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