As user demand increases, then update the current GPON bandwidth will be a challenge, since the MAC address Media Access Control protocol is very complicated. Since TDM-PON (Time Division Multiplex Passive Optical Networks) having a wavelength just upstream and downstream wavelength that limit the average bandwidth per user is a few tens of Mbps  This results in a waste of available bandwidth of the fiber . Moreover, the existing TDM-PON tree topologies missing features, such as the protection and restoration. 
The current TDM-PON architecture limits the versatility of the network and the users require the installation of a completely new PON capacity to serve up to 32 users. A number of new pages added tax included in the OLT in the central office (CO) is only to serve more users. It may not be an economical solution, since it involves complex wiring and supply. 
4.2 MIGRATION TO HYBRID PASSIVE OPTICAL NETWORK (HPON)
In order to narrow the preceding TDM-PON, such as alternative architecture TDM / WDM PON-based hybrid used. WDM components are less expensive than the demand for bandwidth-intensive applications continues to grow, so HPON migration is inevitable in the near future. WDM technology allows efficient sharing of resources, a reconfiguration and scalability. Thus, WDM-PON not only eliminates the need for multiple protocols and time but also provides a virtual point to point, transparently allows the data and safety.
The HPON architecture uses following mechanisms to minimize the total system cost:
Uncooled components are employed at Optical Network Units (ONUs).
Use of array waveguide gratings (AWGs) doubles the number of ONUs supported by fiber splitter based network.
Tunable lasers and receivers used at Optical Line Terminal (OLT) to minimize total number of transceivers, thus ONUs can modulate their upstream traffic onto the optical carriers generated by OLT.
4.3 ARCHITECTURE OF HPON
The overview of the architecture HPON including TDM-PON and WDM-PON subsystem as shown in Fig.  The basic topology is a star slip ring is passive (PON) is credited. The slip ring is connected to the remote node (RN), which are the centers of the stars. The load is connected to the RN on the west side of the ring to talk and listen to the radio transmitter and receiver on the west side of the OLT. Similarly, the UN RN is connected to the east side of the ring to talk and listen to the eastern part of the tax OLT. There is a point-to-point connection between the OLT and the RN each logical level. The collector ring wavelength not again. In the event of a fiber cut, the total RNA as the signal loss is detected, the transceiver at the other side of OLT for continuous operation. 
Figure 4.1: Architecture of Hybrid (WDM/TDM PON)
The basic components forming the functional units of HPON architecture are Remote Node (RN), Optical Network Unit (ONU) and Optical Line Terminal (OLT) which are discussed as under:
4.4.1 Remote Node (RN)
RN basic structure shown in Fig. All but two of an NxN AWG output ports are connected to the distribution fibers connecting the load associated with the RN. The band's three-port device RN separating thin-film filter performs add / drop functionality.  The first band dividing the left side (LHS) of the allocated Waveguide Grating (AWG) by insertion / extraction of the UN on the right side (RHS), and the second separator bar on the right hand side (RHS) of the grid arrayed waveguide (AWG) by add / drop, the UN left. The wavelength of a group / band by two delimiters must be different. , so that each ONU own dedicated wavelengths for transmission of upstream and downstream in the DWDM grid communicate ol . Since the insertion of a AWG is about 6 dB, regardless of the number of ports, one of the more than eight ports can be used to enjoy a better power budget than a passive splitter. Each RN is generally used ONU 16-64  .
Figure 4.2: Remote Node with AWG 
4.4.2 Optical Network Unit (ONU)
The relatively high stability of the specifications required upstream as AWG wavelength optical filters and out of the light path. The easiest way to implement the UN tax-stabilized laser source is used. However, this results in the light and electron bulky power hungry and thus the impact of the cost of the network. The other approach is to use a tunable laser UN, but this would require a control function in the OLT and the wavelength tunable lasers are quite expensive. A second approach is that of another semiconductor optical amplifier (SOA) as a UN modulator shown in FIG. Responsibility for ambient light created by the OLT . It is not necessary to stabilize the wavelength of the United Nations, and the signal can be amplified to increase the quality and the transmission distance  . Thus, the United Nations has no local optical source and an optical modulator instead of modular optical continuous wave (CW) breaks received OLT for upstream transmission. It should be noted that the tunable receiver for the UN. The AWG the remote node removes unwanted wavelength and allows only a specific wavelength to reach each ONU. The UN receiver only needs enough bandwidth for dense wavelength division multiplexing optical any kind (DWDM) channels used in the network. The MAC block in the United Nations not only to check the connection between the upstream and downstream transmissions, but also to coordinate the scheduling of the vote by the OLT and reporting mechanisms.
