<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
flit	B-Protocol
(	O
flow	B-Protocol
control	I-Protocol
unit	I-Protocol
or	O
flow	B-Protocol
control	I-Protocol
digit	I-Protocol
)	O
is	O
a	O
link-level	B-Protocol
atomic	O
piece	O
that	O
forms	O
a	O
network	B-Protocol
packet	I-Protocol
or	O
stream	B-Architecture
.	O
</s>
<s>
The	O
first	O
flit	B-Protocol
,	O
called	O
the	O
header	O
flit	B-Protocol
holds	O
information	O
about	O
this	O
packet	B-Protocol
's	O
route	O
(	O
namely	O
the	O
destination	O
address	O
)	O
and	O
sets	O
up	O
the	O
routing	B-Protocol
behavior	O
for	O
all	O
subsequent	O
flits	B-Protocol
associated	O
with	O
the	O
packet	B-Protocol
.	O
</s>
<s>
The	O
header	O
flit	B-Protocol
is	O
followed	O
by	O
zero	O
or	O
more	O
body	O
flits	B-Protocol
,	O
containing	O
the	O
actual	O
payload	O
of	O
data	O
.	O
</s>
<s>
The	O
final	O
flit	B-Protocol
,	O
called	O
the	O
tail	O
flit	B-Protocol
,	O
performs	O
some	O
book	O
keeping	O
to	O
close	O
the	O
connection	O
between	O
the	O
two	O
nodes	O
.	O
</s>
<s>
A	O
virtual	B-Protocol
connection	I-Protocol
holds	O
the	O
state	O
needed	O
to	O
coordinate	O
the	O
handling	O
of	O
the	O
flits	B-Protocol
of	O
a	O
packet	B-Protocol
.	O
</s>
<s>
At	O
a	O
minimum	O
,	O
this	O
state	O
identifies	O
the	O
output	O
port	O
of	O
the	O
current	O
node	B-Protocol
for	O
the	O
next	O
hop	O
of	O
the	O
route	O
and	O
the	O
state	O
of	O
the	O
virtual	B-Protocol
connection	I-Protocol
(	O
idle	O
,	O
waiting	O
for	O
resources	O
,	O
or	O
active	O
)	O
.	O
</s>
<s>
The	O
virtual	B-Protocol
connection	I-Protocol
may	O
also	O
include	O
pointers	O
to	O
the	O
flits	B-Protocol
of	O
the	O
packet	B-Protocol
that	O
are	O
buffered	O
on	O
the	O
current	O
node	B-Protocol
and	O
the	O
number	O
of	O
flit	B-Protocol
buffers	O
available	O
on	O
the	O
next	O
node	B-Protocol
.	O
</s>
<s>
The	O
growing	O
need	O
for	O
performance	O
from	O
computing	O
systems	O
drove	O
the	O
industry	O
into	O
the	O
multi-core	B-Architecture
and	O
many-core	B-General_Concept
arena	O
.	O
</s>
<s>
One	O
straightforward	O
approach	O
is	O
the	O
bus	B-General_Concept
based	I-General_Concept
interconnect	I-General_Concept
,	O
a	O
group	O
of	O
wires	O
connecting	O
all	O
the	O
processors	O
.	O
</s>
<s>
Hence	O
,	O
a	O
scalable	O
high	O
performance	O
interconnect	B-General_Concept
network	O
lies	O
at	O
the	O
core	O
of	O
parallel	B-Operating_System
computer	I-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
The	O
primary	O
expectation	O
of	O
an	O
interconnection	O
network	O
is	O
to	O
have	O
as	O
low	O
a	O
latency	O
as	O
possible	O
,	O
that	O
is	O
the	O
time	O
taken	O
to	O
transfer	O
a	O
message	O
from	O
one	O
node	B-Protocol
to	O
