O PTICAL WDM N ETWORKS - 全光分波多工封包交換都會環狀網路之高效能服務品質保證媒介存取控制技術

CHAPTER 1. INTRODUCTION

1.1 O PTICAL WDM N ETWORKS

1.1.1 An Overview

For long-haul backbone networks, optical wavelength division multiplexing (WDM) [1,2] has been shown successful in providing virtually unlimited bandwidth to support a large amount of steady traffic based on the optical circuit switching (OCS) paradigm. Future optical metropolitan area networks (MANs) [3,4], on the other hand, are expected to cost-effectively satisfy a wide range of applications having time-varying and high bandwidth demands and stringent delay requirements.

Nevertheless, today’s metropolitan area networks are mostly SONET/SDH ring networks. These networks are circuit-switched networks. The SONET/SDH technology offers data transmission only at specific rates from a prescribed set of rates. The main drawback of SONET/SDH networks is that due to their time-division multiplex operation in conjunction with a circuit set-up time on the order of several weeks or months [5], they accommodate packet traffic only inefficiently [6], especially when the traffic is highly variable, giving rise to the so called metro gap.

Such facts bring about the need of exploiting the optical packet-switching (OPS) [4,7,8] paradigm that takes advantage of statistical multiplexing to efficiently share wavelength channels among multiple users and connections. Note that the OPS technique studied here excludes the use of optical signal processing and optical buffers, which are current technological limitations OPS faces. Numerous topologies and architectures [3,4,7-16] for OPS-based WDM metro networks have been proposed. Of these proposals, the structure of slotted rings [9-16] receives the most attention. Essentially, these slotted-ring networks offer high-performance access and

efficient bandwidth allocation by means of medium access control (MAC) [17-20]

schemes.

Regarding the design of the WDM networks, we first consider two of the important issues: node architectures [21,22] and bandwidth reuse [14,16]. In the WDM networks, the nodes are equipped with number of transmitters and receivers to transmit and receive data. The transmitters and receivers are either fixed-tuned to a particular wavelength (denoted as FT/FR) or tunable to any wavelength (denoted as TT/TR). The systems are first designed by a non-scalable architecture which is equipped with the same number of FT/FR as that of the wavelengths [23]. The main advantage of this system is that concurrent transmissions on distinct channels are possible at a given node. While this architecture requires as many wavelengths as there are nodes in the network, and this severely limits the scalability of such a network. Further, the nodes are further designed with advanced optical devices, such as TT-FR and TT-TR structures. Systems based on TT-FR is still incurred a scalability problem, since each node or a group of nodes is assigned a home channel to receive data. Once there is no data to transmit to a particular node, the bandwidth of its home channel is then wasted. Except the throughput degrades due to the static assignment (poor statistic multiplexing gain), the maximum number of nodes is also limited by the number of available channels. While systems based on the TT-TR structure are the most flexible in accommodating a scalable user population but with a most challenging issue in designing and implementing a high-speed photonic hardware component (TR).

We further observe that a ring network with spatial bandwidth reuse achieves much better throughputs than in star topology [24,25], where bandwidth reuse is not possible. Indeed, the advantage of spatial bandwidth reuse is one of the main reasons why the structure of slotted rings receives the most attention. Generally, the spatial

reuse includes source- and destination-stripping schemes. In the case of source-stripping operation, the transmitting node is responsible for marking the slot empty after it has completed an entire ring loop. With destination stripping, the destination node receives the packets and removes them from the ring, making the slot reusable earlier than in the previous scheme. The network capacity of unidirectional ring networks can be increased with destination stripping where multiple simultaneous transmissions can take place on each wavelength. For uniform traffic, the mean distance between source and destination is half the ring circumference. As a consequence, two simultaneous transmissions can take place at each wavelength on average, resulting in a network capacity that is twice as large as that of unidirectional rings with source stripping. However, in this thesis, we propose a new notion which is referred to as server-stripping. Only a few numbers of nodes in the network is capable of removing the data from the ring. The associated network architecture will be shown to be most cost-effective for bandwidth reuse.

