Author Topic: Dynamic Contention Resolution Protocol for OBS Networks  (Read 2632 times)

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Dynamic Contention Resolution Protocol for OBS Networks
« on: December 13, 2011, 09:07:43 am »
Author : Wael Hosny Fouad Aly, Halima El-Biaze
International Journal of Scientific & Engineering Research Volume 2, Issue 11, November-2011
ISSN 2229-5518
Download Full Paper : PDF

Abstractó Optical burst switching (OBS) is a switching technique that was proposed as a hybrid switching technology to support the next generation Internet. In OBS, incoming IP packets are assembled into super-sized packets called data bursts. Burst contention is a well-known challenging problem in Optical Burst Switching (OBS) networks. Burst contention can be resolved using several approaches, such as wavelength conversion, buffering based on fiber delay line (FDL) or deflection routing Retransmission technique is used to reduce the Burst Loss Ratio (BLR) by deflecting dropped bursts. Segmentation also resolves contention by dividing the contended burst into smaller parts called segments. Combining deflection routing technique and retransmission technique outperforms both pure deflection and pure retransmission techniques to improve the performance. Previous work uses only static combination of retransmission and deflection of bursts to reduce contention. This paper proposes a dynamic protocol to resolve contention based on combining deflection, retransmission and delaying bursts to improve the OBS performance. Experiments were conducted to test the proposed protocol. The proposed technique was tested on complex models such as NSFNET and COST238 topologies. Results show that the proposed protocol outperforms existing techniques in terms of burst lost ratio.

Index Termsó OBS networks, contention resolution techniques, retransmission techniques. 
Optical Burst Switching (OBS) ‎[1] is a promising technology that is used to handle bursty and dynamic Internet Protocol traffic in optical networks effectively.
In OBS networks, user data that could be sent in terms of IP packets is assembled as a huge segment called data bursts, which is sent using one-way resource reservation. The burst is preceded in time, called offset time, by a control packet, called Burst Header Packet (BHP). BHP is sent on a separate control wavelength and requests resource allocation at each switch.
When the control packet arrives at a switch, the capacity is reserved in the cross-connect for the burst. If the needed capacity can be reserved at a given time, the burst can then pass through the cross-connect without the need of buffering or processing in the electronic domain.
Since data bursts and control packets are sent out without waiting for an acknowledgment, the burst could be dropped due to resource contention or due to insufficient offset time if the burst catches up the control packet. Thus, it is clear that burst contention resolution approaches play an essential role to reduce the Burst Loss Ratio (BLR) in OBS networks ‎[3].
Burst contention can be resolved using several approaches, such as wavelength conversion, buffering based on fiber delay line (FDL) or deflection routing. Another approach, called burst segmentation, resolves contention by dividing the contended burst into smaller parts called segments, so that a segment is dropped rather than the entire burst.

Deflection routing is an attractive solution to resolve the contention in OBS networks since it does not require extra cost in terms of physical components and uses the available spectral domain. However, as the load increases, deflection routing could lead to performance degradation and network instability ‎[4]. Since deflection cannot eradicate the burst loss, retransmission at the OBS layer has been suggested by Torra et al. ‎[5].
Several studies have performed to improve OBS perfor-mance, an implementation of TCP Vegas for OBS was experimented ‎[17]‎[14][16]. A congestion detection scheme for TCP over OBS is studied in ‎[20]‎[10]. Several issues, solutions and challenges related to TCP over OBS networks are studied in ‎[12]. Some collaboration was performed to improve the TCP based on a dynamic approach for contention loss notification‎[2]‎[13]. Different TCP characteristics were studied in ‎[15] and their effect on OBS including a responsive rate control for the TCP when used over OBS network. Improving fairness for optical burst switching networks is studied in ‎[16]. Studying cluster processing for OBS taking into consideration the signal processing receivers are discussed in ‎[18].
A static combination of deflection and retransmission has been proposed by Son-Hong Ngo et. al.‎[6] ‎[6]. They have proposed a Hybrid Deflection and Retransmission (HDR) algorithm ‎[6] which combines deflection routing and retransmission. Simulation results show that HDR gives bad performance in terms of BLR since it first applies deflection even if the load is high. To overcome this shortcoming, Son-Hong Ngo et. al have developed another mechanism called Limited Hybrid Deflection and Retransmission (LHDR) that controls the deflection.

This paper introduces a dynamic protocol called Dynamic Contention Resolution Protocol (DCRP) to combine deflection routing, retransmission or delay dynamically. A decision is made dynamically to select whether to use deflection, re-transmit, or delay of bursts. The decision is based on a local knowledge about network condition. The offset time is also adapted by using an adaptive decision threshold. In order to make the local knowledge feasible, DCRP algorithm exploits sending and receiving of Positive Acknowledgement (ACK) and Negative Acknowledgement (NACK) messages to advertise useful statistics about network conditions stored by all nodes.
This paper is organized as follows. Section ‎2 describes the proposed Dynamic Contention Resolution Protocol (DCRP). Section ‎3 presents the experimental results. Finally, Section ‎4 contains the conclusion and the future work.

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