Medium access control unit 3-33

MEDIUM ACCESS CONTROL
PROTOCOLS FOR WIRELESS
SENSOR NETWORKS
1

MAC
• Shared access of the channel requires the MAC protocol
among the sensor nodes
• Regulate access to the shared wireless medium
• Performance requirements of the underlying application
are satisfied
• DLL with 2 Layers
– LLC
– MAC
2

OSI reference model and data link
layer architecture
3

Basic functions:
• Assembly of data into a frame for transmission
• Appending a
• Header field containing addressing information
and
• Trailer field for error detection (PHY-DLL-N/W)
Contd..4

Basic functions:
• Disassembly of a received frame to
– Extract addressing to perform address recognition
– Error control information and error detection and
recovery
• The regulation of access to the shared
transmission medium as per the performance
requirements of the supported application
5

MAC
• Two main factors Influence aggregate
behavior of a distributed MAC
– Intelligence of the decision made by the access
protocol and
– Overhead involved
6

Performance Metrics
–Delay
–Throughput:
–Robustness
–Scalability
–Stability
–Fairness
–Energy Efficiency
7

Delay
1. Delay : Amount of time spent by a data packet in the
MAC layer before it is transmitted successfully
• Delay depends on
– Network traffic load
– Design choices of the MAC protocol
• For Time-critical applications
– MAC protocol should support delay-bound guarantees
necessary to meet their QoS requirements
Contd.. 8

Delay
• Two types of delay
– Probabilistic
– Deterministic
• Probabilistic delay typically characterized by
– Expected value
– Variance
– Confidence interval Contd.. 9

Delay
• Deterministic delay guarantees
– Predictable number of state transitions between
message arrival and message transmission
– Guarantee an upper bound for the access time
– Crucial requirement in a real-time environment
Contd.. 10

Throughput
2. Throughput: The rate at which messages are
serviced by a communication system
• Measured as
– Messages per second
OR
– Bits per second
Contd.. 11

Throughput
• Throughput
– Increases initially
– After the load reaches a certain threshold it may start
to decrease
• Objective of a MAC protocol
“Maximize the channel throughput while
minimizing message delay”
12

Robustness
• 3. Robustness : Reliability+ Availability +
Dependability
• Reflects the degree of the protocol
insensitivity to errors and misinformation.
Contd.. 13

Robustness
• Robustness address issues such as
– Error confinement,
– Error detection and masking,
– Reconfiguration, and restart
Difficult for WSN
• Failure of links
• Communicating nodes
14

Scalability
4. Scalability ability of a communications system to
meet its performance characteristics regardless of
the size of the network or the number of
competing nodes
Contd.. 15

Scalability
• A common approach to achieve scalability
– Avoid relying on globally consistent network states
– Localize interactions among the communicating
nodes
• Hierarchical structures and information aggregation
strategies
• Grouping sensor nodes into clusters
– Allows the design of shared medium access protocols which are
highly scalable
– Aggregating information
16

Stability
5. Stability ability of a communications system to
handle fluctuations of the traffic load over
sustained periods of time
A stable MAC protocol handles instantaneous
loads exceeding the maximum sustained load
(with in maximum capacity of the channel)
Contd…17

Stability
• A MAC protocol is stable
– If the message waiting time is bounded (delay) and
if the throughput does not collapse as the load
increases
• Load fluctuations + stability is difficult to
achieve in WSNs
• Approach : Careful scheduling of bursty traffic
18

Fairness
6. Fairness Allocating channel capacity evenly
among the competing communicating nodes
without reducing the network throughput
• Network is with
– Various traffic sources
– Different traffic generation patterns
– Wide range of QoS requirements.
Contd…19

Fairness
• To accommodate heterogeneous resource
demands
– Assign different weights to reflect their relative
resource share
• Proportional fairness is then achieved based
on the weights assigned
Contd…20

Fairness
• Fair resource allocation in wireless networks
is difficult to achieve
– Global information required to coordinate access
to the communication medium
– The time-varying characteristics of the wireless
links
21

Energy Efficiency
Energy usage – Sensing+ communication + computation
7. Energy Efficiency
a) Energy waste is collision : Occurs when two or more
sensor nodes attempt to transmit simultaneously
 The need to retransmit a packet that has been
corrupted by a collision increases energy
consumption
Contd…22

