<s>
In	O
telecommunication	O
,	O
a	O
distributed-queue	B-Protocol
dual-bus	I-Protocol
network	I-Protocol
(	O
DQDB	B-Protocol
)	O
is	O
a	O
distributed	O
multi-access	O
network	O
that	O
(	O
a	O
)	O
supports	O
integrated	O
communications	O
using	O
a	O
dual	O
bus	O
and	O
distributed	O
queuing	O
,	O
(	O
b	O
)	O
provides	O
access	O
to	O
local	B-General_Concept
or	O
metropolitan	O
area	O
networks	O
,	O
and	O
(	O
c	O
)	O
supports	O
connectionless	O
data	O
transfer	O
,	O
connection-oriented	O
data	O
transfer	O
,	O
and	O
isochronous	O
communications	O
,	O
such	O
as	O
voice	O
communications	O
.	O
</s>
<s>
IEEE	O
802.6	O
is	O
an	O
example	O
of	O
a	O
network	O
providing	O
DQDB	B-Protocol
access	O
methods	O
.	O
</s>
<s>
The	O
DQDB	B-Protocol
Medium	B-Protocol
Access	I-Protocol
Control	I-Protocol
(	O
MAC	B-Protocol
)	O
algorithm	O
is	O
generally	O
credited	O
to	O
Robert	O
Newman	O
who	O
developed	O
this	O
algorithm	O
in	O
his	O
PhD	O
thesis	O
in	O
the	O
1980s	O
at	O
the	O
University	O
of	O
Western	O
Australia	O
.	O
</s>
<s>
To	O
appreciate	O
the	O
innovative	O
value	O
of	O
the	O
DQDB	B-Protocol
MAC	B-Protocol
algorithm	O
,	O
it	O
must	O
be	O
seen	O
against	O
the	O
background	O
of	O
LAN	B-General_Concept
protocols	O
at	O
that	O
time	O
,	O
which	O
were	O
based	O
on	O
broadcast	O
(	O
such	O
as	O
ethernet	O
IEEE	O
802.3	O
)	O
or	O
a	O
ring	O
(	O
like	O
Token	B-Device
Ring	I-Device
IEEE	B-Device
802.5	I-Device
and	O
FDDI	B-Protocol
)	O
.	O
</s>
<s>
The	O
DQDB	B-Protocol
may	O
be	O
thought	O
of	O
as	O
two	O
token	B-Device
rings	I-Device
,	O
one	O
carrying	O
data	O
in	O
each	O
direction	O
around	O
the	O
ring	O
.	O
</s>
<s>
This	O
improves	O
reliability	O
which	O
is	O
important	O
in	O
Metropolitan	O
Area	O
Networks	O
(	O
MAN	O
)	O
,	O
where	O
repairs	O
may	O
take	O
longer	O
than	O
in	O
a	O
LAN	B-General_Concept
and	O
Wi-Fi	O
because	O
the	O
damage	O
may	O
be	O
inaccessible	O
.	O
</s>
<s>
The	O
DQDB	B-Protocol
standard	O
IEEE	O
802.6	O
was	O
developed	O
while	O
ATM	O
(	O
Broadband	B-Protocol
ISDN	I-Protocol
)	O
was	O
still	O
in	O
early	O
development	O
,	O
but	O
there	O
was	O
strong	O
interaction	O
between	O
the	O
two	O
standards	O
.	O
</s>
<s>
ATM	O
cells	O
and	O
DQDB	B-Protocol
frames	O
were	O
harmonized	O
.	O
</s>
<s>
In	O
the	O
DQDB	B-Protocol
algorithm	O
,	O
a	O
distributed	O
queue	O
was	O
implemented	O
by	O
communicating	O
queue	O
state	O
information	O
via	O
the	O
header	O
.	O
</s>
<s>
Each	O
node	O
in	O
a	O
DQDB	B-Protocol
network	O
maintains	O
a	O
pair	O
of	O
state	O
variables	O
which	O
represent	O
its	O
position	O
in	O
the	O
distributed	O
queue	O
and	O
the	O
size	O
of	O
the	O
queue	O
.	O
</s>
<s>
The	O
headers	O
on	O
the	O
reverse	O
bus	O
communicated	O
requests	O
to	O
be	O
inserted	O
in	O
the	O
distributed	O
queue	O
so	O
that	O
upstream	O
nodes	O
would	O
know	O
that	O
they	O
should	O
allow	O
DQDB	B-Protocol
cells	O
to	O
pass	O
unused	O
on	O
the	O
forward	O
bus	O
.	O
</s>
<s>
Currently	O
DQDB	B-Protocol
systems	O
are	O
being	O
installed	O
by	O
many	O
carriers	O
in	O
entire	O
cities	O
,	O
with	O
lengths	O
that	O
reach	O
up	O
to	O
with	O
speeds	O
of	O
a	O
DS3	O
line	O
(	O
44.736	O
Mbit/s	O
)	O
.	O
</s>