1.1.2 Existing MAC Schemes on Single-Channel Rings

Before assessing the OPS WDM slotted-ring networks, we first examine some formerly proposed MAC schemes for ring networks. These schemes can generally be categorized as quota-based or rate-based. In the quota-based schemes, each node is allocated a quota that is the maximum transmission bound within a variable-length cycle. Most of the research work focuses on the dynamic adjustment of the cycle length. In the following, we introduce two of the well-known quota-based schemes:

ATMR [3,26] and MetaRing [3,27]. And, we also introduce a rate-based scheme: RPR (IEEE 802.17 Resilient Packet Ring) [28].

The ATMR protocol adopts a quota-based scheme on single/dual- ring network.

It provides fairness control with a cycle reset mechanism. The mechanism allocates

each node a maximum transmission bound (quota) within a cycle, and it re-starts a new cycle by sending a reset signal from the last active node. If the last active node detects inactivity of all other nodes, it generates a reset which is sent to all nodes as soon as the node itself stops sending. Monitoring of inactivity is performed as each active node overwrites a busy address field in the header of each cell with its own address. So any node which receives a slot with its own busy address assumes that all other nodes are inactive because none of them has overwritten the field. The reset is responsible for the distributed fairness control and causes a node to set up its window counter to the initial window size. The counter is decremented each time the node fills a free slot with data. By counting it down to zero it is guaranteed that within a reset period, i.e. the time between two consecutive resets, each station uses a maximum number of cells. As the window counter expires, the node is forced into the inactive state. In this state it cannot send any data until the next reset activates once more. If a station has no more data to send the node will pass over to the inactive state, but it may become active again without receiving the next reset on arriving data at the transmit queue. The primary disadvantage of this scheme is that a node cannot send any packet before receiving the reset signal. In other words, there is an idle gap between two consecutive reset periods. Therefore, the bandwidth is waste and system utilization downgrades. Another disadvantage is the determination of the value of quota, which relates to the network throughput and the maximum delay time. Since the reset signal has to run at least one round trip time, the quota can not be set too small causing the maximum delay time is above one round trip time.

MetaRing deploys a quota-based fairness scheme on dual-ring network. This mechanism works with a hardware control message, called SAT-signal. This is very short, and on a dual counter rotating ring it circulates in the opposite direction to the data which it controls. The signal has preemptive resume priority, i.e. at any time it

can be inserted into the data flow. If a station gets the SAT-signal and it is satisfied, it sends the signal immediately to the neighboring node. Otherwise it keeps the signal until it becomes satisfied. A node can transmit its local traffic whenever it has not exhausted its quota. When sending the signal to the neighboring station, the slot counter is reset to zero. That is, the quota of a node is renewed every time SAT-signal visits the node. The major drawback of this global fairness is that quotas can only be renewed when a node receives SAT-signal, and which may need several of ring times depending on the value of quota. Therefore, the maximum access delays are within the order of round trip times. When the ring network is overloaded, the access delays seen by each node will oscillate between zero and the maximum value depending on when a packet comes in relative to the recent SAT-signal visit.

The standard, IEEE 802.17, Resilient Packet Ring (RPR) deploys a rate-based fairness algorithm. Current RPR networks are single-channel systems (i.e., each fiber carries a single wavelength channel) and are expected to be primarily deployed in metro edge and metro core areas. It adopts destination stripping enables nodes in different ring segments to transmit simultaneously, resulting in spatial reuse and increased bandwidth utilization. RPR provides a three-level class-based traffic priority scheme. As a rate-based MAC, an RPR station implements several traffic shapers to smooth and control the rate of each traffic class. The three-level classes: class A (divided into A0, A1) for a low-latency low-jitter class, class B (BCIR, B-EIR) for a class with predictable latency and jitter, and class C be a best effort transport class.

The two traffic classes C and B-EIR are called fairness eligible (FE), because such traffic is controlled by a fairness algorithm. The shapers for classes A0, A1, and B-CIR are preconfigured; the bandwidth for class A0 is called reserved. And, the downstream shaper, set to the unreserved rate (other than class A0), ensures that the total transmit traffic from a station does not exceed the unreserved rate. While the FE

shaper is dynamically adjusted by the fairness algorithm for control class B-EIR and class C. RPR also includes a local fairness algorithm to solve the unfairness among the contending stations.

