M OTIVATION AND O BJECTIVES

Numerous topologies and architectures [7-32] for OPS WDM MANs have been proposed in recent years. Of these proposals, the structure of slotted rings [7-28]

receives the most attention. While most of the work [13,14,16-26] is simulation driven, only a handful [7-11,25-32] undertakes experimental prototypes. Two key challenges pertaining to OPS-based WDM networks are the header control and medium access controls. The header control can be in-band [25-32], where both header and payload are modulated and transported via the same wavelength, or out-of-band [7-24], where control headers are carried via a dedicated control wavelength. While both control methods have their merits, from a carrier’s perspective, an out-of-band control system appears impractical due to the additional cost of a fixed transceiver on each node.

In addition, in-band control has several advantages over out-of-band control.

First, in a mesh network, a layer-two wavelength switch can switch the header and payload together to an output port without examining the header. With out-of-band control, complicated control signal processing and routing are needed (e.g., optical burst switching) because the header and payload are carried on different wavelengths.

Second, in-band headers can be used as a performance monitoring signal. Physical impairments on the payload can be monitored by detecting header signal's quality.

Third, because all wavelengths’ control information has to fit within a time slot, out-of-band control requires a costly high-data-rate control wavelength in order to support more channels, resulting in a scalability problem. Thus, this work focuses on

the in-band header control and processing.

There are three basic in-band header control techniques: subcarrier multiplexed (SCM) [25-28,33-50], orthogonal modulation [51-64], and time-domain-multiplexing (TDM) [26-28,65-67]. With the SCM technique, the header information can be carried on a subcarrier frequency that is separated from the baseband payload frequency.

SCM requires stringent wavelength accuracy and stability if a fixed optical notch filer (e.g., a fiber Bragg grating) is used to remove the header at each node. Most traditional SCM methods cannot potentially scale up well with the payload data rate because an expanding baseband may eventually overlap with the subcarrier frequency.

The optical carrier suppression and separation (OCSS) technique [33], however, was shown to be able to generate the header at very high subcarrier frequency and with high bit rate and extinction ratio. Nevertheless, SCM still requires stringent wavelength accuracy and stability while using a fixed optical filter to remove the header at each node.

The orthogonal modulation technique, which includes amplitude shift keying (ASK) [38], frequency shift keying (FSK) [54], ASK/differential phase shift keying (DPSK) [55,56], and DPSK/FSK [57], exhibits severe transmission system penalty due to the inherently low extinction ratio of a high-speed payload signal. Finally, with the TDM technique, the header and payload are serially connected in the time domain, interspaced with an optical guard time to facilitate header extraction and modification.

The bit rates of the header and payload can either be the same [65], or different [66,67]. Generally, traditional TDM-based approaches require an extremely precise control timing and alignment to perform header erasing and rewriting operations. The first goal of this work is to propose a simple and highly efficient TDM-based optical header processing scheme. As will be demonstrated, in our system optical headers can also be easily modified by taking advantage of the particularly notable MAC design.

Another key performance-enhancing feature pertaining to OPS-based networks is the design of the medium access control (MAC) mechanism. The MAC scheme should be designed to offer fair and versatile bandwidth allocation, achieving satisfied throughput and delay performance under a wide range of traffic loads and burstiness.

Moreover, the MAC protocol should take into account the scalability problem with respect to the number of wavelengths. While numerous MAC protocols for OPS-based slotted-ring networks have been proposed in the literature [10-14], our second goal is to explore a variant of a reservation-based mechanism, IEEE 802.6 Distributed Queue Dual Bus (DQDB) [72], for the multi-channel WDM metro networks. In single-channel DQDB, each node must issue a reservation request prior to the transmission. To ensure that packets are sent in the order they arrived at the network, DQDB requires each node to maintain a distributed queue via a Request (RQ) and a CountDown (CD) counters. DQDB was shown to achieve superior throughput and delay performance, nevertheless undergoes the unfairness problem due to long propagation delay under heavy traffic conditions.

In this WDM-DQDB line of work, the WDM Access (WDMA) [13] protocol simply extends the basic single-channel DQDB to the multi-channel case, namely, each node maintains a single distributed queue for all of the wavelengths. Due to the use of a tunable transceiver, WDMA adopts the retransmission mechanism if the receiver contention problem occurs. With such a simple design, WDMA unfortunately results in access unfairness and inefficiencies for multi-channel networks under varying traffic patterns and burstiness. The hybrid optoelectronic ring network (HORNET) [10] employs a distributed queue bidirectional ring (DQBR) protocol.

Due to the use of fixed-tuned receivers, HORNET statically assigns each node a wavelength as the home channel for receiving packets. Such static wavelength

assignment results in poor statistical multiplexing gain and bandwidth efficiency. As a result of the home-channel design, DQBR treats wavelengths independently and requires each node to maintain a distributed queue for each wavelength. Moreover, each node is allowed to issue multiple independent single-slot requests. With DQBR, HORNET achieves acceptable utilization and fairness at the expense of high control complexity for maintaining the same number of counter pairs as that of wavelengths.

Such a design gives rise to a scalability problem. Moreover, the design of permitting unlimited multiple requests with single slot granularity per request unfortunately results in unfairness problems.

A novel OPACS (optical-header processing and access control system) for a 10-Gb/s optical packet-switched [26-28] WDM metro ring network is presented in this dissertation. OPACS has two prominent features that set it apart from existing related work. First, OPACS is designed for a dual unidirectional ring network using in-band signaling control. Each control header is time-division-multiplexed with its corresponding data packet within a slot. By making use of signal gating and wavelength-time conversion techniques, OPACS enables the optical headers across all parallel wavelengths to be efficiently received, modified, and re-transmitted. Second, taking diverse traffic patterns and burstiness into account, OPACS employs a variant of the DQDB scheme, referred to as the distributed multi-granularity and multi-window reservation (DMGWR) scheme. By “multi-granularity”, DMGWR permits each node to reserve different amounts of bandwidth (slots) at a time. By

“multi-window”, DMGWR allows each node to have multiple outstanding reservations within the window size. From numerical results that pit the DMGWR network against two other existing networks (WDMA-based and HORNET), we show that the OPACS network outperforms both networks with respect to throughput, access delay, and fairness under various traffic patterns. Experimental results

demonstrate that all optical headers are removed and combined with the data in a fully synchronous manner, justifying the viability of the system.