Lecture 4, Wireless Networks: Medium Access Control
35
S2001, COM3525 Wireless Networks Lecture 4, 1 Wireless Networks: Medium Access Control Slides adapted from “Mobile Communications” by J. Schiller Guevara Noubir
Transcript of Lecture 4, Wireless Networks: Medium Access Control
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
1
Wire
less
Net
wor
ks:
Med
ium
Acc
ess
Con
trol
Slid
es a
dapt
ed fr
om “
Mob
ile C
omm
unic
atio
ns”
by J
. Sch
iller
Gue
vara
Nou
bir
S20
01, C
OM
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Wire
less
Net
wor
ksLe
ctur
e 4,
2
Mot
ivat
ion
•C
an w
e ap
ply
med
ia a
cces
s m
etho
ds fr
om fi
xed
netw
orks
?
•E
xam
ple
CS
MA
/CD
–C
arrie
r Sen
se M
ultip
le A
cces
s w
ith C
ollis
ion
Det
ectio
n
–se
nd a
s so
on a
s th
e m
ediu
m is
free
, lis
ten
into
the
med
ium
if a
co
llisi
on o
ccur
s (o
rigin
al m
etho
d in
IEE
E 8
02.3
)
•P
robl
ems
in w
irele
ss n
etw
orks
–si
gnal
str
engt
h de
crea
ses
prop
ortio
nal t
o th
e sq
uare
of t
he d
ist
ance
–th
e se
nder
wou
ld a
pply
CS
and
CD
, but
the
colli
sion
s ha
ppen
at t
he r
ecei
ver
–it
mig
ht b
e th
e ca
se th
at a
sen
der
cann
ot “
hear
” th
e co
llisi
on,
i.e.,
CD
doe
s no
t w
ork
–fu
rthe
rmor
e, C
S m
ight
not
wor
k if,
e.g
., a
term
inal
is “
hidd
en”
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wor
ksLe
ctur
e 4,
3
•H
idde
n te
rmin
als
–A
sen
ds to
B, C
can
not r
ecei
ve A
–
C w
ants
to s
end
to B
, C s
ense
s a
“fre
e” m
ediu
m (
CS
fails
)–
colli
sion
at B
, A c
anno
t rec
eive
the
colli
sion
(C
D fa
ils)
–A
is “
hidd
en”
for
C
•E
xpos
ed te
rmin
als
–B
sen
ds to
A, C
wan
ts to
sen
d to
ano
ther
term
inal
(no
t A o
r B
)–
C h
as to
wai
t, C
S s
igna
ls a
med
ium
in u
se–
but A
is o
utsi
de th
e ra
dio
rang
e of
C, t
here
fore
wai
ting
is n
ot
nece
ssar
y–
C is
“ex
pose
d” to
B
Mot
ivat
ion
-hi
dden
and
exp
osed
te
rmin
als
BA
C
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ksLe
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e 4,
4
•T
erm
inal
s A
and
B s
end,
C r
ecei
ves
–si
gnal
str
engt
h de
crea
ses
prop
ortio
nal t
o th
e sq
uare
of t
he d
ist
ance
–th
e si
gnal
of t
erm
inal
B th
eref
ore
drow
ns o
ut A
’s s
igna
l–
C c
anno
t rec
eive
A
•If
C fo
r ex
ampl
e w
as a
n ar
bite
r fo
r se
ndin
g rig
hts,
term
inal
B w
ould
dr
own
out t
erm
inal
A a
lread
y on
the
phys
ical
laye
r•
Als
o se
vere
pro
blem
for
CD
MA-n
etw
orks
-pr
ecis
e po
wer
con
trol
ne
eded
!
Mot
ivat
ion
-ne
ar a
nd fa
r te
rmin
als
AB
C
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Wire
less
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wor
ksLe
ctur
e 4,
5
Acc
ess
met
hods
S
DM
A/F
DM
A/T
DM
A•
SD
MA
(S
pace
Div
isio
n M
ultip
le A
cces
s)–
segm
ent s
pace
into
sec
tors
, use
dire
cted
ant
enna
s
–ce
ll st
ruct
ure
•F
DM
A (
Fre
quen
cy D
ivis
ion
Mul
tiple
Acc
ess)
–as
sign
a c
erta
in fr
eque
ncy
to a
tran
smis
sion
cha
nnel
bet
wee
n a
sen
der
and
a re
ceiv
er
–pe
rman
ent (
e.g.