Energy Efficiency
b) Energy waste is idle listening: A sensor node is listening for a
traffic that is not sent
c) Energy waste is overhearing : Occurs when a sensor node
receives packets that are destined to other nodes
c) Energy waste is caused by control packet overhead : Control
packets to regulate access to the transmission channel
 A high number of control packets than of data packets
delivered indicates low energy efficiency
Contd…23

Energy Efficiency
e) Frequent switching between different
operation modes may result in significant
energy consumption.
Solution :
– Limiting the number of transitions between sleep
and active modes
– Energy-efficient link-layer protocols achieve
energy savings by controlling the radio
– Using comprehensive energy management
schemes
24

Common Protocols
• The choice of the MAC method is the major
determining factor in the performance of a
WSN
• Major categories of MAC Protocols
– Fixed assignment
– Demand assignment
– Random assignment
25

Fixed-Assignment Protocols
• Each node is allocated a predetermined fixed amount of
the channel resources regardless of current Comm. need
• Each node uses its allocated resources without competing
with other nodes
• Typical protocols
– Frequency-division multiple access (FDMA)
– Time-division multiple access (TDMA)
– Code-division multiple access (CDMA) Spread Spectrum based
(FHSS or DSSS)
26

FDMA
• FDMA
-used by radio systems to share the radio
spectrum.
• Based on this scheme- the available
bandwidth is divided into sub channels.
• Multiple channel access is achieved by
allocating communicating nodes with different
carrier frequencies of the radio spectrum.
28

FDMA
• The bandwidth of each node’s carrier is
constrained within certain limits
- no interference /overlap occurs between
communicating nodes.
*Requires frequency synchronization
*Communication is achieved by – having the
receiver tune to the channel used by the
transmitter.
29

TDMA
• Digital transmission technology
• Allows a no of communicating nodes to access a
single radio frequency channel without
interference.
• Achieved by dividing the radio frequency into
time slots and then allocating unique time slots
to each communicating node.
• Nodes takes turns in round robin
fashion(transmitting and receiving)
• Only one node is using the channel at any given
time for the duration of time slot.
30

CDMA
• Spread spectrum based scheme
• Allows multiple communicating nodes to transmit
simultaneously
• Radio frequency modulation technique – radio energy
is spread over a much wider bandwidth that that
needed for the data rate.
• Transmit an information signal by combining it with a
noise like signal of much larger bandwidth to generate
a wideband signal.
• Signal transmitted occupies larger bandwidth than that
normally required to transmit the original information.
31

CDMA
• Using wideband noiselike signals makes it hard
to detect,intercept or demodulate the original
signal.
• Uses either FHSS/ DSSS.
• Hybrid systems use combination
32

Demand Assignment Protocols
• Improve channel utilization by allocating the
capacity of the channel to contending nodes in an
optimum or near-optimum fashion
• Ignore idle nodes and consider only nodes that are
ready to transmit
• Polling (master control device and slave node)
• Reservation (With reservation messages)
33

Random Assignment Protocols
Random assignment strategies do not exercise any
control
All backlogged nodes must contend to access the
transmission medium
Collision occurs when more than one node
attempts to transmit simultaneously
To deal with collisions, the protocol must include a
mechanism to detect collisions 34

Random Assignment Protocols
Pure ALOHA
Slotted ALOHA
CSMA ( carrier-sense multiple access)
CSMA/CD (carrier-sense multiple access with collision detection)
CSMA/CA (carrier-sense multiple access with collision avoidance)
35

ALOHA
• A shared communication system like ALOHA requires a
method of handling collisions that occur when two or
more systems attempt to transmit on the channel at
the same time.
• In the ALOHA system, a node transmits whenever data
is available to send.
• If another node transmits at the same time, a collision
occurs, and the frames that were transmitted are lost.
• However, a node can listen to broadcasts on the
medium, even its own, and determine whether the
frames were transmitted.
36

• There are two different versior.s/types of
ALOHA:
• (i) Pure ALOHA
(ii) Slotted ALOHA
37

