Abstract— mobile laptops and PDA’s but we have

Abstract—
A mobile
ad-hoc network (MANET) consists of a group of mobile nodes and it enables
communications between participating nodes without the burden of any base
stations. To increase
the capacity of wireless network, multiple transceivers can be used. Multiple transceivers
increase the cost of the equipment. So generally for data transmissions, a
single transceiver is used in each node. But single transceiver is difficult to
implement in multichannel environment. This problem can be solved by Ad-hoc
Multichannel Negotiation Protocol (AMNP). For improving reliability, further
Reliable Broadcast Algorithm (RBA) is introduced. Simulation analysis in NS-2
based on the combination of AMNP – RBA  
gives comparatively a better performance.

Keywords — MANET,
Multihop, MAC, Multichannel, AMNP and RBA.

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I.     Introduction

      Nowadays there is tremendous increase in
usage of mobile laptops and PDA’s but we have only limited amount of radio
spectrum. Within the available radio spectrum we have to effectively
communicate between the nodes. Existing works have dedicated to using multiple
channels to increase the capacity of wireless communication by dividing the
radio spectrum into number of channels.

      Most of the mobile devices are equipped
with single transceivers and it operates in single-channel mode hence more
amount of bandwidth is wasted. To mitigate this problem, all mobile nodes have
to be equipped with multiple transceivers. 
Enhancement of the present MAC protocol can give better performance on
multichannel with single transceiver.

     In 1 Jain proposes a CSMA based medium
accesses control protocol for multihop wireless network. In which channel
selection is based on signal to interference and noise ratio at the receiver.
Although this method increases the throughput up to 50% there is delay in
performance due to high packet transmission. In 2, Nasipuri propose a new
CSMA protocol for ad-hoc networks. In which the CSMA protocol divides the
available bandwidth into several channels and selects the channel randomly. It
employs “soft channel reservation” that gives preference to the channel that
was used for last successful transmission.

    In 11 Chen
proposes a AMNP protocol that reduces the collision and interruption
probabilities, and it uses the same frame format of IEEE 802.11 with some
slight modifications but it lacks in reliable broadcast transmission. In 12
Lou proposes RBA (Reliable broadcast Transmission) with selected forward nodes
to avoid broadcast storm and reduce broadcast redundancy.

II.  Problem statement

A. 
Single
transceiver constraint

      In IEEE 802.11 DCF the MAC protocol is
designed for sharing a single channel between the nodes. Nowadays most of the
wireless devices are equipped with one
half-duplex transceiver to transmit or to receive data. The transceiver can
operate on multiple channels dynamically but it can either transmit or receive
data from one channel at a time. So a node cannot communicate with other nodes
when is communicating with another node in another channel concurrently. While using
multiple channels IEEE 802.11 DCF will not be suitable because it may
dynamically switch channels.

 

B. 
Multichannel
hidden terminal problem

The node which cannot hear the radio signal from the transmitter node
and may disturb the ongoing data transmission is called hidden terminal nodes.
Even though IEEE 802.11 provides RTSCTS handshaking signals, in multichannel
environment the nodes still may collide with each other.

C. 
Broadcast
transmission problem

     
Broadcasting is an important activity in multi hop MANET. Broadcasting a
message in single channel is easy, because all the mobile nodes in a network
use a single channel so the message can be delivered.  But in multichannel environment a node may
miss the broadcast frame when is currently transmitting or receiving data from
other nodes.

iii. Amnp – RBA  implementation

      In IEEE 802.11 the sender and the
receiver should perform a four way handshaking mechanism: Request-to-send
/clear-to-send (RTS/CTS), data, and acknowledgment (ACK) when they have data to
transmit in the same channel.

Fig.1 An illustration of AMNP

            In
fig.1, which C0 represents the contention/reservation channel and C1
and C2 represent the data channels. The identifier BB represents the
broadcast beacon, the BWT represents the broadcast waited time and the CST is
the channel switching/settling time, respectively.

Fig. 2 Frame format of MRTS, MCTS and CRI control frames

      If mobile nodes equip with only one
transceiver, some nodes will never communicate with each other at the same
time. As a result, few data frames will be transmitted in the multichannel
environment. If we assign mobile nodes to access channels dynamically, a
complicated and distributed channel scheduling mechanism has to be provided for
MANETs. It will be more difficult in the MANET. 

      Instead of employing
such complicated scheme, AMNP allocates a dedicated contention or broadcast
channel for all mobile nodes to contend. The remaining channels are served as
data channels permanently. Fig 1 illustrates the channel usage of
AMNP in which channels C1– Cn-1 represent data channels,
and channel C0 serves as the dedicated contention channel or
broadcast channel. Since there is no stationary node for supporting centralized
multichannel control in MANETs, the distributed negotiation protocol, which can
provide ad hoc multichannel transmission, is needed. To solve the
above-mentioned problems, we employ the concept of IEEE 802.11 RTS/CTS
handshaking mechanism to fulfill the multichannel negotiation and transmission
mechanism in multi-hop MANETs. We name the RTS/CTS mechanism as MRTS/MCTS in
the AMNP. Unlike IEEE 802.11 RTS/CTS mechanism, we need more information to
indicate the usage of other data channels.