, rad
io b
road
cast
), s
low
hop
ping
(e.
g., G
SM
), fa
st h
oppi
ng (
FH
SS
, F
requ
ency
Hop
ping
Spr
ead
Spe
ctru
m)
•T
DM
A (
Tim
e D
ivis
ion
Mul
tiple
Acc
ess)
–as
sign
the
fixed
sen
ding
freq
uenc
y to
a tr
ansm
issi
on c
hann
el b
etw
een
a se
nder
and
a
rece
iver
for
a ce
rtai
n am
ount
of t
ime
•T
he m
ultip
lexi
ng s
chem
es a
re n
ow u
sed
to c
ontr
ol m
ediu
m a
cces
s!
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ksLe
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e 4,
6
FD
D/F
DM
A -
gene
ral s
chem
e,
exam
ple
GS
Mf
t
124 1 124 1
20 M
Hz
200
kHz
890.
2 M
Hz
935.
2 M
Hz
915
MH
z
960
MH
z
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less
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wor
ksLe
ctur
e 4,
7
TD
D/T
DM
A -
gene
ral s
chem
e,
exam
ple
DE
CT
12
311
121
23
1112
tdo
wnl
ink
uplin
k
417
µs
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wor
ksLe
ctur
e 4,
8
Fre
quen
cy D
ivis
ion
Mul
tiple
Acc
ess
•C
once
pt:
–as
sign
diff
eren
t fre
quen
cy b
ands
to d
iffer
ent u
sers
–no
sha
ring
of a
freq
uenc
y ba
nd b
etw
een
two
user
s–
user
sep
arat
ion
usin
g ba
nd-pas
s fil
ters
–co
ntin
uous
flow
–tw
o-w
ay: t
wo
freq
uenc
y ba
nds
or T
ime
Div
isio
n D
uple
x (T
DD
)
•A
dvan
tage
s: s
impl
e re
ceiv
ers
–lo
nger
sym
bol d
urat
ion:
no-n
eed
for
equa
lizat
ion
•lo
w in
ter-
sym
bol i
nter
fere
nce
•e.
g., 5
0kb/
s Q
PS
K =
>40 µ
s >
> 1
-10µ
s de
lay
spre
ad
•D
raw
back
s:–
freq
uenc
y gu
ard
band
s, c
ostly
tigh
t RF
ban
d-f
ilter
s,
–lo
ng fa
ding
dur
atio
n: n
eed
slow
freq
uenc
y ho
ppin
g–
may
nee
d sp
atia
l div
ersi
ty (
mul
tiple
ant
enna
s/be
am fo
rmin
g) R
x/T
x
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Wire
less
Net
wor
ksLe
ctur
e 4,
9
Fre
quen
cy S
elec
tion
•F
requ
ency
man
agem
ent:
–F
ixed
(ce
llula
r ph
ones-
base
sta
tions
): r
euse
fact
or
–O
n de
man
d (c
ellu
lar
phon
es-mob
ile te
rmin
als)
–D
ynam
ic (
cord
less
/WLA
N):
bas
ed o
n se
nsin
g in
terf
eren
ce le
vels
–P
robl
ems:
con
gest
ion
man
agem
ent,
dyna
mic
load
, …
•A
nten
na im
plic
atio
ns:
–H
igh
ante
nnas
(e.
g., 5
0m):
hig
her
cove
rage
but
hig
her
inte
rfer
ence
bet
wee
n ba
se
stat
ions
(ne
ed fo
r sy
nchr
oniz
atio
n)
–Lo
w a
nten
nas:
hig
her
atte
nuat
ion,
low
er c
over
age,
bet
ter
reus
e
•C
oncl
usio
n:
–P
ure
FD
MA
is o
nly
inte
rest
ing
for
sim
ple
cord
less
sys
tem
s (C
T-2
)
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Wire
less
Net
wor
ksLe
ctur
e 4,
10
Tim
e D
ivis
ion
Mul
tiple
Acc
ess
•C
once
pt:
–us
e th
e sa
me
freq
uenc
y ov
er n
on-ove
rlapp
ing
perio
ds o
f tim
e
•A
dvan
tage
s:–
sim
ple
filte
rs (
win
dow
)
–tr
ansm
it an
d re
ceiv
e ov
er th
e sa
me
freq
uenc
y ch
anne
l
•D
raw
back
s:–
user
s m
ust b
e sy
nchr
oniz
ed w
ith B
S (
mas
ter
cloc
k ov
er a
BC
H)
–gu
ard
times
: com
mon
30-50µ
s, m
ay b
e le
ss in
rec
ent s
yste
ms
–sh
ort s
ymbo
l dur
atio
n: n
eed
for
equa
lizat
ion,
trai
ning
seq
uenc
es...
