Enhancement of Backoff Algorithm in
CSMA/CA Protocols for Broadband PLC
Ievgenii Tsokalo
Chair for Telecommunications
Technische Universität Dresden
Dresden, Germany
Email: tsokalo@ifn.et.tu-dresden.de
Rico Radeke
Chair for Telecommunications
Technische Universität Dresden
Dresden, Germany
Email: rico.radeke@tu-dresden.de
Ralf Lehnert
Chair for Telecommunications
Technische Universität Dresden
Dresden, Germany
Email: ralf.lehnert@tu-dresden.de
Abstract—Improvement of communication protocols
for energy management purposes has become a challeng-
ing part of smart grid systems development. Powerline
communications (PLC) has many advantages for being
used in this area. Many PLC communication proto-
cols use CSMA/CA channel access schemes on Multiple
Access Protocol (MAC) layer. This paper presents an
enhancement of the CSMA/CA broadband PLC protocols
operation by application of special method for backoff
generation. The proposed improvement is applicable to
different CSMA/CA protocols. The resulting CSMA/CA
schemes maintain interoperability with the original ones.
The performance of the improved algorithm is shown by
means of a mathematical model and by simulation.
Index Terms—Broad band Powerline communications,
CSMA, collision probability, backoff algorithm.
I. INTRODUCTION
The development of the smart grid with introduction
of PLC to the existing communication media is highly
supported by the European Union and many state
authorities [1], [2], [3]. Known Broadband PLC (BB-
PLC) protocols use synchronized CSMA/CA channel
access scheme in the MAC protocol exclusively or in
combination with other channel access schemes, e.g.
HomePlug AV [12] and G.hn [13] use a combination
of TDMA and CSMA/CA, HomePlug GreenPHY [12]
- only CSMA/CA, IEEE P1901 [14] - TDMA and
CSMA/CA. Therefore the improvement of CSMA/CA
channel access scheme is a key to increase the per-
formance of PLC networks. The main functional unit
of CSMA/CA is the backoff algorithm, which was
already studied in numerous works [4]–[10]. These
improvements concern the computational steps of min-
imal and maximal contention window (CW) width.
Nevertheless the backoff counter is always considered
to be generated based on a uniform distribution from
the range of values in [1, CW ]. This paper investigates
the application of non-uniform distributions for backoff
counter generation, which shows a considerable per-
formance gain of the synchronized CSMA/CA based
protocols.
In section II the general description of the proposed
improvements on backoff algorithm performance is
given. Section III describes possible ways for collision
probability reduction in CW. Section IV describes
the simulation model for comparison of the observed
backoff algorithm modifications. Section V discusses
the obtained simulation results. The conclusions sec-
tion summarizes the application area of the proposed
improvement idea.
II. COLLISION PROBABILITY FUNCTION
The performance of the CSMA/CA protocol can be
enhanced with application of certain algorithms that
minimize the collision probability in the contention
window. In accordance to our approach the collision
probability is investigated as a function of the backoff
generation distribution. A collision occurs if two or
more nodes (hereinafter the communication modem
is referred to as the node) generate the same value
of backoff t ∈ [1, CW ] and this value occurs to be
minimal (t ≡ Back
min
) in current contention window.
Below we define the collision probability (P
C
) for
Back
min
∈ [1, CW ]:
P
C
(t, CW, n) = P (A|B), (1)
where
• A – event that in time slot t more than one
node finishes its backoff counter;
• B – event that in interval [1, t − 1] no one
node finishes its backoff counter;
• n – number of nodes that attempt to access
the channel in the CW.
The event B consists of possible events B
i
. Each
event B
i
interprets the case that a node (independently
from other nodes) does not finish its backoff counter
exactly in slot i, independently of events in all other
slots. Probability of event A is binomial distributed
and the success event can be denoted as
B
i
. With such
approach the events A and B can be considered as
independent. Taking this into regard:
P
C
(t, CW, n) =
P
n
i=2
(1 − P (B
t
)
i
P (B
t
)
n−i
)
×
×
Q
t−1
i=1
P (B
i
).
(2)
2013 IEEE 17th International Symposium on Power Line Communications and Its Applications
978-1-4673-6016-6/13/$31.00©2013 IEEE