another	O
should	O
be	O
minimal	O
,	O
while	O
allowing	O
a	O
large	O
number	O
of	O
such	O
transactions	O
to	O
take	O
place	O
concurrently	O
.	O
</s>
<s>
The	O
basic	O
building	O
blocks	O
of	O
an	O
interconnection	O
network	O
are	O
its	O
topology	B-Architecture
,	O
routing	B-Protocol
algorithm	I-Protocol
,	O
switching	B-Protocol
strategy	I-Protocol
and	O
the	O
flow	O
control	O
mechanism	O
.	O
</s>
<s>
Topology	B-Architecture
:	O
This	O
refers	O
to	O
the	O
general	O
infrastructure	O
of	O
the	O
interconnection	O
network	O
;	O
the	O
pattern	O
in	O
which	O
multiple	O
processors	O
are	O
connected	O
.	O
</s>
<s>
This	O
pattern	O
could	O
either	O
be	O
regular	O
or	O
irregular	O
,	O
though	O
many	O
multi-core	B-Architecture
architectures	O
today	O
use	O
highly	O
regular	O
interconnection	O
networks	O
.	O
</s>
<s>
Routing	B-Protocol
Algorithm	I-Protocol
:	O
This	O
determines	O
which	O
path	O
the	O
message	O
must	O
take	O
in	O
order	O
to	O
ensure	O
delivery	O
to	O
the	O
destination	O
node	B-Protocol
.	O
</s>
<s>
There	O
are	O
many	O
different	O
routing	B-Protocol
algorithms	I-Protocol
,	O
providing	O
different	O
guarantees	O
and	O
offering	O
different	O
performance	O
trade-offs	O
.	O
</s>
<s>
Switching	B-Protocol
Strategy	I-Protocol
:	O
The	O
routing	B-Protocol
algorithm	I-Protocol
only	O
determines	O
the	O
path	O
that	O
a	O
message	O
must	O
take	O
to	O
reach	O
its	O
destination	O
node	B-Protocol
.	O
</s>
<s>
The	O
actual	O
traversal	O
of	O
the	O
message	O
within	O
the	O
network	O
is	O
the	O
responsibility	O
of	O
the	O
switching	B-Protocol
strategy	I-Protocol
.	O
</s>
<s>
There	O
are	O
basically	O
two	O
types	O
of	O
switching	O
strategies	O
,	O
a	O
circuit	B-Protocol
switched	I-Protocol
network	I-Protocol
is	O
a	O
network	O
where	O
a	O
path	O
is	O
reserved	O
and	O
blocked	O
off	O
from	O
other	O
messages	O
,	O
till	O
the	O
message	O
is	O
delivered	O
to	O
its	O
destination	O
node	B-Protocol
.	O
</s>
<s>
A	O
famous	O
example	O
of	O
circuit	B-Protocol
switched	I-Protocol
network	I-Protocol
is	O
the	O
telephone	O
services	O
,	O
which	O
establish	O
a	O
circuit	O
through	O
many	O
switches	O
for	O
a	O
call	O
.	O
</s>
<s>
The	O
alternative	O
approach	O
is	O
the	O
packet	B-Protocol
switched	I-Protocol
network	I-Protocol
where	O
messages	O
are	O
broken	O
down	O
into	O
smaller	O
compact	O
entities	O
called	O
packets	B-Protocol
.	O
</s>
<s>
Each	O
packet	B-Protocol
contains	O
a	O
part	O
of	O
data	O
in	O
addition	O
to	O
a	O
sequence	O
number	O
.	O
</s>
<s>
This	O
implies	O
that	O
each	O
packet	B-Protocol
can	O
now	O
be	O
transferred	O
individually	O
and	O
assembled	O
at	O
the	O
destination	O
based	O
on	O
the	O
sequence	O
number	O
