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					        DIFFERENT MAC PROTOCOLS FOR NEXT GENERATION
                   WIRELESS ATM NETWORKS

                                                Sami A. El-Dolil
             Dept. of Electronic and Electrical Comm. Eng., Faculty of Electronic Eng, Menoufya Univ.
                                            Msel_dolil@yahoo.com


                                                 ABSTRACT
                This paper presents a comparison between three proposed Medium Access Control
                (MAC) Protocols for next generation multimedia wireless ATM (WATM)
                networks. To support the ATM CBR, VBR, ABR services to end users, a MAC
                protocol must be able to provide bandwidth on demand with suitable performance
                guarantee. The protocols have been proposed to efficiently integrate multiple ATM
                traffics over the wireless channel while achieving high channel utilization. The
                objective of the comparison is to highlight the merits and demerits of the three
                proposed protocols.

               Keywords: Medium access control protocol and ATM network.


1   INTRODUCTION                                            wireless ATM networks. The three protocols are as
                                                            follow.
     Asynchronous transfer mode (ATM) was                     1. Dynamic Allocation TDMA MAC Protocol for
recommended            by        the        International        Wireless ATM Networks.
Telecommunication Union (ITU-T) to be the transfer            2. An Intelligent MAC Protocol for next generation
protocol of the broadband integrated services digital            Wireless ATM Networks.
network (B-ISDN). The concept of wireless ATM                 3. Contention and Polling based Multiple Access
(WATM) was introduced to extend the capabilities of              Control with minimum Piggybacking for
ATM to wireless arena in [1].                                    Wireless ATM Network.
     A major issue of WATM network is the                   Three performance metrics, namely cell loss
selection of a medium access control (MAC)                  probability, average cell delay, and throughput, are
protocol that will efficiently allocate the scarce radio    considered. Section II gives an overview and
resources among the competing mobile stations               description of the proposed protocols. In section III,
while satisfying the QoS required for each admitted         the source models are identified. Section IV,
connection.                                                 describes the resource allocation algorithm. An
     Several MAC protocols are proposed for                 evaluation of the performance of the proposed
wireless ATM network [2] – [9]. In [2], a novel             protocols is presented in section V. Finally, section
predictive approach is used to estimate the current         VI concludes the paper.
requirements for the connections. The variable bit
rate (VBR) traffic is divided into guaranteed and           2   SYSTEM DESCRIPTION
best effort traffic while the time to expiry algorithm
is adapted for voice and VBR slot allocation. In [3],       2.1 Air Interface Frame Structure
the leaky bucket algorithm with priority as well as             The proposed protocols use frequency division
the cell train concept achieves a fair and efficient        duplex (FDD) with a fixed frame length of
slot allocation. In [4], Packet Reservation Multiple        2 m sec. used for the uplink (UL) and the downlink
Access with Dynamic Allocation (PRMA/DA)                    (DL) channel. Fig. 1 illustrates the frame structure
MAC protocol adopts dynamic allocation algorithm            for the uplink channel. The channel bit rate is 4.9
in order to resolve the contention situation quickly        Mbps and the data slot size is 53 bytes. The number
and avoid the waste of bandwidth that occurs when           of slots per frame is 24 slots. The uplink frame is
there are several unneeded request slots. However           divided into control and data transmission periods,
the drawback is that this protocol does not use mini-       each consisting of integer number of slots. Slots
slots for the access request. In [6] the use of             assigned for control purpose are further subdivided
piggybacking information from VBR connection                into four control mini-slots with each mini-slot
improves the slot allocation for VBR traffic and            accommodating reservation mini-packet.
enhances the overall protocol performance.
  The current paper introduces a quantitative
comparison of three proposed MAC Protocol for


                     Ubiquitous Computing and Communication Journal                                             1
                         Control period                     Data Transmission
                                                                  period
                                    Figure: 1 the frame structure.

