3rd Edition, Chapter 5
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Transcript 3rd Edition, Chapter 5
Multiple Access
ECS 152A
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Concepts
Multiple access vs. multiplexing
Multiplexing allows several transmission sources
to share a larger transmission capacity. Often
used in hierarchical structures.
Multiple access: two or more simultaneous
transmissions share a broadcast channel. Often
used in access networks
sometimes interchangeable
Bandwidth (bps) vs. bandwidth (Hz)
bps: data rate
Hz: frequency in physical carrier
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Multiple Access protocols
Point-to-point vs. broadcast channel
Broadcast link can have multiple sending and receiving nodes
all connected to the same, single, shared broadcast channel.
single shared broadcast channel
two or more simultaneous transmissions by nodes:
interference
collision if node receives two or more signals at the same time
multiple access protocol
distributed algorithm that determines how nodes
share channel, i.e., determine when node can transmit
communication about channel sharing must use channel
itself!
no out-of-band channel for coordination
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Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at
rate R.
2. When M nodes want to transmit, each can send at
average rate R/M
3. Fully decentralized:
no special node to coordinate transmissions
no synchronization of clocks, slots
4. Simple
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MAC Protocols: a taxonomy
Three broad classes:
Channel Partitioning
divide channel into smaller “pieces” (time slots,
frequency, code)
allocate piece to node for exclusive use
Random Access
channel not divided, allow collisions
“recover” from collisions
“Taking turns”
Nodes take turns, but nodes with more to send can take
longer turns
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Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency
frequency bands
bands 2,5,6 idle
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FDMA
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FDM of Three Voiceband
Signals
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FDMA: example
AMPS (Advanced Mobile Phone System)
The first cellular system in US
Forward link 869-894 MHz
Reverse link 824-847 MHz
Example: An operator with 12.5MHz in each
simplex band. Bg is the guard band allocated at
the edge of the allocated spectrum band. Bc is the
channel bandwidth.
Bg=10KHz
Bc=30kHz
N= (12.5M-2x10K)/30K =416 simultaneous users!
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Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
access to channel in "rounds"
each station gets fixed length slot (length = pkt
trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6
idle
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TDMA
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Example
GSM
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Code Division Multiple Access (CDMA)
used in several wireless broadcast channels
(cellular, satellite, etc) standards
unique “code” assigned to each user; i.e., code set
partitioning
all users share same frequency, but each user has
own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping
sequence)
decoding: inner-product of encoded signal and
chipping sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes
are “orthogonal”)
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CDMA Encode/Decode
sender
d0 = 1
data
bits
code
Zi,m= di.cm
-1 -1 -1
1
-1
1 1 1
-1 -1 -1
slot 1
-1
slot 1
channel
output
1
-1
1 1 1 1 1 1
1
d1 = -1
1 1 1
channel output Zi,m
-1 -1 -1
slot 0
1
-1
-1 -1 -1
slot 0
channel
output
M
Di = S Zi,m.cm
m=1
received
input
code
receiver
1 1 1 1 1 1
1
-1 -1 -1
-1
1 1 1
1
-1
-1 -1 -1
-1
1 1 1
-1 -1 -1
slot 1
M
1
1
-1
-1 -1 -1
slot 0
d0 = 1
d1 = -1
slot 1
channel
output
slot 0
channel
output
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CDMA: two-sender interference
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Multiplexing
Sharing network resources
Bandwidth, router
buffer
The cost of deploying high
bandwidth transmission line
is more economical
Exploit the statistical
behavior of users
(a)
A
A
B
B
C
C
(b)
A
B
C
A
Trunk
group
MUX
MUX
B
C
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Frequency Division Multiplexing
The bandwidth is divided into
frequency slots
Each frequency slot is
allocated to a different user
FDM was first introduced in
the telephone network
Other examples – broadcast
radio and cable television
(a) Individual signals occupy W Hz
A
f
W
0
B
0
f
W
C
0
f
W
(b) Combined signal fits into channel
bandwidth
A
B
C
f
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Frequency Division Multiplexing
Useful bandwidth of medium exceeds
required bandwidth of channel
Each signal is modulated to a different
carrier frequency
Carrier frequencies separated so signals do
not overlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
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Frequency Division Multiplexing
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FDM System
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Analog Carrier Systems
AT&T (USA)
Hierarchy of FDM schemes
Group
12 voice channels (4kHz each) = 48kHz
Range 60kHz to 108kHz
Supergroup
60 channel
FDM of 5 group signals on carriers between 420kHz and 612
kHz
Mastergroup
10 supergroups
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Time Division Multiplexing
Separate bit streams are
multiplexed into a high-speed
digital transmission line
Transmission is carried out in
terms of frames which are
composed of equal sized slots
which are assigned to users
Demultiplexing is done by
reading the data in the
appropriate slot in each frame
(a)
Each signal transmits 1
unit every 3T seconds
A1
0T
A2
B1
B2
6T
3T
0T
C1
0T
t
6T
3T
C2
6T
3T
t
t
(b) Combined signal transmits 1
unit every T seconds
A1 B1
0T 1T 2T
C1
A2
3T 4T
B2
5T
C2
t
6T
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Time Division Multiplexing
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TDM System
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SONET Digital Hierarchy
DS1
DS2
Low-Speed
Mapping
Function
CEPT-1
DS3
44.736
CEPT-4
139.264
ATM
150 Mbps
STS-1
51.84 Mbps
Medium
Speed
Mapping
Function
HighSpeed
Mapping
Function
HighSpeed
Mapping
Function
STS-1
STS-1
STS-1
STS-1
STS-1
STS-1
STS-1
STS-3c
OC-n
STS-n
Mux
Scrambler
E/O
STS-3c
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Statistical TDM
In Synchronous TDM many slots are
wasted
Statistical TDM allocates time slots
dynamically based on demand
Multiplexer scans input lines and collects
data until frame full
Data rate on line lower than aggregate
rates of input lines
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Wave Division Multiplexing
Optical-domain version of FDM
Different information signals are modulated to different
wavelengths and the combined signals sent through the fiber
Prism and difffraction gratings are used to combine/split signals
1
2
m
Optical
MUX
Optical
deMUX
1 2 .