Pure ALOHA
• 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.
• If the frame is destroyed because of collision the station waits for a
random amount of time and sends it again. This waiting time must
be random 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. 38

Slotted ALOHA
• 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.
42

• In slotted ALOHA, if any station is not able to
place the frame onto the channel at the
beginning of the slot i.e. it misses the time slot
then the station has to wait until the
beginning of the next time slot.
• • In slotted ALOHA, there is still a possibility of
collision if two stations try to send at the
beginning of the same time slot as shown in
fig.
• • Slotted ALOHA still has an edge over pure
ALOHA as chances of collision are reduced to
one-half.
43

Persistence Methods
• What should a station do if the channel is busy?
• What should a station do if the channel is idle?
Contd…46

CSMA/CD
• A station needs to be able to while receive Or transmitting to detect
a collision
• If no collision, the station receives one signal: its own signal.
• If collision, the station receives two signals: its own signal and the
signal transmitted by a second station.
• The received signals in these two cases must be significantly
different.
48

Energy level during transmission,
idleness, or collision
49

CSMA/CD
• The CSMA method does not specify the procedure
following a collision.
• CSMA/CD augments the algorithm to handle the collision.
• A station monitors the medium after it sends a frame to
see if the transmission was successful.
• If so, the station is finished If there is a collision, the
frame is sent again
50

Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)
• Three strategies are used to avoid Collision
In CSMA/CA
– Interframe Space (IFS)
– Contention Window
– Acknowledgment
52

CSMA/CA
• Interframe Space (IFS)
When an idle channel is found, the station does
not send immediately
 Waits for a period of time called the Interframe
space or IFS
(Distant station may have already started transmitting)
IFS can also be used to define the priority of a
station or a frame.
53

CSMA/CA
• Contention Window
 The contention window is an amount of time divided into slots.
 A station that is ready to send chooses a random number of slots as
its wait time.
 The number of slots in the window changes according to the binary
exponential back-off strategy.
 set to one slot the first time and then doubles each time the
station cannot detect an idle channel after the IFS time.
Contd…54

CSMA/CA
 The station needs to sense the channel after each time slot
 If the channel is busy, it just stops the timer and restarts it when
the channel sensed as idle.
 This gives priority to the station with the longest waiting time.
• Acknowledgment
• Still there may be a collision and data may be corrupted during the
transmission
• Hence positive acknowledgment and the time-out timer
55

Problems with CSMA/CA
• Referred to as the
•Hidden node problem
•Exposed node problem
58

Hidden Terminal Problem
59
A hidden node is defined as a node that is within the range of the
destination node but out of range of the transmitting node
B is within the transmission range of nodes A and C
A and C are outside their mutual transmission ranges
Transmission from either of the two nodes will not reach the other
node.
Eg : A---------B<-----------C collision at B

Exposed Terminal Problem
• An exposed node is a node that is within the range of
the sender but out of the range of the destination
• B is within the transmission range of nodes A and C
• A and C are outside their mutual transmission ranges
• D is within the transmission range of node C.
• Eg: B----A and C--D(finds channel busy)
60

Solutions
• 1) Busy-tone approach:
Busy-tone signal
 Not too weak not to be heard by a node within the range of a receiver
 Not too strong to force more nodes than necessary to suppress their
transmissions
 Advantage : solves both hidden node and exposed node problem
 Disadvantage: Need Duplex mode (Comm. Complexity)
61

Solutions
• 2) Handshake
(Ready to- send (RTS), clear-to-send (CTS))
62

Collision avoidance using RTS/CTS
handshake
63
Start timer
Start timer

Collision avoidance failure using
RTS/CTS handshake
64

MAC PROTOCOLS FOR WSNs
• Conserve energy is the critical issue in the design of
scalable and stable MAC layer protocols for WSNs
• Factors contribute to energy waste
– Idle listening
– Packet collisions
– Overhearing
– Exchange of control and synchronization
65

MAC for WSN
• Two main groups of MAC-layer protocols
– Schedule Based
– Contention Based
• Schedule-based protocols
– Deterministic MAC layer protocols
– Access to the channel is based on a schedule
– Channel access is limited to one sensor node at a time
– Pre allocation of resources to individual sensor nodes
Contd…66

