Optical network design with mixed line rates

Abstract

Future telecommunication networks employing optical wavelength-division multiplexing (WDM) are expected to be increasingly heterogeneous and support a wide variety of traffic demands. Based on the nature of the demands, it may be convenient to set up lightpaths on these networks with different bit rates. Then, the network design cost could be reduced because low-bit-rate services will need less grooming (i.e., less multiplexing with other low-bit-rate services onto high-capacity wavelengths) while high-bit-rate services can be accommodated on a wavelength itself. Future optical networks may support mixed line rates (say over 10/40/100 Gbps). Since a lightpath may travel a long distance, for high bit rates, the effect of the physical impairments along a lightpath may become very significant (leading to high bit-error rate (BER)); and the signal’s maximum transmission range, which depends on the bit rate, will become limited.

In this study, we propose a novel, cost-effective approach to design a mixed-line-rate (MLR) network with transmission-range (TR) constraint. By intelligent assignment of channel rates to lightpaths, based on their TR constraint, the need for signal regeneration can be minimized, and a “transparent” optical network can be designed to support all-optical end-to-end lightpaths. The design problem is formulated as an integer linear program (ILP). A heuristic algorithm is also proposed. Our results show that, with mixed line rates and maximum transmission range constraints, one can design a cost-effective network.

Introduction

Traffic flowing through optical backbone networks–which typically employ wavelength-division multiplexing (WDM)–has been increasing steadily, and becoming more heterogeneous as well. As a result, a future-proof optical network needs to be designed which can support mixed line rates (MLR) over different wavelength channels. In such a network, a low-bit-rate service may need minimal or no grooming (i.e., less multiplexing with other low-bit-rate services onto high-capacity wavelengths), while a high-bit-rate service can be set up over a single wavelength [1]. Thus, a MLR network can be a cost-effective heterogeneous optical network design.

In optical WDM backbone networks, MLR can be facilitated by having different sets of wavelengths that can support different rates. Thus, the routing and wavelength-assignment (RWA) problem is modified to the routing, wavelength, and rate assignment (RWRA). In [2], the design of an opaque MLR network is proposed, where each node has electronic regeneration (which can also support wavelength conversion, grooming, etc.). This work also assumed that all wavelengths on a link run at the same rate, but different links have different rates. Based on the distance it needs to travel, a lightpath is routed in such a way that it requires minimum regeneration.

In this work, (1) we consider a more general approach where a single link can have a combination of bit rates, and each physical link in the network may support a combination of bit rates, each on a separate wavelength, and (2) our aim is to design a transparent MLR network that will significantly reduce the amount of electronic signal processing at the nodes.

Note that the maximum transmission distance of a signal decreases with increasing bit rate based on a threshold bit-error rate (BER) [3]. If we know the network topology (including fiber link lengths) and the traffic demands that need to be carried by it, then we could determine the physical length of each lightpath based on a routing algorithm; then, based on the set of line rates available, we can choose only those paths whose maximum transmission distance is less than the physical length of the route. Hence, it may be beneficial to support different lightpaths at different rates, leading to a MLR network. Such a network is called “transparent” if its end-to-end traffic demands flow over all-optical lightpaths with no electronic regeneration.

In this study, we propose and investigate the characteristics of a method to design a transparent MLR network. We compare our design with single-line-rate (SLR) networks (where all wavelength channels run at the same bit rate), and we study the corresponding cost savings on network design, measured by the cost of line cards. In this context, the maximum transmission range (TR) could make some of the high-bit-rate paths infeasible and we may lose out on the volume discount that a high-bit-rate path provides over several low-bit-rate paths. Thus, placement of signal regenerators at a few nodes may improve the cost scenario as studied in our related work [4].

The rest of the paper is organized as follows. In Section 2,¹ we discuss how the BER is estimated based on the physical layer impairments. Section 3 presents our mathematical formulation of the design problem which turns out to be an integer linear programme (ILP). In Section 4, a heuristic algorithm for the cost-effective MLR network design is presented. Section 5 provides illustrative results. Finally, Section 6 concludes the paper.