m
Optical
fiber
1
2
m
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Wavelength Division
Multiplexing
Multiple beams of light at different frequency
Carried by optical fiber
A form of FDM
Each color of light (wavelength) carries separate data channel
1997 Bell Labs
100 beams
Each at 10 Gbps
Giving 1 terabit per second (Tbps)
Commercial systems of 160 channels of 10 Gbps now available
Lab systems (Alcatel) 256 channels at 39.8 Gbps each
10.1 Tbps and Over 100km
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WDM Operation
Same general architecture as other FDM
Number of sources generating laser beams at different
frequencies
Multiplexer consolidates sources for transmission over single fiber
Optical amplifiers amplify all wavelengths
Typically tens of km apart
Demux separates channels at the destination
Mostly 1550nm wavelength range
Was 200MHz per channel
Now 50GHz
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Dense Wavelength Division
Multiplexing
DWDM
No official or standard definition
Implies more channels more closely spaced
that WDM
200GHz or less
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Random Access Protocols
When node has packet to send
transmit at full channel data rate R.
no a priori coordination among nodes
two or more transmitting nodes “collision”,
random access MAC protocol specifies:
how to detect collisions
how to recover from collisions (e.g., via delayed
retransmissions)
Examples of random access MAC protocols:
slotted ALOHA
ALOHA
CSMA, CSMA/CD, CSMA/CA
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Slotted ALOHA
Assumptions
all frames same size
time is divided into
equal size slots, time to
transmit 1 frame
nodes start to transmit
frames only at
beginning of slots
nodes are synchronized
if 2 or more nodes
transmit in slot, all
nodes detect collision
Operation
when node obtains fresh
frame, it transmits in next
slot
no collision, node can send
new frame in next slot
if collision, node
retransmits frame in each
subsequent slot with prob.
p until success
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Slotted ALOHA
Pros
single active node can
continuously transmit
at full rate of channel
highly decentralized:
only slots in nodes
need to be in sync
simple
Cons
collisions, wasting slots
idle slots
nodes may be able to
detect collision in less
than time to transmit
packet
clock synchronization
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Slotted Aloha efficiency
Efficiency is the long-run
fraction of successful slots
when there are many nodes,
each with many frames to send
Suppose N nodes with
many frames to send,
each transmits in slot
with probability p
prob that node 1 has
success in a slot
= p(1-p)N-1
prob that any node has
a success = Np(1-p)N-1
For max efficiency
with N nodes, find p*
that maximizes
Np(1-p)N-1
For many nodes, take
limit of Np*(1-p*)N-1
as N goes to infinity,
gives 1/e = .37
At best: channel
used for useful
transmissions 37%
of time!
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Pure (unslotted) ALOHA
unslotted Aloha: simpler, no synchronization
when frame first arrives
transmit immediately
collision probability increases:
frame sent at t0 collides with other frames sent in [t0-1,t0+1]
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Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] .
P(no other node transmits in [p0-1,p0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
Even worse !
= 1/(2e) = .18
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CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
If channel sensed busy, defer transmission
Human analogy: don’t interrupt others!
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CSMA collisions
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation
delay in determining collision
probability
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CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
colliding transmissions aborted, reducing channel
wastage
collision detection:
easy in wired LANs: measure signal strengths,
compare transmitted, received signals
difficult in wireless LANs: receiver shut off while
transmitting
human analogy: the polite conversationalist
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CSMA/CD collision detection
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“Taking Turns” MAC protocols
channel partitioning MAC protocols:
share channel efficiently and fairly at high load
inefficient at low load: delay in channel access,
1/N bandwidth allocated even if only 1 active
node!
Random access MAC protocols
efficient at low load: single node can fully
utilize channel
high load: collision overhead
“taking turns” protocols
look for best of both worlds!
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“Taking Turns” MAC protocols
Token passing:
Polling:
control token passed from
master node
one node to next
“invites” slave nodes
sequentially.
to transmit in turn
token message
concerns:
concerns:
polling overhead
latency
single point of
failure (master)
token overhead
latency
single point of failure (token)
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Summary of MAC protocols
What do you do with a shared media?
Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division
Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing: easy in some technologies (wire), hard
in others (wireless)
• CSMA/CD used in Ethernet
• CSMA/CA used in 802.11
Taking Turns
• polling from a central site, token passing
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