This is about my thesis regarding wpan 802.15.6 protocol

1. Presentation on IEEE 802.15.4 MAC Protocol
Based on the Paper (A Renewal Theory Based Analytical
Model for the CAP of IEEE 802.15.1 MAC)

2. IEEE 802.15 Working Group
IEEE 802 LAN/MAN Standards Committee
802.1
Higher Layer
LAN Protocols
Working Group
…
802.11
Wireless Local
Area Network
Working Group
… …
802.15
Wireless Personal
Area Network
Working Group
TG4
WPAN Low Rate
Task Group
TG3
WPAN High Rate
Task Group
TG2
Coexistence
Task Group
UWB Zigbee
TG1
WPAN/Bluetooth
Task Group
TG5
Mesh Networking
Task Group
802.22
Wireless Regional Area
Networks

3. IEEE 802.15.4
• IEEE 802.15.4 task group began to develop a
standard for LR-WPAN.
• The goal of this group was to provide a
standard with ultra-low complexity, cost, and
power for low-data-rate wireless connectivity
among inexpensive fixed,portable, and
moving devices.

4. IEEE 802.15.4 in ISO-OSI layered
network model

5. Data link layer.
• IEEE 802 splits DLL into MAC and LLC
sublayers.
• LLC is standardized and is common in
802.3,802.11,802.15.1.
• features of the IEEE 802.15.4 MAC are
association and disassociation, acknowledged
frame delivery, channel access mechanism,
frame validation, guaranteed time slot
management, and beacon management.

6. IEEE 802.15.4 Operational Modes

7. Non beacon-enabled mode:
• nodes in the network use a non-slotted CSMA
with collision avoidance (CSMA/CA) mechanism
to contend for channel access.
• If the channel is sensed to be idle, the trans-
mission of a frame will begin immediately;
otherwise the node will backoff and try to
access the channel in a future slot.
• Pros: Node's receiver does not have to regularly
power-up to receive the beacon.
• Cons: does not provide any guarantee to deliver
data frames, specifically within a certain deadline.

8. 05 2004 Marco Naeve, Eaton Corp. Slide 8
IEEE 802.15.4 Device Classes
• Full function device (FFD)
– Any topology
– PAN coordinator capable
– Talks to any other device
– Implements complete protocol set
• Reduced function device (RFD)
– Limited to star topology or end-device in a peer-to-peer
network.
– Cannot become a PAN coordinator
– Very simple implementation
– Reduced protocol set

9. 05 2004 Marco Naeve, Eaton Corp. Slide 9
IEEE 802.15.4 Definitions
• Network Device: An RFD or FFD implementation
containing an IEEE 802.15.4 medium access control
and physical interface to the wireless medium.
• Coordinator: An FFD with network device
functionality that provides coordination and other
services to the network.
• PAN Coordinator: A coordinator that is the principal
controller of the PAN. A network has exactly one
PAN coordinator.

10. Beacon-enabled mode
• In the beacon-enabled mode, a personal area
network (PAN) coordinator transmits a beacon
periodically to form the so-called
“superframe” time structure.
• It comprises of 4 parts
• A Beacon that enables the beacon enabled mode
• Contention Access Period
• Contention Free Period
• Inactive portion(Optional)
Active Portion

11. 05 2004 Marco Naeve, Eaton Corp. Slide 11
Super Frame Structure
15ms * 2n
where 0  n  14
GTS 3
GTS
2
Network
beacon
Transmitted by PAN coordinator. Contains network information,
frame structure and notification of pending node messages.
Beacon
extension
period
Space reserved for beacon growth due to pending node messages
Contention
period
Access by any node using CSMA-CA
Guaranteed
Time Slot
Reserved for nodes requiring guaranteed bandwidth [n = 0].
GTS
1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Slot
Battery life
extension
Contention Access Period Contention Free Period

12. 05 2004 Marco Naeve, Eaton Corp. Slide 12
Optional Frame Structure
• Superframe may have inactive period
15ms * 2BO
where SO  BO  14
15ms * 2SO
where 0  SO  14
SO = Superframe order
BO = Beacon order
Inactive Period

13. Super Frame Structure
• To

14. CFP(Contention Free Period)
• Period where the channel is reserved and can be
used exclusively by the reserving node.
• It is activated upon request from a node to the
PAN coordinator for allocating time slots
depending on the node’s requirements. Upon
receiving this request, the PAN coordinator
checks whether there are sufficient resources
and, if possible, allocates the requested time
slots. These time slots are called Guaranteed
Time Slots (GTSs) and constitute the CFP.

