19-06-2014, 12:43 PM
Recycling of Bandwidth in Metropolitan Area
Networks(MAN)
Recycling of Bandwidth.pdf (Size: 359.22 KB / Downloads: 38)
Abstract
In case of variable bite rate application, the subscriber station requires bandwidth for downlink and
uplink of data transmission. As subscriber station cant estimate how much data it wants and to ensure
the QOS guaranteed services, It may reserve more bandwidth than its demand. As a result, the reserved
bandwidth may not be fully utilized all the time. Hence there is a wastage of bandwidth. This paper
consists of a scheme named as ‘Bandwidth Recycling’, to recycle the unused bandwidth without changing
the existing bandwidth reservation. In this scheme the subscriber station will used the available unused
bandwidth. By this system throughput can also increase while maintain the same quality of services. In
this scheme we use mathematical analysis and simulation. And this results in the scheme can recycle
35% of unused bandwidth on average. The extension for this project can also be showed by the three
scheduling algorithms. Thus the simulation results to improve overall throughput by 40% in a steady
network.
Introduction
The worldwide interoperability for microwave Access (Wimax) which is based on
IEEE 802.16 standard, is designed to facilitate services with high transmission rate for data and
multimedia application in metropolitan areas. The physical (PHY) and Medium Access Control
(MAC) layers of WIMAX have been specified in the IEEE 802.16 standards. Many advanced
communication technologies such as Orthogonal Frequency Division Multiple
Access(OFDMA) and Multiple-input & Multiple-output(MIMO) are also present in the
standard. Supported by these modern technologies, WIMAX is able to provide a large services
coverage, high data rates & QOS guaranteed services. Because of these features WIMAX is
considered as a promising alternative for last mile broadband wireless access (BWA).
In order to provide QOS guaranteed services, the subscriber statics (ss) has to reserve
necessary bandwidth from the base station(ss) has to reserve necessary bandwidth from the
base station(BS) before any data transmission. In order to serve variable bit rate(VBR)
applications, the SS tends to keep the reserve bandwidth to maintain the QOS guaranteed
International Journal of Computer Science & Information Technology (IJCSIT) Vol 3, No 6, Dec 2011
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services. AS a result, the reserved bandwidth transmission data may be more than the amount
of transmission data and the reserved bandwidth may not be fully utilized all the times.
Although the amount of reserved bandwidth is adjustable via. Making bandwidth request(BR),
the adjusted bandwidth is applicable to the next coming frame because of this the unused
bandwidth in the current frame has no change to be utilize.
It is very difficult to adjust the amount of reserved bandwidth. Thus the SS may be
exposed to the risk of degrading the QOS. To improve the bandwidth utilization while
maintaining the same QOS guaranteed services, our research objective is two fold:
1. The existing bandwidth reservation is not changed to maintain the same QOS
guaranteed services.2. Our research work focuses on increasing the bandwidth utilization by
utilizing the unused bandwidth.
The concept in our scheme, “Recycling the bandwidth in the current frame” is that the
remaining SSs other than the transmitting station will get a chance to utilize the bandwidth
which remained unused. The unused bandwidth is not supposed to occur regularly, our scheme
allow SSs with non-real time applications, which have more flexibility of delay requirements to
recycle the unused bandwidth. Thus, our objective is clear that the maximum throughput with
the unchanged QoS.
IEEE 802.16 standard says that each SS in order to transmit data in the current frame,
has to be scheduled on the Uplink map. We called SSs as the TSS(Transmission SSs) in this
paper. Later it is the work of the BS to schedule a SS to each TS, so that the unused bandwidth
of the TS is utilized by the backup SS. So, we call the backup SS as the complementary station
(ss).
In the IEEE 802.16 standard, BR’s are made in pre-connection basis. The BS allocates
bandwidth in per SS basic. It gives the SS flexibility to allocate the granted bandwidth to each
connection locally. Therefore, the unused bandwidth is defined as the granted bandwidth which
is still available after serving all connections running on the SS. The TS informs the CS about
the unutilized bandwidth through a special message called Releasing message(RM). However,
because of the variety of the geographical distance between TS and CS and the transmission
power of the TS, the CS may not receive the RM. Our theoretical analysis shows that this
probability is least 42% which is confirmed by our simulation. But still there are some factors
that may effect this scheme:
1. The CS cannot receive the RM
2. The CS does not have non-real time data to transmit while receiving a RM.
To mitigate those factors, additional scheduling algorithms are proposed algorithm
further improve the average throughput by 4% in a steady N/W.
