<s>
A	O
ring	B-Architecture
network	I-Architecture
is	O
a	O
network	B-Architecture
topology	I-Architecture
in	O
which	O
each	O
node	O
connects	O
to	O
exactly	O
two	O
other	O
nodes	O
,	O
forming	O
a	O
single	O
continuous	O
pathway	O
for	O
signals	O
through	O
each	O
node	O
–	O
a	O
ring	O
.	O
</s>
<s>
Rings	O
can	O
be	O
unidirectional	O
,	O
with	O
all	O
traffic	O
travelling	O
either	O
clockwise	O
or	O
anticlockwise	O
around	O
the	O
ring	O
,	O
or	O
bidirectional	O
(	O
as	O
in	O
SONET/SDH	B-Protocol
)	O
.	O
</s>
<s>
Because	O
a	O
unidirectional	O
ring	B-Architecture
topology	I-Architecture
provides	O
only	O
one	O
pathway	O
between	O
any	O
two	O
nodes	O
,	O
unidirectional	O
ring	B-Architecture
networks	I-Architecture
may	O
be	O
disrupted	O
by	O
the	O
failure	O
of	O
a	O
single	O
link	O
.	O
</s>
<s>
In	O
response	O
,	O
some	O
ring	B-Architecture
networks	I-Architecture
add	O
a	O
"	O
counter-rotating	O
ring	O
"	O
(	O
C-Ring	O
)	O
to	O
form	O
a	O
redundant	O
topology	B-Architecture
:	O
in	O
the	O
event	O
of	O
a	O
break	O
,	O
data	O
are	O
wrapped	O
back	O
onto	O
the	O
complementary	O
ring	O
before	O
reaching	O
the	O
end	O
of	O
the	O
cable	O
,	O
maintaining	O
a	O
path	O
to	O
every	O
node	O
along	O
the	O
resulting	O
C-Ring	O
.	O
</s>
<s>
7	O
(	O
SS7	O
)	O
,	O
Spatial	B-Protocol
Reuse	I-Protocol
Protocol	I-Protocol
,	O
Fiber	B-Protocol
Distributed	I-Protocol
Data	I-Protocol
Interface	I-Protocol
(	O
FDDI	B-Protocol
)	O
,	O
and	O
Resilient	O
Packet	O
Ring	O
.	O
</s>
<s>
IEEE	B-Device
802.5	I-Device
networks	O
–	O
also	O
known	O
as	O
IBM	B-Device
Token	I-Device
Ring	I-Device
networks	O
–	O
avoid	O
the	O
weakness	O
of	O
a	O
ring	B-Architecture
topology	I-Architecture
altogether	O
:	O
they	O
actually	O
use	O
a	O
star	O
topology	B-Architecture
at	O
the	O
physical	O
layer	O
and	O
a	O
media	B-Device
access	I-Device
unit	I-Device
(	O
MAU	O
)	O
to	O
imitate	O
a	O
ring	O
at	O
the	O
datalink	O
layer	O
.	O
</s>
<s>
7	O
(	O
SS7	O
)	O
,	O
and	O
some	O
SONET/SDH	B-Protocol
rings	O
have	O
two	O
sets	O
of	O
bidirectional	O
links	O
between	O
nodes	O
.	O
</s>
<s>
Ring	B-Architecture
Protection	I-Architecture
reconfiguration	O
for	O
line	O
faults	O
of	O
bidirectional	O
rings	O
can	O
be	O
very	O
fast	O
,	O
as	O
switching	O
happens	O
at	O
a	O
high	O
level	O
,	O
and	O
thus	O
the	O
traffic	O
does	O
not	O
require	O
individual	O
rerouting	O
.	O
</s>
<s>
Circuits	O
are	O
set	O
up	O
with	O
out-of-band	O
signalling	O
protocols	O
,	O
whereas	O
packets	O
are	O
usually	O
carried	O
via	O
a	O
Medium	B-Protocol
Access	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
MAC	O
)	O
.	O
</s>
<s>
The	O
purpose	O
of	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
is	O
to	O
determine	O
which	O
station	O
transmits	O
when	O
.	O
</s>
<s>
As	O
in	O
any	O
MAC	B-Protocol
protocol	I-Protocol
,	O
the	O
aims	O
are	O
to	O
resolve	O
contention	O
and	O
provide	O
fairness	O
.	O
</s>
<s>
There	O
are	O
three	O
main	O
classes	O
of	O
media	O
access	O
protocol	O
for	O
ring	B-Architecture
networks	I-Architecture
:	O
slotted	O
,	O
token	O
and	O
register	O
insertion	O
.	O
</s>
<s>
The	O
slotted	O
ring	O
treats	O
the	O
latency	O
of	O
the	O
ring	B-Architecture
network	I-Architecture
as	O
a	O
large	O
shift	O
register	O
that	O
permanently	O
rotates	O
.	O
</s>
<s>
The	O
pre-eminent	O
example	O
of	O
the	O
slotted	O
ring	O
is	O
the	O
Cambridge	B-General_Concept
Ring	I-General_Concept
.	O
</s>
<s>
"	O
Token	B-Device
Ring	I-Device
is	O
an	O
example	O
of	O
a	O
ring	O
topology.	O
"	O
</s>
<s>
802.5	B-Device
(	O
Token	B-Device
Ring	I-Device
)	O
networks	O
do	O
not	O
use	O
a	O
ring	B-Architecture
topology	I-Architecture
at	O
layer	O
1	O
.	O
</s>
<s>
As	O
explained	O
above	O
,	O
IBM	B-Device
Token	I-Device
Ring	I-Device
(	O
802.5	B-Device
)	O
networks	O
imitate	O
a	O
ring	O
at	O
layer	O
2	O
but	O
use	O
a	O
physical	O
star	O
at	O
layer	O
1	O
.	O
</s>
<s>
It	O
is	O
true	O
that	O
there	O
are	O
no	O
collisions	O
on	O
an	O
IBM	B-Device
Token	I-Device
Ring	I-Device
,	O
but	O
this	O
is	O
because	O
of	O
the	O
layer	O
2	O
Media	B-Protocol
Access	I-Protocol
Control	I-Protocol
method	O
,	O
not	O
the	O
physical	B-Architecture
topology	I-Architecture
(	O
which	O
again	O
is	O
a	O
star	O
,	O
not	O
a	O
ring	O
.	O
)	O
</s>
<s>
Token	O
passing	O
is	O
a	O
way	O
of	O
managing	O
access	O
to	O
the	O
cable	O
,	O
implemented	O
at	O
the	O
MAC	B-Protocol
sublayer	I-Protocol
of	O
layer	O
2	O
.	O
</s>
<s>
Ring	B-Architecture
topology	I-Architecture
is	O
the	O
cable	O
layout	O
at	O
layer	O
one	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
do	O
token	O
passing	O
on	O
a	O
bus	O
(	O
802.4	O
)	O
a	O
star	O
(	O
802.5	B-Device
)	O
or	O
a	O
ring	O
(	O
FDDI	B-Protocol
)	O
.	O
</s>
