A Novel Communication Structure for Wireless Sensor Network

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A Novel Communication Structure for Wireless Sensor Network


Wenjin Xu
,
Jianfeng Liu


College of Information Scien
ce&Technology of
Qingdao University of Scien
ce&Technology

Naval Aeronautical Engineering Academy Qingdao Branch,

Qingdao city China 266061

qd_ekid@hotmail.com


Abstract


Due to their general applicability, standard
middleware approaches, especially in

connection with
serial communication systems, tend to be too
sophisticated for

Wireless Sensor Network(WSN). This
paper describes

the characte
ristics and the
application

of

a

modular

communication
infrastructure for

WSN.
The core of the
communication infrastructure is the

Serial Bus
Wireless Sensor Network

(
SB
-
WSN
)

object server,
which provides for modular real
-
time communication

services wi
thin the application

of

WSN
.



1.

Introduction


The motivation for the development of a new,
flexible communication

infrastructure is based on the
special requirements imposed by

realtime WSN
communication esp
ecially in tough environment
[1]

.
This structural

feature leads to a number of favourable
properties as high structural stiffness, high

accuracy
and low moved masses
.


For performing complex high
-
speed handling and
assembly processes high

performance is necessary.
Every task performed by
WSN

includes
get
ting
information from

the

environment
.

A flexible
communication infrastructure implies that the
information

has to be
got

by a
serirs bus

which is able
t
o
connect some
sub
-
processors of the WSN
[2]

.All
these interactions and computations are to be proces
sed
during each single

information

cycle. The coupling of
all
processors

necessitates a
rather
centrally

organized
communication
l architecture which accounts for cyclic
peripheral data

transmission every
information

cycle.

So, set values on the torque lev
el can begenerated
and transmitted.

A
dditionally, there are some
middleware

systems which transparently organize data
flows

and user data access in such systems.

This
infrastructure consists of a Middleware for WSN based
on IEEE 1394(FireWire) communicati
on standard.

These
demands on the new communication sturcture
can be summarized as:

. high and scalable computational power

.
an efficient communication concept to satisfy high
realtime requirements

. the flexibility to exchange information within the
WSN

nodes



2.

Communication
Structure


In centrally organized
communication
applications,
distributed system components like

intelligent sensors,
actors and drives have to be incorporated into the

communication process
[2].

Especially in
communication

applicatio
ns where information has to

be computed and distributed in real
-
time, the
communication infrastructure has to

ensure cycle times, which are independent
of the
transferred data amount
[3]
.
Figure 1.

Th
r
ee main requirements for the

communication
within such a

complex
WSN
system for high
-
dynamic
requirements are derived from the necessity to control a
complex

ad
-
hoc

communication

while transmitting data
each 125its cycle
[1]
.









208
















Figure1. SB
-
WSN Communication Structure



WSN Node




2.1
.


Communication System


T
ransaction layer and defines

additional real
-
time
services.

Although there are a number of serial bus
systems used for real
-
time

communication, today,
either the available bandwidth and the determinism

is
not
sufficient for real
-
time applications with cycle
times of a few hundred

microseconds, or there is not
yet an industrial communication protocol available.

Besides the requirements, figure 1 shows some of the
main characteristics of the

IEEE1394 communicat
ion
standard
[1]
, which we utilized as the external

communication standard for our robot application.
Intersections with the

equirement axes point out the
qualification of the IEEE1394a and IEEE1394b

version of the communication standard:

.
cycle tim
es down to 125gs (8kHz)

.
bandwidth of 400 (800) Mbit/s

.
synchronisation accuracy of

l
μ
s


.
up to 63 participants on one bus

.
up to 1023 buses


The need for a suitable communication protocol on
top of the IEEE1394

[1]

to allow an implementation of
communication services in the desired way led to

the
development of a new protocol. It uses

the existing
physical and the

link layer of the IEEE1394 standard,
implements the



2.2
.

Middleware

SB
-
WSN


In the following paragraphs, the middleware part of
the communication infrastructure for our
SB
-
WSN

system will be described in detail.
























Figure 2
: Layers
o
f the communication
Struct
ure



Figure
2

shows the software layers of the
communication
Structure
. The

communication
infrastructure consists of a highly adapted IEEE1394
driver, the

real
-
time middlew
are
SB
-
WSN
a
s

the communication protocol. Together with the
communication infrastructure, all

the application
modules (summarised in the application block) are
hosted on a

central control unit, where
SB
-
WSN

offer
its
services to

applications higher in the
protocol stack.

Application Layer

Software Isochronous Transfer Asynchronous
Transfer

Application

IAP

SB
-
MSN


IEEE 1394 Driver

Hardware

L
ink


layer

cycle

contr
o
lolol

Physica

layer
l

Pack

send
r

Pack

Rec

Medi
um in

Arbit
ration

En/D
ecode
r


Bus management Layer

Tr
ansaction

Layer





Main
Process
or

Sub
-
Proce
ssor

Sub
-
Proce
ssor




Sub
-
Proce
ssor


Sub
-
Proce
ssor



































































Connection using IEEE 1394 Protocal






SB
-
WSN

object
server


209


Together they facilitate fast real
-
time

interconnections
between local processes on the central control unit and
external

WSN

modules
.


2
.3
.

Organization of Determinisitic Data Flow
and data Processing using SB
-
WSN


The idea of
SB
-
WSN
Mi
ddleware for
WSN

communication
a
nd Process Control Applications
was
originally developed to suit the communication
demands of
large wireless network located in large
area.