•hi
gh in
ter-s
ymbo
l int
erfe
renc
e
•e.
g., 5
0Kbp
s, Q
PS
K, 8
use
rs:
–5
µs s
ymbo
l dur
atio
n
–de
lay
spre
ad: 1µ
s (c
ordl
ess)
, upto
20µ
s fo
r ce
llula
r
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Wire
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wor
ksLe
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e 4,
11
FD
MA
/TD
MA
•F
irst c
hann
el a
lloca
tion:
–ra
ndom
acc
ess
chan
nel (
RA
CH
) to
sen
d sh
ort r
eque
sts
–A
LOH
A ty
pe p
roto
col o
ver
the
RA
CH
•O
ne c
an u
se b
oth
FD
MA
and
TD
MA
–ex
ampl
es: G
SM
sys
tem
, D-AM
PS
M9
M5
M6
M1
M2
M3
M4
M9
M5
M6
M1
M2
M3
M4
Tim
e
Fre
quen
cy
cycl
e
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Wire
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Net
wor
ksLe
ctur
e 4,
12
Acc
ess
met
hod
CD
MA
•CD
MA
(C
ode
Div
isio
n M
ultip
le A
cces
s)–
all t
erm
inal
s se
nd o
n th
e sa
me
freq
uenc
y pr
obab
ly a
t the
sam
e ti
me
and
can
use
the
who
le
band
wid
th o
f th
e tr
ansm
issi
on c
hann
el–
code
s ge
nera
te s
igna
ls w
ith “
good-c
orre
latio
n” p
rope
rtie
s–
sign
als
from
ano
ther
use
r ap
pear
as
“noi
se”
(use
spr
ead
spec
trum
tech
nolo
gy)
–si
gnal
s ar
e sp
read
ove
r a
wid
eban
d us
ing
pseu
do-n
oise
seq
uenc
es (
e.g.
, eac
h se
nder
has
a
uniq
ue r
ando
m n
umbe
r, th
e se
nder
X
OR
sth
e si
gnal
with
this
ran
dom
num
ber)
–th
e re
ceiv
er c
an “
tune
” in
to th
is s
igna
l if i
t kno
ws
the
pseu
do
rand
om n
umbe
r, tu
ning
is d
one
via
a co
rrel
atio
n fu
nctio
n
•Dis
adva
ntag
es:
–hi
gher
com
plex
ity o
f a r
ecei
ver
(rec
eive
r ca
nnot
just
list
en in
to
the
med
ium
and
sta
rt r
ecei
ving
if
ther
e is
a s
igna
l)–
all s
igna
ls s
houl
d ha
ve th
e sa
me
stre
ngth
at a
rec
eive
r (n
ear
-far
effe
ct)
•Adv
anta
ges:
–
all t
erm
inal
s ca
n us
e th
e sa
me
freq
uenc
y =
> n
o pl
anni
ng n
eede
d;
mac
rodi
vers
ity–
huge
cod
e sp
ace
(e.g
. 232)
com
pare
d to
freq
uenc
y sp
ace
–in
terf
eren
ces
(e.g
. whi
te n
oise
) is
not
cod
ed–
forw
ard
erro
r co
rrec
tion
and
encr
yptio
n ca
n be
eas
ily in
tegr
ated
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Wire
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wor
ksLe
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e 4,
13
CD
MA
in th
eory
•S
ende
r A
–
send
s A d
= 1
, key
Ak
= 0
1001
1 (a
ssig
n: „
0“=
-1, „
1“=
+1)
–se
ndin
g si
gnal
A s=
Ad
* A
k=
(-1
, +1,
-1, -
1, +
1, +
1)
•S
ende
r B
–se
nds B
d=
0, k
ey B
k=
110
101
(ass
ign:
„0“
= -1
, „1“
= +
1)
–se
ndin
g si
gnal
B s=
Bd
* B
k=
(-1
, -1,
+1,
-1, +
1, -1
)
•B
oth
sign
als
supe
rimpo
se in
spa
ce
–in
terf
eren
ce n
egle
cted
(no
ise
etc.
