Vudogar For a more detailed and comprehensive description of QoS provisioning techniques for FiWi access networks the interested reader is referred to [ 59 ]. More fiw, the OLT manages the assignment of traffic to a large number of subcarriers, which travel all along the optical distribution network to be demultiplexed only at the user side and vice versa. The fiber optic network becomes a means for speedy handoff between base stations kpportunities serve the mobile users. However, availability-aware routing is less bandwidth-efficient acccess the shortest path routing due to an increased mean number of hops. Apart from realizing low-cost microcellular radio networks, optical fibers can also be used to support a wide variety of other radio signals.
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There are wireless links between any two wireless nodes if they are within the transmission range of each other. For any two nodes, and , if they are in the transmission range of each other. In any two ONU nodes, , , the direct communication with infinite capacity see Figure 2 can be placed because the bandwidth capacity provided by the PON subnetwork is much higher than the capacity provided in the wireless subnetwork.
If this communication happens in the PON subnetwork, there will be no wireless interference, and the interference set of such links is set to be. If two gateways in traditional WMNs are within transmission range of each other, they can communicate with each other through wireless transmissions. If they are not covering each other with their transmission ranges, there is no direct communication between them.
There may be other types of architectures as well. In the work presented in [ 5 ], a unique node representing the wired network is introduced alongside a unidirectional link from each gateway to the unique node with infinite capacity. This communication has no wireless interference, and the interference set of such types of links is set to be. For the interference model, we can apply the same model as is presented in the works like [ 1 , 6 ] as because the set of wireless links that interfere with is denoted by Later in this paper, we will use these notations and sample architecture to describe the network traffic and routing issues.
Here, we will know in brief how these technologies operate. RoF networks provide transparency against modulation techniques and are able to support various digital formats and wireless standards in a cost-effective manner. These networks use optical fiber as an analogue transmission medium between a central control office and one or more RAUs radio antenna units.
The CO is in charge of controlling access to both optical and wireless media. However, using an optical distribution system in wireless networks may have a negative impact on the performance of medium access control MAC protocols. The additional propagation delay may exceed certain timeouts of wireless MAC protocols, and the network performance would become worse.
Protocol translation must be done at the interface of optical and wireless segments by an appropriate optical-wireless device, such as optical network unit-base station ONU-BS. In this case, wireless MAC frames do not have to travel along the optical fiber to be processed at the central control office but simply have to traverse their associated access point and remain in the wireless mesh network.
A Closer Look at the Works on Fi-Wi In the last few years, Fi-Wi network structures have been used extensively in many areas, and some researchers have been focusing on architectural issues like finding the optimal ONU placement to maximize the network throughput, reconfiguration of the PON structures, and routing algorithms in the wireless subnetwork of these networks.
The strategies of the network throughput and the gateway deployment in wireless mesh subnetwork play significant roles. Among various published works on these aspects, we will discuss some of the works in this section to have a better understanding of the topic. Reference [ 7 ] presents an idea on how to protect Fi-Wi network structure from multiple segment failures. The authors term their schemen as RPMF ring-based protection considering multiple failures.
In this scheme, a backup ONU is designated in each segment of the network and all the segments are clustered in the network. Then, in each cluster, the authors deploy backup fibers among the backup ONUs in different segments to build a protection ring. This scheme guarantees that the segments in each cluster have enough residual capacity to carry the interrupted traffic in the worst failure scenario such that the network can tolerate the simultaneous failures of multiple segments.
If a segment fails, the scheme allows the failed segment to transfer its traffic into any other available segments in the same cluster with two alternative paths along the protection ring. Thus, this scheme can tolerate the simultaneous failures of any one segment and its backup fiber. An integrated flow assignment and fast resequencing algorithm are suggested in [ 6 ] to reduce the out-of-order probability of packets departing from the OLT with virtually zero resequencing delay.
