Annual Report 2010 - Fachgruppe Informatik an der RWTH Aachen ...

Designing and analyzing communication protocols under the goal of maximizing network
lifetime has become increasingly popular and many classical problems like broadcast, data
gathering, or scheduling have been examined in this model. However, previous theoretical
analyses assume perfect knowledge about ideal batteries with linear charge/discharge
properties. Real-world battery characteristics (like e.g. rate capacity and recovery effects) are
not taken into account.
In our model we allow for a greater versatility in battery characteristics by assuming only
very limited knowledge about battery capacities. Every battery has a given maximum
capacity, but when discharging a battery by one unit of energy it loses a variable amount of its
capacity. This amount is allowed to change in an online fashion every time the battery is used.
We are only given a bound on the allowed interval.
This project aims to design efficient algorithms that achieve best possible competitive-ratios
against a worst-case opponent. Although competitiveness in this online setting can be
severely bounded from below, algorithms with non-trivial competitive-ratios compared to the
optimal offline algorithm are possible.
Competitive Buffer Management for QoS Switches
K. Al-Bawani
joint work with A. Souza (Humboldt University of Berlin)
In this project, we design and analyze online algorithms for the problem of buffer
management in QoS networks. In this kind of networks, each data packet is guaranteed a level
of service corresponding to its class of service (CoS). This concept of quality of service (QoS)
is abstracted by attributing each packet with a non-negative value that represents its CoS, such
that a higher value means higher priority of transmission.
We consider a basic model of a network switch that consists of multiple buffers (queues) and
one output port. Packets, each with a certain value, arrive at the input ports of the switch, are
temporarily stored in the queues, and are eventually extracted from the queues and sent out of
the switch. Queues are of limited capacity, and packets are sent in the order of their arrival
(FIFO). Furthermore, at each time step, only one packet can be transmitted through the output
port. Consequently, in cases of congestion, queues may overflow and thus some arriving
packets are lost. Our problem arises from the need to decide which packets to insert into the
queues and which ones to drop, so that the total value of the transmitted packets (i.e., the
throughput) is maximized. Moreover, at times of transmission, we would also like to decide
which queue to serve.
Recently, we studied a variant of the multi-queue model based on the concept of class
segregation: Given m packet values, the switch consists of m queues, such that each queue
stores packets of only one value. We analyzed a natural greedy algorithm that accepts packets
as long queues are not full, and always serves the non-empty queue of the highest packet
value. We showed an upper bound of 2 on the approximation ratio of this algorithm, i.e., for
any sequence of packets, the optimum throughput is at most 2 times the throughput of the
greedy algorithm.
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