)
–A
s+
Bs
= (
-2, 0
, 0, -
2, +
2, 0
)
•R
ecei
ver
wan
ts to
rec
eive
sig
nal f
rom
sen
der
A–
appl
y ke
y Ak
bitw
ise
(inne
r pr
oduc
t)•
Ae
= (
-2, 0
, 0, -
2, +
2, 0
) •A
k=
2 +
0 +
0 +
2 +
2 +
0 =
6
•re
sult
grea
ter
than
0, t
here
fore
, orig
inal
bit
was
„1“
–re
ceiv
ing
B•
Be
= (
-2, 0
, 0, -
2, +
2, 0
) •B
k=
-2
+ 0
+ 0
-2
-2
+ 0
= -6
, i.e
. „0“
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Wire
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wor
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e 4,
14
CD
MA
on
sign
al le
vel I
data
A
key
A
sign
al A
data
⊕ke
y
key
sequ
ence
A
10
1
10
01
00
10
00
10
11
00
11
01
10
11
10
00
10
00
11
00
Ad
Ak
As
Rea
l sys
tem
s us
e m
uch
long
er k
eys
resu
lting
in a
larg
er d
ista
nce
betw
een
sing
le c
ode
wor
ds in
cod
e sp
ace.
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ksLe
ctur
e 4,
15
CD
MA
on
sign
al le
vel I
I
sign
al A
data
B
key
Bke
yse
quen
ce B
sign
al B
As
+ B
s
data
⊕ke
y
10
0
00
01
10
10
10
00
01
01
11
11
10
01
10
10
00
01
01
11
Bd
Bk
Bs
As
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wor
ksLe
ctur
e 4,
16
CD
MA
on
sign
al le
vel I
II
Ak
(As
+ B
s)
* A
k
inte
grat
orou
tput
com
para
tor
outp
ut
As
+ B
s
01
0
10
1da
ta A
Ad
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Wire
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wor
ksLe
ctur
e 4,
17
CD
MA
on
sign
al le
vel I
V
inte
grat
orou
tput
com
para
tor
outp
utBk
(As
+ B
s)
* B
k
As
+ B
s
01
1
10
0da
ta B
Bd
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wor
ksLe
ctur
e 4,
18
CD
MA
on
sign
al le
vel V
com
para
tor
outp
ut(1
)(1
)?
wro
ng
key
K
inte
grat
orou
tput
(As
+ B
s)
* K
As
+ B
s
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Wire
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wor
ksLe
ctur
e 4,
19
CD
MA
: Dire
ct S
eque
nce
•E
ach
mob
ile is
allo
cate
d a
PN
seq
uenc
e:
•T
he e
lem
ents
of t
he P
N-seq
uenc
e ar
e ca
lled
chip
s
•T
o tr
ansm
it on
e bi
t 1/0
sen
d th
e P
N-S
eq/in
v-P
N-S
eq
•E
xam
pleH
adam
ardM
atrix
:–
Eac
h m
obile
use
s on
e ro
w
–T
he r
ows
are
orth
ogon
al
���� ��
���� ��
+−
−+
−−
++
−+
−+
++
++
=
11
11
11
11
11
11
11
11
4H
Pow
er
Freq
uenc
ysp
read
ed s
igna
l
non-spread signal
user
1
user
4
user
3
user
2
10
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ksLe
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e 4,
20
•M
echa
nism
–ra
ndom
, dis
trib
uted
(no
cen
tral
arb
iter)
, tim
e-m
ultip
lex
–S
lotte
d A
loha
add
ition
ally
use
s tim
e-s
lots
, sen
ding
mus
t alw
ays
star
t at s
lot b
ound
arie
s
•A
loha
•S
lotte
d A
loha
Alo
ha/s
lotte
d al
oha
send
er A
send
er B
send
er C
colli
sion
send
er A
send
er B
send
er C
colli
sion
t t
S20
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Wire
less
Net
wor
ksLe
ctur
e 4,
21
Car
rier
Sen
se P
roto
cols
Use
the
fact
that
in s
ome
netw
orks
you
can
se
nse
the
med
ium
to c
heck
whe
ther
it
is c
urre
ntly