MAC for WSN
• Contention-based MAC-layer protocols
– Avoid pre allocation of resources to individual sensors
– A single radio channel is shared by all nodes and allocated on
demand
– Simultaneous attempts to access the communications medium
results in collision
• The main objective of contention-based MAC layer protocols
– To minimize, rather than completely avoid, the occurrence of
collisions
– To reduce energy consumption
Contd…67

Schedule-Based Protocols
• Schedule-based MAC protocols for WSNs
• Regulates access to resources to avoid contention
between nodes
• Typical resources include
– Time
– Frequency band
– CDMA code
Contd…68

Schedule-Based Protocols
• The objective of schedule based
– To achieve a high level of energy efficiency
(To prolong the network lifetime)
– Scalability
– Adaptability to changes in
• Traffic load
• Network topology
• Most of the scheduled-based protocols for
WSNs use a variant of a TDMA scheme
Contd…69

TDMA Scheme
Contd…70
A set of N contiguous slots (N : system parameter, logical frame)
Logical frame repeats cyclically over time
Sensor nodes are assigned a/set of specific time slots
Sensor node operates in each logical frame in the assigned time slot
The schedule can be either
Fixed
Constructed on demand (requirements of sensor nodes and traffic pattern)
 Hybrid (Structure varies over different time scales and sensor behavior)

TDMA Scheme
Advantages
• No collisions
• Energy efficient — easily support low duty cycles
Disadvantages
• Difficult to accommodate node changes
• Requires strict time synchronization
• Could limit available throughput
71

Mode of Operation
• Based on its assigned schedule, a sensor alternates between
two modes of operation:
• Active mode
• Sleep mode
• Active mode:
– Sensor uses its assigned slots within a logical frame to transmit and
receive data frames
– Outside their assigned slots, sensor nodes move into sleep mode
• Sleep mode :
– Sensor nodes switch their radio transceivers off to conserve energy.
72

Self-Organizing Medium Access Control for
Sensornets (SMACS)
• SMACS Enable
– Formation of random network topologies
– No need of global synchronization among the network
nodes
• Referred to as non synchronous scheduled communication
• No need for costly exchange of global connectivity
information or time synchronization.
Contd…73

SMACS
• Each node
– Regularly execute a neighborhood discovery procedure
– Establishes a link to each neighbor discovered
– Maintains a TDMA-like frame (superframe) to
maintains its own time slot schedules with all its
neighbors to communication
Contd…74

SMACS
– The length of a superframe is fixed and is divided
into smaller frames.
– Talk to neighbors in different time slots
– Use a hybrid TDMA/FH
– Each link consists of a pair of time-slots that operate
on fixed frequencies
Contd…75

SMACS
– Random wake up schedule during connection
phase and nodes sleep during idle time slots.
– Nodes tune their radios
• To proper frequency channel
OR
• CDMA code to achieve communication
76

Bluetooth MAC
• Centralized TDMA-based protocol
• Bluetooth operates in the 2.45-GHz ISM frequency band
• Physical layer is based on a frequency-hopping scheme
• Hopping frequency of 1.6 kHz
• A set of 79 hop carriers are defined with 1-MHz spacing
• Data rate supported is 1 Mbps.
Contd…77

Bluetooth MAC
• A group of devices sharing a common channel and called piconet
• Access to the channel is regulated using a slotted time-division duplex (TDD)
• Master unit controls access to the channel in each piconet
• Each channel is divided into 625-ms slots
• Each piconet is assigned a unique frequency-hopping pattern determined by
the master’s Bluetooth device address (48 bits) and clock.
• Master assigns each slave device a unique internal address of 3 bits.
Contd…78

Bluetooth MAC
• The master polls the slave devices continuously for
communication.
• Different piconets use different hopping sequences,
and hence guaranteeing their coexistence.
• Piconets can be interconnected, via bridge nodes, to
form larger ad hoc networks, known as scatter nets.
Contd…79

Operational Modes in Bluetooth
• Bluetooth specifies four operational modes:
 Active
 Sniff
 Hold
 Park
• Active mode:
– Slave listens for packet transmission from the master.
– Checks the address and packet length field of the packet header
– If the packet does not contain its own address, the slave sleeps for the duration of
the remaining packet transmission.
– The intended slave remains active and receives the packet payload in the reserved
slot.
Contd…80