In summary, ATMR allows the last active node to initialize a reset-signal rotating on the ring to inform all nodes to re-start a new cycle. MetaRing uses a token-based signal circulating around the ring. When a node receives the token, it either forwards the token and thus starts a new cycle immediately, or holds the token until the node has no data to send or the quota of previous cycle expires. These schemes were shown to achieve high network utilization and great fairness. However, they cause cycle lengths to prolong several ring times, resulting in a large maximum delay bound and delay jitter, and thus poor bursty-traffic adaptation. In the rate-based schemes, RPR (IEEE 802.17 Resilient Packet Ring) is based on a pre-determined leaky bucket rate to transmit data, in combination with a local-fairness algorithm to resolve the potential congestion problem. Comparing with the quota-based schemes, the rate-based scheme was shown to reduce the maximum delay bound [29]. However, the leaky rate is modified only after receiving the feedback from the downstream nodes when congestion occurs. As a result, due to using the pre-determined rate and the slow response to rate changes, the scheme yields poor statistical multiplexing gain and dissatisfying delay-throughput performance especially under the high-burstiness fluctuating traffic condition. The goal of this thesis is to present a quota-based MAC scheme that tackles the performance problem from a perspective of the determination of the quota rather than the cycle length.

1.1.3 A Survey on WDM Ring Networks

There have been numerous OPS WDM slotted-ring networks proposed in the literature [3]. In the following, we assess three well-known prototyping networks that

are most relevant to our work. First, Hybrid Optoelectronic Ring NETwork (HORNET) [9] is a bi-directional WDM slotted ring network in which each node is equipped with a tunable transmitter and a fixed-tuned receiver. It employs a MAC protocol, called Distributed Queue Bidirectional Ring (DQBR), which is a modified version of IEEE 802.16 Distributed Queue Dual Bus (DQDB) protocol [30]. DQBR requires each node to maintain a distributed queue via a pair of counters per each wavelength to ensure that packets are sent in the order they arrive at the network.

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. Moreover, due to the use of fixed-tuned receiver, 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 thus throughput deterioration.

The second prototyping network, called Ring Optical Network (RingO) [10], which is a unidirectional WDM slotted ring network with N nodes where N is equal to the number of wavelengths. Each node is equipped with an array of fixed-tuned transmitters and one fixed-tuned receiver operating on a given home wavelength that identifies the node. Such a design gives rise to a scalability problem. RingO employs a MAC protocol, called a synchronous round robin with reservations (SR³) [11], which is a combination of the synchronous round-robin (SRR), token-control quota based (Multi-MetaRing), and slot-reservation mechanisms. The scheme was shown to achieve high utilization and fairness. As for the fairness-scheme, Multi-MetaRing, it inherits all the pros and cons from the MetaRing. Specifically, there are W numbers of tokens rotating on W wavelengths. The scheme encounters an additional problem in which a node may hold several tokens at the same time due to the fact that only one data packet can be sent per slot time. The problem results in an increase in access

delay and throughput degradation.

The metro network of the European IST Data And Voice Integration over DWDM (DAVID) [12,13] attempted to address the overall efficiency of ring-to-ring traffic, and fairness and QoS control inside a metro ring. DAVID is structured to be comprised of several independent fiber rings interconnected via a buffer-less SOA-based packet switch, i.e., the hub node. The hub node is responsible for forwarding data packets among different rings of the network in the optical domain via an available wavelength. Due to having multiple rings, the hub requires each node to make slot reservation prior to the transmissions and has to resolve a feasible wavelength-to-wavelength permutation [15] at all times. Within each ring, the Multi- MetaRing scheme is employed to ensure the fairness control. Each active node is equipped with a tunable transmitters, a tunable receiver, and an SOA-based slot eraser, enabling high slot reuse but at the expense of prohibitive system cost.