In the uplink channel, control slots provide a               where,
communication mechanism for a mobile station to
send a reservation request during the contention             TF: Frame duration (2 m-sec).
phase of the connection. The data slots are provided         Tint: Average inter-arrival time of ABR data message
with contention-free mechanism during the data               (100 m-sec).
transmission phase. An uplink control packet is sent
whenever a mobile station needs to inform the base           Int: largest integer value.
station with its traffic characteristics and source          Contention period is set to a constant number of
status.                                                      control mini-slots and this number is chosen to
Feedback for the uplink control packets is sent in the       satisfy the required QoS for voice traffic. The
downlink control packets.                                    contention process is divided to four stages:

2.2 Contention Access Scheme                                 First Stage: When the connection becomes active it
     The first and second protocols use the same                          randomly selects one of the 4
contention scheme and the length of the control                           subsequent frames to send its request
period is dynamically adjusted as a function of                           during the contention period.
contention traffic load. The control mini-slots are          Second Stage: If the connection exhibits collision in
used by the mobile stations to send their reservation                       the first stage it randomly selects one
requests in contention mode using slotted Aloha                             of the 3 subsequent frames to send its
protocol. To reduce the access time of real-time                            request during the contention period.
connection, which greatly affects the QoS of the
real-time services, we separate the control mini-slots       Third Stage: If the connection exhibits collision in
assigned to real-time and non real-time connections.                        the second stage it randomly selects
The number of control mini-slots assigned to real-                          one of the 2 subsequent frames to
time and non real-time connections is adaptively                            send its request during the contention
allocated with the collision status. The total number                       period.
of uplink control mini-slots ranges from 4 to 12             Fourth Stage: If the connection exhibits collision in
mini-slots. A priority is given to real-time                                the third stage it sends its request in
connections by assigning their control mini-slots                           every frame until the base station
first according to the number of collisions occurred                        successfully receives its request.
in the previous frame.
     In the third protocol, the control period is further    In every stage the connection randomly select one of
divided into contention and polling periods. Control         the available contention mini-slots in the selected
slots assigned in the control period are further             frame to send its reservation request. If the
subdivided into four control mini-slots, some of them        connection request is correctly received during any
used as contention mini-slots and the others used as         stage the connection exit from the contention process
polling mini-slots. A fixed number of control mini-          and the base station periodically allocate slots to the
slots are allocated for contention and polling access.       connection until the end of talk-spurt.
The contention mini-slots are used by voice                  The described contention process aims to reduce the
connections to send their reservation requests in            contention load during the contention period in each
contention mode at the beginning of talk-spurt, while        frame, increase the probability of successfully
the poling mini-slots are used by ABR connections            accessing the network, decreasing the probability of
to send their buffer length status to the base station.      collision, reduce the access delay time and at the
The number of polling mini-slots are chosen such             same time minimize the number of used contention
that the polling period will be less than or equal to        mini-slots and utilize them efficiently. Decreasing
the average inter-arrival time of ABR data message           the number of available frames for selection in each
(100 m-sec).                                                 subsequent stage aiming to reduce the access delay
Number of polling mini-slots ≥ int (number of ABR            time of the connections and hence reduce the cell
users * (TF/Tint)).                                          loss probability.




