Multiple Access Techniques

Multiple Access
Techniques
Inayat ur rahman

Multiple Access Techniques(1)
• Single shared communication channel
• Two or more simultaneous transmissions
by nodes
– Collision / interference
– Only one node can send successfully at a
time
• To allow many users to share
simultaneously a finite amount of radio
spectrum resources

Multiple Access Techniques (2)
• One way to look at it as
• The transmission from the BS in the
downlink can be heard by each and every
mobile user in the cell, and is referred as
broadcasting.
• Transmission from the mobile users in the
uplink to the BS is many-to-one, and is
referred to as multiple access.

Multiple Access
Multiple
Access
Random
Access
Controlled
Access
Channelization
Aloha CSMA CSMA/CD CSMA/CA Reservation Polling
Token
passing
FDMA TDMA
CDMA

Random Access
• No station is superior to another
• None is assigned control over other
• No schedule time for a station to transmit
• Transmission is random among the station
• Collision may occur in case if two stations
tries to transmit at the same time

Random Access
Aloha:
• Earliest random access method
• Developed at university of hawaii in 1970
• Designed for wireless LANs
• But can be used by any shared medium
• Pure and slotted aloha

Random Access
Pure Aloha:
• The original aloha protocol is called pure aloha
• Any station can transmit when they have
something to send
• Requires an ACK
• A frame needs to be retransmitted in case of no
ACK
• In case of collision, each station has to wait for
random amount of time before retransmission.

Random Access
Pure Aloha vulnerable time

Random Access
Slotted aloha:
• Fixed size transmission slots
• A station only transmit at the beginning of the
slot
• Collision can occur if two station tries to send in
same time slot
• In case of collision stations has to wait for a
random amount of time
• Random time is calculated using backoff
algorithm

Random Access
Backoff Algorithm:
• In case of collision an algorithm is devised for
retransmission
• In case of collision random number N is selected
between 0..[2K-1], where k is the number of
collision
• That random number N is then multiplied with
maximum propagation delay Tp
• Time to wait = N * Tp
• If k=1 then N =0,1
• If k=2 then N = 0,1,2,3

Binary Exponential Backoff (cont’d)
slot length = 2 x end-to-end delay = 50 ms
A B
t=0ms: Assume A and B collide (kA = kB = 1)
A, B choose randomly from 21 slots: [0,1]
Assume A chooses 1, B chooses 1
t=100ms: A and B collide (kA = kB = 2)
A, B choose randomly from 22 slots: [0,3]
Assume A chooses 2, B chooses 0
t=150ms: B transmits successfully
t=250ms: A transmits successfully

Random Access
• Maximum number of collisions allowed are
16
• After collision number 10 the random
number selected between 0..1023
• After 16 collision the transmission is
aborted and can be retransmitted some
other time
• The typical time for Tp can be around 51.2
microseconds

Random Access
CSMA:
• Carrier sense multiple
access
• Listen to the medium first
• Sense before transmit
• Listen before talk
• Reduce collisions but
cannot eliminate it
• Can be persistent and non
persistent

The Effect of Propagation Delay
on CSMA
A B
carrier sense = idle
Transmit a packet
Collision
packet

Propagation Delay and CSMA
• Contention (vulnerable) period in Pure ALOHA
– two packet transmission times
• Contention period in Slotted ALOHA
– one packet transmission time
• Contention period in CSMA
– up to 2 x end-to-end propagation delay
Performance of CSMA >
Performance of Slotted ALOHA >
Performance of Pure ALOHA

Random Access
CSMA:
• There are several types of CSMA
protocols:
– 1-Persistent CSMA
– Non-Persistent CSMA
– P-Persistent CSMA

Random Access
1-Persistent :
1. If the medium is idle, transmit otherwise goto 2
2. if the medium is busy continue to listen until
channel is sensed idle, then transmit
immediately.
- Highest chance of collision because two
stations might find the medium idle and might
transmit simultaneously
The protocol is called 1-persistent because the host transmits with a probability
of 1 whenever it finds the channel idle.