Mixed integer programming MIP Model [ 8 ] and simulated annealing SA algorithm [ 9 ] are designed to minimize the average distance of any wireless mesh router to its neighbor ONU. In order to improve bandwidth utilization, a dynamic reconfiguration algorithm in WDM wavelength-division multiplexing PONs is proposed in the work [ 10 ]. Delay-aware routing algorithm DARA [ 11 ] and capacity and delay aware routing algorithm CaDAR [ 12 ] have been suggested to highlight the issue of network routing in the Fi-Wi network.
Moreover, the study in [ 13 ] proposed a centralized integrated routing to accomplish the load balancing at ONUs and to maximize the network throughput. Reference [ 14 ] proposes a new scheduling algorithm called modified deficit round robin scheduling MDRR to increase the throughput of TCP and to ensure possible in-order packet delivery at the destination.
In MDRR, the quantum value given to the queues is based on the weight associated with them. It provides better quality of service and eventually leads to changing the throughput, latency, jitter, and packet overflow at the queues. The work presented in [ 15 ] shows that in a wireless network with nodes, the per-node throughput is using the random node placement and communication mode. It becomes using the optimal node placement and communication pattern. The joint channel assignment and routing algorithms are proposed [ 5 , 16 ] in multiradio, multichannel WMNs to maximize the network throughput under certain fairness constraints.
The work in [ 17 ] tries to attain the fairness guarantee among different traffic flows and to maximize the network throughput. Some works like [ 18 — 23 ] address signal processing issues for better performance of the links in Fi-Wi structure. The gateway deployment scheme of wireless networks has been studied as well. The authors adopt a hop count limitation mechanism to avoid throughput degradation caused by increased wireless interference and effects of cross traffic in the WMN.
Moreover, a way to deploy the minimal number of gateways in WMNs is pointed out in [ 25 , 26 ] so that the QoS Quality of Service requirements are fulfilled. Algorithms for gateway placement under different wireless models have been proposed in [ 27 ], and the algorithm is also accounted for the traffic load variance from different users. Reference [ 29 ] presents how to add new gateways into existed WMNs to achieve more network capacity. However, these works are only focused on the traffic to the Internet, not for peer-to-peer communications.
Reference [ 30 ] points out the impact brought by the wireless-optical-wireless mode on the network throughput in Fi-Wi networks under certain ONU placement. In [ 30 ], the performance improvements of peer-to-peer communications between two mesh routers in the same Fi-Wi network have been investigated, and the use of optical lines in PONs instead of numerous wireless multihop connections in WMN for the peer-to-peer communication between distantly positioned source and destination mesh routers has been proposed.
According to [ 30 ], utilizing PONs for communication between source and destination mesh routers mitigates the throughput reduction and end-to-end delay, caused by wireless interference which is increased by many hops involved in a WMN.
After going through all these works mentioned above, we have found that only a few works talk about Fi-Wi network architectures that provide alternative routing for peer-to-peer communications, which can utilize the high bandwidth provided by PON subnetwork and could ease interferences in wireless subnetwork to increase the network throughput. In the next section, we will look into the case of throughput enhancement in such network as this is the main facility for which Fi-Wi networks are considered to be appealing.
Quality-of-Service QoS Issues Quality-of-service QoS plays a key role in running various multimedia applications and services over Fi-Wi networks because QoS techniques are used to measure bandwidth, to detect changing network conditions such as congestion or availability of bandwidth, and to prioritize network traffic.
In fact, better QoS of Fi-Wi is the main attractive point in choosing such kind of network for the implementation over other alternatives. QoS technologies can be applied to prioritize traffic for latency-sensitive applications such as voice or video and to control the impact of latency-insensitive traffic.
However, the variability of wireless channel conditions and mobility of wireless end users might change the quality of service, which requires quick response time.
Recently, various QoS techniques were developed for Fi-Wi networks. QoS support is achievable by managing the network layer. Different techniques might be used for the design of hybrid Fi-Wi networks to deliver QoS-enabled services for end-users. The simulation results show an improved network throughput and end-to-end delay for different QoS demands.
