This document discusses various terminology related to the MAC sublayer, including: 1. The station model consisting of independent stations that generate frames for transmission. 2. The single channel assumption where a single channel is available for all communication. 3. The collision assumption where if two frames are transmitted simultaneously they will overlap and be garbled. It then covers concepts such as carrier sensing, hidden and exposed terminals, and near and far terminals that create challenges for wireless networks. Finally, it introduces various multiple access methods including SDMA, FDMA, TDMA, and CDMA.

1.  Motivation
Hidden and exposed terminals,
Near and far terminals
 SDMA
 FDMA
 TDMA
CDMA
UNIT-2

2. Terminology Of MAC Sublayer
1. Station Model: The model consists of N
independent stations, each with a program or user
that generates frames for transmission.
Once a frame has been generated, the station is
blocked and does nothing until the frame has been
successfully transmitted.

3. Terminology Of MAC Sublayer
2.Single Channel Assumption: A single
channel is available for all communication. All
stations can transmit on it and all can receive from
it.
As far as the hardware is concerned, all stations
are equivalent, although some protocol software
may assign priorities to them.

4. Terminology Of MAC Sublayer
3.Collision Assumption: If two frames are
transmitted simultaneously, they overlap in time
and the resulting signal is garbled. This event is
called a collision.
All stations can detect collisions. A collided frame
must be transmitted again alter. There are no
errors other than those generated by collisions.

5. Terminology Of MAC Sublayer
4a. Continuous Time: Frame transmission can
begin at any instant. There is no master clock
dividing time into discrete intervals.
4b. Slotted Time: Time is divided into discrete
intervals (slots). Frame transmissions always
begin at the start of a slot. A slot may contain 0, 1,
or more frames, corresponding to an idle slot, a
successful transmission, or a collision,
respectively.

6. Terminology Of MAC Sublayer
5a. Carrier Sense: Stations can tell if the
channel is in use before trying to use it. If the
channel is sensed as busy, no station will attempt
to use it until it goes idle.
5b. No Carrier Sense: Stations cannot sense
the channel before trying to use it. They just go
ahead and transmit. Only later can they determine
whether or not the transmission was successful.

7. Pure ALOHA
Nodes transmit on a common channel
Transmit frame of fixed length
When two transmissions overlap, they garble
each other (collision)
The central node acknowledges the correct
frames it receives
When a node does not get an acknowledgment
within a specific timeout, it assumes that its frame
collided
When a frame collides, the transmitting node
schedules a retransmission after a random delay

8. • In pure ALOHA, the stations transmit frames
whenever they have data to send.
• When two or more stations transmit
simultaneously, there is collision and the frames
are destroyed.
• In pure ALOHA, whenever any station transmits a
frame, it expects the acknowledgement from the
receiver.
• If acknowledgement is not received within
specified time, the station assumes that the frame
(or acknowledgement) has been destroyed.
•
Pure ALOHA

9. otherwise same frames will collide again and
again.
• Therefore pure ALOHA dictates that when time-
out period passes, each station must wait for a
random amount of time before resending its frame.
This randomness will help avoid more collisions.
• Figure shows an example of frame collisions in
pure ALOHA.

11. ● Frames are transmitted at completely arbitrary times
Pure ALOHA

12. Slotted ALOHA was invented to improve
the efficiency of pure ALOHA as chances of
collision in pure ALOHA are very high.
• In slotted ALOHA, the time of the shared
channel is divided into discrete intervals
called slots.
• The stations can send a frame only at the
beginning of the slot and only one frame is
sent in each slot.
Slotted ALOHA

14. Pure ALOHA and slotted ALOHA
time
Nodes can start transmitting at any time.
pure ALOHA
time
slotted ALOHA
slot
● Nodes must start their transmissions at the beginning of
a time slot

15. Carrier Sense Multiple Access Protocols
With slotted ALOHA the best channel utilization
that can be achieved is 1/e. This is hardly
surprising, since with stations transmitting at
will, without paying attention to what other stations
are doing, there are bound to be many collisions
In local area networks, however, it is possible for
stations to detect what other stations are doing,
and adapt their behavior accordingly
Protocols in which stations listen for a carrier (i.e.
a transmission) and act accordingly are called
carrier sense protocols

