SYSTEM SIMULATIONS ICTS2005 The Proceeding
Information and Communication Technology Seminar, Vol. 1 No. 1, August 2005
ISSN 1858-1633 2005 ICTS 142
standardization is to include in push-MAs a few bytes for standardization fields so that all providers, if they
desire, can find out what the others are doing. In doing so, it would regulate fairness in the faded information
field.
Therefore, some algorithms are required to choose nodes for the faded information field. In all the
following algorithms, it is assumed that the service provider is aware of all the nodes in the field, in
addition to the costs to travel to these nodes. This can be achieved by simply sending a message to any node
in the field asking for the latest information about the field. The costs are used as a measure of the distance
to the node for the purpose of creating a localized faded information field.
• Sorting – The available nodes in the field are sorted in an order by a certain cost, such as delay by
each service provider. For a field size of S nodes, it will choose the first S nodes that are closest to it in the
sorted list. It can thus determine the delay to the furthest node it requires, and imposes a travel
restriction on its push-MAs based on this delay. The push-MAs will be required to keep updating the nodes
in the field until they reach the travel restriction set by the SP. This could be in the form of travel time, or
time to live after which the MAs cease to exist. Alternatively the push-MAs can be multicast to the
required nodes in the field. The sorting technique is inherently resource intensive and it must be re-
employed to account for cost updates in the network as and when it occurs. However, the advantage is that
all the SPs are expected to have the same number of nodes in the field, since each selects the same number
of nodes from the sorted lists. • Step Size – In this case, the service provider
determines the distances of the closest and furthest nodes, and then divides that distance by the number of
nodes to get the average inter-nodal ‘hop’ distance of a push-MA. The equation for the algorithms i as
follows:
D = maxd – mind N – 1 S + mind Where D
= step
distance
d = distance from SP to node N = number of nodes
S = required field size The SP will now send push-MAs that only travel the
required distance in the field. Alternatively, the server would determine which nodes fall in the required
distance and multicast the push-MAs to those nodes. The step size must be recomputed every time there is
an update in the system; however, finding the maximum and minimum distances in the field is the
major computation, and is much faster than sorting. • Averaged Step Size – The inter-nodal distances
are likely to be skewed towards the high end in a random simulation run. In order to account for this
skewing an average distance is computed that is used to determine the step size instead. In this method, the
determination of the maximum distance is replaced by finding the total distance to all the nodes from the SP.
D = ∑d N – 1 – mind N – 1 2
S + mind Depending on geographical circumstances, an SP can
be far away from the rest of the nodes in the FIF. In this case, the computation gives an unbalanced result
because the initial distance is large. So the minimum distance is subtracted from the average to find the
inter-nodal step distance, and then added again at the end to account for the time from the SP to the first
node
• Modified Averaged Step Size – The averaged step size algorithm still gets affected by the skewing
of nodes. The algorithm was modified by computing the average step size of all the costs excluding the
minimum and maximum costs. In this method, the total distance has to be computed and the maximum
and minimum distance from the SP has to be determined. The maximum and minimum are
subtracted from the total as shown below:
D = ∑d – Nmin mind – Nmax maxd
N – 1 – Nmin – Nmax – min2d N – 1 – Nmin – Nmax 2
S – Nmin + min2d where
Nmin = number of nodes at the minimum distance
Nmax = number of nodes at the maximum distance
min2d = distance of the second closest node As mentioned in the Averaged Step Size algorithm, it
is possible that a node is very far away from the rest of the field. In order to remove this initial distance, the
minimum distance should be subtracted. However, in the Modified Averaged Step Size algorithm, the
minimum distance is not included in the computation; so instead, the distance that is second from minimum
is subtracted instead.