                      Ubiquitous Computing and Communication Journal                                              2
2.3 Traffic Integration Strategy                         4.1 Dynamic Allocation TDMA MAC Protocol for
                                                              Wireless ATM Networks
     As different wireless ATM services share the
                                                              4.1.1 Slot Allocation Algorithm for Voice
same resources, an effective interaction between the
                                                              traffic
allocation algorithms is needed to maximize the
                                                              The voice connections have the higher priority.
utilization efficiency of the shared resources. In the
                                                         At the beginning of a talk-spurt, the mobile sends a
first and second protocol , the voice connections
                                                         control packet. When the base station knows that the
have the highest priority and the VBR connections
                                                         connection becomes active the base station
have the next higher priority. The ABR connections
                                                         periodically allocates slots to the connection until
have the lowest priority.
                                                         the end of talk spurt. At the end of the talk-spurt, the
     In the third protocol, the available transmission
                                                         mobile sets a flag in the last voice packet to inform
slots are assigned first to active voice connections,
                                                         the base station that the connection is no longer
then a minimum assigned slots are allocated to ABR
                                                         active.
traffic, then VBR traffic slots are allocated, and
                                                         4.1.2 Slot Allocation Algorithm for VBR traffic
finally, the remaining slots are distributed between
                                                              VBR connections have the next highest priority.
ABR connections according to the buffer length of
                                                         They only contend (send a control packet) at the
each connections.
                                                         session beginning. Next, all the control information
                                                         is piggybacking on the data packets, which reduces
3 SOURCE MODELS
                                                         the contention over the real-time mini-slots. At the
                                                         base station, a token pool of certain size is
3.1 Voice Source Model
                                                         introduced for each VBR connection. Tokens are
     A voice source generates a signal that follows a
                                                         generated at a fixed rate that is equal to the mean
pattern of talk-spurts separated by silent gaps. A
                                                         cell rate. A token is removed from the
speech activity detector can be used to detect this
                                                         corresponding pool for every slot allocated to the
pattern. Therefore, an ON/OFF model can describe a
                                                         connection
voice source: the source alternates between the ON
                                                         After slot allocation for voice connections the base
state where the source generates packets at rate 8
                                                         station allocates one slot for each VBR connection
kbps, and the OFF state where no packets are
                                                         to send one of their cells and also to piggyback the
generated. Durations of talk-spurts and silent gaps
                                                         current traffic parameter (e.g. buffer length, cell
are modelled as exponential distributions with mean
                                                         delay) of the connection. Then the base station
values of 1 and 1.35 sec, respectively.
                                                         allocates slots for each connection .The number of
     If a voice packet is not sent within its maximum
                                                         slots allocated for a connection is the minimum of
transfer delay (MTD), it should be dropped The
                                                         the buffer length and the number of tokens in the
MTD is set to be 16 m-sec.
                                                         pool such as;
3.2 VBR Source Model
                                                         Nv= min (Av, Bv) .
     The source rates are modelled as truncated
                                                         where
Gaussian distribution between (128 – 384 kbps)
                                                                Nv : number of slots allocated for the VBR
with mean rate of 256 kbps. The rate of the source
                                                         connection.
varies every 33 m-sec (the duration of image frame)
                                                                Av : number of tokens in the pool.
and the MTD of the VBR packet is set to be 50 m-
                                                                Bv : number of the packets in the mobile
sec.
                                                         station buffer.
                                                         Each connection cannot send greater than 12 cells in
3.3 ABR Source Model
                                                         the frame. Within the frame, priority is given to the
    It resembles a data source with messages of
                                                         connection with minimum time-of-expiry to send
certain length. The length of the message is
                                                         their cells earlier.
exponentially distributed with mean 2 k bit, and the
                                                         4.1.3 Slot Allocation Algorithm for ABR traffic
inter-arrival time between messages is negatively
                                                                The base station records the buffer length
exponential distributed with mean of 100 m-sec.
                                                         status of each connection using the control
The MTD of the ABR packet is set to be 6 sec.
                                                         information transmitted by the mobile. When a
                                                         message arrives at a mobile, it sends the number of
4 BANDWIDTH ALLOCATION ALGORITHM                         packets in the new message either piggybacked to a
                                                         data packet or in a control packet.
     The bandwidth allocation for uplink                 Like VBR connections a token pool is introduced
transmission is only considered since the downlink       for each ABR connection. ABR connections have
transmission can be scheduled in the same manner         lower priority than voice and VBR connections. The
as in a wired ATM switch.                                number of slots allocated for a connection is the
                                                         minimum of the buffer length and the number of
                                                         tokens in the pool such as;