.	O
</s>
<s>
Flow	O
control	O
:	O
Note	O
that	O
we	O
have	O
previously	O
established	O
the	O
fact	O
that	O
multiple	O
messages	O
can	O
flow	O
through	O
the	O
interconnect	B-General_Concept
network	O
at	O
any	O
given	O
time	O
.	O
</s>
<s>
Each	O
message	O
to	O
be	O
transferred	O
can	O
be	O
broken	O
down	O
into	O
smaller	O
chunks	O
of	O
fixed	O
length	O
entities	O
called	O
packets	B-Protocol
.	O
</s>
<s>
Packets	B-Protocol
may	O
in	O
turn	O
be	O
broken	O
down	O
into	O
message	O
flow	B-Protocol
control	I-Protocol
units	I-Protocol
or	O
flits	B-Protocol
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
flits	B-Protocol
represent	O
logical	O
units	O
of	O
information	O
,	O
while	O
phits	O
represent	O
the	O
physical	O
domain	O
,	O
that	O
is	O
,	O
phits	O
represent	O
the	O
number	O
of	O
bits	O
that	O
can	O
be	O
transferred	O
in	O
parallel	O
in	O
a	O
single	O
cycle	O
.	O
</s>
<s>
It	O
has	O
an	O
interconnection	O
network	O
which	O
uses	O
flit	B-Protocol
level	O
message	O
flow	O
control	O
wherein	O
each	O
flit	B-Protocol
is	O
composed	O
of	O
eight	O
16-bit	O
phits	O
.	O
</s>
<s>
That	O
means	O
its	O
flit	B-Protocol
size	O
is	O
128bits	O
and	O
phit	O
size	O
is	O
16bits	O
.	O
</s>
<s>
It	O
also	O
uses	O
the	O
flit	B-Protocol
level	O
message	O
flow	O
control	O
,	O
but	O
its	O
flit	B-Protocol
size	O
is	O
equal	O
to	O
its	O
phit	O
size	O
,	O
which	O
is	O
set	O
to	O
8	O
bits	O
.	O
</s>
<s>
Note	O
that	O
the	O
message	O
size	O
is	O
the	O
dominant	O
deciding	O
factor	O
(	O
among	O
many	O
others	O
)	O
in	O
deciding	O
the	O
flit	B-Protocol
widths	O
.	O
</s>
<s>
Keeping	O
the	O
size	O
of	O
each	O
packet	B-Protocol
small	O
,	O
in	O
which	O
case	O
the	O
number	O
of	O
packets	B-Protocol
will	O
increase	O
,	O
thus	O
increasing	O
the	O
total	O
number	O
of	O
transactions	O
,	O
while	O
decreasing	O
the	O
size	O
of	O
each	O
individual	O
transaction	O
.	O
</s>
<s>
Keeping	O
the	O
size	O
of	O
each	O
packet	B-Protocol
large	O
,	O
in	O
which	O
case	O
the	O
number	O
of	O
packets	B-Protocol
will	O
decrease	O
,	O
thus	O
decreasing	O
the	O
total	O
number	O
of	O
transactions	O
,	O
while	O
increasing	O
the	O
size	O
of	O
each	O
individual	O
transaction	O
.	O
</s>
<s>
Based	O
on	O
the	O
size	O
of	O
the	O
packets	B-Protocol
,	O
the	O
width	O
of	O
the	O
physical	O
link	O
between	O
two	O
routers	O
has	O
to	O
be	O
decided	O
.	O
</s>
<s>
Meaning	O
,	O
if	O
the	O
packet	B-Protocol
size	O
is	O
large	O
,	O
the	O
link	O
width	O
also	O
has	O
to	O
be	O
kept	O
large	O
,	O
however	O
,	O
a	O
larger	O
link	O
width	O
implies	O
more	O
area	O
and	O
higher	O
power	O
dissipation	O
.	O
</s>
<s>
The	O
link	O
width	O
(	O
which	O
also	O
decides	O
the	O
phit	O
