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Token Based Medium Access Control in Wireless Networks by sdsdfqw21

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									                   Token Based Medium Access Control in Wireless Networks

                                   Yi-Sheng Liu and Fambirai Takawira
                               School of Electrical and Electronic Engineering
                                             University of Natal
                                                   Durban
                                                     4041
                                                South Africa
                                    E-mail: {liuy1, ftakaw} @nu.ac.za
                                           Tel: +27-31-260-2730
                                           Fax: +27-31-260-2740

Abstract - Most of the medium access control (MAC)            (e.g. CSMA [1]), and sensing before and during
protocols proposed for mobile Ad-Hoc networks are             transmission (e.g. CSMA/CD [1]). The majority of the
based on the carrier sense multiple access (CSMA)             ad hoc wireless networks implement the network based
schemes. This paper introduces a new MAC protocol             on a modified CSMA scheme. It is known from [2] that
that is based on a token passing scheme with the              with the CSMA based scheme, the performance is
incorporation of code division multiple access                degraded by the hidden terminal problem. In a wireless
(CDMA). The proposed protocol uses a single token             ad-hoc network there is no guarantee that some
that is constantly circulating the network from node          terminals may not be hidden from the other terminals.
to node, and the node that holds it has permission to         To alleviate or solve the problem, many protocols based
transmit. The advantage of the proposed protocol is           on RTS/CTS handshakes have been proposed, [3-8]. It
that even under heavy traffic condition, each node            has been shown [8] that when the traffic load is heavy, a
still satisfies the proposed data rate and delay level        packet may still encounter collision with probability as
guarantee QoS requirements. This paper presents               high as 60% due to loss of RTS or CTS packets.
simulation results, demonstrating the stability of the
proposed protocol.                                                It is well known [1,9] that token-passing MAC
                                                              schemes outperform CSMA schemes. The two most
                                                              commonly implemented token passing schemes are: the
              I.        INTRODUCTION1                         IEEE 802.5 [13] standard and FDDI [14]. Both of them
I  T is well known that the ad hoc network is constructed
   by mobile hosts and can be rapidly deployed without
   any     established     infrastructure    or     central
                                                              are based on the token ring protocol, the only difference
                                                              between the two is the physical implementation. In these
                                                              schemes, the token is passed to the next node in the
administration. For the ad hoc network, due to its self-
                                                              polling cycle according to a polling order table. For a
organizing characteristic, it’s very challenging to design
                                                              wireless network, it is important to note that whenever
an efficient and effective medium access control (MAC)
                                                              the token is passed, the address of the next recipient
protocol. The currently existing MAC protocols can be
                                                              must be explicitly given, since all the nodes will receive
categorized into two major categories: scheduling and
                                                              the transmission. Our proposed protocol is also based on
random access, where scheduling can be again sub-
                                                              the token passing scheme, with some modifications like
divided into other two types as fixed assignment (e.g.
                                                              the node will not withhold the token while it is
TDMA [1]) and demand assignment. The demand
                                                              transmitting data packets. The detailed description of
assignment can be further classified as central control
                                                              the scheme is discussed in section II.
(e.g. polling [1]) and distributed control (e.g. Token
passing [1]). The random access scheme can be
                                                                  The remainder of this paper is organized as follows.
basically classified into three main categories: no
                                                              Section II describes the proposed MAC protocol in
sensing (e.g. ALOHA [1]), sensing before transmission
                                                              detail. The simulation model and the parameter are
                                                              described in section III. Simulation results are in section
1
 This work was partially sponsored by Alcatel Altech          IV and conclusions are drawn in section V.
Telecoms and Telkom SA as part of the Center of
Excellence Programme.
       II.      PROPOSED MAC PROTOCOL                             and the delay level guarantee is implemented
                   DESCRIPTION                                    using a delay level selector. Detailed descriptions
A. CDMA Techniques                                                of the QoS requirements are discussed in section
    In spread spectrum communication, a transmitter               B4.
spreads a transmission in a wide frequency spectrum by
using a spreading code, which is independent of the data    B2. Token Structure
packet being sent. A receiver uses the same code to de-        In this paper, the token consists of the source
spread the received signal and retrieve the data. This is   address, destination address, number of codes available,
known as code division multiple access (CDMA) as it         and network parameters as shown in Fig. 2.
allows multiple receivers to simultaneously receive
packets from different transmitters when the                    Source     Destination     NOC       Network
communication overlap in time and space domains [10].           ID         ID                        parameters
CDMA based schemes have been shown to offer                 Fig. 2. Token structure
improved performance than the TDMA based schemes;
among these are the large capacity and the graceful         l     Source ID: the address of the node who passed the
degradation. As a result, spread spectrum techniques are          token (predecessor’s address).
applied in standard ad-hoc networks like bluetooth [11]     l     Destination ID: the address of the node that is
and IEEE 802.11 [12] based protocols.                             currently holding the token (successor’s address).
                                                            l     Number of Codes Available (NOC): this
B. Proposed MAC Protocol                                          parameter is used to limit the number of
                                                                  simultaneous transmissions in the network; the
                                                                  detailed explanation of this parameter is discussed
                                                                  in section B3.
                                                            l     Network parameters: this field is used to pass
                                                                  other network management information to other
                                                                  nodes.

