Author: Yafeng Wu, John A. Stankovic, Tian He, and Shan Lin

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21 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Realistic and Efficient Multi
-
Channel

Communications in Wireless Sensor Networks

Author: Yafeng Wu,
John A. Stankovic, Tian
He, and Shan Lin

Outline


References


Related Work


WSN Reality


The TMCP


PMIT Algorithm


Results


Introduction


A number of MAC protocols exist that utilize a single
channel, but the throughput in these are insufficient to the
needs of a high data rate network, such as the multi
-
media variety.


Current multi
-
channel protocols are not suitable for
general WSN’s due to the small number of channels that
are generally available.


Tree
-
based multi
-
channel protocol (TMCP) segregates
the nodes into trees, delegating a channel to each tree.


The TMCP attempts to realistically solve the problem of
multi
-
channel nodes in a WSN.


The TMCP uses the 802.15.4, or Zigbee, protocol.

Related Work


MMSN
--

Multi
-
Frequency Media Access Control for Wireless Sensor
Networks


TMMAC


TDMA based MAC protocol


MCMAC


Multi
-
Channel MAC protocol


All three of these are node
-
based and attempt to synchronize the
channels between upstream and downstream nodes. All 3 use time
slots to coordinate their transmissions. They face practical problems
in real WSN’s:


A large number of orthogonal channels are needed in dense
networks


Require precise time synchronization at the nodes.


Must account for channel switching delay and scheduling
overhead


Complex protocols that call for more resources at the motes.

Multi
-
Channel Reality


First, find the number of available orthogonal channels. With 3 motes
in a line, and 8 channels available to each mote, test the interference
caused by the motes between channels.


Next, determine the interference of 802.11 networks.


By evaluating the packet reception ratio on all channels, it is
determined that only three of the eight channels have a reasonably
high packet reception ratio.


Furthermore, when the RSSI, or Received Signal Strength Indication
dropped below
-
77dB the packet reception rate dropped dramatically.


Time synchronization errors play a crucial role in time synchronous
protocols. For example, the Micaz mote, is considerably bad at time
keeping and keeping it synchronized across nodes takes a
considerable amount of overhead, without sufficient time
synchronization the packet delivery ratio can drop by an incredible
80%.

The TMCP

TMCP Revealed


Firstly, partition the network into multiple vertex
-
disjoint subtrees all rooted at the base.


Since each tree has a single channel there is not a
need for time synchronization.


Use coarse
-
grained channel assignment, which uses
much fewer channels.


Simple channel coordination scheme.


reduce packet losses by eliminating inter
-
tree
interferences and exploiting spatial reuses of parallel
transmissions among subtrees.

TMCP Revealed (continued).


TMCP has 3 components


Channel Assignment


Given k orthogonal channels this module segregates the
nodes into k subtrees and assigns one unique channel
to each subtree. By using orthogonal channels the
intranode interference is greatly reduced.


Channel Detection


Data Communication


The DC component manages the data through each
subtree.

Interference view

PMIT Algorithm


Definition 1: INT
(
u
) =
{v|v


D
(
v, I
v
)
}
, where
D
(
v, I
v
) is the
interference disk with node
v
in its center and radius
I
v
, and the
interference value of a node
u
is defined as
int
(
u
) =
|INT
(
u
)
|


I
v

= (1+
α
)
×
R
v
, and
α >
0 implies that all of
u
’s neighbors
belongs to
INT
(
u
).


Definition 2:
The intra
-
tree interference value of a tree
T
is
defined as
int
(
T
) = max
{int
(
u
) :
u
is a non
-
leaf of
T}


Given k orthogonal channels, partition the network into k vertex
-
disjoint trees with minimizing the maximum intra
-
tree
interference value of all trees, or PMIT.


This problem is NP
-
complete.

Results


Between 1.6 and 2.7 times the throughput compared
to the spanning tree protocol with a single channel.


Decreases collisions.


Greater scalability.


2.8 times greater aggregate throughput with 42%
less latency.


When the workload increases the performance of the
TMCP becomes unstable.