15. CFP(Contention Free Period)
• In the CFP, the network coordinator alone con-
trols entirely the contention-free channel
access by assigning guaranteed time slots
(GTS) to those nodes with their GTS requests
granted. The assignment of the GTS to those
nodes is determined by the scheduling
scheme adopted by the network coordinator.

16. 16
Superframe with Inactive Part (cont’d)
• There are two parameters
– SO (Superframe Order): to determine the length of the active period
– BO (Beacon Order): to determine the length of the beacon interval
• In CFP, a GTS may consist of multiple slots, all of which are assigned to a
single device, for either transmission (t-GTS) or reception (r-GTS)
– GTS = guaranteed time slots
• In CAP, the concept of slots is not used
– Instead, the whole CAP is divided into smaller “contention slots”
– Each “contention slot” is of 20 symbols long
• This is used as the smallest unit for contention backoff
– Then devices contend in a slotted CSMA/CA manner

17. 17
Slotted CSMA/CA Algorithm
• The backoff period boundaries of every device
in the PAN shall be aligned with the
superframe slot boundaries of the PAN
coordinator
– i.e. the start of first backoff period of each device
is aligned with the start of the beacon
transmission
• The MAC sublayer shall ensure that the PHY
layer commences all of its transmissions on
the boundary of a backoff period

18. 18
Slotted CSMA/CA Algorithm (cont’d)
• Each device shall maintain three variables for each
transmission attempt
– NB: number of slots the CSMA/CA algorithm is required to
backoff while attempting the current transmission
– BE: the backoff exponent which is related to how many
backoff periods a device shall wait before attempting to
assess a channel
– CW: (a special design)
• Contention window length, the number of backoff slots that needs
to be clear of channel activity before transmission can commence
• It is initialized to 2 and reset to 2 if the channel is sensed to be
busy
– So a station has to detect two CCA before contending

19. 19
Slotted CSMA/CA
optional

20. CAP(Contention Access Period)
• The active portion of the superframe
structure, is divided into 16 equally-spaced
slots. Each one of this slots is further
decomposed into smaller slots of length 320s
called "backoff periods“.

21. Slotted CSMA/CA
Flowchart describing the slotted version of the CSMA/CA
mechanism. The algorithm works as follows; all the nodes
are synchronized and transmissions can begin only at the
boundaries of the backoff slots. Then a node which has a
packet ready for transmission follows the next procedure:
• Step 1. Initialization of the algorithm variables: NB = 0 and
CW = 2. If macBattLifeExt is set true, BE is initialized to
min(2;macMinBE), otherwise it is initialized to
macMinBE=3.
• Step 2. Backoff procedure: the node backsoff for a random
number of backoff slots, chosen uniformly between 0 and
2𝐵𝐸 − 1, before sensing the channel. This random backoff
serves to reduce the probability of collisions among
contending nodes.

22. Slotted CSMA/CA
• Step 3. Clear Channel Assessment: After the backoff counter
expires, the node verifies if the medium is idle or not performing a
CCA.
• If this CCA reported an idle channel, CW is decremented by 1 and if
CW = 0, then the message is transmitted.
• If CW ≠ 0, the algorithm jumps again to Step 3 with the new value
of CW. If the CCA reported a busy channel, then BE and NB are
incremented by 1 and the algorithm starts again in Step 2.
• This mechanism is repeated until either BE equals the parameter
aMaxBE (which has a default value of 5), at which point it is frozen
at aMaxBE, or until a certain maximum number of permitted
random backoff stages is reached, at which point an access failure is
declared to the upper layer. The maximum number of permitted
random backoff stages is determined by the parameter
macMaxCSMABackoffs, which has a default value of 5.

23. • Step 4. Acknowledgment (optional): If the acknowledged
mode is activated, the successful transmission is followed
by an acknowledgement packet (ACK) from the receiver of
length 11 bytes. This acknowledgment frame is transmitted
without using the slotted CSMA/CA mechanism to access
the channel. However, if the transmitting node does not
receive the ACK packet within MacAckWaitDuration, it
declares a collision or a packet loss. Then the same data
packet is retransmitted with the initial backoff parameters.
The maximum number of retransmissions collision or
packet loss is limited to aMaxFrameRetries. If the number
of retransmissions reaches aMaxFrameRetries, the packet is
discarded.

24. 802.15.4 protocol Diagram

25. A Renewal Theory Based Analytical Model for the
Contention Access Period of IEEE 802.15.4 MAC
• In this paper, a simple and yet accurate
analytical model for the IEEE 802.15.4 MAC
protocol is proposed. Instead of modeling the
channel, the writer models the behavior of an
individual node based on a novel concept of
three-level renewal process, which can be
solved by the fixed-point technique [12].