Note that both RingO and DAVID adopt Multi-MetaRing as their fairness control scheme. Recall that MetaRing is a quota-based scheme, thereby most relevant to our work. In MetaRing, a control message SAT-signal (which stands for SATisfied) rotates around the ring, and the quota of a node is renewed every time SAT-signal visits the node. In Multi-MetaRing, it is simply designed by independent multiple MataRing with one separate SAT-signal for each channel (i.e. W number of SAT-signals for W number of data channels). In other words, there are multiple token-like signals rotating on the multiple channels to ensure the fairness among nodes. The quota of a particular channel of a node is renewed only when the node receives the token on that channel. Therefore, Multi-MetaRing inherits all the disadvantages from the MetaRing. That is, the maximum access delays are within the order of round trip times. When the ring network is overloaded, the access delays seen by each node will oscillate between zero and number of round trip times. This

outcome is especially unsuitable for bursty metro traffic and real-time traffic. When applying to WDM networks, Multi-MetaRing encounters an additional problem in which a node may hold several tokens at the same time due to the fact that only one data packet can be sent per slot time (due to the fact that each node is equipped with only one transmitter). The problem results in an increase in access delay and throughput degradation.

1.1.4 Existing MAC Schemes with QoS assurance in WDM Networks

For future optical Wavelength Division Multiplexing (WDM) networks, OPS WDM networks have been envisioned as a future framework for next-generation Internet (NGI), which is expected to support integrated multimedia services with various quality-of-service (QoS) requirements [31-42]. Expected supported services include constant bit rate (CBR), variable bit rate (VBR), and available bit rate (ABR).

The real-time traffic, such as CBR and VBR traffic, referred to as high priority data, is subject to a centralized/distributed call admission control (CAC) [43-48] that accepts connections if all demands are guaranteed to be satisfied. While the ABR traffic which is referred to as low priority data takes advantages of all the remaining bandwidth. Pertaining to such OPS WDM networks, one of the most interesting and challenging issues is to design an efficient medium access control (MAC) that flexibly accommodates maximal real-time traffic with remarkable QoS performance while still sustaining exceptional aggregate system throughput.

Most existing MAC schemes with QoS provision primarily focus on two major challenges, the reservation mechanism and the accommodation of real-time traffic. In single-channel networks, such as IEEE 802.16 Distributed Queue Dual Bus (DQDB) protocol [30], Asynchronous Transfer Mode (ATM), and IEEE 802.17 Resilient Packet Ring (RPR) [28], support QoS by rate-basis reservation, which allocates the

required rates of bandwidth for real-time traffic. However, in WDM networks, they adopt slot-basis reservation schemes [31-34], where they clearly specify which data slots are reserved for real-time traffic. Since in most of the current WDM networks, each node is equipped with only one receiver, bringing in a receiver-contention problem (two packets destined for the same node are prohibited at the same slot time).

If the real-time traffic is only rate-reserved, it may fail to transmit due to the receiver-contention problem. As to regard the accommodation of real-time traffic, most approaches focus on the bandwidth requirements estimation, such as guaranteed bandwidth (peak rate), effective bandwidth, and dynamic measurement bandwidth [43-48]. Both guaranteed bandwidth and effective bandwidth are often over-estimated, thereby resulting in poor system utilization. While the measurement-based bandwidth is too complex and difficult to be properly predicted, it is either over-estimated or under-estimated (poor QoS guarantee). In this thesis, we simply tackle the problem from the perspective of a given proportion of bandwidth left over for the bursty traffic rather than the actual bandwidth estimation.

In WDM ring networks, existing researches propose QoS provision by slot-basis reservation but either in inflexible or over aggressive reserved manner, thereby causing poor statistical multiplexing gain for real-time traffic or system utilization degradation. The methodologies in [31,32] make reservation at their corresponding preferential frame-based slots, which were pre-assigned either on a per-source-destination basis [31] or per-destination basis [32] to suit the hardware limitations imposed by their network architectures. Since each node is equipped with one fixed-tuned receiver tuned to its home channel, the reservation can only be done at some pre-assigned wavelength and at some pre-assigned slot times. While [32]

solves the scalability problem, thus the number of the nodes is greater than the number of wavelengths. These schemes indeed satisfy the QoS requirement. However,

because they make reservations only at particular slots, they are rather inflexible and inefficient, leading to poor statistical multiplexing gain for real-time traffic. Another scheme [33] makes high-priority-marks at the control channel and shares with all nodes whenever a node fails to transmit any high priority data. Although the share

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