Analysis of Multi-Hop Traffic Grooming in WDM Mesh Networks Wang Yao * , Gokhan Sahin † , Mengke Li * , and Byrav Ramamurthy * * Dept. of Computer Science and Engineering University of Nebraska-Lincoln Lincoln, Nebraska 68588–0115, USA Email: {wyao, mli, byrav}@cse.unl.edu † Dept. of Electrical and Computer Engineering Miami University Oxford, Ohio 45056, USA Email: sahing@muohio.edu Abstract— Traffic grooming is an essential functionality of WDM optical networks to provision multi-granularity subwave- length connections. Depending on the number of lightpaths allowed in a connection route, traffic grooming can be classified as single-hop traffic grooming (SH-TG) and multi-hop traffic grooming (MH-TG). MH-TG is more general and resource- efficient than SH-TG, because it allows connections from different source-destination pairs to share the bandwidth of a lightpath. In this paper, we propose a MH-TG algorithm, namely the fixed-order multi-hop (FOMH) grooming algorithm, based on the fixed-alternate routing approach. We introduce the grooming node selection (GNS) problem in MH-TG and propose three grooming policies, namely exhaustive sequential (ES), limited-hop sequential (LHS) and load sharing (LS) policies, to address the GNS problem. Given that the analysis of MH-TG is a relatively unexplored area, we propose an analytical model to evaluate the blocking performance of MH-TG using FOMH and the LS grooming policy. To address the multi-layered routing and multi- rate connection characteristics of traffic grooming, we introduce a novel multi-level decomposition approach in our analytical model which decomposes traffic at four different levels, namely alternate path, connection route, lightpath and link levels. The Erlang fixed-point approximation method is used to solve the analytical model. Numerical results show that analytical results matches well with simulation results. We also evaluate the effect of the grooming policies, the number of virtual hops (lightpaths) within a connection route and the number of alternate paths on the performance of the grooming algorithm. I. I NTRODUCTION THE rapid increase of the Internet demands large vol- umes of bandwidth. Wavelength division multiplexing (WDM) technology has the potential to meet this need by allowing simultaneous transmission of traffic on multiple wavelengths in a fiber. A wavelength-routed network (WRN) based on WDM technology is deemed as a promising candidate for the core network of the next-generation Internet. Traffic grooming addresses the gap between the bandwidth capacity of wavelengths and the bandwidth requirement of connections. With the advances in optical technology, the capacity of a single wavelength has reached OC-192 (10Gbps). On the other hand, the bandwidth of a connection request (such as SONET circuits, IP/MPLS label switched paths) OXC DXC DXC OXC DXC OXC DXC OXC Node 1 Node 2 Node 3 Node 4 L1 L2 C1 Fiber Transceiver Lightpaths: L1, L2 Connection: C1 Fig. 1. Illustration of lightpaths and a connection in traffic grooming. Lightpath L 1 traverses fiber links (1,2) and (2,3), lightpath L 2 traverses fiber link (3,4), and connection C 1 uses a two-hop path including L 1 and L 2 . may be less than that, possibly OC-3 (155Mbps) or even lower. To make efficient use of the wavelength bandwidth, traffic grooming [1]-[6] is needed to pack connections at sub- wavelength granularities effectively onto wavelength channels. In a WRN, the physical topology is a set of OXC nodes connected by fiber links. A wavelength path is referred to as a lightpath, which may span several fiber links in the physical topology. A lightpath uses a wavelength on each fiber link along its path. All the lightpaths form the virtual topology. The multi-granularity subwavelength connections are carried over the virtual topology. A connection may traverse several lightpaths along its path and uses a portion of the bandwidth of each lightpath it traverses. Fig. 1 illustrates two lightpaths and a connection in a SONET over WDM optical network. Note that a connection must originate and end in the electronic domain, which is at the digital cross-connect (DXC) in this case. In IP over WDM networks, the DXCs in Fig. 1 are replaced with IP/MPLS routers. Traffic grooming can be classified as single-hop traffic grooming (SH-TG) and multi-hop traffic grooming (MH-TG). SH-TG restricts a connection to use a single lightpath. There- fore, a lightpath can only be used by connections belonging to the same source and destination pair. MH-TG, on the other hand, allows a connection to use multiple lightpaths. Thus, the bandwidth of a lightpath can be shared by connections from different source and destination pairs. MH-TG is the general case of SH-TG. It is more resource- efficient than SH-TG. For instance, to satisfy three connections 177 0-7803-9277-9/05/$20.00/©2005 IEEE