The document discusses media access control (MAC) protocols for wireless networks. It explains that standard MAC schemes from wired networks often fail in wireless scenarios due to signal attenuation over distance and the hidden terminal problem. It provides examples of the hidden terminal, exposed terminal, and near-far terminal problems that can occur in wireless networks. It then summarizes several MAC protocols used in wireless networks, including CSMA/CA, TDMA, FDMA, and ALOHA/Slotted ALOHA.

1. WIRELESS NETWORKS
CH3: MEDIA ACCESS CONTROL

2.  This chapter aims to explain why special MACs are
needed in the wireless domain and why standard
MAC schemes known from wired networks often
fail.
 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 the sender is the one detecting
collisions.
 This is not a problem using a wire, as more or less
the same signal strength can be assumed all over
the wire if the length of the wire stays within certain
often standardized limits.
 If a collision occurs somewhere in the
wire, everybody will notice it.

3.  The situation is different in wireless networks.
 the strength of a signal decreases proportionally to the
square of the distance to the sender.
 The sender may now apply carrier sense and detect
on idle medium.
 The sender starts sending – but a collision happens at
the receiver due to a second sender.
 The sender detects no collision and assumes that the
data has been transmitted without errors, but a
collision might actually have destroyed the data at the
receiver.
 Collision detection is very difficult in wireless scenarios
as the transmission power in the area of the
transmitting antenna is several magnitudes higher
than the receiving power.
 So, this MAC scheme from wired network fails in a
wireless scenario.

4. HIDDEN AND EXPOSED TERMINALS SCENARIO
 The transmission range of A reaches B, but not C.
 The transmission range of C reaches B, but not A.
Finally, the transmission range of B reaches A and
C, i.e., A cannot detect C and vice versa.

5.  A starts sending to B, C does not receive this
transmission.
 C also wants to send something to B and senses the
medium.
 The medium appears to be free, the carrier sense fails.
C also starts sending causing a collision at B.
 But A cannot detect this collision at B and continues with
its transmission. A is hidden for C and vice versa.
 While hidden terminals may cause collisions, the next
effect only causes unnecessary delay.
 Now consider the situation that B sends something to A
and C wants to transmit data to some other mobile
phone outside the interference ranges of A and B.
 C senses the carrier and detects that the carrier is busy.
 C postpones its transmission until it detects the medium
as being idle again.
 But as A is outside the interference range of C, waiting is
not necessary.

6. NEAR AND FAR TERMINALS SCENARIO
 A and B are both sending with the same
transmission power.
 As the signal strength decreases proportionally to
the square of the distance, B’s signal drowns out
A’s signal.
 As a result, C cannot receive A’s transmission.

7.  Now think of C acts as a base station coordinating
media access.
 In this case, terminal B would already drown out
terminal A on the physical layer.
 C in return would have no chance of applying a fair
scheme as it would only hear B.
 The near/far effect is a severe problem.
 All signals should arrive at the receiver with more
or less the same strength.
 Otherwise a person standing closer to somebody
could always speak louder than a person further
away.
 Precise power control is needed to receive all
senders with the same strength at a receiver.

8. SDMA
 Space Division Multiple Access (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 which frequencies (FDM), time
slots (TDM) or code (CDM) are still available.
 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 cells and
sectorized antennas which constitute the infrastructure
implementing space division multiplexing

9. FDMA
 Frequency division multiple access (FDMA)
comprises all algorithms allocating frequencies to
transmission channels according to the frequency
division multiplexing (FDM) scheme.
 Allocation can either be fixed or dynamic.
 Channels can be assigned to the same frequency at all
times, i.e., pure FDMA, or change frequencies according
to a certain pattern i.e., FDMA combined with TDMA.
 FDM is often used for simultaneous access to the
medium by base station and mobile station in cellular
networks.
 Here the two partners typically establish a duplex
channel, i.e., a channel that allows for simultaneous
transmission in both directions. The two
directions, mobile station to base station and vice versa
are now separated using different frequencies.
 This scheme is then called frequency division duplex
(FDD).