                    Ubiquitous Computing and Communication Journal                                             3
Na= min (Aa , Ba).                                         introduced for all VBR connections. Tokens are
where,                                                     generated at a fixed rate that is equal to the mean
       Na : number of slots allocated for the ABR          cell rate per connection multiplied by the number of
connection.                                                VBR connections. A token is removed from the
       Aa : number of tokens in the pool.                  corresponding pool for every slot allocated to any
       Ba : number of the packets in the mobile            VBR connection. The cell delay is piggybacking on
station buffer.                                            the data packets.
     The connection with higher number of tokens in        The number of slots allocated for a VBR connection
its pool sends their cells earlier within the frame. If    depends on the cell delay and the number of token
there are remaining slots inside the frame, the base       in the pool such as;
station allocates them fairly between ABR and VBR          Nvj=int ( Kv* ( Dvj/Dv))
connections.                                               where
                                                                 Nvj : number of slots allocated for the
4.2 An Intelligent MAC Protocol for next                   connection number j
generation Wireless ATM Networks                                 Kv : number of tokens in the pool
  4.2.1 Slot Allocation Algorithm for Voice and                  Dvj : delay time of the last transmitted cell
  CBR traffic                                                   from connection number j
       The voice connections have the highest                    Dv : total cell delay of all VBR connections
priority. At the beginning of a talk-spurt, the mobile
sends a control packet to inform the base station that     4.2.3 Slot Allocation Algorithm for ABR traffic
the connection become active.                                   The base station records the buffer length status
At the base station, a token pool is introduced for        of each connection using the control information
each active voice connection and each token is             transmitted by the mobile. When a message arrives
increased by a fixed amount equal to Tv every frame        at a mobile, it sends the number of packets in the
to indicate the number of cells generated in the           new message either piggybacked to a data packet or
mobile station buffer and decreased by one for every       in a control packet.
slot allocated to the corresponding connection. Then       Like VBR connections one token pool is introduced
the voice connections are arranged according to the        for all ABR connections. ABR connections have
content of its token and slots are allocated to the        lower priority than voice and VBR connections. The
connection with higher value in its token first. At        number of slots allocated for an ABR connection
the end of the talk-spurt, the mobile sets a flag in the   depends on the buffer length and the number of
last voice cell to inform the base station that the        token in the pool such as;
connection is no longer active.                            Naj= Ka* ( Baj/Ba)
Tv=Tf /Tp                                                  Where
 where,                                                          Naj : number of slots allocated for the
        Tf : frame duration (2msec).                       connection number j.
        Tp : packetization time of the ATM cell of               Ka : number of tokens in the pool.
voice connection (48m-sec).                                      Baj : number of cells in the buffer of the
The number of slots allocated for voice connection         connection (buffer length).
in each frame should not exceed Lv .                             Ba : summation of the buffer lengths of all
where                                                      ABR connections.
Lv = number of voice connection*( Tf / Tp).                If there are remaining slots inside the frame, the
                                                           base station allocates them between ABR and VBR
The token poll has two advantages:                         connections such that Ψ % for VBR connections and
    First: it indicates the number of packets              the rest for ABR connections where;
         generated at the mobile station buffer.                  D avg
     Second: it indicates the amount of delay of the       Ψ= (           )*100.
generated packet.                                                 T oe
This helps in deciding which voice connection
                                                           where
should send its packet early and leads to reducing
                                                              Davg : average delay of VBR connections.
the average delay of the voice connections.
                                                              Toe : time of expiry of VBR cells (50 m-sec).
4.2.2 Slot Allocation Algorithm for VBR traffic
                                                           4.3 Contention and Polling based Multiple Access
     VBR connections have the next higher priority.
                                                               Control with minimum Piggybacking for
They only contend (send a control packet) at the
                                                               Wireless ATM Network
session beginning. Next, all the control information
                                                             4.3.1 Slot Allocation Algorithm for Voice Traffic
is piggybacking on the data packets, which reduces
the contention over the real-time mini-slots. At the           At the beginning of talk spurt the voice
base station, one token pool of certain size is            connection sends a reservation request through the