Background Information
IEEE 802.16 is written by a working group established by IEEE Standards Board in
1999 to develop standards for the global deployment of broadband ‘Wireless Metropolitan Area
Networks’. Then although the 802.16 family of standards is officially called wireless MAN in
IEEE, it has been commercialized under the name “WIMAX”.
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In the IEEE 802.16 standard the physical (PHY) and Medium Access Control (MAC)
layers of wimax have been specified. AND in this three types of transmission mediums
supported as the physical layer (PHY). They are single channel (SC) i.e., Using single signal at
a given frequency and bandwidth, Orthogonal Frequency-Division Multiplexing (OFDM) i.e., it
is essentially identical to coded OFDM (COFDM) and it is a frequency division multiplexing
(FDM) scheme utilized as a digital multi –carrier modulation method orthogonal frequencydivision
multiple access (OFDMA) is a multi user version of the popular orthogonal
frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is
achieved in OFDMA by assigning subsets of subcarriers to individual users. Hence OFDMA
assume as the PHY in our analytical model since it is employed to support mobility in IEEE
802.16e standard (the true mobile wimax standard of 802.16e is divergent from fixed wimax)
and the scheme working in this should also work in others. Various types of modulations
supported by OFDMA is BSPK (binary phase shift keying modulation) QPSK (quadrature
phase shift keying modulation), 16-QAM (quadrature amplitude modulation) 64-QAM.
Here focused on point to multi point(PMP) made in which the ss will not allow to
communicate with another ss but the bs directly. Depending on the transmission direction, bs
and ss communication classified into downlink(DL) and uplink(UL) transmissions. Hence the
former transmission is BS TO SS. Conversely SS to BS.
As IEEE 802.16 supports two types of transmission modes. Both UL and DL
transmissions can not be operated simultaneously in time division duplex(TDD) but it can in
frequency division duplex (FDD) mode. In this paper we focused on the TDD mode. BS is
responsible for scheduling both UL and DL transmissions in wimax. All these behavior is
expreddes in a MAC frame.
In IEEE 802.16 standard the structure of a MAC frame contains two parts, UL and DL
subframe for UL transmissions and DL transmissions respectively. In this IEEE 802.16
networks is coordinated by the BS. All the required details including burst profiles and offsets
is in the DL and UL maps, which are broadcasted at the beginning of a MAC frame. This is a
connection-oriented. It gives the advantage of having better control over network resource to
provide QOS guaranteed services. It support wide variety of applications, the IEEE 802.16
standard classifies traffic into five scheduling classes : Unsolicited Grant Service(UGS) i.e., is a
service flow in which the transmission system automatically and periodically provides a defines
number of timeslots and fixed packet size that is used by a particular receiver., Real Time
Polling Service(rtPS) i.e., It is designed to support real-time service flows that generate variable
size data packets on a periodic basis, such as MPEG video,, Non-Real Time Polling
service(ntrPS) i.e., It is designed to support non –real time service flows that require variable
size Data Grant Burst Types on a regular basis, such ashigh bandwidth FTP(File Transfer
Protocol), Best Effort(BE), Extended Real Time Polling Service(ertPS) i.e., it is a scheduling
mechanism that builds on the efficiency of both UGS and rtps. It is designed for realtime traffic
with variable data rate over the WIMAX. Each application is classified inti one of the
scheduling classes and establish a connection with the BS based on its scheduling class. The BS
assigns a connection ID(CID) to each connection. The bandwidth reservation is made based on
the CID via sending a BR when receiving a BR, the BS can either grant or reject the BR
depending on its available resourse and scheduling polices.
International Journal of Computer Science & Information Technology (IJCSIT) Vol 3, No 6, Dec 2011
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SS may request bandwidth using two different types of BRs: incremental BR and
Aggregate BR. The former allow the SS to indicate the extra bandwidth required for a
connection. SS uses aggregate BR to specify about its connection. The BS resets its perception
of that service’s needs upon receiving the request. Consequently, the reserved bandwidth may
be decreased.
Protocol
IEEE 802.16 standard specifies that the unused reserved bandwidth has to be
recognized with some value. For this we use a padding value called stuff byte value(SBV) i.e.,
0xFF. If the size of the unused region is at least the size of a MAC header, the entire unused
region is initialized as a MAC PDU. The padding CID is used in the CID field of the MAC
PDU header. In this research, we intend to recycle the unused space for data transmissions.
