Multiple Access
ECS 152A
1
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
2
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

no out-of-band channel for coordination
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!
3
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
4
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
5
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
6
FDMA
7
FDM of Three Voiceband
Signals
8
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!
9
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
10
TDMA
11
Example
 GSM
12
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”)
13
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
14
CDMA: two-sender interference
15
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
16
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
17
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
18
Frequency Division Multiplexing
19
FDM System
20
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

21
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
22
Time Division Multiplexing
23
TDM System
24
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
25
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
26
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
27
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

28
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
29
Dense Wavelength Division
Multiplexing
 DWDM
 No official or standard definition
 Implies more channels more closely spaced
that WDM
 200GHz or less
30
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
31
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
32
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
33
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!
34
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]
35
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
36
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!
37
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
38
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
39
CSMA/CD collision detection
40
“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!
41
“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)
42
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
43