10.  The two frequencies are also known as
uplink, i.e., from mobile station to base station.
 downlink, i.e., from base station to mobile station
or from satellite to ground control.
 All uplinks use the band between 890.2 and 915
MHz, all downlinks use 935.2 to 960 MHz.

11.  the base station, shown on the right side, allocates
a certain frequency for up- and downlink to
establish a duplex channel with a mobile phone.
 Up- and downlink have a fixed relation.
 If the uplink frequency is
fu = 890 MHz + n·0.2 MHz,
the downlink frequency is
fd = fu +45 MHz, i.e.,
fd = 935 MHz + n·0.2 MHz
for a certain channel n.

12. TDMA
 Compared to FDMA, time division multiple
access (TDMA) offers a much more flexible
scheme, which comprises all technologies that
allocate certain time slots for
communication, i.e., controlling TDM.
 listening to different frequencies at the same time is
quite difficult, but listening to many channels
separated in time at the same frequency is simple.
 synchronization between sender and receiver can
be achieved in the time domain by allocating a
certain time slot for a channel, or by using a
dynamic allocation scheme.

13. FIXED TDM
 The simplest algorithm for using TDM is allocating time
slots for channels in a fixed pattern.
 This results in a fixed bandwidth and is the typical
solution for wireless phone systems.
 MAC is quite simple, as the only crucial factor is
accessing the reserved time slot at the right moment.
 If this synchronization is assured, each mobile station
knows its turn and no interference will happen.
 The fixed pattern can be assigned by the base
station, where competition between different mobile
stations that want to access the medium is solved.
 Fixed access patterns fit perfectly well for connections
with a fixed bandwidth.
 Furthermore, these patterns guarantee a fixed delay –
one can transmit, e.g., every 10 ms.
 TDMA schemes with fixed access patterns are used for
many digital mobile phone systems like IS-54, IS-
136, GSM, DECT, PHS, and PACS.

14.  Assigning different slots for uplink and downlink
using the same frequency is called time division
duplex (TDD).
 the base station uses one out of 12 slots for the
downlink, whereas the mobile station uses one out
of 12 different slots for the uplink.
 Up to 12 different mobile stations can use the same
frequency without interference using this scheme.

15.  Classical Aloha
 Slotted Aloha
 Carrier sense multiple access
 Demand assigned multiple access
 PRMA packet reservation multiple access
 Reservation TDMA
 Multiple access with collision avoidance
 Polling
 Inhibit sense multiple access
 CDMA
 Spread Aloha multiple access

16. ALOHA: PURE ALOHA
 The basic idea of an ALOHA system is simple: let users
transmit whenever they have data to be sent.
 There will be collisions, of course, and the colliding frames
will be damaged.
 However, due to the feedback property of broadcasting, a
sender can always find out whether its frame was
destroyed by listening to the channel.
 If listening while transmitting is not possible for some
reason, acknowledgements are needed.
 If the frame was destroyed, the sender just waits a random
amount of time and sends it again.
 The waiting time must be random or the same frames will
collide over and over, in lockstep.

17.  Systems in which multiple users share a common
channel in a way that can lead to conflicts are
widely known as contention systems.

18.  If the first bit of a new frame overlaps with just the
last bit of a frame almost finished, both frames will
be totally destroyed and both will have to be
retransmitted later.
 Let t be the time required to send a frame. If any
other user has generated a frame between time t0
and t0 + t, the end of that frame will collide with the
beginning of the shaded one.
 In pure ALOHA a station does not listen to the
channel before transmitting, it has no way of
knowing that another frame was already underway.

19.  any other frame started between t0 + t and t0 + 2t
will bump into the end of the shaded frame.

20. SLOTTED ALOHA
 Slotted ALOHA was invented to improve the
efficiency of pure ALOHA.
 In slotted ALOHA we divide the time into slots of T
and force the station to send only at the beginning
of the time slot.