• Sniff mode
– Intended to reduce the duty cycle of a slave’s listen activity
– A slave in sniff mode listens for the master transmissions
only during the specified time slots for any possible
transmission to it
• Hold mode
– A slave goes into the sleep mode for a specified amount of
time
– Returns to the active mode when the hold time expiresContd…81

• Park mode
– The slave stays in the sleep state for an
unspecified amount of time
– The master has to awake the slave explicitly and
bring it into the active mode at a future time
Contd…82

Communication in Bluetooth
• Four types of communication between nodes within and
across piconets:
– Intra piconet unicast:
• Slave-to-slave communication within a piconet
– Intra piconet broadcast
• Broadcasting by a slave to all participants within its piconet
– Inter piconet unicast
• Piconet-to-piconet communications
– Inter piconet broadcast
• Piconet-to-all scatternet node communications
Contd…83

Intra piconet unicast communication
– Slave writes its MAC address in the corresponding field
of the data packet
– Sets the forward field to 1 and the destination address
– Master checks the forward field
– The master replaces the MAC address field with its MAC
address
– Master sends the message to the intended slave device
indicated by the destination address
Contd…84

Intra piconet broadcast communication
• Slave writes its own MAC address
• Sets the forward field to 1 and the destination
address to 000
• The master notices that the forward field is set
• Master replaces the MAC address with its own
address
• Sends the message to all nodes in its piconet
Contd…85

Inter piconet unicast communication
– Source device sends the data packet with
• Own MAC address
• Sets the forward field to 1
• Broadcast field to 1
• Destination address to the relay of the next piconet.
– Source device sets the routing vector field (RVF)
of the packet (path to the targeted destination)
Contd..86

Inter piconet unicast communication
– The RVF is a sequence of tuples of the form (LocId,
Mac_Addr)
• LocId represents the identity of the local master
• Mac_Addr its corresponding piconet MAC address.
– The master forwards it to the relay node
– The relay extracts from the RVF the local identity and
the MAC address of the master
– The process is repeated till the destination device has
been reached.
Contd…87

Inter piconet broadcast communication
 Source device creates a packet containing its own MAC address
 Sets the forward and broadcast fields of the packet to 1
 Sets the destination address to 000
 The packet is sent to the master
 The master sends the packet to all the slaves within its piconet,
including relay nodes.
 Relay node receives the broadcast packet and forwards it to all
masters connected, except the one from which it came.Contd…88

Low-Energy Adaptive Clustering
Hierarchy (LEACH)
• A hierarchical approach and organizes nodes
into clusters
• Cluster head rotation
• TDMA within each cluster
– Static TDMA frame
• Node only talks to cluster head
• CDMA between clusters
• Only cluster head talks to base station Contd…89

Hierarchical clusters
Contd…90

LEACH
• Base station send out synchronization pulses to the all the nodes
• Communication between a node and its cluster head are achieved using
direct-sequence spread spectrum (DSSS)
• Each cluster is assigned a unique spreading code
• Spreading codes are assigned to cluster heads on a first-in, first-served
basis
• Nodes adjust their transmit powers to reduce interference with nearby
clusters
• Advantages of TDMA:
– Cluster nodes turn off their radio components till its allocated time slots.
Contd…91

LEACH
• Cluster head aggregates the data (of cluster
nodes) before sending them to the base station.
• The communication between a cluster head and a
base station is achieved using fixed spreading
code and CSMA.
• Cluster head delays the data transmission until
the channel becomes idle.
Contd…92

Advantages of LEACH
• Contention-free, eliminate energy waste caused
by collisions.
• Nodes turn their radios during slots to data
transmission or received.
• Sensor node can turn off its radio, thereby
avoiding overhearing
– Extend the network lifetime significantly.
Contd…93

Disadvantages of LEACH
• Hierarchical structure restricts nodes to
communicate only with their cluster head.
• Peer-to-peer communication cannot be supported
because of sleep mode.
• Achieving time synchronization among distributed
sensor nodes is difficult and costly.
Contd…94