Random Access
Non persistent CSMA
1. If the medium is idle, transmit otherwise
goto 2
2. If the medium is busy,wait for random
amount of time and repeat step 1
- Problem , capacity is wasted since
medium is idle following the end of
transmission

Tradeoff between 1- and Non-
Persistent CSMA
• If B and C become ready in the middle of A’s
transmission,
– 1-Persistent:
• B and C collide
– Non-Persistent:
• B and C probably do not collide
• If only B becomes ready in the middle of A’s
transmission,
– 1-Persistent:
• B succeeds as soon as A ends
– Non-Persistent:
• B may have to wait

Random Access
P-Persistent CSMA
• Optimal strategy: use P-Persistent CSMA
• Assume channels are slotted
• One slot = contention period (i.e., one
round trip propagation delay)

Random Access
P-Persistent CSMA (cont’d)
1. Sense the channel
– If channel is idle, transmit a packet with probability p
• if a packet was transmitted, go to step 2
• if a packet was not transmitted, wait one slot and go to step 1
– If channel is busy, wait one slot and go to step 1.
2. Detect collisions
– If a collision occurs, wait a random amount of time and go
to step 1

P-Persistent CSMA (cont’d)
• Consider p-persistent CSMA with p=0.5
– When a host senses an idle channel, it will
only send a packet with 50% probability
– If it does not send, it tries again in the next
slot.

Comparison of CSMA and ALOHA
Protocols
(Number of Channel Contenders)

CSMA/CD
• In CSMA protocols
– If two stations begin transmitting at the same time,
each will transmit its complete packet, thus wasting
the channel for an entire packet time
• In CSMA/CD protocols
– The transmission is terminated immediately upon the
detection of a collision
– CD = Collision Detect

Random Access
CSMA/CD
• Carrier sense multiple access with collision
detection
• Station monitors the medium after it sends the
frame
• If successful then station’s job done
• Retransmit the frame otherwise (collision)
• Transmission and sensing of the medium
simultaneously
• In case of collision a jam signal is sent to notify
all the station about the collision

Random Access
CSMA/CD
detection
1. If the medium is idle, transmit otherwise goto 2
2. If the medium is busy, transmit using persistent
tech.
3. If a collision occurs during transmission
a. transmit a jam signal
b. wait for random time and re-attempt(16 times)
c. random time generated according to
exponential backoff algorithm

CSMA/CD (cont’d)
• Carrier sense
– reduces the number of collisions
• Collision detection
– reduces the effect of collisions, making the
channel ready to use sooner

Collision detection time
How long does it take to realize there has been a
collision?
Worst case: 2 x end-to-end prop. delay
A B
packet

Random Access
CSMA/CD
Minimum Frame Size:
• Minimum frame restriction apply
• Before sending last bit of the frame,
sending station must detect a
collision(ifAny)
• Once the frame is sent, sending station
doesn’t keep the copy
• Tfr = 2Tp

Random Access
Difference between aloha and CSMA/CD
• In addition to the persistent process the
medium needs to be sensed first
• In aloha first the frame is transmitted and
then wait for ACK. In CSMA/CD station
receives and transmit continuously (on two
different ports)
• Sending of the jam signal that enforce the
collision in case other stations have not
yet sensed the collision.

Random Access
Question:
How can the station detect the medium?

Random Access
Energy Levels:
• Three values
• 0, normal and abnormal
• 0 means line is idle
• Normal means successful capture of the
channel for transmission
• Abnormal means that collision occurs

Random Access
CSMA/CA
avoidance
• Three strategies
Interframe Space (IFS)
• Sense the channel, if it is idle don’t transmit wait
for period of time called interframe space
• If the medium is idle even after IFS, still needs to
wait for time equal to contention window
• IFS can also be used to define the priority of a
station or a frame
• For example, a station that is assigned a shorter
IFS has a higher priority

Random Access
Contention Window:
• Amount of time divided into slots
• A station that is ready to send choose a random number
of slot as its wait time
• The time slot in the window changes according to the
backoff algorithm
• Station needs to sense after each time slot
• If station finds the channel idle then continue, if busy,
then halt and continue when idle
• Station needs to sense the channel after each time slot
• If the station finds the channel busy, it doesn’t restart the
process ; it just stops the timer and restart it when the
channel is sensed as idle.
ACK:
• ACK is needed for ensuring the successful delivery of
the transmission

Controlled Access
Reservation:
• a station needs to make a reservation before sending
data
• Time is divided into intervals
• In each interval a reservation frame is proceeds the data
frames sent in that interval
• If there are N stations in the system there are exactly N
reservation minislots in the reservation frame.
• Each mini slot belongs to a station
• When a station needs to send data it makes reservation
into its own mini slot
• The station that have made reservation can send their
data frames after the reservation frames

