Integration of EPON and WiMAX for Fixed Mobile Convergence Networks
Broadband access has seen a tremendous growth in today’s world with vast number of applications that require high data rates and good QoS with bandwidth sensitive parameters for these applications. For example, IPTV and Video on Demand  require large bandwidth and high data rates along with appreciable quality of service. These types of services are offered on the fixed broadband services through fiber, since fiber satisfies all the previously mentioned requirements for high end-user applications, for example, Verizon FiOS.
But today, the broadband era is undergoing changes, by which there is a demand for providing broadband services while on the go, i.e., mobility. Presently, WiMAX base stations are being installed (have been installed in many regions in the U.S.) and the users may enjoy data rates up to 75 Mbps/40 Mbps on the downlink/uplink when mobile on the 20 MHz channel . In order to provide speeds up to these standards, the backhaul networks must be robust and should include components that can support these high data rates. EPON supports bandwidths up to 1 Gbps in both downlink and uplink directions and can be shared by multiple Optical Network Units (ONU) .
Perhaps combining mobility with immense speeds suggests a solution to merge Ethernet Passive Optical Networks (EPON) with WiMAX. The EPON may act as backhaul and WiMAX base stations can be used to provide connectivity to the users. By this, EPON satisfies the criteria to support high bandwidth due to the use of fiber and WiMAX can support the conditions for non-line of sight (NLOS) operations for mobile users . This integration also provides an expected solution for Fixed Mobile Convergence (FMC) networks.
2. Integration of EPON and WiMAX architectures
The integration of EPON and WIMAX is one of the best solutions to achieve the pros dealing with QoS and efficient usage of bandwidth at high speeds. Four possible architectures will be discussed, with each architecture improving from its predecessor and evolving with greater benefits. Certain issues about the compatibility of EPONs with WiMAX also exist, which is seen to be resolved to merge both these highly advanced technologies. Fig.1 shows these architectures diagrammatically.
2.1 Independent Architecture
In this architecture, both EPON and WiMAX operate independently. The ONU is attached to the WiMAX base station and they can be interconnected provided they have the same standard interface such as Ethernet between them .
If this architecture is selected, then the ONU is unable to view details about packet forwarding, packet schedules for subscriber stations (SS) etc. taking place in the WiMAX base station and the WiMAX BS is unable to view details relating to the upstream flow from the ONU to the OLT . Considering an overall scenario, this may restrict the users to gain efficient use of bandwidth of the whole system and the ONU and BS are required to be placed at the boundary, which may result in the increase of cost .
In the independent architecture, each ONU can be interfaced with homes, thereby resulting in FMC for wired access .
2.2 Hybrid Architecture
As seen in fig.1, the hybrid architecture comprises of ONU and WiMAX BS that are combined together and collectively known as ONU-BS. There are three CPUs and other SS that are connected to the ONU-BS. This arrangement is done such that by integrating both, the hardwares and softwares existing in the ONU-BS can work together collectively without the need to be connected by extra fiber (as seen in independent architecture). CPU-1 consist of EPON protocols for data communications and CPU-3 consist of WiMAX protocols required for data communications. CPU-2 is in contact with the rest of the CPUs and works such that bandwidth allocations and requests made by each CPU corresponding to EPON and WiMAX are co-coordinated by this CPU unit (CPU-2). The CPU-2 sends a command to CPU-1 and CPU-3 to request upstream information from the SS and then allocate bandwidth to them on the downstream.
Fig.2  shows the hardware and functional module layouts for the hybrid architecture. Fig.2 (b) shows the functional modules depicted in correspondence to their respective elements in fig.2 (a). The CPU-1 comprises of EPON packet scheduler, priority queues and packet classifiers. CPU-2 consists of WiMAX packet re-constructors and upstream scheduler . The ONU-BS central controller is the CPU-2 which controls the rest of the CPUs and generates commands to send/receive data on uplink and downlink to and from the subscriber stations.
The cost on the equipment is saved since everything comprises in a single integrated box and an improvement in QoS and throughput may be seen in contrast to independent architecture .
2.3 Unified Connection-Oriented Architecture
The transmission technique in WiMAX is connection-oriented, for example bandwidth allocation and requests are connection-oriented. Aggregated bandwidth in WiMAX is allocated to each SS and their respective services. This is not the case in EPON where the transmission technique is queue-oriented . The aggregated bandwidth is allocated to each ONU. The ONU grants bandwidth up to eight different priority queues .
The QoS obtained in the connection-oriented WiMAX is better than EPON, whereas, operational scalability is better in EPONs as compared to WiMAX . This architecture indicated a unified solution for both the architectures so that the interconnection between both are maintained efficiently and the services and applications to the users are not affected either. This is done by encapsulating the Ethernet frames into the WiMAX MAC PDU (Protocol Data Unit) with a Logical Link Identifier (LLID) used for addressing purposes  as shown in fig.3 .
In order to diminish this step, a common platform or standard must be set so that the conversion from a frame to a MAC PDU can be eliminated. The Ethernet frames are encapsulated in the WiMAX MAC PDU since the QoS can be implemented better even though WiMAX protocols can operate under EPON techniques (since QoS is an issue with EPON).
2.4 Microwave-over-Fiber (MoF) Architecture
As shown in fig.4, MoF architecture is divided into WiMAX BS and OLT modules. The WiMAX BS consists of Macro-BS which processes frames or data packets from the micro-cell . The Macro-BS comprises of multiple WiMAX BS units and a macro-BS central controller/coordinator . Bandwidth allocation and packet scheduling is also done by the Macro-BS  for all multiple WiMAX BS units.
The EPON part consists of ONU units and a dumb antenna used to receive WiMAX signals. Considering fig.4, in the WiMAX BS part, the WIMAX signal is modulated on a wireless carrier frequency” . Then the two signals are multiplexed and modulated collectively by the optical carrier frequency to send the resulted signal over fiber. “The modulation of WiMAX carrier frequency over an optical frequency is termed microwave-over-fiber” . The optical frequencies can be set differently to distinguish the different dumb antennas at the central node.
The WiMAX subcarriers are modulated over the 2.5 GHz frequency band. If a 1:16 splitter ratio is deployed, as shown in fig.4, then there are 16 optical carriers which also include 16 dumb antennas.
3. Bandwidth Allocation and QoS
Bandwidth allocation and the QoS are major aspects that must be looked into in order to lay an integrated architecture with EPON and WiMAX (excluding independent architecture). Both these technologies use the poll/request/grant mechanism for the allocation of bandwidth .
At EPON’s end, the ONU knows the attributes of the WiMAX BS and also knows the bandwidth grant information requirements such that it may request the required bandwidth from the OLT depending upon the WiMAX BS’s requirements. The WiMAX BS allocates the bandwidth within subscriber stations and different service flows. One thing must be kept in mind that in the discussed integrated architecture, EPON acts as a backhaul network.
As discussed in section 2.3, WiMAX is connection oriented whereas EPON is queue-oriented priority service. WiMAX defines the QoS parameters by following the integrated service (IntServ) mode (for point to multipoint WiMAX also uses DiffServ) whereas EPON supports QoS in the DiffServ mode . EPON has eight different priority queues established in each ONU for QoS and WiMAX specifies QoS levels . A challenge is to establish communication between IntServ and DiffServ modes of services for QoS.
4. Handover operation for the integrated architecture
For the handover process to take place, a dedicated control channel must be reserved by the central handover controller so as to establish connection with the WiMAX BSs at every remote terminal. The WiMAX BS at each remote node sends a signal to the old WiMAX BS to terminate the connection and asks the new BS to start a new connection for the user, in order to carry out the handover process .
Considering the different types of architectures, the MoF architecture is best suited to the handover operation. The MoF architecture consists of a WiMAX macro-BS which does not require a dedicated control channel as it is closely associated with the micro-BSs and central controller in order to carry out the handover process. The macro-BS monitors traffic in such a way that it keeps track of forwarding packets from every dumb antenna . Therefore, the optical frequency must be taken into consideration as different dumb antennas operate at different optical frequencies and is a major aspect to be considered for the hand-off process.
The handover process is a major attribute for Fixed Mobile Convergence, such that in FMC handover can be done from the fixed broadband network with the wireless broadband access network.
5. Future Perspectives
Instead of using EPON, as discussed in this paper, GPON and WDM PONs may also be used , which consists of different wavelengths being offered to different connections. In order to get the benefit of higher data rates, adaptive MIMO antenna techniques can also be used at the WiMAX BSs.
FMC is a significant area of research these days and with the introduction of similar integrated architectures, the users may be able to enjoy services requiring high data rates and efficient bandwidth allotment with ease of handovers and uninterrupted operation of the applications being used.
The paper  proposed four different solutions for integrated architecture for EPON and WiMAX technologies. The compatibility between these different network architectures and communication between their protocols with each other to provide good QoS and easy handover abilities were also seen. One of the main benefits of this architecture is reduction in operational costs and network design .
Using the wired technologies as backhaul (due to high bandwidth and high data rate), wireless connectivity and high speeds can be experienced by the users while they are mobile and this may also result in a favorable solution for Fixed Mobile Convergence networks.
 “Fixed Mobile Convergence Architectures for Broadband Access: Integration of EPON and WiMAX”, Gangxiang Shen; Tucker, R.S.; Chang-Joon Chae;
Communications Magazine, IEEE ; Volume: 45 , Issue: 8
Digital Object Identifier: 10.1109/MCOM.2007.4290313
Publication Year: 2007