                     Ubiquitous Computing and Communication Journal                                             4
contention mini-slot. When the base station                  of VBR allocated slots becomes lower than Vmean to
successfully receives the request, it periodically           decrease the counter.
allocates slots to the connection up to the end of talk
                                                             • Each VBR connection could not have
spurt. At the end of talk spurt the connection set a
                                                             lower than one allocated slot per frame.
one bit flag in the last transmitted cell to indicate that
the connection is no longer active.                               As we suggested before the delay threshold can
                                                             be set at a fixed value and its value have a significant
4.3.2 Slot Allocation Algorithm for VBR Traffic
                                                             effect on the allocation process and the achieved
     Initially the base station allocates one slot for       QoS of VBR traffic. During the simulation at fixed
each active VBR connection and then broadcast a              delay threshold we take its fixed value equals to 0.5
delay threshold value to all VBR connections every           maximum CTD of VBR cell (25msec) as an
frame. One bit flag is used to indicate the delay            appropriate value and evaluate the performance of
status of the buffer and is piggybacked to the data          the allocation process in this case.
packet (cell). Each VBR connection checks its buffer
                                                             Table 1: Dynamic adjustment of the delay threshold
and sets the flag to one when the packet delay
exceeds the delay threshold, and to zero when the
packet delay is lower than the delay threshold. The             Counter                    Delay Threshold values
slot allocation procedures are performed as follow:                                               (m-sec)
• The base station increases the assigned slots by one          Counter ≤4                            15
for a VBR connection each time its
                                                                4 < counter ≤ 8                       20
 packet delay is greater than the delay threshold
(piggybacking flag equal to one).                               8 < counter ≤12                       25
• At the base station a counter is introduced. The              12 < counter ≤15                      30
counter incremented by one when the number of
                                                                15 < counter ≤18                      35
slots allocated for VBR traffic in the frame is greater
than Vmean and decremented by one when it is lower              18 < counter ≤ 20                     40
than Vmean.
                                                                20 < counter                          45
 where; Vmean: the mean number of cells
generated from all VBR connections per
frame according to the mean cell generation                  4.3.3 Slot Allocation Algorithm for ABR Traffic
rate per connection.                                              Polling control mini-slots are used by ABR
• The delay threshold can be set to a fixed value or         connections to send their buffer length to the base
dynamically adjusted to control the slot allocation          station. The number of polling mini-slots is selected
process for VBR traffic. The counter can be used to          such that the polling period should be lower than or
dynamically adjust the delay threshold by increasing         equal to the inter-arrival time between ABR data
the delay threshold value when the counter value is          messages (100 m-sec) to enable the base station to
increased and decreasing the delay threshold value           efficiently monitor the buffer length status of each
when the counter is decreased. Since the increase in         connection.
the counter value indicates the increase in the                 Initially a minimum number of slots are allocated
allocated bandwidth (slots), so we need to reduce it by      to ABR traffic. So that, each ABR connection has an
increasing the delay threshold value which in turn           allocated bandwidth equivalent to 50 % of its
decreases the piggybacking and hence decreases the           average cell generation rate. The base station
number of allocated slots and vice versa. Table.1            controls this minimum assigned bandwidth by
shows the dynamic delay threshold values using the           maintaining a leaky bucket for every ABR
dynamic adjustment.                                          connection. Tokens added to the bucket at constant
• When some of the connection reserved slots are not         rate equals to 50% of the average cell generation rate.
used by the connection for transmitting its packets          Every time a slot is allocated to the connection, a
(number of generated packets become lower than the           token is removed from the bucket. So, in each frame
number of allocated slots) the base station release this     the connection with non empty leaky bucket has
slot and decrement the number of the reserved slots          allocated slots equal to the number of tokens in its
for that connection in the subsequent frames by one.         bucket. After allocating the VBR traffic slots, the
                                                             remaining slots are allocated to ABR connections.
• When the counter becomes greater than the upper            ABR connections are arranged according to their
limit value (25) the base station release some of the        buffer length where the connection with higher
reserved slots for the connections that have no              buffer length has its required slots allocated first.
piggybacking in the previous frame until the number