26. Cont.
• The new modeling approach significantly
simplifies the mathematical analysis, where the
important performance metrics of MAC
throughput and average frame service time can
be directly obtained.
• The proposed model is in fact a general analytical
framework which enables us to analyze different
protocol variants of either single or double
sensing, in a saturated or unsaturated case, under
a general traffic arrival distribution, and with
various backoff policies.

27. Renewal Process example
• Many applications of the renewal theory involve rewards or costs (which
can be simply seen as a negative reward). For example, consider the
classical example of a renewal process: a machine component gets
replaced upon failure or upon having operated for T time units. Then the
time the n-th component is in service is given by Yn = min{Xn, T }, where
Xn is the life of the component. In this example, one might be interested in
the rate of the number of replacements in the long run.
• An extension of this basic setup is as follows. A component that has failed
will be replaced at a cost c f , while a component that is replaced while still
being operational (and thus at time T) costs only c < c f . In this case, one
might be interested in choosing the optimal time T that minimises the
long-run operational costs. The solution to this problem involves the
analysis of renewal processes with costs and rewards.

28. Renewal examples cont.
• let {N(t), t > 0} be a renewal process with interarrival times Xn, n > 1, and denote
the time of the n-th renewal by S n = X1 + · · · + Xn. Now suppose that at the time
of each renewal a reward is received; we denote by Rn the reward received at the
end of the n-th cycle.
• We further assume that (Rn, Xn) is a sequence of i.i.d. random variables, which
allows for Rn to depend on Xn. For example, N(t) might count the number of rides
a taxi gets up to time t. In this case, Xn is the length of each trip and
one reasonably expects the fare Rn to depend on Xn. As usual, we denote by (R, X)
the generic bivariate random variable that are distributed identically to the
sequence of rewards and interarrival times (Rn, Xn).
• In the analysis, together with the standard assumption in renewal processes that
the interarrival times have a finite expectation E[X] = τ, we will further assume that
E[ |R| ] < ∞. The cumulative reward up to time t is given by R(t) = PN(t)
n=1 Rn, where the sum is taken to be equal to zero in the event that N(t) = 0.

29. Chapter 7 29
Poisson Process:
Counting process
iid exponential times
between arrivals
Continuous Time Markov
Chain: Exponential times
between transitions
Renewal Process:
Counting process
iid times between arrivals
Relax
counting
process
Relax
exponential
interarrival
times

30. Chapter 7 30
Counting Process
A stochastic process {N(t), t  0} is a counting process if N(t)
represents the total number of events that have occurred in
[0, t]
Then {N(t), t  0} must satisfy:
N(t)  0
N(t) is an integer for all t
If s < t, then N(s)  N(t)
For s < t, N(t) - N(s) is the number of events that occur in the
interval (s, t].

31. Chapter 7 31
Renewal Process
A counting process {N(t), t  0} is a renewal process if for each n,
Xn is the time between the (n-1)st and nth arrivals and {Xn, n  1}
are independent with the same distribution F.
The time of the nth arrival is
with S0 = 0.
Can write
and if m = E[Xn], n  1, then the strong law of large numbers says
that
1
, 1,
n
n i
i
S X n

 

   
max : n
N t n S t
 
as 1
n
S
P n
n
m
 
   
 
 
Note: m is now a
time interval, not a rate;
1/ m will be called
the rate of the r. p.

32. Chapter 7 32
Renewal Reward Processes
Suppose that each time a renewal occurs we receive a reward.
Assume Rn is the reward earned at the nth renewal and {Rn, n 
1} are independent and identically distributed (Rn may depend
on Xn).
The total reward up to time t is  
 
1
N t
n
n
R t R

 

33. Renewal Reward Processes
Interpretation
• In the context of the above interpretation of the holding
times as the time between successive malfunctions of a
machine, the "rewards" W1,W2… (which in this case
happen to be negative) may be viewed as the successive
repair costs incurred as a result of the successive
malfunctions.
• An alternative analogy is that we have a magic goose which
lays eggs at intervals (holding times) distributed as Si.
Sometimes it lays golden eggs of random weight, and
sometimes it lays toxic eggs (also of random weight) which
require responsible (and costly) disposal. The
"rewards" Wi are the successive (random) financial
losses/gains resulting from successive eggs (i = 1,2,3,...)
and Yt records the total financial "reward" at time t.

34. 3–level Renewal
• The link layer activities (channel sensing and frame
transmissions) of any node over a given time interval is
a renewal process since the node resets its backoff
parameters to the default initial value after each
transmission trial (regardless of the result) or when it
senses a busy channel at the end of the last backoff
stage.
• Over a larger time scale, the end of each transmission
trial is also a renewal point of the frame service
process.
• If the time scale is even larger, the renewal point can
also be set at the end of each successful transmission.