                                                            B3. Channel Access Control
                                                                The channel access control is based on the token
                                                            continuously circulating in the network following a
                                                            predetermined order. This order can be dynamically
                                                            adjusted but the algorithm for this scheme is outside the
                                                            scope of this paper. Channel access control scheme is
                                                            described as follow:
                                                            l     When a node is visited by a token, it simply
Fig. 1. System model for the proposed protocol
                                                                  forwards the token if it’s still busy transmitting
                                                                  data packets or if it has just finished transmitting
B1. Node Structure
                                                                  and still has a code/channel. In this later case it
l    Each node is equipped with two full-duplex
                                                                  will also release the code and increment the NOC
     transceivers; one set is used specifically for token
                                                                  value.
     transmission and reception, and the other set is
                                                            l     If it’s neither busy transmitting nor has a code
     used for data packet transmission and reception.
                                                                  channel, then it captures the token if the delay
l    Each node is also equipped with a MUD (multiple
                                                                  constraint of the QoS requirement is met. This is
     user detector) in order to receive more than one
                                                                  explained in section B4. Under the scenario that
     transmission at the same instance.
                                                                  the token is not captured, the node passes the
l    Each node maintains an updated list of its
                                                                  token to its neighbor according to the pre-defined
     neighbours in order to know where should the
                                                                  order.
     token be passed to.
                                                            l     Once the token is captured (delay level QoS
l    Each node consists of two types of buffer as
                                                                  requirement is satisfied), the node will:
     shown in Fig. 1; they are packet and permit buffer
                                                                  a. Increment the NOC by one
     respectively.
                                                                  b. If there are packets in the packet buffer, up to
l    The node supports 2 QoS guarantees; data rate
     and delay level guarantees respectively. To                       σ packets will get transmitted where σ is
     incorporate these two guarantees in to a node, it                 number of permits inside the permit buffer at
     can be seen from Fig. 1 that data rate guarantee is               the time when token is captured by the node.
     implemented using a permit generation system
B4. QoS Incorporation                                             The graphical illustration of the scenarios is shown
    Quality of Service (QoS) is a measure of the             in Fig. 3. A token circulates in the network, by passing
satisfaction experienced by a user using the service. In a   from node to node, and a token detection scheme is
wireless communication system, there are numerous            created for monitoring token’s activity. Node 1
ways to formulate the QoS objectives quantitatively. For     transmits the token and monitors the channel to observe
the proposed scheme, it is decided to incorporate two        whether node 2 has forwarded the token to node 3 or
quality of services into the protocol: data rate guarantee   not. It is achieved by monitoring the token transmission
and the delay level guarantee. The descriptions of the       activity. If after some predefined time-out period node 1
services are shown below.                                    still has not detected token transmission activity, it
                                                             assumes “token lost” or “node falls out of network”
1. Data rate guarantee:                                      scenario had occurred. To resolve the situation, node 1
   To provide this service, a permit generation system is    regenerates the token and passes it to node 3. This
implemented; each node has a permit buffer for storing       action is performed to avoid the generation of looping
the generated permits. The permit generation rate ( ,        effect.
permits/s) is proportional to the data rate ( ),
                                                                        III.      SIMULATION MODEL
αi = k • βi                                           (1)       The proposed MAC protocol can be modeled as
                                                             shown in Fig. 1. An ON-OFF model is used for each
   The proportional constant k is the same for all the       source. The mean ON time is set to 10ms, and mean
users. If the token has been captured then the packet        OFF time is set to 90ms.
buffer is emptied up to the level where the amount of
packets removed is equal to the number of permits in             The other simulation parameters are shown as
the permit buffer (scheme is gated). The maximum             follows:
amount of packets that can transmit is dependent on the       l    A wireless channel with link speed/channel rate
quantity of the permits in the permit pool. If the permit          128kbits/s is considered.
pool is empty when the token arrives, the transmission        l    The network parameter is set to be 1km by 1km,
request is then denied in order to meet the QoS               l    The quantity of the nodes in the network is set to
requirement.                                                       20 nodes.
                                                              l    4 traffic types.
2. Delay level guarantee:                                     l    Two different sets of data rates (β1, β2)
   This service is implemented using the channel              l    Two delay levels specified in terms of (N1, N2).
reservation technique. For channel reservation; each          l    There are thus 4 classes of traffic types which is
delay class, i, has associated with a channel reservation          defined by the pair {βi, Ni}
parameter Ni where 0        Ni     C, C is the maximum        l    Each class contains 5 nodes with the same data
                                                                   rate and the delay level setting.
number of codes used in the network. When a node              l    Class 1 (Low Data Rate, High Priority)
receives a token, the node will only capture it if the        l    Class 2 (Low Data Rate, Low Priority)
parameter Ni   NOC is met.                                    l    Class 3 (High Data Rate, High Priority)
                                                              l    Class 4 (High Data Rae, Low Priority)
B5. Token Lost And Node Out of Network Scenario               l    The token is set to 40 bits
                                                              l    Packet length for a packet is set to 160 bits.
                                                              l    Assume 10 CDMA channels/codes are available.