16. CSMA with collision detection
(CSMA/CD)
If two stations sense the channel to be idle and
begin transmitting simultaneously, they will both
detect the collision immediately
Abort a transmission as soon as they detect a
collision. Quickly terminating damaged frames
saves time and bandwidth
After a station detects a collision, it aborts its
transmission, waits a random period of time, and
then tries again, assuming that no other station
has started transmitting in the meantime
Used in the LANs in the MAC sub Layer

17. Motivation for specialized MAC
17
Can we apply media access methods from fixed networks?
Example CSMA/CD
Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision occurs
(legacy method in IEEE 802.3)
Problems in wireless networks
signal strength decreases proportional to the square of the distance
the sender would apply CS and CD, but the collisions happen at the receiver
it might be the case that a sender cannot “hear” the collision, i.e., CD does not
work
furthermore, CS might not work if, e.g., a terminal is “hidden”
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18. 18
CSMA/CD fails in wireless N/w because CSMA/CD is not really
interested in collisions at the sender , but rather in those at the
receiver.
The signal should reach the receiver without collisions. But sender is
the one detecting collisions
This is not a problem using wire, as more or less the same signal
strength can be assumed all over the wire
The strength of a signal in wireless N/w decreases proportionally to
the square of the distance to the sender
The sender start sending but a collision happens at the receiver due to
a second sender.
Pallepati Vasavi

19. Pallepati Vasavi19
Motivation - hidden and exposed
terminals

20. Motivation - hidden and exposed terminals
20
Hidden terminals
 A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium (CS fails)
 collision at B,A cannot receive the collision (CD fails)
 A is “hidden” for C ,and vice versa.
BA C
Pallepati Vasavi

21. Exposed terminals
 B sends to A, C wants to send to another terminal (not A or B) outside range
 C senses the carrier and detects that carrier is busy
 C postpones its transmission until it detects the medium as being idle again
 but A is outside the radio range of C, therefore waiting is not necessary
 C is “exposed” to B
Hidden terminals cause collisions, where as Exposed
terminals causes unnecessary delay
Pallepati Vasavi21
BA C

22. Consider the situation below .A and B are both sending with
the same transmission power
Pallepati Vasavi22
Motivation - near and far
terminals
A B C

23. Motivation - near and far
terminals
23
 Signal strength decreases proportional to the square of the distance
 So, B’s signal drowns out A’s signal making C unable to receiveA’s transmission
 If C is an arbiter for sending rights, B drown out A’s signal on the physical layer
making C unable to hear out A.
A B C
Pallepati Vasavi
The near/far effect is a severe problem of wireless networks using CDM.
 All signals should arrive at the receiver with more or less the same
strength for which Precise power control is to be implemented.

24. Pallepati Vasavi24

25. Access methods SDMA/FDMA/TDMA/CDMA
25
SDMA (Space Division MultipleAccess)
segment space into sectors, use directed antennas
cell structure
FDMA (Frequency Division MultipleAccess)
assign a certain frequency to a transmission channel between a sender and a
receiver
permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS,
Frequency Hopping Spread Spectrum)
TDMA (Time Division MultipleAccess)
assign the fixed sending frequency to a transmission channel between a sender
and a receiver for a certain amount of time
CDMA (Code Division Multiple Access)
 Same bandwidth is occupied by all the users however they are all assigned separate code
Pallepati Vasavi

26. 26
SDMA is used for allocating a separated space to users in wireless networks.
A typical application involves assigning an optimal base station to a mobile
phone user
The mobile phone may receive several base stations with different quality.
A MAC algorithm could now decide which base station is best, taking into
account with frequencies (FDM), time slots(TDM) or code(CDM) are still
available(depending on technology)
Typically SDMA is never used in isolation but always in combination with
one or more other schemes
The basis for the SDMA algorithm is formed by cell and sectorized antennas
which constitute the infrastructure implementing SDM
Pallepati Vasavi
SDMA

27. FDMA ( Frequency Division Multiple Access)
27
FDMA assigns individual channels to individual users
Each user is allocated a unique freq., band or channel
These channels are assigned on demand to users who request service
During the period of the call, no other user can share the same
channel
The FDMA channel carries only one phone circuit at a time.
If an FDMA channel is not in use, then it sits idle and can not be used
by other users.
FDMA requires tight RF filtering to minimize adjacent channel
interference
Pallepati Vasavi

