’s) network connectivity from one Base Station (BS) to another
BS. Providing the support for ongoing video call, or Voice over IP (VoIP), conversations for
mobile users makes it necessary to make the handoff process as fast as possible. Therefore, in
order to decrease handoff interruption time and unnecessary call drops it is important to
implement different handoff optimization methods. Providing different Quality of Service (QoS)
support such as high speed data transmission, low handoff latency, a reduced amount of packet
loss in Mobile WiMAX network, and various handoff enhancement approaches have been
proposed.
For the past few years of Mobile WiMAX technology many researchers have explained the
handoff process in Mobile WiMAX along with different handoff techniques. Some proposed
effective algorithms for improving handoff methods; while others proposed vertical handoff
schemes with the aim of reducing handoff signaling overhead on the wireless backbone and
providing a low handoff delay to mobile nodes [3]. Other researchers have introduced Secure
Internet Protocol (IPSec) based end-to-end securing solutions for real-time services, which
provide a secure and fast seamless handoff solution while preserving the QoS and security when
moving between heterogeneous access networks [4]. Again, various fast handoff techniques have
also been suggested in literature to meet QoS requirements. In general, we found that different
researchers worked on different aspects but with the same general motto, to improve handoff
performance in Mobile WiMAX. All of the handoff techniques were implemented in such a way
that they always supported short interruption time, low handoff latency, high speed, and were
cost effective. Still, a great deal of research is being conducted in order to determine a more
efficient handoff process to reach the desired performance. If we want to improve the
performance of the handoff methods we need to first analyze the present condition of the
performance of the implemented handoff processes in Mobile WiMAX. The performance of the
handoff processes are regulated by the parameters of the WiMAX module. Therefore, we need to
adjust those parameters in order to increase handoff performance by achieving shorter handoff
duration time. The velocity of the MS may also have some impact on the performance of the
handoff process. Therefore, our main focus is to analyze the performance of the handoff methods
in Mobile WiMAX by investigating different performance related parameters with varied MS
velocity.
This paper is organized as follows. After the introduction, the handoff in Mobile WiMAX is
described in Section 2. Simulation results are presented and discussed in Section 3. Based on the
simulation results, conclusions are given in Section 4.
2. HANDOFF IN MOBILE WIMAX
Mobility is one of the most important features in wireless cellular communication. When a
mobile node changes its location from one place to another, to provide mobility support, the
minimum requirements are reliable, efficient, and continuous data transmission across the air
interface. Generally, this continuous data transmission service can be achieved by implementing

87
a handoff from one cell to another. Handoff is a process with the intention of changing the
network access point of a mobile node without any data loss or disturbance of the current
connection while a call is in progress. Therefore, in mobile WiMAX, the basic meaning of
handoff process is to provide uninterrupted connectivity when a MS transfers from the air-
interface of one BS to the air-interface of another BS. Though handoff is usually understood as a
change of physical connection of a serving BS, it does not mean that the BS must be changed.
Sometimes handoff can occurr within different channels, this means changing from one
frequency to another while the serving BS remains the same. In mobile WiMAX, it is called
intra-cell handoff while the second option is called inter cell handoff. Again, in some other
cases, handoff can occur within different network technologies. When handoff occurs within a
single network it is known as horizontal handoff, while handoff between different networks is
known as vertical handoff. To implement a handoff process requires at least two BSs, the
currently serving BS and target BS, and a MS within the range of the two BSs [5] [6].
2.1. Reasons of Handoff
Telecommunication reasons for conducting handoff can vary. Here we have mentioned some of
these cases.
• When the cellular phone is moving away from the area covered by the serving BS, the device
eventually goes outside the range of the serving BS. So in order to avoid call termination, the
call needs to be transferred to an area covered by another BS [5].
• When the signal strength is not enough to maintain a proper call at the edge of a serving cell,
the call needs to be transferred to another cell [6].
• When the capacity for connecting new calls of a cell in a BS becomes full or more traffic is
pending, capacity must be made available for users who can only be connected on that cell.
Therefore, the existing calls or the new calls from a phone that is located in an area that is
overlapped by both cells can be transferred from the first cell to the second cell [5] [6].
• In non-CDMA networks several phones use different cells but the same channel. As a channel
is being used by several phones, disturbing co-channel interference comes from another phone.
Therefore, in order to avoid interference, the call is transferred to a different channel in the same
cell or to a different channel in another cell [5].
• In vertical handoff, a faster network is occasionally available. So the phone changes its
network to the cheaper one [6].
2.2. Types of Handoff
Handoff is divided roughly into two broad categories: hard and soft handoffs with different
variants based on the technology used. They are also known as “break before make” and “make
before break” handoffs. Generally, in hard handoffs old connections are broken before new
connections are created; in soft handoffs, both existing and new connections are used during the
handoff process. IEEE 802.16e divided the handoff process into three major classifications Hard
Handoff (HHO), Macro Diversity Handoff (MDHO), and Fast Base Station Switching (FBSS),
while HHO is mandatory in WiMAX systems, the other types of handoff are optional [7].