                      Ubiquitous Computing and Communication Journal                                               5
5     PERFORMANCE EVALUATION AND                            of its VBR Resource allocation algorithm is lower than
      SIMULATION RESULTS                                    the other two protocols.
      A comparison has been made to evaluate the                 For ABR traffic, Fig. 4 and Fig. 7 show that the
performance of the three proposed protocols using the       reduction of data transmission bandwidth of the third
same simulation parameters.                                 protocol by 3.85% due to the contention and polling
      Fig. 2 through Fig. 7 illustrates the performance     periods significantly reduces the available bandwidth
with integrated traffic. There are 30 voice                 for ABR traffic which make the cell losses start early
connection and 12 VBR connections in the network,           before 40 ABR connection and the average cell delay
while the ABR connections are added gradually to            significantly increases with ABR connections since a
the network. All results are presented as a function        considerable part of ABR resource allocations takes
of the number of ABR connection.                            place after VBR slot allocation. The first and second
      For voice traffic, Fig. 2 and Fig. 5 show that a      protocols achieve good QoS for ABR traffic but the
good QoS is achieved by the three protocols in term of      first protocol achieve slightly better performance in
cell loss probability (lower than 10-4) and average cell    term of cell loss probability and average cell delay.
delay (lower than 5 m-sec). it is clear that,                    At 45 ABR connection the first and second
approximately, the first and the second proposed            protocol achieve approximately 94% data transmission
protocols achieve better performance than the third one     throughput and 98.5% total channel utilization while
as they use the same contention access scheme, due to       providing the acceptable QoS required for each traffic
using lower number of control mini-slots for                category. For the third protocol, at 36 ABR connection
contention access. It is worth to mention that the third    91% data transmission throughput and 98.5% total
protocol uses 8.3% of the bandwidth (8 control mini-        channel utilization are achieved while preserving the
slots) for contention and polling access, while the first   required QoS for each ATM traffic type.
and second protocol uses 4.45% of the bandwidth
(approximately 4 control mini-slots) which make the
available data transmission bandwidth for the third
protocol lower by 3.85 % than that of the first and
second protocol.
      For VBR traffic, Fig. 3 shows that with low VBR
traffic up to 45 connections, the second protocol
achieves the best performance in term of cell loss
probability as its resource allocation algorithm depends
on the cell delay at each connection buffer, so that the
connection with higher delay allocated more slots than
that with lower delay which leads to reduce the
probability that the cell delay exceeds the maximum
CTD (cell transfer delay) and then lost. This decreases
the cell loss probability. On the other hand this
increases the average cell delay that becomes higher
than the average cell delay caused by the first protocol
as indicated in Fig. 6.The third protocol achieves
higher loss probability than the second protocol (Fig.          Figure 2: Cell loss probability of Voice connections
3), and the highest average cell delay (Fig. 6) since the       as a function of the number of ABR connections (12
dynamic delay threshold adjustment process produces             VBR and 30 Voice connection)
more average delay than the other two protocols. When
the number of ABR connections becomes greater than
45, the offered traffic becomes higher than the
available bandwidth. The third protocol achieves the
lowest cell loss probability and lower average cell
delay than the second one since a considerable part of
ABR slot allocation takes place after VBR slot
allocation, and the VBR slot allocation is controlled by
the value of delay threshold which has upper limit
value. So increasing the number of ABR connection
has low significant effect on VBR slot allocation. The
first protocol achieves the lowest performance in terms
of cell loss probability (Fig. 3), while achieves the
lowest average cell delay with all number of ABR
connections, (Fig. 6). This indicates that the efficiency