                                                                        IV.       SIMULATION RESULTS
                                                                 Three results are examined: the throughput for each
                                                             defined class and the entire network, the delay for each
                                                             class and the entire network, and the effectiveness of the
                                                             delay level /channel reservation QoS guarantee. Fig. 4
                                                             plots the throughput for the entire network and the two
                                                             classes with higher priority, and Fig. 5 plots the
                                                             throughput for the other two lower priority classes. Fig.
                                                             6 plots the delay for the entire network and for each
                                                             class. Fig. 7 plots the effectiveness of delay level QoS
Fig. 3. Graphical display of the scenario                    by varying the value Ni with fixed data rate 16kbits/s for
the lower data rate class and 64kbits/s for the higher
data rate class.
                                                                       30000

                                                                       25000

                                                                       20000
                           Overall
  12                       Class 1                                     15000
                           Class 3
  10                                                                   10000

   8                                                                    5000

   6                                                                         0
                                                                                      8     12         16         24       32
   4
                                                                                          Lower Data Rate (kbits/s)
   2                                                                          Overall               Class 1               Class 2
   0                                                                          Class 3               Class 4
        0       1000       2000          3000      4000
                                                                   Fig. 6    Delay performance for the network and each
                  Arrival Rate (kbits/s)                           class.

Fig. 4 Average code utilization for the network with
the two higher priority classes.                                     16000
                                                                     14000
                                                                     12000
                                                                     10000
                                                                      8000
            3                                                         6000
       2.5                                                            4000
                                                  Class 2             2000
            2                                     Class 4                0
       1.5                                                                        0         1           4             5      7
                                                                                                            Ni
            1
       0.5                                                                  Class 1       Class 2           Class 3       Class 4
            0
                                                                   Fig. 7     Monitoring the effectiveness of the delay
       -0.5 0       1000          2000          3000        4000   level QoS by observing the delay performance of each
                       Arrival Rate (kbits/s)                      class with different delay classes.

                                                                       Fig. 4 shows the code utilization for the network
Fig. 5     Average code utilization for the two lower
                                                                   with two higher priority classes. It can be seen that for
priority classes.
                                                                   overall network throughput performance, the maximum
                                                                   value of 10 is reached as predicted since 10 CDMA
                                                                   channels are distributed in the network for the
                                                                   simulation. For both higher priority classes 1, and 3,
                                                                   class 3 reaches maximum throughput limit earlier than
                                                                   class 1, this is also as predicted since it generates
                                                                   fourfold data packets as class 1.
                                                                      Fig. 5 shows the code utilization for the two lower
                                                                   priority classes 2 and 4. The two classes with low
                                                                   priority have similar throughput performance with the
                                                                   higher priority classes when the arrival rate is low.
                                                                   However, as soon as the arrival rate increases, the
                                                                   throughput of the lower classes drops in order to satisfy
                                                                   the delay level QoS guarantee.
   Fig. 6 shows the delay performance of the proposed               [12] IEEE Standards Department. Wireless LAN Medium Access
                                                                    Control (MAC) and Physical Layer (PHY) Specifications. IEEE
scheme. It can be seen that the delay for each class is
                                                                    Standard 802.11-1997, 1994.
relatively low if the data rate is low, however, the large          [13] IEEE Standards 802.5-1989. Token Ring Access Method and
delay deviation between classes is observed when the                Physical Layer Specifications. The Institute of Electrical and
data rate is increased to 16kbits. This is due to the fact          Electronic Engineers, Inc., 1989.
                                                                    [14] ANSI Standard X3T9.5, Fiber Distributed Data Interface (FDDI)
that the pre-defined channel rate cannot provide
                                                                    – Token Ring Medium Access Control (MAC), May, 1987.
sufficient amount of channel capacity for data rates of
16kbits/s, therefore the delay for both classes is
increased. Similar tendencies can also be observed for
the higher priority classes with data rates of 32kbits/s.

    For monitoring the effectiveness of the delay level
QoS guarantee, it is observed from Fig. 7 that as the
value of Ni increases, more codes are reserved for higher
priority classes. The delay for the low priority classes 2
and 4 are therefore increased; whereas the delay for the
higher priority classes 1 and 3 remain unaffected. As
predicted, this occurs when the delay level QoS
guarantee gets more stringent, the lower priority classes
have fewer opportunities for transmission.

                 V.      CONCLUSION
    In this paper, a MAC protocol based on token
passing scheme for wireless network is proposed. From
the simulation results shown in section IV, it can be
seen that under heavy traffic load, the QoS requirements
are still maintained.

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