## 2 Optical Access-Metro Networks
### Scheduling and Resource Management  
Each RPR node may consist of one or two transit queues: a primary transit queue (PTQ) and a secondary transit queue (STQ), whereby the PTQ is mandatory and the STQ is optional. In the dual-queue mode, traffic class A is buffered in the PTQ, while the STQ stores in-transit traffic of classes B and C. Control packets and packets buffered in the PTQ and STQ are transfered according to the following access policies. In general, PTQ traffic is given priority over STQ traffic. If the PTQ is empty, local traffic is served until the STQ reaches a certain threshold; once the threshold is reached, priority is given to STQ traffic. These mechanisms, under stable conditions, assure that in-transit packets are not dropped by intermediate nodes. It is worthwhile to note that traffic class A experiences only the propagation delay and, occasionally, queuing delay due to a local packet transmission. Packets subject to fairness control belong only to B-EIR and C traffic classes, also known as fairness eligible (FE) classes. RPR nodes are not allowed to use reclaimable or reserved bandwidth for FE packets. In EPON, different queues of an ONU store different classes of traffic with different QoS requirements. To supprt QoS in EPON, two scheduling methods are considered: \((i)\)**intra-ONU scheduling**, where the scheduling is performed at a given ONU according to its priority queues, and \((ii)\)**inter-ONU scheduling**, where the scheduling of each queue is performed at the OLT. The following two strategies were proposed to implement these two scheduling paradigms [12]:  
* Direct (single-level) scheduling: allows the OLT to perform both inter-ONU and intra-ONU scheduling [13]. In this approach, a scheduler located in the OLT receives information from and individually schedules each queue located in the ONUs. As the information about each queue is collected in one place, the centralized scheduler can easily ensure that the required service guarantees arepreserved and that the excess bandwidth is fairly divided among backlogged queues. The OLT can generate multiple grants, each for a specific traffic class, to be sent to the ONUs using GATE messages [14]. Using a unique logical link identifier (LLID) for each queue allocated by the scheduler eliminates any need for low-level schedulers or ingress shapers in the ONU and gathers all the intelligence in the OLT. * Hierarchical (multi-level) scheduling: allows the OLT to perform inter-ONU scheduling while each ONU performs intra-ONU scheduling [13]. In this scheme, each ONU requests the OLT to allocate bandwidth based on its buffer occupancy status. The OLT only allocates the requested bandwidth to each ONU while it has no information about the internal queue structure of each ONU. Each ONU divides the allocated bandwidth among its different service classes based on their QoS requirements and schedules the transmissions of different priority queues within the allocated transmission window. In the hierarchical EPON scheduling scheme, the OLT receives a single REPORT message from each ONU and sends a single GATE message to each one. As a result, this scheme is scalable with the number of queues. The important task of intra-ONU scheduling in providing QoS is class-based bandwidth allocation. Class-based bandwidth allocation is achieved by collecting the REPORT messages from all ONUs before making decisions. In strict priority intra-ONU scheduling, the OLT assigns fixed bandwidth to expedited forwarding (EF) traffic [15]. The leftover bandwidth is granted to assured forwarding (AF) requests. The remaining bandwidth after serving EF and AF traffic is distributed among best effort (BE) requests. The so-called light-load penalty is the major problem of this scheme where fixed bandwidth allocation for EF traffic may penalize AF and BE traffic by increasing frame delay and long report collection time. The following two methods were proposed to solve the light-load penalty in EPONs:  
1.