<s>
The	O
Collection	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
CTP	O
)	O
is	O
a	O
routing	B-Protocol
protocol	I-Protocol
for	O
wireless	B-Architecture
sensor	I-Architecture
networks	I-Architecture
.	O
</s>
<s>
The	O
number	O
of	O
expected	O
transmissions	O
needed	O
to	O
send	O
data	O
between	O
two	O
nodes	O
,	O
ETX	B-Protocol
,	O
is	O
used	O
as	O
the	O
routing	O
metric	O
.	O
</s>
<s>
This	O
assumes	O
packets	O
are	O
retransmitted	O
at	O
the	O
link	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
In	O
a	O
route	O
that	O
includes	O
multiple	O
hops	O
,	O
the	O
metric	O
is	O
the	O
sum	O
of	O
the	O
ETX	B-Protocol
of	O
the	O
individual	O
hops	O
.	O
</s>
<s>
Each	O
node	O
sends	O
its	O
data	O
to	O
the	O
tree	O
root	O
to	O
which	O
it	O
is	O
nearest	O
,	O
that	O
is	O
,	O
the	O
tree	O
root	O
from	O
which	O
it	O
is	O
separated	O
by	O
the	O
smallest	O
ETX	B-Protocol
.	O
</s>
<s>
A	O
tree	O
root	O
always	O
has	O
an	O
ETX	B-Protocol
of	O
zero	O
.	O
</s>
<s>
Each	O
node	O
only	O
keeps	O
the	O
smallest	O
ETX	B-Protocol
(	O
to	O
the	O
nearest	O
tree	O
root	O
)	O
.	O
</s>
<s>
The	O
collection	O
of	O
ETX	B-Protocol
values	O
is	O
known	O
as	O
a	O
gradient	O
,	O
and	O
messages	O
are	O
only	O
sent	O
down	O
the	O
gradient	O
from	O
nodes	O
with	O
higher	O
ETX	B-Protocol
to	O
nodes	O
with	O
smaller	O
ETX	B-Protocol
.	O
</s>
<s>
This	O
kind	O
of	O
forwarding	O
is	O
common	O
to	O
many	O
algorithms	O
and	O
protocols	O
in	O
wireless	B-Architecture
sensor	I-Architecture
networks	I-Architecture
.	O
</s>
<s>
Rapidly	O
changing	O
link	O
qualities	O
,	O
for	O
example	O
in	O
sensor	B-Architecture
networks	I-Architecture
with	O
moving	O
nodes	O
,	O
cause	O
routing	O
information	O
to	O
become	O
outdated	O
which	O
can	O
lead	O
to	O
routing	O
loops	O
.	O
</s>
<s>
Each	O
packet	O
contains	O
the	O
ETX	B-Protocol
from	O
the	O
sender	O
to	O
the	O
root	O
.	O
</s>
<s>
If	O
a	O
node	O
receives	O
a	O
packet	O
with	O
ETX	B-Protocol
lower	O
than	O
its	O
own	O
this	O
indicates	O
an	O
inconsistency	O
in	O
the	O
tree	O
.	O
</s>
<s>
The	O
goal	O
is	O
to	O
have	O
the	O
sender	O
of	O
the	O
packet	O
receive	O
the	O
beacon	O
frame	O
and	O
adjust	O
its	O
ETX	B-Protocol
accordingly	O
.	O
</s>
<s>
However	O
,	O
sending	O
beacons	O
more	O
frequently	O
leaves	O
less	O
bandwidth	O
for	O
application	B-Application
level	I-Application
data	O
and	O
uses	O
more	O
energy	O
.	O
</s>