28. Uplink frequency:824-849MHZ
Downlink frequency:869-894MHZ
832 channels spaced 30KHZ apart(3 users/channel)
48.6 kbps bitrate
Used in AMPS(Analog cellular telephone systems)
Used FDD(Frequency Division Duplexing)
Pallepati Vasavi28

29. 29
Channel1
Channel2
ChannelN-2
ChannelN-1
ChannelN
……
Time
Freq
Code
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30. Main features
 Continuous transmission
Narrow bandwidth
Low overhead
 Simple hardware at mobile unit and BS : (1) no digital
processing needed (2) ease of framing and synchronization.
FDMA can be used with both analogue and digital signal.
 FDMA requires high-performing filters in the radio hardware, in
contrast toTDMA and CDMA.
 FDMA is not vulnerable to the timing problems thatTDMA has.
 Due to the frequency filtering, FDMA is not sensitive to nearfar
problem.
Pallepati Vasavi30

31. Advantages
If channel is not in use, it sits idle
Channel bandwidth is relatively narrow (30kHz)
Simple algorithmically, and from a hardware standpoint
Fairly efficient when the number of stations is small and the
traffic is uniformly constant
 Capacity increase can be obtained by reducing the
information bit rate and using efficient digital code
 No need for network timing
 No restriction regarding the type of baseband or type of
modulation
Pallepati Vasavi31

32. Disadvantages
The presence of guard bands
 Requires right RF filtering to minimize adjacent channel
interference
 Maximum bit rate per channel is fixed
 Small inhibiting flexibility in bit rate capability
 Does not differ significantly from analog system
Pallepati Vasavi32

33. FDD/FDMA - general scheme, example GSM
33
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
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34. TDMA (Time Division Multiple Access)
34
 TDMA (Time Division Multiple Access) System divide the ratio
spectrum into time slots.
 In each slot only one user is allowed to either transmit or receive
 Each user occupies a cyclically repeating time slot
 transmission for any user is non continuous
 Listening to different frequencies at the same time is quite difficult
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35. 35
Freq
Slot Code
Time
.
.
.
.
Channel N
Channel 2
Channel 1
Tim
e
Slots
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36. Uplink frequency:824-849MHZ
Downlink frequency:869-894MHZ
832 channels spaced 30KHZ apart(3 users/channel)
48.6 kbps bitrate
Used in Digital AMPS(Advanced Mobile Phone systems)
UsedTDD(Time Division Duplexing)
Pallepati Vasavi36

37. MAIN FEATURES
Shares single carrier frequency with multiple users.
Non-continuous transmission.This results in low battery
consumption since the subscriber transmitter can be turned
OFF when not in use.
Slots can be assigned on demand in dynamicTDMA.
TDMA uses different time slots forTx and Rx, thus
duplexers are not required.
 Global Systems for Mobile communications (GSM) uses the
TDMA technique
Pallepati Vasavi37

38. Advantages
It carry data rates of 64 kbps to 120 Mbps .
It provides the user with extended battery life and talk time.
It is the most cost effective technology to convert an
analogue system to digital.
TDMA technology separates users according to time, it
ensures that there will be no interference
TDMA allows the operator to do services like fax, voice band
data, and SMS as well as bandwidth-intensive application such
as multimedia and videoconferencing.
Pallepati Vasavi38

39. Disadvantages
 Each user has a predefined time slot.When moving from
one cell to other, if all the time slots in this cell are full the
user might be disconnected.
It is subjected to multipath distortion.A signal coming from a
tower to a handset might come from any one of several
directions. It might have bounced off several different
buildings before arriving.
Pallepati Vasavi39

40. TDD/TDMA - general scheme, example DECT
40
1 2 3 11 12 1 2 3 11 12
t
downlink uplink
417 µs
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41. Aloha/slotted aloha
41
Mechanism
random, distributed (no central arbiter), time-multiplex
Slotted Aloha additionally uses time-slots, sending must always start at slot
boundaries
Aloha
SlottedAloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t
Pallepati Vasavi

42. DAMA - Demand Assigned Multiple Access
42
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha
(assuming Poisson distribution for packet arrival and packet length)
Reservation can increase efficiency to 80%
a sender reserves a future time-slot
sending within this reserved time-slot is possible without collision
reservation also causes higher delays
typical scheme for satellite links
Examples for reservation algorithms:
Explicit Reservation according to Roberts (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA
Pallepati Vasavi

