## 3 Wireless Access-Metro Networks
### Scheduling and Resource Management  
The resource management and scheduling of LTE are monitored by the link layer which consists of three sublayers [33]: \((i)\) packet data convergence protocol (PDCP) sublayer, which is responsible for handover, IP header compression, and ciphering; \((ii)\) radio link control (RLC) sublayer, which performs data segmentation and concatenation; and \((iii)\) MAC sublayer, which is responsible of resource management and scheduling. Compared to third generation (3G) cellular networks, the link layer protocols of LTE are designed to enhance QoS by increasing their reliability, security, and integrity. In LTE networks, each end-user applies the scheduling request (SR) mechanism to send a request to access the channel. In the SR mechanism, an end-user reports the amount of data buffered in its queues, known as buffer status report (BSR). Typically, the following two mechanisms are defined to perform resource management by means of SR: \((i)\) dedicated SR (D-SR), where each end-user should send its SR on a dedicated resource over the physical uplink-control channel (PUCCH) to the BS, called enhanced NodeB (eNB), and \((ii)\) random access-based SR (RA-SR), where the end-user should use a four-phase contention-based random access procedure [33]. We note that RA-SR is used if no PUCCH resources for D-SR are assigned to the end-user. Unlike conventional cellular networks, e.g., global system for mobile communications (GSM), where the channels are dedicated to high-priority QoS-enabled end-users, in LTE end-users apply a shared uplink and a shared downlink channel. In [35], a combined admission control and time-frequency domain scheduling framework was proposed to support QoS and service differentiation for the LTE uplink. It is important to note that no specific scheduling and resource assignment scheme was standardized. The deployed scheduling algorithm in the eNB is responsible to meet the QoS requirements of different traffic classes according to instantaneous channel conditions and/or fairness allocation policies [36]. The scheduler selects both the appropriate end-user and radio bearer for downstream, while in upstream, the uplink scheduling grants are assigned to end-users without specifying the radio bearer. Since the end-user sends BSRs for pre-alloctaed active radio bearers, the eNB ensures that users with high-priority data are given preference and obtain the assigned QoS. The scheduling grants are carried to end-users on the physical downlink-control channel (PDCCH) [33]. For deploying LTE under realistic wireless channel conditions, a two-layered retransmission scheme, called hybrid-automatic repeat request (HARQ) protocol, is proposed to handle occasional retransmission errors. The MAC and RLC sublayers deploy the HARQ protocol to provide a highly reliable selective-repeat ARQ. This protocol aims at reliably transferring traffic with low latency and low overhead [17]. Furthermore, LTE is able to perform channel-dependent scheduling in both time and frequency domains in order to synchronize itself with rapid channel-quality variations (_i.e._, fading) [37]. For low data rate applications, e.g., voice-over-IP (VoIP), where the additional channel-dependent scheduling overhead is not efficient, LTE uses space-frequency block coding (SFBC) for transmit diversity [17]. No specific scheduling algorithm is standardized to support different types of data flows and QoS for WiMAX SSs. The requested services of each SS are first registered during the initialization phase and connections are established subsequently. If a given SS changes its services, additional connections can be established in the network. Each connection is associated with a service flow (SF). An SF is defined based on available scheduling services and includes a set of QoS parameters, an SF identifier (SFID), and a CID [21]. To implement WMNs, two scheduling types are used: \((i)\) centralized and \((ii)\) distributed.