As it is the key functionality of all commercial
middleware

approaches
SB
-
WSN
was
used as an
object server to

t
ransparently handle data and
procedure requests in robot control systems
[4]
. In order

to meet hard
-
real
-
time requirements derived from the
application within complex

WSN

systems with
communication cycle frequencies of up to 8kH
z

which
is defined by external communication standard IEEE
1394.
, The new infrastructure

SB
-
WSN
was developed
in order to organize all event
-
driven, soft
-

and

hard
-
real
-
time (cyclic) communication channels together
with the necessary

process scheduling on
a single
central computer.

In the next paragraphs, the message type
descriptions

operational phase of
SB
-
WSN

will follow.


2.4
.

Message Passing and Message Types


Local application processes on the central control
unit transmit and receive their

data using

two
different
mechanisms. The first

communication mechanism uses

message passing, where the application threads act as
clients or servers,

respectively, shows the general
message types involved while

communicating
between servers
(Sub
-
Processors)
[2]
, clients
(WSN
nods)

and the object server
(Main Processors)
. All
enquiries sent to

the
SB
-
WSN

object server are routed
to their ultimate target and back if an answer

appears to
be necessary. These enquiries are user event driven and
thus not related
[5]
.


.
request / answer
-

a client issues an enquiry and
waits for the reply from

a subsequent server. This is a
blocking communication operation.


.
command
-

a client issues an enquiry without
waiting for a reply from a

server. This communication
operation is n
on
-
blocking for the client.


.

config


these messages are uesd to re
gister
shared memory regions or messages(in case of a server
application)

.
control
-

these messages are used to obtain
statistical information about

messages and ongoing system status di
rectly from
the object server

In order to evaluate the efficiency of the message
transmission mechanism,

statistical measurements were
performed on a AMD Athlon XP 2400+ computer.

For
a transfer data size of up to 100 bytes, they exhibit
processing times f
or

request/answer messages of less
than 10gs while command messages even require

less
than 8gs.

The other communication mechanism is intended for
high
-
speed data transfers

between application
processes, directly. Here, the
SB
-
MSN
object server
sets up

shar
ed memory regions without being involved
in the actual communication

tasks. Interested processes
on the control unit thus achieve an individual memory

pointer and can read/write fast inter
-
process data.
For each shared memory region


the processes act as
p
ublishers and subscribers, respectively. In the case of
multiple

application processes accessing the same
shared memory region, data integrity has to be ensured.
At the same time real
-
time processes have to be
scheduled with

resvect to cycle times and prio
rity.

B
oth of these require
ments are met by the
implementa
i
on of an addi
tio
nal
real
time scheduler for
user threads
[6]
, using QNX condition variables. With
respect to

the
individual priority setup and possible
dynamic priority changes the
diff
erent

thteads
achieve
and release a scheduler
-
token, which permits user
functionali
ilities to operate without interruption.


3.
Operation
of SB
-
WSN



When s
tarting
SB
-
WSN
, an initial set of real
-
time application processes is set up,

according to
the desired priorities
defined in an initialisation
file. This set of real
-
time processes can be
modified and adjusted at any time of operation.
The
SB
-
WSN

services are now accessible to all
connected user application processes (as parts

of the whole robot control system). Using

config messages, the server processes

register
their services and variables with the
SB
-
WSN

object server. Additionally,

shared memory
regions are registered for fast data exchange
between processes

and threads.

During run
-
time, there are two parallel lev
els of
operation for the user application

module
communication: event
-
driven and cyclic operation.

On the event
-
driven

level the

SB
-
WSN

object server
receives command and request messages, checks

for
the appropriate servers registered for these messages


210


and forwards the

messages to the servers. If the client
expects an answer it is computed by the server

and sent
back to the object server, which forwards it to the
requesting client.

Internally, the object server keeps
track of all open client requests. I
t completes

them as
soon as the answers arrive from the servers. Using an
internal request list,

SB
-
WSN

ensures the consequent
separation of clients and servers in order to

avoid
deadlocks
[6]
. Before the object server forwards a
message it checks for the

o
perational state of the
recipient, thus ensuring correct transmission.
Additionally,

SB
-
WSN

implements a watchdog
functionality that cancels open event
-
driven

transactions when a previous request was not answered
within a specified time.

The cyclic operati
on level of
SB
-
WSN

can be coupled with different signal

sources
in order to adapt the control cycle time. Within this
operation level, shared

memory regions are used to
communicate process data between the real
-
time

processes (or threads) without embracing

the

SB
-
WSN

object server to route

messages.





4.
Conclusion


I
n this paper, a novel communication structure SB
-
WSN is introduced. Using IEEE 1394 standsrds
[1]

as
serial comm
unication bus , the new mi
ddleware SB
-
WSN is developed.
T
he appli
cation of the structure for
complicated WSN meets realtime commu
ni
cation
requirements enableing a 8KHz cycle.
In future , there
would be a more powerful standard for WSN
communication in large area using
complex wire a
nd
wireless information system.



5.
References


[1]

Judi

Romijn1


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Leader Election Protocol

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esign

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Netherlands

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-
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[5]
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Blum, B.M.



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kovic, J.A.


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[6]
Sohrabi

K.


Pottie, G.J.




Dept. of Electr. Eng., California Univ.,
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