43. Access method DAMA: Explicit Reservation
43
Explicit Reservation (ReservationAloha):
two modes:
 ALOHA mode for reservation:
competition for small reservation slots, collisions possible
 reserved mode for data transmission within successful reserved slots (no collisions possible)
it is important for all stations to keep the reservation list consistent at any point in
time and, therefore, all stations have to synchronize from time to time
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t
Pallepati Vasavi

44. Access method DAMA: PRMA
44
Implicit reservation (PRMA - Packet Reservation MA):
a certain number of slots form a frame, frames are repeated
stations compete for empty slots according to the slotted aloha principle
once a station reserves a slot successfully, this slot is automatically assigned to this
station in all following frames as long as the station has data to send
competition for this slots starts again as soon as the slot was empty in the last
frame
frame1
frame2
frame3
frame4
frame5
1 2 3 4 5 6 7 8 time-slot
collision at
reservation
attempts
A C D A B A F
A C A B A
A B A F
A B A F D
A C E E B A F D
t
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservation
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45. Access method DAMA: Reservation-TDMA
45
ReservationTime Division Multiple Access
every frame consists of N mini-slots and x data-slots
every station has its own mini-slot and can reserve up to k data-slots using
this mini-slot (i.e. x = N * k).
other stations can send data in unused data-slots according to a round-
robin sending scheme (best-effort traffic)
N mini-slots N * k data-slots
reservations
for data-slots
other stations can use free data-slots
based on a round-robin scheme
e.g. N=6, k=2
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46. MACA - collision avoidance
46
 MACA (Multiple Access with CollisionAvoidance) uses short signaling packets for
collision avoidance
 RTS (request to send): a sender request the right to send from a receiver with a short RTS
packet before it sends a data packet
 CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
 Signaling packets contain
 sender address
 receiver address
 packet size
 Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed
FoundationWireless MAC)
Pallepati Vasavi

47. MACA examples
47
 MACA avoids the problem of hidden terminals
 A and C want to
send to B
 A sends RTS first
 C waits after receiving
CTS from B
 MACA avoids the problem of exposed terminals
 B wants to send to A, C
to another terminal
 now C does not have
to wait for it cannot
receive CTS from A
A B C
RTS
CTSCTS
A B C
RTS
CTS
RTS
Pallepati Vasavi

48. MACA variant: DFWMAC in IEEE802.11
48
idle
wait for the
right to send
wait for ACK
sender receiver
packet ready to send; RTS
time-out;
RTS
CTS; data
ACK
RxBusy
idle
wait for
data
RTS; RxBusy
RTS;
CTS
data;
ACK
time-out ∨
data;
NAK
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
time-out ∨
NAK;
RTS
Pallepati Vasavi

49. Polling mechanisms
49
If one terminal can be heard by all others, this “central” terminal
(a.k.a. base station) can poll all other terminals according to a certain
scheme
now all schemes known from fixed networks can be used (typical mainframe -
terminal scenario)
Example: RandomlyAddressed Polling
base station signals readiness to all mobile terminals
terminals ready to send can now transmit a random number without collision
with the help of CDMA or FDMA (the random number can be seen as dynamic
address)
the base station now chooses one address for polling from the list of all random
numbers (collision if two terminals choose the same address)
the base station acknowledges correct packets and continues polling the next
terminal
this cycle starts again after polling all terminals of the list
Pallepati Vasavi

50. ISMA (Inhibit Sense Multiple Access)
50
Current state of the medium is signaled via a “busy tone”
the base station signals on the downlink (base station to terminals) if the medium
is free or not
terminals must not send if the medium is busy
terminals can access the medium as soon as the busy tone stops
the base station signals collisions and successful transmissions via the busy tone
and acknowledgements, respectively (media access is not coordinated within this
approach)
mechanism used, e.g.,
for CDPD
(USA, integrated
intoAMPS)
Pallepati Vasavi

51. CDMA
51
 CDMA (Code Division MultipleAccess)
There is no restriction on time and frequency in this scheme.
 Parallel communication without collision and whole bandwidth can
be used
Users are separated by code not by time slot and frequency slot
All terminals send on the same frequency probably at the same time
and can use the whole bandwidth of the transmission channel
Each sender has a unique random number, the sender XORs the
signal with this random number
The receiver can “tune” into this signal if it knows the pseudo
random number, tuning is done via a correlation function
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52. Pallepati Vasavi52

