2094 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 10, OCTOBER 2000 The Multitoken Interarrival Time (MTIT) Access Protocol for Supporting Variable Size Packets Over WDM Ring Network James Cai, Student Member, IEEE, Andrea Fumagalli, Member, IEEE, and Imrich Chlamtac, Fellow, IEEE Abstract—One of the approaches currently pursued to provide an optical network architecture for supporting the next generation Internet consists of transmitting the Internet protocol (IP) packets directly over the wavelength division multiplexing (WDM) layer. The challenge of this approach is to make the optical bandwidth directly accessible to IP with the statistical multiplexing necessary to support the bursty nature of IP traffic in a cost effective way. This paper proposes a WDM ring architecture with a mul- titoken interarrival time (MTIT) access protocol designed to achieve a bandwidth efficient multiplexing technique in local and metropolitan network applications. The MTIT protocol achieves efficient multiplexing of all-optically transmitted packets by means of multiple control tokens whose rotation speed along the ring is regulated via the “token interarrival time,” as opposed to the “token rotation time” adopted in existing standards. The proposed WDM architecture and protocol provide a unique medium access control that allows one to maintain the channel access delay at a node within a limited range of values, possibly reaching average values that are below the roundtrip propagation time of the ring. In the paper it is shown that the MTIT protocol efficiency grows with the number of wavelengths—in agreement with today’s WDM technology trends—and it is not a function of the packet size—a mandatory feature to support IP traffic. Index Terms—Access protocols, token networks, wavelength di- vision multiplexing. I. INTRODUCTION A MONG TODAY’S available telecommunication tech- nologies, optical networking is the only solution that can support the fast growth of bandwidth demand occurring in the Internet. The number of channels provided in a single fiber has been increasing dramatically over the past few years, with this trend possibly continuing in the foreseeable future. However, most of the current optical networking devices are applied to provide transmission capabilities, while (packet) switching and multiplexing functions are still realized in the electronic do- main. Due to the huge difference of processing speed between electronics and optics, and the electronic multilayer approach of extant implementations, digital switches and routers have become a potential bottleneck in the optical network, strongly limiting the practical availability of bandwidth to the end user. Manuscript received November 11, 1999; revised May 15, 2000. This work was supported by NSF under Contracts NCR-9628189 and NCR-9596242. The authors are with the University of Texas at Dallas, Erik Jonsson School of Engineering and Computer Science, Center of Advanced Telecommunications Systems and Services, Richardson, TX 75083-0688 USA (e-mail: {jcai, andreaf, chlamtac}@utdallas.edu). Publisher Item Identifier S 0733-8716(00)09018-1. The primary objective in designing modern optical networks is thus to remove the electronic bottleneck caused by digital switches/cross-connects/routers. In this endeavor, the network designer is facing two technical challenges: reducing the complexity of multilayer architectures and, where possible, providing all-optical packet switching. The first challenge consists of efficiently supporting packet switching, an intrinsic feature of the Internet protocol (IP), directly over the wavelength division multiplexing (WDM) [11] layer. Although SONET/SDH and ATM [8] provide a basic infrastructure for network management and quality of service (QoS) support, they were not specifically designed to efficiently support IP traffic, mainly because of the fixed length of SONET/SDH frames and ATM cells. SONET/SDH framing is based on an 8 kHz voice synchronized time sample and the frame embeds the header information within the payload. One of the main benefits of SONET/SDH over the past decade has been its fixed time division multiplexing (TDM) capability to groom tributary traffic ranging from slow to high rate connections. Nowadays,similar grooming capabilities are, however, provided also by emerging gigabit switches and routers. ATM cells were introduced to provide statistical TDM in support of broadband integrated services. Due to the relative high header-to-payload ratio of the short cell, the ATM bandwidth overhead is, however, a practical concern in several applications. In addition to the overheads of a multilayer architecture, in terms of both band- width efficiency and the equipment cost of each layer, actual ATM and SONET/SDH equipment allows one to provision only fixed TDM. Statistical multiplexing of traffic remains thus a task for the upper layers, e.g., IP. The straightforward advantages of interfacing IP directly over WDM, as opposed to extant solutions that rely on intermediate layers such as SONET/SDH and ATM, include overall reduction of equipment cost and management complexity, as well as improved bandwidth efficiency. The second technical challenge consists of achieving all-op- tical packet switching, i.e., completely removing the electronic processing of in-transit packets at the intermediate nodes. Cur- rently, optical packet switching and buffering technology is still in its experimental phase [21], [22] and there are not clear signs that it will become of practical use in the near future. Elec- tronic switching and multiplexing thus remain an essential in- gredient in designing current networks and cannot be altogether eliminated from wide area networks (WANs). Hence, at least in the near future, WANs will have to rely on both electronic packet switching and optical circuit switching to provide the full range of connectivity and services [25]. The reduced topology 0733–8716/00$10.00 © 2000 IEEE