21.  Because a station is allowed to send only at the
beginning of the synchronized time slot, if a station
misses this moment, it must wait until the beginning
of the next time slot.
 This means that the station which started at the
beginning of this slot has already finished sending
its frame.
 Of course, there is still the possibility of collision if
two stations try to send at the beginning of the
same time slot.
 However, the vulnerable time is now reduced to
one-half, equal to Tfr.

22. • The throughput for slotted ALOHA is S = G x e -G.
• The maximum throughput Sma x = 0.368 when
G = 1.

23. CARRIER SENSE MULTIPLE ACCESS (CSMA)
 To minimize the chance of collision
and, therefore, increase the performance, the
CSMA method was developed.
 The chance of collision can be reduced if a station
senses the medium before trying to use it.
 In other words, CSMA is based on the principle
"sense before transmit" or "listen before talk."
 CSMA can reduce the possibility of collision, but it
cannot eliminate it.
 The possibility of collision still exists because of
propagation delay.
 when a station sends a frame, it still takes time for
the first bit to reach every station and for every
station to sense it.

24.  a station may sense the medium and find it idle, only
because the first bit sent by another station has not yet
been received.
 At time t1, station B senses the medium and finds it
idle, so it sends a frame.
 At time t2 (t 2 > tl), station C senses the medium and
finds it idle because, at this time, the first bits from
station B have not reached station C. Station C also
sends a frame.
 The two signals collide and both frames are destroyed.

25.  Vulnerable Time :
 The vulnerable time for CSMA is the propagation time
Tp. This is the time needed for a signal to propagate
from one end of the medium to the other.
 But if the first bit of the frame reaches the end of the
medium, every station will already have heard the bit
and will refrain from sending.
 Persistence Methods
 What should a station do if the channel is busy?
 What should a station do if the channel is idle?

26. 1-PERSISTENT
 The 1-persistent method is simple and
straightforward.
 In this method, after the station finds the line idle, it
sends its frame immediately (with probability 1).
 This method has the highest chance of collision
because two or more stations may find the line idle
and send their frames immediately.

27. NONPERSISTENT
 A station that has a frame to send senses the line. If
the line is idle, it sends immediately.
 If the line is not idle, it waits a random amount of
time and then senses the line again.
 The nonpersistent approach reduces the chance of
collision because it is unlikely that two or more
stations will wait the same amount of time and retry
to send simultaneously.
 However, this method reduces the efficiency of the
network because the medium remains idle when
there may be stations with frames to send.

28. P-PERSISTENT
 The p-persistent method is used if the channel has time slots
with a slot duration equal to or greater than the maximum
propagation time.
 It reduces the chance of collision and improves efficiency. In
this method, after the station finds the line idle it follows these
steps:
 1. With probability p, the station sends its frame.
 2. With probability q = 1 - p, the station waits for the beginning
of the next time slot and checks the line again.
 a. If the line is idle, it goes to step 1.
 b. If the line is busy, it acts as though a collision has occurred
and uses the back- off procedure.

29. DEMAND ASSIGNED MULTIPLE ACCESS
 A general improvement of Aloha access systems can
also be achieved by reservation mechanisms and
combinations with some (fixed) TDM patterns.
 These schemes typically have a reservation period
followed by a transmission period.
 During the reservation period, stations can reserve
future slots in the transmission period.
 While, depending on the scheme, collisions may occur
during the reservationperiod, the transmission period
can then be accessed without collision.
 These schemes cause a higher delay under a light load
but allow higher throughput due to less collisions.