88
The HO procedures can be divided into two major phases known as network topology
acquisition parts and the actual HO process. The network topology acquisition phase can be
divided into 3 sub procedures: network topology advertisement, MS scanning of neighbor BSs,
and association procedure. These three sub-procedures are executed before HO through the
backbone network. Again, the actual HO phase can be divided into other sub phases: cell
selection, handoff decision and initiation, synchronizing with new downlink and obtain
parameters, obtaining uplink parameters, ranging and uplinking parameter adjustment, MS re-
authorization, re-registering, and termination with the serving BS [6]. All of these sub-phases are
executed during HO.
3. SIMULATION AND RESULTS
The main goal of these simulations was to investigate the impact of different parameters of the
WiMAX module during handoff as well as examine the properties of Mobile WiMAX. A simple
and basic scheme was designed in order to keep the simulation process simple and trouble-free.
To implement handoff in Mobile WiMAX we have used the Network Simulator 2 (NS-2 Version
2.29), which forms a basis for the simulations. Additionally, in order to support Mobile
WiMAX, we have to use two other particular modules known as WiMAX and Mobility module
from the NIST project [8]. Again, the additional modules also lack some functionality to support
the implementation of Mobile WiMAX. Therefore, we tried to get the results and measurements
as accurately as possible from the simulation according to the simulator’s perspective.
The simulation scenario consists of three 802.16e BSs (BS0, BS1, BS2) and one MS; where the
BSs are aligned in a line and the MS travels through the coverage areas of these three BSs. The
BSs are located in such a way that there will be some overlap within the coverage areas of the
neighboring BSs. We selected constant values for some of the elements such as cell size,
transmission power of BSs, and the route of the MS. Our main concern was to adjust the
parameters of the WiMAX module to achieve faster handoff times in the NS-2. We simulated
the whole scenario with speeds of 10 meters/second to 40 meters/second with 1 meter/second
steps. The first handoff will occur when the MS reaches the edge of the coverage area of BS0
and it needs to change its connection access point from BS0 to BS1 for continuous service. The
second handoff will occur when the MS reaches edge of the coverage area of the BS1 where it
needs to once again change its connection access point from BS1 to BS2.
Figure 1. Simulation Scenario

89
In simulation, our main goal was to find out the parameters that have the greatest impact on the
performance of the handoff process in Mobile WiMAX. According to the scenario, we have
measured the time for the two handoffs.
3.1. Adjusted Parameters
In order to achieve less handoff time we have adjusted the list of parameters of the WiMAX
module. When we changed the value of a parameter we ran the simulation and measured the
handoff time for each adjustment. We changed the values of some parameters from the example
file of the WiMAX module which is programmed with predefined values. Then we adjusted the
rest of the parameters that are offered by the MAC/ 802.16 in NS-2 and have some impact on the
handoff time. Some of the parameters were set as their default value in the simulation code.
When we completed the adjustment successfully, we found that some of the parameters did not
have any impact on handoff latency, while other parameters had a great influence. We will
discuss these parameters in brief description in the simulation result section.
3.2. Constant Parameters
As the implementation process of handoff in Mobile WiMAX is quite complex we want to keep
the simulation process simple. Therefore, we have used constant values for some of the
parameters that are related to the configuration of the BSs such as the BS coverage area,
transmission power, and operating frequency. By keeping these parameters constant we could
adjust other parameters to achieve less handoff duration time for both handoffs. For each
adjustment we used the same simulation scenario, where the number of BSs, location of the BSs,
and the route of the MS were mentioned. The bit rate for transmitting data was also kept constant
around 1.4 Mbps. From literature, we found that to send a MPEG-1 file a data transmission bit
rate of 1.5 Mbps is required [9]. Comparing this value with the used data bit rate of this
simulation can be considered as a moderate bit rate. The packet size and the duration time for
sending a packet were also kept constant, 1600 bytes and 15 ms respectively.
3.3. Simulation Result
Our main focus of this simulation was to adjust the values of the different parameters with varied
MS speed during the two handoffs. After completing the adjustment of all the parameters, we
found that the handoff times were 36.49 ms for the first handoff and 37.76 ms for the second
handoff, both of which are less than 50 ms which is the standard for the handoff latency defined
by the WiMAX forum. The following section will discuss the influence of the parameters that
we adjusted in the simulation, and later on we will discuss the impact of varied MS speeds on
handoff times.
3.3.1. Lgd_factor
The Link Going Down Factor is one of the most important parameters of the WiMAX module,
as it determines the sensitivity of detecting a falling link. We need to set this factor to generate a
Link Going Down. When the received power of a signal is less than factor RXThresh, a trigger
is generated to initiate scanning for neighbor BSs.
For a certain coverage area, at the edge, RXThresh value defines the limit of that area that means
outside of that area the data packet will be discarded. The higher the factor the sooner the trigger
is generated. If we set the factor value to 1, it does not detect any falling link. The optimal factor
value ranged from 1.3 to 1.6 as shown in Figure 2, so we chose the best value as 1.5.