      When two nodes
communicate, first a node has to complete a MRTS/MCTS handshaking in
the contention channel to acquire the access right of the expected data channel
if it has a packet to transmit. The main purpose of the MRTS control frame is
to inform its direct receiver and neighbours the preselected data channel to
indicate a virtual carrier sensing delay named network allocation vector (NAV)
this will prevent the exposed and hidden node problems in the preselected
channel. Likewise, the MRTS also carries the newest status information of data
channels to notify other mobile nodes within its transmitting range for
information updating.

      The frame format of
MRTS is shown in Fig 2 where the frame control, receiver address, transmitter address and
frame check sequence fields are the same as the description in the IEEE 802.11
standard. In order to be compatible with the IEEE 802.11 standard, we use the
reserved value Type = 01 and Subtype = 0011 as indicated in the frame control
field to represent the MRTS control frame. The original duration field is
eliminated since the channel C0 is for contention and broadcast use only.
Therefore the NAV will not be used in C0 when contending for the channel
access. The additional fields selected channel (SC), channel usage indication
(CUI) and the nth used channel’s offset are described as follows. The SC field
indicates which channel that the sender prefers to transmit data with the
receiver.

     The preferred channel
(selected) is not compulsory for the receiver depending on the availability of
the channel on the receiver’s side. The CUI
field length is one octet long and the content of CUI indicates the status of
the usage in each channel.  The bit will
be set to 0 if the corresponding data channel is not in use; the bit will be
set to 1, if the corresponding data channel is in use.

      When a node has received a MRTS frame, it
will compare the SC field of the MRTS with its channel status and then check
whether it can satisfy the request. If the preselected channel is also
available in receiver’s side, the receiver will grant the transmission request
and reply the MCTS frame back to the sender immediately. Otherwise, the
preselected channel cannot be granted to use since the preselected data channel
in receiver’s side is not free. The receiver then reselects another available
channel according to comparing with the status of channel usage of the sender.
The reselection rules are as follows:

1)  If
the sender has another free data channel and the channel is also available in
receiver’s side. The receiver will select the common available channel to
receive data frames.

2)  If
there is no available free channel in the side of the sender or receiver now,
the receiver will compare all data channels of both sender and receiver and
then select a common channel which will be earliest released.

     
Channel information from both sides are taken in order to prevent the
hidden node problem. After the checking process, the receiver will reply a MCTS
frame back to the sender to make the final decision. The MCTS frame contains
the current the usage status of data channels.

     
Taking Fig. 3 for example, assuming there are 5 mobile nodes in the ad
hoc network. Node c and d are the exposed terminal of node a and b, and node e is the
hidden terminal of node b. Initially
node e finishes its backoff count
down and then sends an MRTS frame to request the channel 1 for transmitting
data. The receiver node d approves
the request since the channel 1 is also available in side of d. After the negotiation of node d and e, node a finishes its
backoff count down and sends an MRTS to node b to ask channel 1 for transmitting data. Since channel 1 has been
reserved by node d and e, the request could not be accepted.
Node b compares channel statuses of
node a with node b and then selects an available channel
2 in this example and sends MCTS back to node a. After receiving an MCTS from node b, node a is notified
that channel 1 would not be accepted and the agreed channel is channel 2. Node a will resend an MRTS to refresh the
reservation information (to node c in
this example).

Fig. 3 Transmission of MRTS/MCTS
frames to select a channel

D. 
Broadcast
casting in AMNP

                   The broadcast operation is an important activity in ad-hoc
networks. Broadcasting is done to achieve routing information exchanges,
address resolution protocol and message advertisement etc. Broadcasting can be
done easily when there is a single channel but in multichannel environment, a node may miss the broadcast frame when is currently transmitting or
receiving data from other nodes. Here a single transceiver constraint is
chosen. To solve this problem, AMNP uses a designated control frame named
broadcast beacon (BB) to announce to its neighbouring nodes of an upcoming
broadcast transmission.

Fig. 4 Frame format of Broadcast Beacon

     All nodes which received the BB will stay
in the contention channel and wait a broadcast
waiting time (BWT) to receive this frame even though it has made
a successful reservation. All the scheduled reservations will be delayed a SIFS
+ BWT + SIFS + broadcast frame length + SIFS period.

     Several problems
remain by adopting this transmission of the broadcast frame after a SIFS
interval. The following four cases are considered as shown in Fig 5, to
describe the broadcast problems occurred in the multichannel environment.

Case 1: After finishing the transmission where
the sender and the receiver will return to the contention channel during the
time period of the beginning of the BB and before the broadcast frame.

Fig. 5 Broadcast problems in Multichannel environment

Case 3: At a finished transmission where the
sender and the receiver will return to the contention channel in the broadcast
frame.

Case 4: At a finished transmission where the
sender and the receiver will return to the contention channel after the
broadcast frame.