35. Concept of 3-level renewal process
O: a success of CCA x

36. Level-1
• level-1 renewal cycle is defined as the period between two adjacent
time instants where the tagged node starts a stage 0 backoff.
• In this context, the number of sensing attempts R conducted by the
tagged node can be viewed as a reward associated with the level-1
renewal cycle of length X.
• A level-1 renewal cycle can be of either type X1 or X2, as shown in
Fig. 2. Type X1 is a cycle that includes no transmission from the
tagged node due to M consecutive failures in sensing the channel
idle, which is marked by the symbol “×” in the figure.
• Type X2 is a cycle that contains a period of frame transmission from
the tagged node immediately after sensing an idle channel (marked
as“o”). Note that the transmission in an X2 cycle may be a
successful transmission or a collision.

37. Level-1
• Level-1 =No Transmission due to M
consecutive failure+Transmission
(Failure/Success)
• Level-1 = X1+ X2
• X1=

38. Level-2
• Bigger Scale than level-1,
• Transmission Transmission
• A level-2 renewal cycle Y is from the end of an X2
level-1 cycle to the end of the next X2 cycle.
• As shown in Fig. 2, there can be j (j ≥ 0) X1 cycles
before the X2 cycle.
• Depending on the result of transmission in the
X2-cycle, a level-2 cycle can be of either type Y1,
in which the transmission results in a collision, or
type Y2, in which the transmission succeeds.

39. Level-3
• Bigger Scale than level-2
• Success Success
• Finally, a level-3 renewal cycle Z is from the end of a Y2
level-2 cycle to the end of the next Y2 cycle.
• Similarly, there can be k, k ≥ 0, Y1 cycles before the Y2
cycle.
• Therefore, the successful transmission of a frame in the
Z cycle can be viewed as the reward for the level-3
renewal cycle.
• The throughput of the tagged node can thus be
obtained as the average reward in a Z cycle.

40. 40
L-1Renewal points for 802.15.4
X1 X2

41. MAC Analysis
This paper uses Renewal process to analyze
• 1) the throughput and
• 2) Avg Frame service time
To address this issue we need to solve two parameters α
and τ
• α= channel sensing failure prob.(can be obtained by
observing the transition probability among the channel
state)
• τ= sensing attempt rate=sensing probability =
𝑨𝒗𝒈.𝒔𝒆𝒏𝒔𝒊𝒏𝒈 𝒂𝒕𝒕𝒆𝒎𝒑𝒕𝒔
𝑨𝒗𝒈.𝒍𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝒍𝒆𝒗𝒆𝒍−𝟏 𝒄𝒚𝒄𝒍𝒆

42. α = f(τ),
τ= f(α)
These two equations can be solved by fixed
point techniques.
and

43. Evaluating τ
• τ=
𝐸[𝑅]
𝐸[𝑋]
• α=sensing failure probability
• 1- α= probability that a station succeeds in
sensing the channel idle and transmit the
frame by one single attempt.
• α(1- α) = prob. in transmitting in two attempts
• E[R]= Avg. number of sensing attempt for one
station in level-1 cycle

44. • E[R]= (1 - α) + 2α(1 - α) + 3α2(1 - α) + · · ·+ (M - 1)αM-2(1 - α) + MαM-1
= 𝑚=0
𝑀−1
αM
• Expected value of a variable=P(x)*N(N is the
number of trials and P(x) is the probability of
success)
1st attempt 2nd attempt Mth attempt

45. Single Sensing Case(One CCA)
b0,b1,b2,b3 are no of retrials attempt upto maximum M no of attempts.

46. Finding Avg. Length of L-1 cycle, E[X]
Avg. Length of a level-1 renewal cycle
E[X]= E[X1]+E[X2]
• E[X]= (1 - α) (b0 + 1 + L) + α(1 - α) (b0 + b1 + 2 + L) + ・ ・ ・+ αM-1 (1 − α) ( 𝑚=0
𝑀−1
(bm + 1) + L)
E[X2]
+ αM
𝑚=0
𝑀−1
(bm + 1)
E[X1]
=
𝑚=0
𝑀−1
αM(bm + 1)+(1−αM)𝐿
Here,

47. • Now τ=
𝐸[𝑅]
𝐸[𝑋]
• τ = 𝑚=0
𝑀−1 αM
𝑚=0
𝑀−1 αM(bm + 1)+(1−αM)𝐿
which is a function of α

48. Evaluating α
Pi= Prob. Of a channel being idle
Pb=1- Pi =Prob. Of a channel being busy
Using Conditional probability
Pi=
𝑃
(
𝑏
,
𝑖
)
1+𝑃
𝑏
,
𝑖 −
𝑃
(
𝑖
,
𝑖
)