90
Figure 2. Handoff Times vs Link Going Down Factors
3.3.2. Scan_iteration
Scan iteration defines the requested number of iterating scanning interval by an MS, which
means how many times the MS will complete the scanning procure. We have found that the
more iteration time provided, the longer handoff duration time. We have seen from Figure 3 that
the best value is two iterations.
Figure 3. Handoff Times vs. Scan Iteration
10
20
30
40
50
60
70
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5
HandoffTimes(ms)
Link Going Down Factors
Handoff Times with Different Link Going
Down Factors
10
20
30
40
50
60
70
1 2 3 4 5 6 7
HandoffTimes(ms)
Scan Iteration
Handoff Times with Different Scan Iteration

91
3.3.3. Scan_duration
Scan duration is another important parameter of the WiMAX module, which defines the length
of the scanning period in frames. In order to make a successful scan, long duration time is
desirable. On the other hand, short duration time for scanning also produces moderate elapsed
time. We have found the best value is six frames with frame duration of 5 ms.
3.3.4. Interleaving_interval
Interleaving interval is the parameter that defines the time duration between the normal operation
and scanning periods of the MS in frames. We have seen from Figure 4 that if we set the value
less than or equal to twenty frames it will affect the handoff time with a small variation.
However, increasing the value to more than twenty frames causes longer handoff times. We got
the best value at nine frames.
Figure 4. Handoff Times vs. Interleaving Periods
3.3.5. T21_timeout
T21 timeout is the parameter that defines the timeout value for the MS for searching the DL-
MAP message, which means the MS needs to find a DL-MAP message within this period. We
found the same handoff time when the timeout was set between 5 to 27 ms, as shown in Figure
5. However, if we increase 1 ms after 27 ms it increases the handoff time drastically. We set this
timeout value to 20 ms.
3.3.6. Frame_duration
Frame duration defines the length or the size of a frame. From literature, we have found that a
suitable value for the frame duration is 4 or 5 ms. We chose 5 ms for our simulation.
10
30
50
70
90
110
130
150
1 3 5 7 9 11 13 15 17 19 21 23 25
HandoffTimes(ms)
Interleaving Periods (frames)
Handoff Times with Different Interleaving
Periods

92
Figure 5. Handoff Times vs. T21 Timeout Values
3.3.7. Lost_dlmap_interval
Lost DL-MAP interval defines the timeout value for last reception of a DL-MAP message in the
MS, which means that when a new DL-MAP message does not arrive to the MS within this time
period the synchronization between the MS and BS will be interrupted.
From the simulation, we have investigated that a value of less than 9 ms causes a longer handoff
time. Sometimes it even increases the handoff time by several seconds. We have seen from
Figure 6 that the best result comes at 15 ms.
Figure 6. Handoff Times vs. Lost Downlink-MAP Interval
10
25
40
55
70
85
100
115
130
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29
HandoffTimes(ms)
T21 timeout (ms)
Handoff Times with Different T21 Timeout
Values
10
25
40
55
70
85
100
115
130
1 3 5 7 9 11 13 15 17 19
HandoffTimes(ms)
Lost Downlink-MAP Interval (ms)
Handoff Times with Different Lost Downlink-
Map Interval