width	O
)	O
now	O
factors	O
into	O
deciding	O
the	O
flit	B-Protocol
width	O
.	O
</s>
<s>
At	O
this	O
point	O
,	O
it	O
is	O
important	O
to	O
note	O
that	O
though	O
inter-router	O
transfers	O
are	O
necessarily	O
constructed	O
in	O
terms	O
of	O
phits	O
,	O
the	O
switching	O
techniques	O
deal	O
in	O
terms	O
of	O
flits	B-Protocol
.	O
</s>
<s>
For	O
more	O
details	O
regarding	O
the	O
various	O
switching	O
techniques	O
refer	O
Wormhole	B-Protocol
switching	I-Protocol
and	O
Cut-through	B-Protocol
switching	I-Protocol
.	O
</s>
<s>
Since	O
the	O
majority	O
of	O
switching	O
techniques	O
work	O
on	O
flits	B-Protocol
,	O
they	O
also	O
have	O
a	O
major	O
impact	O
in	O
deciding	O
the	O
flit	B-Protocol
width	O
.	O
</s>
<s>
Consider	O
an	O
example	O
of	O
how	O
packets	B-Protocol
are	O
transmitted	O
in	O
terms	O
of	O
flits	B-Protocol
.	O
</s>
<s>
In	O
this	O
case	O
we	O
have	O
a	O
packet	B-Protocol
transmitting	O
between	O
A	O
and	O
B	O
in	O
the	O
figure	O
.	O
</s>
<s>
The	O
packet	B-Protocol
transmitting	O
process	O
is	O
happening	O
in	O
the	O
following	O
steps	O
.	O
</s>
<s>
The	O
packet	B-Protocol
will	O
be	O
split	O
into	O
flits	B-Protocol
W	O
,	O
X	O
,	O
Y	O
and	O
Z	O
.	O
</s>
<s>
The	O
transmit	O
buffer	O
in	O
A	O
will	O
load	O
the	O
first	O
flit	B-Protocol
Z	O
and	O
send	O
it	O
to	O
B	O
.	O
</s>
<s>
After	O
B	O
receive	O
Z	O
,	O
B	O
will	O
move	O
the	O
flit	B-Protocol
out	O
of	O
the	O
buffer	O
.	O
</s>
<s>
The	O
transmit	O
buffer	O
in	O
A	O
will	O
then	O
load	O
the	O
next	O
flit	B-Protocol
Y	O
and	O
send	O
it	O
to	O
B	O
.	O
</s>
<s>
Keep	O
performing	O
the	O
above	O
actions	O
until	O
all	O
flits	B-Protocol
has	O
been	O
transmitted	O
to	O
B	O
.	O
</s>
<s>
B	O
will	O
then	O
put	O
together	O
all	O
the	O
flits	B-Protocol
to	O
get	O
the	O
whole	O
packet	B-Protocol
.	O
</s>
<s>
A	O
flit	B-Protocol
(	O
flow	O
control	O
units/digits	O
)	O
is	O
a	O
unit	O
amount	O
of	O
data	O
when	O
the	O
message	O
is	O
transmitting	O
in	O
link-level	B-Protocol
.	O
</s>
<s>
The	O
flit	B-Protocol
can	O
be	O
accepted	O
or	O
rejected	O
at	O
the	O
receiver	O
side	O
based	O
on	O
the	O
flow	O
control	O
protocol	O
and	O
the	O
size	O
of	O
the	O
receive	O
buffer	O
.	O
</s>
<s>
The	O
mechanism	O
of	O
link-level	B-Protocol
flow	O
control	O
is	O
allowing	O
the	O
receiver	O
to	O
send	O
a	O
continuous	O
signals	O
stream	B-Architecture
to	O
control	O
if	O
it	O
should	O
keep	O
sending	O
flits	B-Protocol
or	O
stop	O
sending	O
flits	B-Protocol
.	O
</s>
<s>
When	O
a	O
packet	B-Protocol
is	O
transmitted	O
over	O
a	O
link	O
,	O
the	O
packet	B-Protocol
will	O
need	O
to	O
be	O
split	O
into	O
multiple	O
flits	B-Protocol
before	O
the	O
transmitting	O
begin	O
.	O
</s>