free
–1-
pers
iste
nt C
SM
A
–no
n-pe
rsis
tent
CS
MA
–p-
pers
iste
nt p
roto
col
–C
SM
A w
ith c
ollis
ion
Det
ectio
n (C
SM
A/C
D):
not
app
licab
le to
wire
les
s sy
stem
s
•1-
pers
iste
nt C
SM
A–
whe
n a
stat
ion
has
a pa
cket
:•
it w
aits
unt
il th
e m
ediu
m is
free
to tr
ansm
it th
e pa
cket
•if
a co
llisi
on o
ccur
s, th
e st
atio
n w
aits
a r
ando
m a
mou
nt o
f tim
e
–fir
st tr
ansm
issi
on r
esul
ts in
a c
ollis
ion
if se
vera
l sta
tions
ar
e w
aitin
g fo
r th
e ch
anne
l
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
22
Car
rier
Sen
se P
roto
cols
(Co
nt’d
)
•no
n-pe
rsis
tent
CS
MA
–w
hen
a st
atio
n ha
s a
pack
et:
•if
the
med
ium
is fr
ee, t
rans
mit
the
pack
et•
othe
rwis
e w
ait f
or a
ran
dom
per
iod
of ti
me
and
repe
at th
e al
gori
thm
–hi
gher
del
ays,
but
bet
ter
perf
orm
ance
than
pur
e A
LOH
A
•p-
pers
iste
nt p
roto
col
–w
hen
a st
atio
n ha
s a
pack
et w
ait u
ntil
the
med
ium
is fr
ee:
•tr
ansm
it th
e pa
cket
with
pro
babi
lity p
•w
ait f
or n
ext s
lot w
ith p
roba
bilit
y 1-p
–be
tter
thro
ughp
ut th
an o
ther
sch
emes
but
hig
her
dela
y
•C
SM
A w
ith c
ollis
ion
Det
ectio
n (C
SM
A/C
D)
–st
atio
ns a
bort
thei
r tr
ansm
issi
on w
hen
they
det
ect a
col
lisio
n–
e.g.
, Eth
erne
t, IE
EE
802.
3 bu
t not
app
licab
le to
wire
less
sys
tem
s
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
23
DA
MA
-D
eman
d A
ssig
ned
Mul
tiple
Acc
ess
•C
hann
el e
ffici
ency
onl
y 18
% fo
r A
loha
, 36%
for
Slo
tted
Alo
ha (
assu
min
g P
oiss
on d
istr
ibut
ion
for
pack
et a
rriv
al a
nd p
acke
t len
gth)
•R
eser
vatio
n ca
n in
crea
se e
ffici
ency
to 8
0%–
a se
nder
rese
rve
sa fu
ture
tim
e-slo
t
–se
ndin
g w
ithin
this
res
erve
d tim
e-s
lot i
s po
ssib
le w
ithou
t col
lisio
n
–re
serv
atio
n al
so c
ause
s hi
gher
del
ays
–ty
pica
l sch
eme
for
sate
llite
link
s
•E
xam
ples
for
rese
rvat
ion
algo
rithm
s:–
Exp
licit
Re
serv
atio
n a
cco
rdin
g to
Ro
be
rts
(Re
serv
atio
n-A
LO
HA
)
–Im
plic
it R
ese
rva
tion
(P
RM
A)
–R
ese
rva
tion-T
DM
A
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
24
Acc
ess
met
hod
DA
MA
: Exp
licit
Res
erva
tion
•Exp
licit
Res
erva
tion
(Res
erva
tion
Alo
ha):
–tw
o m
odes
: •
AL
OH
A m
od
efor
res
erva
tion:
com
petit
ion
for
smal
l res
erva
tion
slot
s, c
ollis
ions
pos
sibl
e
•re
serv
ed
mo
defo
r da
ta tr
ansm
issi
on w
ithin
suc
cess
ful r
eser
ved
slot
s (n
o co
llis
ions
po
ssib
le)
–it
is im
port
ant f
or a
ll st
atio
ns to
kee
p th
e re
serv
atio
n lis
t co
nsis
tent
at a
ny p
oint
in
time
and,
ther
efor
e, a
ll st
atio
ns h
ave
to s
ynch
roni
ze fr
om ti
me
to ti
me
Alo
hare
serv
edA
loha
rese
rved
Alo
hare
serv
edA
loha
colli
sion
t
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
25
Acc
ess
met
hod
DA
MA
: PR
MA
•Im
plic
it re
serv
atio
n (P
RM
A -P
acke
t Res
erva
tion
MA
):–
a ce
rtai
n nu
mbe
r of
slo
ts fo
rm a
fram
e, fr
ames
are
rep
eate
d
–st
atio
ns c
ompe
te fo
r em
pty
slot