The integrated QoS-aware dynamic bandwidth allocation DBA scheme proposed in [ 32 ] supports bandwidth fairness at the ONU-BS base station interface which sends optical bandwidth requests to the optical line terminal OLT based on the status of its queues.
The reported results show improved network throughput, delay, and bandwidth utilization. To gain maximum network throughput in both wired and wireless network traffic, key QoS metrics, such as packet loss, bandwidth, delay, jitter, and packet-error ratio, are important. The design of suitable routing algorithms is another mean to improve QoS support in Fi-Wi networks. In order to prevent network congestion and improve throughput, packets of a flow are sent through multiple paths in wireless subnetwork and then sent to the OLT through ONUs in the optical subnetwork.
OLT has responsibility to transmit the packets to their destinations. As all traffic in such network is sent to OLT, OLT works as a convergence node, which can resequence the packets before those are sent to the Internet or to another wireless client inside wireless subnetwork. When OLT sends the packets to their destination, packets may arrive out of order because of the variant delay along multiple paths in Fi-Wi network. When TCP sender receives three duplicate ACKs, sender would assume that a segment has been lost and the network is congested.
The congestion avoidance is performed after the fast retransmission and fast recovery. However, there is a problem when many packets arrive dynamically from different flows with packet reordering at OLT, where the OLT maintains a queue for each flow and the processing is divided into multiple equal time slots.
Each divided time slot can send out only one packet from the OLT to the Internet or another wireless client inside wireless subnetwork. In order to increase the throughput of TCP and to reduce the out-of-order packets from OLT, a suitable packet scheduling algorithm is needed.
Scheduling is done for load balancing and achieving QoS. In Fi-Wi network, the scheduling algorithm is used at OLT to resequence the packets of each flow to provide in-order arrivals at the destination. The resequencing can also be done at the end system. The resequencing at OLT must be done fast so that too much delay would not be introduced to the resequenced packets. The most challenging part of making scheduling decision is to maintain the number of duplicate ACKs for each queue.
The out-of-order packets to the OLT are put to the high priority level and otherwise to the low priority level; both levels are served in the first-in-first-out FIFO order. In FIFO, the oldest process in the ready queue is selected for running when the current running process is executed. The average waiting time is quite long in FIFO scheduling algorithm. Deficit round robin DRR can handle packets of variable size without knowing their mean size.
A maximum packet size number is subtracted from the packet length, and packets that exceed that number are held back until the next visit of the scheduler. The results show MDRR is more efficient in assuring fairness among the different flows and reducing the effect of packet reordering and the required buffer size in the OLT.
Traffic Routing Issues in Fi-Wi The integration of the optical networks and wireless networks offers an attractive and feasible solution to measure the traffic flow and count the number of hops in Fi-Wi network.
Although there are many routing algorithms to derive maximum network throughput, linear-programming- LP- based routing algorithm [ 5 , 33 ] is more suitable because there are multiple ONUs and fiber wired communications between ONUs. Therefore, LP-based routing algorithm is introduced in this section for future study.
It should be noted that LP-based routing technique which is optimal routing scheme is not likely to be used in the practical scenarios as theory often does not directly translate to the practical implementation cases. Therefore, a practical routing scheme can be applied for traffic between wireless peer in Fi-Wi network to achieve more network throughput gain from the wireless-optical-wireless mode.
In this section, Fi-Wi network traffic characteristics and various routing models are discussed, which are adopted from [ 5 , 30 , 33 ]. Multiflow Traffic Demand Multipath routing is utilized in order to increase the total network utilization, higher aggregate bandwidth, less delay, and better load balancing which improve the network throughput and support upstream, downstream, and peer-to-peer communication efficiently. In the network architecture shown in Figure 2 see Section 2.
The first one is the traffic from wireless mesh client to the Internet, and the second one is peer-to-peer communication in Fi-Wi networks. So, let be the traffic flow vector in the network.
Therefore, the flow in the vector.
On the Issues and Challenges of Fiber-Wireless (Fi-Wi) Networks