Instead of padding all portion of the unused bandwidth in our scheme, a TS with unused
bandwidth transmits only a SBV and a RM. The SBV is used to inform the BS that no more
data are coming from the TS. On the other hand, the RM comprises a generic MAC PDU with
no payload. The mapping information between CL and UL map is based on the basic CID of
each SS. The CID field in RM should be filled by the basic CID of the TS. The transmission
range of the TS should be large enough so that the corresponding CS will receive the RM. To
maximize the transmission coverage of the RM, one possible solution is to increase the
transmission power of the TS while transmitting the RM. However,
Analysis
The presence of percentage of potentially unused bandwidth occupied in the reserved
bandwidth is critical for the potential performance gain of our scheme. we investigate this
percentage on VBR traffics which is popularly used today. In order to that, in our scheme, each
TS should transmit a RM to inform its corresponding CS when it has unused bandwidth.
However, the transmission range of the TS may not be able to cover the corresponding CS. It
depends on the location and the transmission power of the TS. There is chance that CS does not
receive the RM, thus results in failure of utilizing unused unused bandwidth. So, the benefit of
our scheme is reduced. In this section, we analyze mathematically the probability of a CS to
receive a RM successfully. As a result probability affects the bandwidth recycling rate
(BBR).BBR stands for the percentage of the unused bandwidth which is recycled. Moreover,
the performance analysis is presented in terms of throughput gain (TG). At the end, we evaluate
the performance of our scheme under different traffic load. All analytical results are validated
by the simulation.
6.1 The Probability of RMs received by the corresponding CSs Successfully:
Let us assume a BS resides at the center of a geographical area. There are n SSs
uniformly distributed in the coverage area of the BS. Since PMP mode is considered, the
transmissions only exits between BS and SSs. Moreover, each SS may be in different locations.
The transmission rate of each SS may be variant depending on the PHY transmission
technology and transmission power. For a given SS, St ,let Rt (B) ,Rt
(Q) ,Rt
(16) and Rt
(16) denote as
the transmission range of BPSK,QPSK,16-QAM and 64-QAM,respectively. In our scheme, the
RM should be transmitted via the most robust modulation. Since it has the largest coverage of
RMs among all modulations supported by the IEEE 802.16 standard without adjusting the
transmission power. Based on the fixed transmission power, the relation of transmission range
between modulations is expressed as:
SIMULATION RESULTS
In this section, we first present our simulation model followed by introducing the
definition of performance metrics used for measuring the network performance. The simulation
results are shown as the third part of this section. At the end, we provide the validation of
theoretical analysis and simulation results.
7.1 Simulation Model
In our model we comprises one BS residing at the center of geographical area and 50
SSs uniformly distributed in the service coverage of BS. The parameters of PHY and MAC
layers used in the simulation are summarized in Table 1. PMP mode is employed in our model.
Since our proposed scheme is used to recycle the unused bandwidth in UL subframe, the
simulation only focuses on the performance of UL transmissions.
Simulation Results
Figure 4 presents the percentage of the unused bandwidth in our simulation traffic
model (i.e., UBR). It shows the room of improvement by implementing our scheme. From the
simulation results, we conclude that the average UBR is around 38%. In the beginning, the
UBR goes down. It is because each connection still requests bandwidth from the BS. As time
goes on, the UBR starts to increase when the connection has received the requested bandwidth.
After 75th second of simulation time, UBR increases dramatically due to the inactivity of real
time connections. The purpose to have inactive real time connections is to simulate a network
with large amount of unused bandwidth and evaluate the improvement of the proposed scheme
in such network status. The evaluation is presented in the later of this section.
CONCLUSIONS
Variable bit rate applications generate data in variant rates. It is very challenging for
SSs to predict the amount of arriving data precisely. Although the existing method allows the
SS to adjust the reserved bandwidth via bandwidth requests in each frame, it cannot avoid the
risk of failing to satisfy the QoS requirements. Moreover, the unused bandwidth occurs in the
current frame cannot be utilized by the existing bandwidth adjustment since the adjusted
amount of bandwidth can be applied as early as in the next coming frame. Our research does
not change the existing bandwidth reservation to ensure that the same QoS guaranteed services
are provided. The proposed scheme has provided a way for utilizing the unused bandwidth in
the current frame itself. In addition to the priority-based scheduling algorithm, other three
additional algorithms have been proposed to improve the recycling effectiveness by considering
various factors that effect our scheme. Further, the mathematical analysis and simulation model
has shown that the proposed scheme is worth enough to provide good throughput and also
ensure the QoS guaranteed services.