s ac
cord
ing
to th
e sl
otte
d al
oha
prin
cipl
e
–on
ce a
sta
tion
rese
rves
a s
lot s
ucce
ssfu
lly, t
his
slot
is a
utom
atic
ally
ass
igne
d to
this
sta
tion
in a
ll fo
llow
ing
fram
es a
s lo
ng a
s th
e st
atio
n h
as d
ata
to s
end
–co
mpe
titio
n fo
r th
is s
lots
sta
rts
agai
n as
soo
n as
the
slot
was
em
pty
in th
e la
st fr
ame fr
ame 1
fram
e 2
fram
e 3
fram
e 4
fram
e 5
12
34
56
78
time-
slot
colli
sion
at
rese
rvat
ion
atte
mpt
s
AC
DA
BA
F
AC
AB
A
AB
AF
AB
AF
D
AC
EE
BA
FD
t
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
rese
rvat
ion
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
26
Acc
ess
met
hod
DA
MA
: R
eser
vatio
n-TD
MA
•Res
erva
tion
Tim
e D
ivis
ion
Mul
tiple
Acc
ess
–ev
ery
fram
e co
nsis
ts o
f N m
ini
-slo
ts a
nd x
dat
a-slo
ts
–ev
ery
stat
ion
has
its o
wn
min
i-s
lot a
nd c
an r
eser
ve u
p to
k d
ata
-slo
ts u
sing
th
is m
ini-s
lot (
i.e. x
= N
* k
).
–ot
her
stat
ions
can
sen
d da
ta in
unu
sed
data
-slo
ts a
ccor
ding
to a
rou
nd-rob
in
send
ing
sche
me
(bes
t-e
ffort
traf
fic)
N m
ini-
slot
sN
* k
dat
a-s
lots
rese
rvat
ions
for
data
-slo
tsot
her
stat
ions
can
use
free
dat
a-s
lots
base
d on
a r
ound
-rob
in s
chem
e
e.g.
N=
6, k
=2
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
27
MA
CA
-co
llisi
on a
void
ance
•M
AC
A (
Mul
tiple
Acc
ess
with
Col
lisio
n A
void
ance
) us
es s
hort
sig
nalin
g pa
cket
s fo
r co
llisi
on a
void
ance
–R
TS
(re
ques
t to
send
): a
sen
der
requ
est t
he r
ight
to s
end
from
are
ceiv
er w
ith a
sh
ort R
TS
pac
ket b
efor
e it
send
s a
data
pac
ket
–C
TS
(cl
ear
to s
end)
: the
rec
eive
r gr
ants
the
right
to s
end
as s
oon
as
it is
rea
dy to
re
ceiv
e
•S
igna
ling
pack
ets
cont
ain
–se
nder
add
ress
–re
ceiv
er a
ddre
ss
–pa
cket
siz
e
•V
aria
nts
of th
is m
etho
d ca
n be
foun
d in
IEE
E80
2.11
as
DF
WM
AC
(D
istr
ibut
ed F
ound
atio
n W
irele
ss M
AC
)
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
28
•M
AC
A a
void
s th
e pr
oble
m o
f hid
den
term
inal
s–
A a
nd C
wan
t to
send
to B
–A
sen
ds R
TS
firs
t–
C w
aits
afte
r re
ceiv
ing
CT
S fr
om B
•M
AC
A a
void
s th
e pr
oble
m o
f exp
osed
term
inal
s–
B w
ants
to s
end
to A
, C
to a
noth
er te
rmin
al–
now
C d
oes
not h
ave
to w
ait f
or it
can
not
rece
ive
CT
S fr
om AM
AC
A e
xam
ples
AB
C
RT
S
CT
SC
TS
AB
C
RT
S
CT
S
RT
S
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
29
MA
CA
var
iant
: DF
WM
AC
in
IEE
E80
2.11
idle
wai
t for
the
right
to s
end
wai
t for
AC
K
send
erre
ceiv
er
pack
et r
eady
to s
end;
RT
S
time-
out;
RT
S
CT
S; d
ata
AC
K
RxB
usy
idle
wai
t for
data
RT
S; R
xBus
yRT
S;
CT
Sda
ta;
AC
K
time-
out ∨
data
; N
AK
AC
K: p
ositi
ve a
ckno
wle
dgem
ent
NA
K: n
egat
ive
ackn
owle
dgem
ent
RxB
usy:
rec
eive
r bu
sy
time-
out ∨
NA
K;
RT
S
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
30
Pol
ling
mec
hani
sms
•If
one
term
inal
can
be
hear
d by
all
othe
rs, t
his
“cen
tral
” te
rmi
nal (
a.k.
