158
Broadband Powerline Communications Networks
Time slots
CW
1
CW
2
CW
3
CW
n
t
Figure 5.32
Principle of dynamic backoff mechanism
Dynamic Backoff Mechanism
The application of dynamic backoff mechanisms is a very simple method for the reduction
of the collision probability, which is also applied in IEEE 802.3 Ethernet-LAN standard.
The principle of the dynamic backoff mechanism can be explained as follows: after a first
unsuccessful transmission (collision), the affected network station sets a contention win-
dow (CW) on a default value (e.g. CW 1, Fig. 5.32). The retransmission of the collided
packet will be carried out in a randomly calculated moment within the CW. If the retrans-
mission is also unsuccessful, the CW is increased and a time for the next retransmission
is calculated within the new CW. This procedure is repeated until the packet is success-
fully transmitted. The transmission of a new packet starts again with the default CW,
or with the last used CW, which depends on the specific variant of an applied dynamic
backoff mechanism.
The increase of the contention window reduces the collision probability, because the
probability that two or more network stations transmit at the same time slot decreases with
the increase of the CW. Even in the case that a higher number of stations are currently
retransmitting the packets (backlogged), the contention window can be increased so far
that the collision probability becomes very small.
The increase of the CW can be carried out according to the exponential backoff mecha-
nism, for example, as described in [Walke99], or any other algorithm. In accordance with
the exponential backoff mechanism, the access to the channel is controlled by an access
probability for each network station. The access probability is determined as
p
= 2
-i
,
where
i is number of collisions for a data packet. Thus, for each retransmission attempt,
the access probability is equally distributed within a time slot interval [1, 2
i
], representing
a contention window CW
i
(Fig. 5.32). In this way, the contention window is extended
for every increase of variable
i, representing a new packet collision.
The application of the dynamic backoff mechanism stabilizes random protocols and
avoids the performance collapse in highly loaded networks. However, the maximum net-
work utilization is not significantly increased. On the other hand, the increase of the
contention window causes longer transmission delays. Therefore, there are some limits
for a maximum CW regarding the transmission delays. Accordingly, realization of QoS
guarantees for time-sensitive telecommunications services seems to be difficult as well.
Calculation of Optimal Retransmission Probability
A further possibility for the collision resolution is the calculation of the transmission/re-
transmission probability for the packets in accordance with the current load situation in
the network. It can be carried out by an estimation of the backlog number of collided
packets and the calculation of an adequate transmission probability to avoid the col-
lisions. Several stabilization algorithms are described in [Walke99]; for example, the