Disadvantages of LEACH
• Schedule-based schemes also require additional
mechanisms such as FDMA or CDMA to overcome inter
cluster communications and interference.
• TDMA-based MAC-layer protocols
– limited scalability
– Not easily adaptable to node mobility and changes in network
traffic and topology.
– As nodes join or leave a cluster, the frame length as well as the
slot assignment must be adjusted.
– Frequent changes may be expensive or slow to take effect.
Contd…95

Random Access-Based Protocols
• No coordination among the nodes accessing the
channel
• Colliding nodes back off for a random duration of
time
• Enhancement of protocols with
– Collision avoidance and
– Request-to-send (RTS) and clear-to-send (CTS)
Make them more robust to the hidden terminal
problem
96

Random Access-Based Protocols
• The energy efficiency of contention-based MAC-layer
protocols remains low due to
– Collisions
– Idle listening
– Overhearing
– Excessive control overhead
• Random access MAC-layer protocols focused on
reducing energy waste 97

Power Aware Multiaccess protocol
with Signaling (PAMAS)
• Avoids overhearing among neighboring nodes by using a separate
signaling channel
• The protocol combines use of
– Busy tone and
– RTS and CTS packets
• Two channels
– Data
– Control
• Upon wakeup, probe control channel for activity related to destination
node
98

99
PAMAS and Channel Probing
• Neighbor answer probe(busy tone) ? Yes, go back to sleep
• Probing eliminated interference with transmission in data
channel
• Nodes currently not actively transmitting / receiving
packets turn off their radio transceivers.
• Disadvantages
– More complex to implement (two channels)
– PAMAS does not reduce idle listening

Timeout-MAC (T-MAC)
• Nodes alternate between sleep and wake-up
modes for predetermined time
• Each node wakes up periodically to
communicate with its neighbors
• A node keeps listening and potentially
transmitting as long as it is in the active period
100

Timeout-MAC (T-MAC)
• Active events include
– Hearing of a periodic frame timer
– Reception of data over the radio
– Sensing of an activity such as collision on the channel,
– End of transmission of a node’s own data packet or acknowledgment
– End of a neighboring node’s data exchange,
(determined through overhearing of prior RTS and CTS packets.)
• At the end of the active period, the node goes into sleep mode.
101

Timeout-MAC (T-MAC)
• Contention-based MAC-layer protocol designed for applications
characterized by
– low message rate
– low sensitivity to latency
(Support burst traffic)
• T-MAC nodes use RTS, CTS, and acknowledgment packets to
communicate with each other. (Avoid collision and ensure
reliable transmission)
• Protocol uses an adaptive duty cycle to
– Reduce energy consumption and
– Adapt to traffic load variations
102

Timeout-MAC (T-MAC)
• Reduce idle listening by transmitting all messages
in bursts of variable length
• Protocol dynamically determines the optimal
length of the active time, based on current load.
• Messages between active times must be buffered
• Buffer capacity determines an upper bound on the
maximum frame time. 103

Sparse Topology and Energy
Management (STEM) Protocol
• The trades latency for energy efficiency
• Use two radio channels
– Data radio channel
– Wake-up radio channel.
• STEM is known as a pseudo asynchronous scheduled scheme
• A node turns off its data radio channel until communication with another node is
desired
• Node begins transmitting on the wake-up radio channel when it has data to
transmit
• The wake-up signal channel acts like a paging signal 104

Sparse Topology and Energy
Management (STEM) Protocol
• The transmission of this signal lasts long enough to ensure that all neighboring
nodes are paged.
• A node can also be awakened to receive all of its pending packets before going
into the sleep mode again.
• The STEM protocol
– Can be used in conjunction with other MAC-layer scheduling protocols
– Effective only in network environments where events do not happen very
frequently.
– If events occur frequently, the energy wasted by continuously transmitting
wake-up signals 105

B-MAC - Berkeley MAC
• B-MAC’s Goals:
– Low power operation
– Effective collision avoidance
– Simple implementation (small code)
– Efficient at both low and high data rates
– Reconfigurable by upper layers
– Tolerant to changes on the network
– Scalable to large number of nodes

Medium access control unit 3-33

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Medium access control unit 3-33