a.
base
sta
tion)
can
pol
l all
othe
r te
rmin
als
acco
rdin
g to
a c
erta
in
sche
me
–no
w a
ll sc
hem
es k
now
n fr
om fi
xed
netw
orks
can
be
used
(ty
pica
l mai
nfra
me -
term
inal
sce
nario
)
•E
xam
ple:
Ran
dom
ly A
ddre
ssed
Pol
ling
–ba
se s
tatio
n si
gnal
s re
adin
ess
to a
ll m
obile
term
inal
s
–te
rmin
als
read
y to
sen
d ca
n no
w tr
ansm
it a
rand
om n
umbe
r w
ithou
tco
llisi
on
with
the
help
of C
DM
A o
r F
DM
A (
the
rand
om n
umbe
r ca
n be
see
n as
dy
nam
ic
addr
ess)
or
with
col
lisio
ns (
over
the
Ran
dom
Acc
ess
CH
anne
l)
–th
e ba
se s
tatio
n no
w c
hoos
es o
ne a
ddre
ss fo
r po
lling
from
the
list
of a
ll ra
ndom
nu
mbe
rs (
colli
sion
if tw
o te
rmin
als
choo
se th
e sa
me
addr
ess)
–th
e ba
se s
tatio
n ac
know
ledg
es c
orre
ct p
acke
ts a
nd c
ontin
ues
poll
ing
the
next
te
rmin
al
–th
is c
ycle
sta
rts
agai
n af
ter
polli
ng a
ll te
rmin
als
of th
e lis
t
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
31
ISM
A (
Inhi
bit S
ense
Mul
tiple
A
cces
s)•
Cur
rent
sta
te o
f the
med
ium
is s
igna
led
via
a “b
usy
tone
”–
the
base
sta
tion
sign
als
on th
e do
wnl
ink
(bas
e st
atio
n to
term
inal
s) if
the
med
ium
is
free
or
not
–te
rmin
als
mus
t not
sen
d if
the
med
ium
is b
usy
–te
rmin
als
can
acce
ss th
e m
ediu
m a
s so
on a
s th
e bu
sy to
ne s
tops
–th
e ba
se s
tatio
n si
gnal
s co
llisi
ons
and
succ
essf
ul tr
ansm
issi
ons
via
the
busy
tone
an
d ac
know
ledg
emen
ts, r
espe
ctiv
ely
(med
ia a
cces
s is
not
coo
rdin
ate
d w
ithin
this
ap
proa
ch)
–m
echa
nism
use
d, e
.g.,
for
CD
PD
(U
SA
, int
egra
ted
into
AM
PS
)
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
32
•A
loha
has
onl
y a
very
low
effi
cien
cy, C
DM
A n
eeds
com
plex
rec
eive
rs to
be
able
to r
ecei
ve d
iffer
ent s
ende
rs w
ith in
divi
dual
cod
es a
t the
sam
e tim
e
•Id
ea: u
se s
prea
d sp
ectr
um w
ith o
nly
one
sing
le c
ode
(chi
ppin
g se
quen
ce)
for
spre
adin
g fo
r al
l sen
ders
acc
essi
ng a
ccor
ding
to a
loha
SA
MA
-S
prea
d A
loha
Mul
tiple
A
cces
s
1se
nder
A0
send
er B
0
1
t
narr
owba
nd
send
for
a sh
orte
r pe
riod
with
hig
her
pow
er
spre
ad th
e si
gnal
e.g
. usi
ng th
e ch
ippi
ng s
eque
nce
1101
01 (
„C
DM
A w
ithou
t CD
“)
Pro
blem
: fin
d a
chip
ping
seq
uenc
e w
ith g
ood
char
acte
ristic
s
1
1
colli
sion
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
33
Com
paris
on
SD
MA
/TD
MA
/FD
MA