30.  demand assigned multiple access (DAMA) also called
reservation Aloha, a scheme typical for satellite systems.
 DAMA has two modes.
 During a contention phase following the slotted Aloha
scheme, all stations can try to reserve future slots.
 For example, different stations on earth try to reserve access
time for satellite transmission.
 Collisions during the reservation phase do not destroy data
transmission, but only the short requests for data
transmission.
 If successful, a time slot in the future is reserved, and no other
station is allowed to transmit during this slot.
 Therefore, the satellite collects all successful requests and
sends back a reservation list indicating access rights for future
slots.
 All ground stations have to obey this list.
 To maintain the fixed TDM pattern of reservation and
transmission, the stations have to be synchronized from time
to time.
 DAMA is an explicit reservation scheme. Each
transmission slot has to bereserved explicitly.

31. PRMA PACKET RESERVATION MULTIPLE
ACCESS
 An example for an implicit reservation scheme is
packet reservation multiple access (PRMA).
 Here, slots can be reserved implicitly according to
the following scheme.

32.  A certain number of slots forms a frame. The frame is
repeated in time
 A base station, which could be a satellite, now
broadcasts the status of each slot to all mobile stations.
 All stations receiving this vector will then know which
slot is occupied and which slot is currently free.
 In the example, the base station broadcasts the
reservation status ‘ACDABA-F’ to all stations, here A to
F.
 All stations wishing to transmit can now compete for this
free slot in Aloha fashion.
 If more than one station wants to access this slot, a
collision occurs.
 The base station returns the reservation status
‘ACDABA-F’, indicating that the reservation of slot seven
failed and that nothing has changed for the other slots.
 Again, stations can compete for this slot.

33.  Additionally, station D has stopped sending in slot
three and station F in slot eight.
 This is noticed by the base station after the second
frame .
 Before the third frame starts, the base station
indicates that slots three and eight are now idle.
 As soon as a station has succeeded with a
reservation, all future slots are implicitly reserved
for this station.
 This ensures transmission with a guaranteed data
rate.
 The slotted aloha scheme is used for idle slots
only, data transmission is not destroyed by collision.

34. RESERVATION TDMA
 In a fixed TDM scheme N mini-slots followedby
N·k data-slots form a frame that is repeated.
 Each station is allotted its own mini-slot and can
use it to reserve up to k data-slots.
 This guarantees each station a certain bandwidth
and a fixed delay.
 Other stations can now send data in unused data-
slots as shown.

35. MULTIPLE ACCESS WITH COLLISION
AVOIDANCE
 Multiple access with collision avoidance
(MACA) presents a simple scheme that solves the
hidden terminal problem, does not need a base
station, and is still a random access Aloha scheme
– but with dynamic reservation.

36.  With MACA, A does not start its transmission at
once, but sends a request to send (RTS) first.
 B receives the RTS that contains the name of
sender and receiver, as well as the length of the
future transmission.
 This RTS is not heard by C, but triggers an
acknowledgement from B, called clear to send
(CTS).
 The CTS again contains the names of sender (A)
and receiver (B) of the user data, and the length of
the future transmission.
 This CTS is now heard by C and the medium for
future use by A is now reserved for the duration of
the transmission.
 After receiving a CTS, C is not allowed to send
anything for the duration indicated in the CTS
toward B.

37.  Still, collisions can occur during the sending of an
RTS.
 Both A and C could send an RTS that collides at B.
RTS is very small compared to the data
transmission, so the probability of a collision is
much lower.
 B resolves this contention and acknowledges only
one station in the CTS.
 No transmission is allowed without an appropriate
CTS.

38. MACA ALSO HELP TO SOLVE THE ‘EXPOSED
TERMINAL’ PROBLEM
 With MACA, B has to transmit an RTS first containing
the name of the receiver (A) and the sender (B).
 C does not react to this message as it is not the
receiver, but A acknowledges using a CTS which
identifies
 B as the sender and A as the receiver of the following
data transmission.
 C does not receive this CTS and concludes that A is
outside the detection range.
 C can start its transmission assuming it will not cause a
collision at A.
 The problem with exposed terminals is solved without
fixed access patterns or a base station.

39.  Problems with MACA is the overheads associated
with the RTS and CTS transmissions – for short
and time-critical data packets, this is not negligible.