93
3.3.8. Client_timeout
This is one kind of timer value that detects the range out of the MS. We have seen that smaller
timer value is not efficient for a MS travelling within a large coverage area. On the other hand,
greater timer values also cause longer delays. We found the best value to be 6 ms.
3.3.9. Lost_ulmap_interval
The functionalities of this parameter are similar to the previous one, but it is used for UL-MAP
message. From Figure 7, we have found that values less than 5 ms cause a longer delay in
handoff. So, the preferable value during simulation is 15 ms.
Figure 7. Handoff Times vs. Lost Uplink-MAP Interval
3.3.10. Ranging_backoff_start
Ranging backoff start is a parameter used during the ranging phase of the handoff. It defines the
size for initial backoff window. We have found the same handoff time for a window size of 2 to
5 slots, which is shown in Figure 8.
If we increase the slot size to more than 5 it increases handoff time. On the other hand, we have
also seen that a slot size of 1 also causes longer handoff time. Therefore, we have used 3 slots as
a backoff window size for initial ranging.
10
25
40
55
70
85
100
115
130
1 3 5 7 9 11 13 15 17 19
HandoffTimes(ms)
Lost UPlink-MAP Interval (ms)
Handoff Times with Different Lost UPlink-
Map Interval

94
Figure 8. Handoff Times vs. Ranging Backoff Window Size
3.3.11. MaxRADelay
This parameter was used to define the maximum delay for sending a reply message to an RS.
Larger values cause larger handoff delays. We have seen from Figure 9 that a value less than or
equal to 6 ms achieves the best results. Therefore, we have selected the value for the maximum
delay as 4 ms.
Figure 9. Handoff Times vs. Maximum RA Delay Values
10
20
30
40
50
60
70
80
90
100
1 2 3 4 5 6 7 8 9 10
HandoffTimes(ms)
Ranging Backoff Window Size (slot)
Handoff Times with Different Ranging Backoff
Window Size
10
20
30
40
50
60
70
80
1 2 3 4 5 6 7 8 9 10
HandoffTimes(ms)
MAX RA Delay (ms)
Handoff Times with Different Maximum RA
Delay

95
3.3.12. MinDelayBetweenRA
This parameter overrides the default value of the minimum interval time between two
consecutive RAs and has a great impact on handoff time.
Figure 10. Handoff Times vs. Minimum RA Delay Values
From Figure 10 we have found that the best handoff time occurs when the delay was at a
minimum of 35 ms.
3.3.13. Queue_length
Queue length parameter is basically used during data transmission from the MS. It defines the
length of the buffer for a sending packet. As the data rate we have used in the simulation is not
very large, it does not require a large buffer. The data rate for packet size 2 was used in the
simulation, which is enough to run the simulation. We found that if we chose a buffer size of 1
packet, some data fell apart during transmission. So the best value we found is 2 packets.
3.3.14. T44_timeout
This parameter defines the timeout value for a successful scanning request during the scanning
phase. We have investigated from the simulation result that this value needs to be more than 5
ms. We chose the default value 10 ms, which was defined in the WiMAX module.
3.3.15. MinRtrAdvInterval
This is one of the most important parameters of the ND module and defines the minimum
interval between consecutive RAs. The large interval value causes longer handoff time. We have
found that the best result comes with the interval period of zero.
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30 35 40 45 50
HandoffTimes(ms)
MIN RA Delay (ms)
Handoff Times with Different Minimum RA
Delay