Controlled Access
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
0 0 0 0 0 1 0 0 0 0 1 0 1 1 0Data
Station1
Data
station4
Data
station3
Data
station1
Reservation Access Method
• In first interval station 1,3 and 4 have made reservations
• In second interval only station 1 has made reservation
Reservation
Frame

Controlled Access
Polling:
• One station must be primary
• All the other are secondary
• Primary control the secondary stations
• All transmission is through primary station
• There are two function
Select and Poll

Controlled Access
Select:
• Used when primary device has something
to send
• Primary station send select frame to alert
the secondary receiver of incoming
transmission
• After select ,, wait for ACK
• After ACK sends data
• Wait for ACK again to ensure successful
delivery

Controlled Access
Poll:
• Used when secondary station needs to
send
• Primary station send poll to ask if there is
anything to send
• Secondary stations might reply with either
NAK(negitive acknolegement) or with data
frames
• If NAK the primary station sends poll to the
next station
• If data frame then primary ACK the receipt

Controlled Access
Token Passing
• Each station has a predecessor and successor
• Token gives right to a station to send data
• When a station has something to send it waits
until it gets the token
• When the station has no data to send it release
the token
• Token management is required for
1. amount of time a station can hold token
2. monitoring to ensure the token is not
destroyed

Priority Transmission: Example
Host B has 1 frame of priority 3 to send to A
Host C has 1 frame of priority 2 to send to A
Host D has 1 frame of priority 4 to send to A
Token starts at host A with priority 0 and circulates
clockwise
Host C is the monitor station

Example (cont’d)
Event Token/Frame AC Field
A generates a token P=0, M=0, T=0, R=0
B grabs the token and sets the
message destination to A P=3, M=0, T=1, R=0
Frame arrives at C, and C reserves
priority level 2. Monitor bit set. P=3, M=1, T=1, R=2
Frame arrives at D, and
D attempts to reserve priority level 4: P=3, M=1, T=1, R=4
Frame arrives at A, and A
copies it P=3, M=1, T=1, R=4
Frame returns to B, so B removes
it, and generates a new token P=4, M=0, T=0, R=0
Token arrives at C, but its priority is
too high. C reserves priority 2. M bit. P=4, M=1, T=0, R=2

Example (cont’d)
Event Token/Frame AC Field
Token arrives at D, and D grabs
it, sending a message to A P=4, M=0, T=1, R=2
Frame arrives at A, and A
copies it P=4, M=0, T=1, R=2
Frame arrives at B, which does
nothing to it P=4, M=0, T=1, R=2
Frame arrives at C, which sets the
monitor bit P=4, M=1, T=1, R=2
Frame returns to D, so D removes
it and generates a new token with P=2 P=2, M=0, T=0, R=0
etc… Attempt to complete this scenario on your own.

Channelization
• Available bandwidth of a link is shared in time,
frequency or through code between different
stations
• FDMA, TDMA, CDMA
FDMA
• Available bandwidth is divided into frequency
bands
• Each station is assigned a band to send data
• i.e. each band is reserved for a specific station
and is there all the time
• To prevent interference the allocated bands are
separated by guard bands

Channelization
FDMA
• FDMA is usually used when fairly large
bandwidth is available like radio, coax or fiber
optic
• Use of guard bands means that in practice
whole range of frequency band cannot be used
• Guard band reduces the efficiency of the system
• Number of bands increase results in width of
each band reduced
• Its also proportional to number of guard band
intervals
• FDMA is conceptually simple system

Channelization
Problems with FDMA
• Each link takes up one frequency band and
require one transmitter and receiver.
• For full duplex link, we need to have active
transmitter and receiver, which makes circuitry
more complex
• If variable data rate transmission is desirable
then two methods,
– Either widen each frequency band
• Result in wastage of resource for those who don’t need fast
links
– Combine several bands
• Increase complexity
• FDMA is ideal where data rate is constant like in
voice and analog TV signals.