40. POLLING
 Where one station is to be heard by all others
(e.g., the base station of a mobile phone network or
any other dedicated station), polling schemes can
be applied.
 Polling is a strictly centralized scheme with one
master station and several slave stations.
 The master can poll the slaves according to many
schemes: round robin ,randomly, according to
reservations etc.
 The master could also establish a list of stations
wishing to transmit during a contention phase.
 After this phase, the station polls each station on
the list.
 Similar schemes are used, e.g., in the Bluetooth
wireless LAN

41. INHIBIT SENSE MULTIPLE ACCESS
 This scheme, which is used for the packet data
transmission service, is also known as digital
sense multiple access (DSMA).
 Here, the base station only signals a busy medium
via a busy tone on the downlink.
 After the busy tone stops, accessing the uplink is
not coordinated any further.
 The base station acknowledges successful
transmissions.
 A mobile station detects a collision only via the
missing positive acknowledgement.
 In case of collisions, additional back-off and
retransmission mechanisms are implemented.

42. CODE-DIVISION MULTIPLE ACCESS (CDMA)
 In CDMA, one channel carries all transmissions
simultaneously.
 CDMA differs from FDMA because only one
channel occupies the entire bandwidth of the link.
 It differs from TDMA because all stations can send
data simultaneously; there is no timesharing.
 CDMA simply means communication with different
codes.

43.  Let us assume we have four stations 1, 2, 3, and 4
connected to the same channel.
 The data from station 1 are d l , from station 2 are
d2, and so on.
 The code assigned to the first station is c1, to the
second is c2, and so on.
 We assume that the assigned codes have two
properties.
 1. If we multiply each code by another, we get O.
 2. If we multiply each code by itself, we get 4 (the
number of stations).
 With these two properties in mind, let us see how
the above four stations can send data using the
same common channel,

44.  Station 1 multiplies its data by its code to get d l . Cl
 Station 2 multiplies its data by its code to get d2 . C2
And so on.
 The data that go on the channel are the sum of all these
terms.
 Any station that wants to receive data from one of the
other three ,multiplies the data on the channel by the
code of the sender.

45.  For example, suppose stations 1 and 2 are talking
to each other.
 Station 2 wants to hear what station 1 is saying.
 It multiplies the data on the channel by cl the code
of station 1.
 Because (c1 . c1) is 4, but (c2 . c1), (c3 . c1), and
(c4 . c1) are all Os, station 2 divides the result by 4
to get the data from station 1.
 data =(dj . Cj + dz . Cz +d3 . C3 + d4 . c4) . C1
 =d j • C1 . Cj + dz. Cz . C1 + d3 . C3 . C1 + d4 . C4'
C1
 =4 X d1

46.  CDMA is based on coding theory. Each station is
assigned a code, which is a sequence of numbers called
chips.
 They are called orthogonal sequences and have the
following properties:
 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
called 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, element by
element, and add the results, we get N, where N is the
number of elements in the each sequence.

47.  This is called the inner product of two equal
sequences. 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 O. This is called
inner product of two different sequences.
 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 00]

48. DATA REPRESENTATION
 We follow these rules for encoding: If a station needs to
send a 0 bit, it encodes it as -1;
 if it needs to send a 1 bit, it encodes it as +1. When a
station is idle, it sends no signal,
 which is interpreted as a O
 Encoding and Decoding
 We assume that stations 1 and 2 are sending a 0 bit and
channel 4 is sending a 1 bit. Station 3 is silent.
 The data at the sender site are translated to -1, -
1, 0, and +1.
 Each station multiplies the corresponding number by its
chip.
 The result is a new sequence which is sent to the
channel.
 For simplicity, we assume that all stations send the
resulting sequences at the same time.

49.  The sequence on the channel is the sum of all four
sequences as defined before
 Now imagine station 3, which we said is silent, is
listening to station 2.
 Station 3 multiplies the total data on the channel by
the code for station 2, which is [+1 -1 +1-1], to get