                                              Figure 3: Cell loss probability of VBR connections as a function of
                                              the number of ABR connections (12 VBR and 30 Voice connection)
                     Ubiquitous Computing and Communication Journal                                6
    The performance with real time traffic is illustrated in       With the third protocol the average cell delay lower
    Fig. 8 through Fig. 11. 12 VBR connections are                than with integrated traffic because in the absence of
    presented while the voice connections are added               ABR traffic if there are remaining slots they will be
    gradually to the system. For voice traffic, Fig. 8 and        given to VBR connections.
    Fig. 10 show that the performance is the same as with            A cell loss probability of 10-3 for VBR traffic
    integrated traffic, where the first and second protocols      achieved by the first protocol at 106 voice connection
    perform better than the third one. For VBR traffic, Fig.      (95% channel utilization), by the second protocol at
    9 and Fig. 11 show that the second protocol achieves          112 voice connection (97% channel utilization), and by
    the lowest cell loss probability. The average cell delay      the third protocol at 103 voice connection (94.7%
    of the first and second protocols is low with slightly        channel utilization).
    different values until 112 voice connection (97%
    channel utilization), after that, the average cell delay of
    the second protocol become significantly higher since
    the efficiency of its resource allocation algorithm in
    reducing cell loss probability results in increasing the
    average cell delay. The third protocol has lower cell
    delay than that with integrated traffic because in the
    absence of ABR traffic, any remaining slots will be
    given to VBR connections.




                                                                      Figure 6: Average cell delay of VBR connections as a
                                                                      function of the number of ABR connections (12 VBR
                                                                      and 30 Voice connections).




Figure 4: Cell loss probability of ABR connections as a
function of the number of ABR connections (12 VBR
and 30 Voice connection)




                                                                      Figure 7: Average cell delay of ABR connections as a
                                                                      function of the number of ABR connections (12 VBR
                                                                      and 30 Voice connections).



    Figure 5: Average cell delay of Voice connections as a
    function of the number of ABR connections, (12 VBR
    and 30 Voice connections).

                          Ubiquitous Computing and Communication Journal                                            7
These results indicate that the second protocol has the   protocol uses the lowest piggybacking overhead. For
most efficient VBR slot allocation algorithm then the     ABR traffic, the reduction of data transmission
first protocol and finally the third protocol, but the    bandwidth of the third protocol by 3.85% reduce the
third protocol uses the lowest piggybacking overhead.     available bandwidth for ABR traffic which make the
                                                          cell losses and the average cell delay significantly
                                                          increases with higher values than the other two
                                                          protocol. Finally the three proposed protocols
                                                          achieve very high channel utilization of
                                                          approximately 98% for the wireless ATM channel
                                                          while respects the required QoS of multimedia ATM
                                                          traffic types.




Figure 8: Cell loss probability of Voice connections as
a function of the number of Voice connections (12 VBR
Connection).




                                                            Figure 10: Average cell delay of Voice connections as a
                                                            function of the number of voice connections (12 VBR
                                                            connections).




Figure 9: Cell loss probability of VBR connections as a
function of the number of Voice connections (12 VBR
Connection).
6 CONCLUSION
We have presented an extensive performance
comparison of three proposed MAC protocols to
highlight the merits and demerits of each of them.
For voice traffic, a good QoS achieved by the three
                                                            Figure 11: Average cell delay of VBR connections as a
protocols. But, the first and second proposed
                                                            function of the number of voice connections (12 VBR
protocols achieve better QoS than the third protocol
                                                            connection)
while using lower number of control mini-slots for
contention access. For VBR traffic, the results
indicate that the second protocol has the most
efficient VBR slot allocation algorithm then the first
protocol and finally the third protocol, but the third



                    Ubiquitous Computing and Communication Journal                                          8
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                     Ubiquitous Computing and Communication Journal                                   9

				
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Description: UBICC, the Ubiquitous Computing and Communication Journal [ISSN 1992-8424], is an international scientific and educational organization dedicated to advancing the arts, sciences, and applications of information technology. With a world-wide membership, UBICC is a leading resource for computing professionals and students working in the various fields of Information Technology, and for interpreting the impact of information technology on society.
UbiCC Journal UbiCC Journal Ubiquitous Computing and Communication Journal www.ubicc.org
About UBICC, the Ubiquitous Computing and Communication Journal [ISSN 1992-8424], is an international scientific and educational organization dedicated to advancing the arts, sciences, and applications of information technology. With a world-wide membership, UBICC is a leading resource for computing professionals and students working in the various fields of Information Technology, and for interpreting the impact of information technology on society.