   In case1 the nodes will receive the broadcast
frame because it stays connected in contention channel after the transmission
so it receives the broadcast frame. In case 2 it is not sure the nodes will
receive the broadcast frame depending upon the physical response time and ready
time.

     Case 3 and case 4 will definitely miss the
broadcast frame, to solve this problem we prefer reliable broadcast algorithm.

E.  Reliable
broadcast Algorithm

        In reliable broadcast algorithm it
requires only selected forward nodes among the 1-hop neighbours to send ACKs to
confirm their receipt of the packet. Forward nodes are selected in such a way
that all senders’ 2-hop neighbour nodes are covered. Moreover, no ACK is needed
for non-forward 1-hop neighbours, each of which is covered by at least two
forward neighbours, one by the sender itself and one by one of the selected
forward nodes. The sender waits for the ACKs from all of its forward nodes. If
not all ACKs are received, it will resend the packet until the maximum times of
retry is reached. If the sender fails to receive all ACKs from the forward
nodes, it assumes that the non-replied forward nodes are out of its range and
chooses other nodes to take their roles as forward nodes.

The forward
nodes are selected based on the following greedy algorithm:

In the sample network shown in Figure 6, N(1)={1,2,3,4,
6} and N2(1)
={1,2,3,4,5,6,7}. When using the
FNSSP, sender node 1 selects nodes 2, 3 and 4 as its forward nodes. Node
3 is selected because there is no node in N(1) to cover it.

Algorithm: Forward Node Set Selection
Process (FNSSP)

Step 1: The forward node set F is
initialized to be empty.

Step 2: Add in F the node that covers the largest number of
2-    hop neighbours that are not yet
covered by current F. A tie is broken by node ID.

Step 3: Repeat step 2 until all 2-hop neighbours are covered.

Fig .6 A sample
network where the sender 1 uses the FNSSP to select its forward nodes.

iv. Simulation results

            The network is varied from 54 to 108
nodes. The mobility model uses the random
waypoint model in a rectangular field. Here each mobile node
starts its journey from a random location to a random destination with a randomly
chosen speed (uniformly distributed between 0–94 m/s).

Simulation Parameters

                     Value

Simulation Area

300m*300m

Transmission range

100 m

Transmission rate

2 Mb/sec

SIFS

10µs

DIFS

50µs

MRTS frame length

variable 160 bits

MCTS frame length

112 bits

ACK frame length

112 bits

MAC header length

34 octets

broadcast frame length

128 octets

 

 

 

 

 

 

 

 

 

 

 

Table 1. Simulation
Configuration Parameter

      In all simulation analysis, one
contention channel and 11 data channels are considered. Simulation area is 300m
x 300m, transmission range is about 100m and transmission rate is about
2Mb/sec.

      In Fig 7 When 54 nodes are considered
AMNP performs better than IEEE 802.11. When frame arrival rate increases to 20,
a significant amount of increase of throughput can be noted.

Fig. 7 Comparison of
Throughput derived by IEEE 802.11 and AMNP

When 108 nodes
are considered AMNP performs better than IEEE 802.11. When frame arrival rate
increases to 20, a considerable amount of increase of throughput can be noted
when compared to IEEE 802.11.

Fig. 8 Comparison of Throughput derived by
IEEE 802.11 and AMNP

MAC delay is the sum of MAC operations
including back-off countdown, channel negotiation and transmission delay.

Fig. 9 Comparison of
Mac delay derived by IEEE 802.11 and AMNP

Fig. 10 Comparison
of Mac delay derived by IEEE 802.11 and AMNP

The Fig. 9 and Fig. 10 show comparative
delay analysis for IEEE 802.11 and AMNP protocol with 54,108 nodes. It is seen
that AMNP protocol has lower MAC delay compared to IEEE 802.11 protocol.

Fig. 11 Comparison
of Mac delay derived by RBA and AMNP-RBA

 

End – to – End delay is the total delay in the
network; AMNP-RBA has higher delay because it is sum of back-off countdown,
channel negotiation, transmission delay and the delay in broadcasting.

Broadcast delivery ratio is the ratio of the
nodes that received the broadcast packets to the number of the network.
AMNP-RBA has lesser BDR compared to RBA because it is used in multichannel
environment.

Fig. 12 Comparison of Broadcast delivery ratio
derived by RBA and    AMNP-RBA

v. Conclusion

            The multi-hop
MANET transmission capacity can be improved by adopting parallel multichannel
access schemes. AMNP protocol addresses the problems like multichannel hidden
terminal problem and the multichannel broadcast problem. This is due to those
mobile nodes that cannot listen to all channels simultaneously. The proposed
new MRTS and MCTS handshaking message conquers the multichannel hidden terminal
problem.  The BB control frame to conquer
the multichannel broadcast problem. The performance analysis shows that there
is an encouraging result. The parameters are compared for 802.11 and AMNP. It
is concluded that the combination of AMNP – RBA gives a reliable broadcast
transmission. Because in AMNP case 3 and case 4 are assumed for reliable
transfer.

References

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