53. Advantages
Potentially larger capacity (more users can communicate
simultaneously) If users don’t use the medium all the time
(e.g., they are just reading email), CDMA will allow much
more users to communicate simultaneously.
CDMA will use the resource (the radio spectrum) more
efficiently.
Provides larger spread spectrum, thus more robust against
noise bursts and multipath frequency selective fading
GSM bandwidth = 200 kHz
 IS-95 bandwidth = 1.25 MHz
W-CDMA (3G) bandwidth = 10MHz
Pallepati Vasavi53

54. Advantages
The transition from one BS to another (handoff) is not
abrupt, as inTDMA, and provides better quality No absolute
limit on the number of users
Easy addition of more users
 Impossible for hackers to decipher the code sent
 Better signal quality
Huge code space (e.g. 232) compared to frequency space
Interferences (e.g. white noise) is not coded
forward error correction and encryption can be easily
integrated
Pallepati Vasavi54

55. Disadvantages:
As the number of users increases, the overall
quality of service decreases
Self-jamming
 Near- Far- problem arises
Higher complexity of a receiver (receiver
cannot just listen into the medium and start
receiving if there is a signal)
All signals should have the same strength at a
receiver
Pallepati Vasavi55

56. CDMA in theory
56
SenderA
sends Ad = 1, key Ak = 010011 (assign: “0”= -1, “1”= +1)
sending signal As = Ad *Ak = (-1, +1, -1, -1, +1, +1)
Sender B
sends Bd = 0, key Bk = 110101 (assign: “0”= -1, “1”= +1)
sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)
Both signals superimpose in space
interference neglected (noise etc.)
As + Bs = (-2, 0, 0, -2, +2, 0)
Receiver wants to receive signal from senderA
apply key Ak bitwise (inner product)
 Ae = (-2, 0, 0, -2, +2, 0) • Ak= 2 + 0 + 0 + 2 + 2 + 0 = 6
 result greater than 0, therefore, original bit was “1”
receiving B
 Be = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e.“0”
Pallepati Vasavi

57. CDMA on signal level I
57
data A
key A
signal A
data ⊕ key
key
sequence A
Real systems use much longer keys resulting in a larger distance
between single code words in code space.
1 0 1
10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1
01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Ad
Ak
As
Pallepati Vasavi

58. CDMA on signal level II
58
signal A
data B
key B
key
sequence B
signal B
As + Bs
data ⊕ key
1 0 0
00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1
11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1
Bd
Bk
Bs
As
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59. CDMA on signal level III
59
Ak
(As + Bs)
* Ak
integrator
output
comparator
output
As + Bs
data A
1 0 1
1 0 1 Ad
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60. CDMA on signal level IV
60
integrator
output
comparator
output
Bk
(As + Bs)
* Bk
As + Bs
data B
1 0 0
1 0 0 Bd
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61. CDMA on signal level V
61
comparator
output
wrong
key K
integrator
output
(As + Bs)
* K
As + Bs
(0) (0) ?
Pallepati Vasavi

62. SAMA - Spread Aloha Multiple Access
62
 Aloha has only a very low efficiency, CDMA needs complex receivers to be able to
receive different senders with individual codes at the same time
 Idea: use spread spectrum with only one single code (chipping sequence) for
spreading for all senders accessing according to aloha
1sender A
0sender B
0
1
t
narrow
band
send for a
shorter period
with higher power
spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)
Problem: find a chipping sequence with good characteristics
1
1
collision
Pallepati Vasavi

63. Comparison SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
Terminals only one terminal can
be active in one
cell/one sector
all terminals are
active for short
periods of time on
the same frequency
every terminal has its
own frequency,
uninterrupted
all terminals can be active
at the same place at the
same moment,
uninterrupted
Signal
separation
cell structure, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
Advantages very simple, increases
capacity per km²
established, fully
digital, flexible
simple, established,
robust
flexible, less frequency
planning needed, soft
handover
Dis-
advantages
inflexible, antennas
typically fixed
guard space
needed (multipath
propagation),
synchronization
difficult
inflexible,
frequencies are a
scarce resource
complex receivers, needs
more complicated power
control for senders
Comment only in combination
with TDMA, FDMA or
CDMA useful
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
63 Pallepati Vasavi