Figure 4.3: Block diagram of the HPON ONU, with optical carrier provided
Externally by OLT 
4.4.3 Optical Line Terminal (OLT)
Figure shows the block diagram of OLT. Tunable components such as fast tunable lasers and tunable filters are employed.
Figure 4.4: Block diagram of OLT for HPON 
Since the average load of the access networks generally low , , and thus assumed statistical multiplexing gain tunable OLT shared components, which minimizes the transceiver count a given number of ONU , and thus reduces the overall system cost . This arrangement is also good in terms of scalability: the number of users increases or excess network traffic, tunable lasers and receivers are added to the OLT. Circulators separate the optical signals upstream of the downstream signals. The programmer controls the operation of tunable transmitters and tunable receivers.  The WDM coupler separates channel CWDM DWDM channels are presented. The CWDM demultiplexer by thin-layer demultiplexing filter is used for upstream traffic.  The outputs of the optical encoder (downstream) of the controlled channel DWDM optical amplifiers, and enters port 1 of the circulator. The data, after a pass through port 2 of the pump and the WDM coupler in the optical access network. DWDM upstream traffic channels through the WDM coupler of the circulator port 2 and port 3 of the circulator. The optical pre-amplifier may be used to increase the receiver sensitivity.   It should be noted that the downstream traffic, tunable transmitters used in OLT and continuous-wave (CW) optical modulation of the current outbreak, the United Nations framework. With this configuration, only half-duplex
the physical layer communication is possible between the OLT and each ONU.  In contrast, a similar architecture  is a two-fiber ring, two light sources and two mux / demux perform full-duplex communication, the success HPON architecture reduces the cost dramatically. In compensation, the MAC protocol requires carefully planned and scheduling algorithms to provide effective communication. 
4.5 MAC PROTOCOL AND FRAME FORMATS FOR SUCCESS
The success of OLT WDMPON check the amount of upstream traffic and the UN to send funds saved, but the form of CW optical burst than what the United Nations to transmit upstream traffic. Since there is no separate control channel or a control message schema embedded escape sequences , the MAC protocol must rely on the in-band signaling, using frame format shown in Fig. 3, in this report, and the fields are set to give the voting process. Here, the "ID" of the 1-bits  was extended past frames 8-bit extension flags: the "Frame Type" flag is used to indicate that the frame of the normal traffic or not.The fields usage is in the 8-bit flags is shown in Table . 
Table 4.1: Bit flags in downstream success WDM PON frame
-No action required
-ONU should report in the corresponding upstream frame
Each ONU computes the amount of traffic waiting in its upstream traffic queue in octets through the report field in an upstream frame when the 'Force Report' field of a received downstream frame is set, and the OLT uses the grant field to indicate the actual size of each grant (also in octets). As shown in Fig. 4.5, the length of the whole CW burst corresponds to that of all upstream Ethernet frames (i.e., the size of grant) plus the report field and the overhead. We use two control parameters to govern the polling process consisting of reporting and granting operations as follows:
ONU TIMEOUT: The OLT maintains a timer for the United Nations and back every time you enter later sent to the United Nations framework. This resets the timer, if the right framework is zero upper field were reported. If the time limit expires after the time of the UN, which means no upstream traffic, which was supported by the United Nations or report message is lost during transmission to the OLT, the OLT sends a new UN survey support the timer again and again. This keeps the polling process parameter passing, if the interrogation message loss and limits the maximum polling cycle. It also affects the average delay of packets upstream traffic when the system is under low load.
MAX GRANT: This parameter limits the maximum size of a grant (i.e., the payload part of the CW burst) for ONU upstream traffic.