3.3.16. MaxRtrAdvInterval
This parameter is similar to the one used above but
found that the best result comes with the interval period of 10 ms.
3.4. Handoff Times with Varied MS Velocity
Investigating the impact of different velocities of the MS is one of our main concerns
research. During the adjustment of the different parameters, we also varied the velo
MS. We have done simulation by changing the speeds of the MS from 10 m/s to 40 m/s with a 1
m/s step. We chose the highest speed 40 m/s as it equals 144 km/h, which is well above to the
standard limit (100 km/h) for a seamless handoff [7].
When we adjusted the parameters we varied the speed and investigated the results of the handoff
times. The impacts of the different velocities of the MS on handoff times
11 and Figure 12. We found that for the MS speed of 10 m/s to 19 m/s, the handoff times varied
consistently by a few milliseconds within the r
ms. After 20 m/s MS speed, the handoff times increased inconsistently and went up to
approximately 150 ms, much larger than the latency limit. But this result can differ for real time
traffic because here the MS speeds were controll
hand, due to the lack of the simulator, we could not implement some of the major features of
Mobile WiMAX like QoS, authentication, and service flow. Therefore, the results can also vary
for those missing features.
Figure 11. Simulated Handoff Times with Varied MS Speeds for First Handoff
0
15
30
45
60
75
90
105
120
135
150
10 12 14 16
HandoffTime(ms)
Handoff 1
lar to the one used above but is used for maximum interval. We have
found that the best result comes with the interval period of 10 ms.
Handoff Times with Varied MS Velocity
different velocities of the MS is one of our main concerns
research. During the adjustment of the different parameters, we also varied the velo
simulation by changing the speeds of the MS from 10 m/s to 40 m/s with a 1
/s step. We chose the highest speed 40 m/s as it equals 144 km/h, which is well above to the
standard limit (100 km/h) for a seamless handoff [7].
mpacts of the different velocities of the MS on handoff times are presented in
found that for the MS speed of 10 m/s to 19 m/s, the handoff times varied
consistently by a few milliseconds within the range of 35 ms to 45 ms, which are
. After 20 m/s MS speed, the handoff times increased inconsistently and went up to
traffic because here the MS speeds were controlled smoothly by the simulator. On the other
Simulated Handoff Times with Varied MS Speeds for First Handoff
18 20 22 24 26 28 30 32 34 36
Velocity (m/s)
Handoff 1 50 ms limit
96
is used for maximum interval. We have
different velocities of the MS is one of our main concerns of this
research. During the adjustment of the different parameters, we also varied the velocity of the
simulation by changing the speeds of the MS from 10 m/s to 40 m/s with a 1
/s step. We chose the highest speed 40 m/s as it equals 144 km/h, which is well above to the
presented in Figure
found that for the MS speed of 10 m/s to 19 m/s, the handoff times varied
less than 50
. After 20 m/s MS speed, the handoff times increased inconsistently and went up to
by the simulator. On the other
Simulated Handoff Times with Varied MS Speeds for First Handoff
38 40

97
Figure 12. Simulated Handoff Times with Varied MS Speeds for Second Handoff
4. CONCLUSION
In this paper the performance of the handoff process in Mobile WiMAX network was
investigated. Simulation was done with the NS-2 simulator. As NS-2 does not have all of the
necessary components to support the implementation of Mobile WiMAX, two additional
modules known as mobility and WiMAX from NIST are used in parallel with it. The parameters
of the WiMAX module were adjusted and investigated to achieve shorter handoff duration time
during both handoffs. After the adjustment of all the parameters, the velocity of the MS was
varied within the range from 10 m/s to 40 m/s. The main objective of this research was to find
out the parameters that have the greatest impact on handoff performance as well as handoff
latency during handoff in Mobile WiMAX. We have seen that some of the parameters have a
direct influence on the handoff latency while others do not.
REFERENCES
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[2] Encyclopedia Britannica, “WiMAX,” <url:
http://www.britannica.com/EBchecked/topic/1017801/WiMax >, retrieved December 2010.
[3] Zhang, Y., “Vertical Hando_ between 802.11 and 802.16 Wireless Access Networks,” Waterloo,
Canada, pp. 3, 2008.
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Handofftime(ms)
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Handoff 2 50 ms limit

98
[4] Diab, B. W.,Tohme, S., “End-to-End Security and Seamless Handover Solution for Real-Time
Communications over 3G Networks,” ACM, New York, pp.1, 2009.
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Journal of Computational Engineering Research, ISSN: 2250–3005, Vol. 2, July-August 2012.
[6] Makelainen, A., “Analysis of Handoff Performance in Mobile WiMAX,” pp.12-68, 2007.
[7] WiMAX Forum: Feb 2006. “Mobile WiMAX – Part I: A Technical Overview and Performance
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[8] NIST Information Technology Laboratory “Seamless and Mobility,” < url:
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[9] ISO. "ISO/IEC 11172-1:1993 - Information technology -- Coding of moving pictures and associated
audio for digital storage media at up to about 1,5 Mbit/s -- Part 1: Systems".<url:
http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=19180> Retrieved
2011-01-10.

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