/CD
MA
Ap
pro
ach
SD
MA
TD
MA
FD
MA
CD
MA
Idea
segm
ent s
pace
into
cells
/sec
tors
segm
ent s
endi
ngtim
e in
to d
isjo
int
time-
slot
s, d
eman
ddr
iven
or
fixed
patte
rns
segm
ent t
hefr
eque
ncy
band
into
disj
oint
sub
-ban
ds
spre
ad th
e sp
ectr
umus
ing
orth
ogon
al c
odes
Ter
min
als
only
one
term
inal
can
be a
ctiv
e in
one
cell/
one
sect
or
all t
erm
inal
s ar
eac
tive
for
shor
tpe
riods
of t
ime
onth
e sa
me
freq
uenc
y
ever
y te
rmin
al h
as it
sow
n fr
eque
ncy,
unin
terr
upte
d
all t
erm
inal
s ca
n be
act
ive
at th
e sa
me
plac
e at
the
sam
e m
omen
t,un
inte
rrup
ted
Sig
nal
sepa
ratio
nce
ll st
ruct
ure,
dire
cted
ante
nnas
sync
hron
izat
ion
inth
e tim
e do
mai
nfil
terin
g in
the
freq
uenc
y do
mai
nco
de p
lus
spec
ial
rece
iver
s
Adv
anta
ges
very
sim
ple,
incr
ease
sca
paci
ty p
er k
m²
esta
blis
hed,
fully
digi
tal,
flexi
ble
sim
ple,
est
ablis
hed,
robu
stfle
xibl
e, le
ss fr
eque
ncy
plan
ning
nee
ded,
sof
tha
ndov
er
Dis
-ad
vant
ages
infle
xibl
e, a
nten
nas
typi
cally
fixe
dgu
ard
spac
ene
eded
(m
ultip
ath
prop
agat
ion)
,sy
nchr
oniz
atio
ndi
fficu
lt
infle
xibl
e,fr
eque
ncie
s ar
e a
scar
ce r
esou
rce
com
plex
rec
eive
rs, n
eeds
mor
e co
mpl
icat
ed p
ower
cont
rol f
or s
ende
rs
Com
men
ton
ly in
com
bina
tion
with
TD
MA
, FD
MA
or
CD
MA
use
ful
stan
dard
in fi
xed
netw
orks
, tog
ethe
rw
ith F
DM
A/S
DM
Aus
ed in
man
ym
obile
net
wor
ks
typi
cally
com
bine
dw
ith T
DM
A(f
requ
ency
hop
ping
patte
rns)
and
SD
MA
(fre
quen
cy r
euse
)
still
face
s so
me
prob
lem
s,hi
gher
com
plex
ity,
low
ered
exp
ecta
tions
; will
be in
tegr
ated
with
TD
MA
/FD
MA
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
34
Thr
ough
puts
of S
ome
Ran
dom
Acc
ess
Pro
toco
ls
G: l
oad
(incl
udes
bot
h su
cces
sful
tran
smis
sion
s an
d re
tran
smis
sion
s)
S: s
ucce
ssfu
l tra
nsm
issi
on
a: r
atio
of p
ropa
gatio
n de
lay
to th
e pa
cket
tran
smis
sion
del
ay
)1(
)2
1(
)1(
)1(
)2
1(
)]2/1(
1[a
Ga
G
aG e
aG
ea
G
ea
GG
aG
GG
S+
−−
+−
++
−−
++
++
+=
)1()
1(
)1(
)1(
]1[
aG
aG
aG
aG
ae
ea
ee
GG
S+
−−
+−
−
+−
−+
−+
=
aG
aG
ea
Ge
S−
−
++
=)
21(
ae
aG
eS
aG
aG +
−=
−
−
1
Pro
toco
lT
hrou
ghpu
t
Pur
e-A
LOH
AS
=G
e-2G
Slo
tted-
ALO
HA
S=
Ge-G
Non
slo
tted
1-pe
rsis
tent
Slo
tted
1-pe
rsis
tent
CS
MA
Non
pers
iste
ntno
n sl
otte
d C
SM
A
Non
pers
iste
ntsl
otte
d C
SM
A
S20
01, C
OM
3525
Wire
less
Net
wor
ksLe
ctur
e 4,
35