Channelization
TDMA
• Stations share bandwidth of a channel in time
• Each station is allocated a time slot during which
it can send data
• Each station transmit its data in its assigned time
slot
• The main problem can be synchronization
• Each station must follow the beginning of its time
slot and its location
• This is difficult because of propagation delay
among distant stations
• To overcome guard time are introduced

Channelization
TDMA
• If a signal is transmitted @ 1000 bits per
second, means 1 bit after each millisecond
arrives
• If we transmit as a burst, there will be no
need to transmit a bit after every
millisecond
• E.g. 10 bit burst could be sent after each
10 millisecond
• In this case we need to pause the
transmission for some time and transmit
after specified time, in the form of bursts

Channelization
Combining TDMA and FDMA
• It means that the bandwidth is divided
frequency wise and every band is further
divided time wise (GSM and D-AMPS
systems etc)
• Used in radio communication
• Used to achieve higher data rate
• To achieve higher data rate only by using
TDMA will result in very complex circuitry,
which contribute towards cost ultimately

Channelization
CDMA
• Code Division Multiple Access
• It is different from FDMA because only one
channel occupies the entire bandwidth of
the link
• It is different than TDMA because all
stations can send data simultaneously
• In CDMA, one channel carries all
transmission simultaneously

Channelization
An analogy
• CDMA simply means communicating with
different codes
• For example in a big room people are sitting.
• In TDMA every body has to speak but on their
own turn
• In FDMA we can think of them speaking to each
others in some small separations
• In CDMA they all can speak to each other
simultaneously but in different language (code)
• E.g. two people talk in French and reject
everything else as noise if its not in French

Channelization
• Lets assume we have four stations 1,2,3 and 4 connected to
the same channel
• Data from station 1 is d1 and so on
• The code assign to the station 1 is c1 and so on
• We assume that the assigned codes have two properties
1. if we multiply each code by another, we get 0
2. if we multiply each code by itself, we get
4 ( the number of stations)
• With these properties in mind, all four stations wants to send
data on a shared channel
• Station 1 multiplies its data by its code to get d1.c1, station 2
gets d2.c2 and so on
• The data that go on the channel is the sum of all these terms
• d1.c1 + d2.c2 + d3.c3 + d4.c4

Channelization
d1.c1+d2.c2+d3.c3+d4.c4
1 2
3 4
Data Common
Channel
d1
d1.c1 d2.c2
d3.c3
d3 d4
d2
d4.c4

Channelization
• E.g. station 2 wants to extract information
sent by station 1
• Station 2 multiplies c1 by the data on the
channel
• Data = (d1.c1 + d2.c2 + d3.c3 + d4.c4).c1
d1.c1.c1 + d2.c2.c1 + d3.c3.c1 + d4.c4.c1
4.d1 as c1.c2=c3.c1=c4.c1=0 and
c1.c1= 4
• Station divide it by 4 to get data.

Channelization
Chips
• CDMA is based on coding theory
• Each station is assigned a code
• That code is a sequence of numbers called
chips
• To code for previous example are
• C1 = +1 +1+1 +1 C2= +1 -1 +1 -1
C3 = +1 +1 -1 -1 C4= +1 -1 -1 +1
• The codes are not selected randomly they are
selected carefully
• The are called orthogonal sequences

Channelization
Properties of orthogonal sequences
1. Each sequence is made of N elements, where N is the number of
stations
2. If we multiply a sequence by a number, every element in the
sequence is multiplied by that element. This is call multiplication of
a sequence by a scalar. For example
2.[+1 +1 -1 -1] = [+2 +2 -2 -2]
3. If we multiply two equal sequences, elements by elements, and
then add the results, we get N, where N is the number of elements
in the each sequence. This is called the inner product of two equal
sequence. For example
[+1 +1 -1 -1].[+1 +1 -1 -1] = 1+1+1+1 =4
4. If we multiply two different sequences, element by element, and
add the results, we get 0. this is called inner product of two
different sequence. For example
[+1 +1 -1 -1].[+1 +1 +1 +1] = 1+1-1-1 = 0
5. Adding two sequences means adding the corresponding elements.
The result is another sequence. For example
[+1 +1 -1 -1] + [+1 +1 +1 +1] = [+2 +2 0 0]

Channelization
Data Representation
• If a station needs to send 0 bit; it encodes it as
-1
• If it is to send 1 bit; it encodes it as +1
• When a station is idle; it sends no signal, which
is interpreted as 0
Data bit 0  -1 Data bit 1  +1 Silence  0

[-1 -1 -3 +1]
1 2
3 4
Data Common
Channel
Bit 0
-1
c1
[+1+1+1+1]
0
Silent
+1
Bit 1
bit 0
-1c2
[+1-1+1-1]
c1
[+1-1-1+1]
c1
[+1+1-1-1]
[-1+1-1+1] d2.c2[-1-1-1-1] d1.c1
[+1-1-1+1] d4.c4[0 0 0 0] d3.c3

Questions
• What is difference of FDM and FDMA ?
• What is difference of TDM and TDMA ?

Multiple Access Techniques

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