Figure 4.5: Frame formats for success WDM-PON MAC protocol 
4.6 FUNCTIONALITY OF HPON
An RN is either a passive network adapter (interface), or within one AWG. If an RN is a passive splitter to separate wavelength DWDM network will be used to further extend the data connected ONU RN. Equivalent responsibility transmitters of Fabry-Perot (FP) laser Wavelength Division send data upstream coarse grid (CWDM) grids. In addition, each has its own wavelength DWDM network dedicated to communicate ONU OLT if RN AWG is shown in Fig. Whereas about 6 dB insertion loss AWG, regardless of the number of ports to ensure that a port can be used with more than eight AWG better balance of power splitter is a passive experiment. Each subject RN WDM-PON ONU 16-64 . The downstream traffic and upstream traffic from the ONUs can use the same wavelength, but in different directions on the same fiber. The advantage of being based on CWDM and DWDM stars TDM, WDM star-based traffic equilibrium. The stars tend to serve businesses WDM, TDM and the stars tend to serve residential areas. Tunable components help reduce OLT tax matters. OLT tunable lasers generate intermediate frames, and both continuous wave (CW) to burst modulated by the United Nations. There is no local light sources are needed by the UN. This arrangement results in the half duplex communication with the OLT and each ONU. Compared with the traditional architecture, with two fiber rings, two light sources and two Mux / Demux devices perform full-duplex operation, this architecture dramatically reduces the implementation costs .
Figure 4.6: Functional architecture of HPON with AWG 
4.7 BANDWIDTH UPGRADATION SCENARIO
The figure shows the gradual establishment of bandwidth in optical access networks HPON based architecture. Figure 4.6 (a) shows the existing TDM-PON tree topology connected to a central office (CO). All TDM-PON OLT own cable inside the CO Figure 4.6 (b) shows the first step in the migration of the existing network infrastructure. Passive interfaces are replaced by the nurses Extrusion processing machines with a thin passive film and add / drop filters, the introduction of CWDM. The fibers in a PON network connection is replaced with a single fiber ring chains served by the RN CO. Note that the distribution fibers affected by this migration. The United Nations in terms of suitability of optical access network is exactly the same, just a short time of inactivity required for the upgrade. Therefore, there is virtually TDM-PON ONU can operate as before without major upgrades. Figure 4.6 (c) describes the second phase of the migration. As the number of users requiring large bandwidth for future broadband applications, RNA, comprising a grid array waveguide (AWG) as a DWDM multiplexer / demultiplexer can be inserted into the network. In this case, there is a dedicated DWDM channel between the OLT and each ONU. If the protection and recovery functions are implemented in the existing RNA using semi-passive switch, inserting a new RN in the network does not interfere with the operation of the network in general. Figure 4.6 (d) shows the possible extension of the network. Given that a dedicated wavelength to the output of the AWG, it can be used as a ring-PON backhaul network topology tree. Connecting the two PON fiber network can be connected to various RNA forms a protection path. To further improve the capacity of the SOA (semiconductor optical amplifier) â€‹â€‹based modulator can be replaced with a stabilized laser source-duplex operation. Thus, the architecture is slightly HPON pure TDM-PON network updates WDM-PON is an economical way .
Figure 4.7: Optical access Network bandwidth up gradation scenario from existing TDMPON to HPON (WDM/TDM based Hybrid PON) 
4.8 SCALABILITY ANALYSIS
In order to analyze the HPON network scalability in the number of users and range of links, two major limitations of the budget and the network connection is available wavelengths. The larger number of users limit characteristics of components, such as the maximum transmission power, receiver sensitivity loss and Paste. Let's look at the typical values â€‹â€‹for common standards in table 4.1 
Table 4.2: Typical specification of optical components 
Output power of FP laser
Insertion loss: Thin film add/drop
Propagation loss: fiber
Insertion loss of AWG
Loss: splicing and aging on the link
Gain of SOA
10 - 25
Passive splitter splitting ratio
4 - 32
Noise figure of SOA
First, assume that network supports only DWDM users, which means every RN has an AWG. We can assume that each RN can support up to 32 ONUs. Let 'Wt' be the total number of ONUs. Then, the number of RNs on the ring to support Wt ONUs, X, is given by
Let 'L' be the maximal fiber length between OLT and any ONU. The power splitting ratio between receiver branch and modulation branch inside the ONU is 6 to 1.3dB.
The downstream traffic power budget must satisfy the following condition in order for signal to be successfully received:
Similarly the power budget for the upstream traffic can be estimated as:
Note that for DWDM users, the upstream traffic has a tighter bond of power budget than that of the downstream traffic. In the same way, we can derive the bound for the number of users and link reach assuming the network only serves TDM-PON, in this scenario. The ONUs are having low power FP Laser, here we derive link reach considering to the upstream power budget: