**3.2 Access network**

Ultra-fast and super-broadband are recognized as becoming increasingly important as demands for bandwidth multiply. Investment in the development of next-generation optical-wireless converged access technologies will enable a future network to be deployed that will radically reduce Fiber-Wireless (FiWi) infrastructure costs by removing local exchanges and potentially much of the metro network. To integrate fiber and wireless technologies, there are important challenges. First, it will be crucial to have mechanism in place to control system load, which will translate into the physical characteristics of the different radio access technologies of wireless systems, the variability of users' requirements and the data rate of on-going wireless connections, complicating the resource management/sharing in FiWi access networks. This raises technical issues such the required protocol interfaces between the resource management entities of tightly coupled networks, and calls for the design of very flexible and effective protocols to allow enhanced routing and link adaptation that makes the best usage of the available resources while dynamically accommodating the users' traffic properties and quality of services requirements.

### **3.2.1 Passive optical network**

There are different topologies for deploying the fiber network from a central exchange station to end-user's premises such as: 1) point-to-point (P-to-P): where individual fibers run from the central station to end-users, 2) point-to-multi-point (P-to-MP) active star architecture: where a single feeder fiber carries all traffic to an remote active node close to the end-users, and from there individual short branching fibers run to the end-users. In this architecture, the fiber network implementation cost is less than that of a point-to-point topology but the main disadvantages of this architecture are a) the bandwidth of the feeder fiber is shared between several end-users and the allocated dedicated bandwidth for each end-user is less than in the point-to-point architecture. b) the requirement for active equipment in a remote node will impose some restrictions on network deployment such as the availability of a reliable and uninterruptable power supply, proper space for installation of active equipment, air conditioning and ventilation, and maintenance costs, 3) point-tomulti-point passive star architecture: in which the active node of the active star topology is replaced by a passive optical power splitter/combiner that feeds the individual short range fibers to end-users. This topology has become a very popular and is known as the passive optical network (PON). In this topology, in addition to the reduction in installation cost, the active equipment is completely replaced by passive equipment avoiding the powering and related maintenance costs, (Koonen, 2006).

Besides the technical issues of implementation, the maintenance and operation cost overhead should be accounted for as it plays a key role in choosing a particular architecture. In the P-to-P architecture, for each end-user, two dedicated optical line terminations (OLT) are needed, while, in the P-to-MP scheme, for each end-user one dedicated OLT is required at the end-user side, another shared OLT at the central station is interfaced between several end-users. When the number of customers increases, the system costs of the P-to-P architecture grow faster than those of the P-to-MP architecture, as more fibers and more line terminating modules are needed. Therefore, sharing the implemented infrastructures between several operators, service providers, technologies, and end users is an essential solution to reduce the infrastructure network cost overhead. . As shown in Fig. 7, the initial cost of P-to-P topology (*CostP-to-P( N*1*)*) for *N*1 users is lower than initial cost of P-to-MP topology (*CostP-to-MP(N*1*)*), while by increasing the duct length at point *L*0, the *CostP-to-P(N*1*)* crosses the *CostP-to-MP(N*1*)* graph and will be greater than it for fibre lengths greater than *L*0. Furthermore, the initial cost of P-to-MP topology for *N*<sup>2</sup> users, where *N*2 > *N*1, is more cost effective than the initial cost of P-to-P topology for *N*<sup>2</sup> users.

Fig. 7. The comparison of systems cost of FTTH different topology networks versus duct length to end-users premises.

multi-point passive star architecture: in which the active node of the active star topology is replaced by a passive optical power splitter/combiner that feeds the individual short range fibers to end-users. This topology has become a very popular and is known as the passive optical network (PON). In this topology, in addition to the reduction in installation cost, the active equipment is completely replaced by passive equipment avoiding the powering and

Besides the technical issues of implementation, the maintenance and operation cost overhead should be accounted for as it plays a key role in choosing a particular architecture. In the P-to-P architecture, for each end-user, two dedicated optical line terminations (OLT) are needed, while, in the P-to-MP scheme, for each end-user one dedicated OLT is required at the end-user side, another shared OLT at the central station is interfaced between several end-users. When the number of customers increases, the system costs of the P-to-P architecture grow faster than those of the P-to-MP architecture, as more fibers and more line terminating modules are needed. Therefore, sharing the implemented infrastructures between several operators, service providers, technologies, and end users is an essential solution to reduce the infrastructure network cost overhead. . As shown in Fig. 7, the initial cost of P-to-P topology (*CostP-to-P( N*1*)*) for *N*1 users is lower than initial cost of P-to-MP topology (*CostP-to-MP(N*1*)*), while by increasing the duct length at point *L*0, the *CostP-to-P(N*1*)* crosses the *CostP-to-MP(N*1*)* graph and will be greater than it for fibre lengths greater than *L*0. Furthermore, the initial cost of P-to-MP topology for *N*<sup>2</sup> users, where *N*2 > *N*1, is more cost effective than the initial cost of P-to-P topology for *N*<sup>2</sup>

( ) *P N*<sup>2</sup> *P to*

( ) *P N*<sup>1</sup> *Pto*

( ) *MP N*<sup>1</sup> *P to*

( ) *L*<sup>0</sup> *N*<sup>1</sup>

Fig. 7. The comparison of systems cost of FTTH different topology networks versus duct

( ) *MP N*<sup>2</sup> *Pto*

related maintenance costs, (Koonen, 2006).

users.

 <sup>0</sup> *Cost* (*N* ) *<sup>P</sup>to<sup>P</sup>* <sup>0</sup> *Cost* (*N* ) *<sup>P</sup>toMP* <sup>0</sup> *Cost* (*N* ) *<sup>P</sup>toMP* <sup>0</sup> *Cost* (*N* ) *<sup>P</sup>to<sup>P</sup>*

length to end-users premises.

In the P-to-P and P-to-MP active star architectures, each fiber link are only carries a data stream between two electro-optic converters, and the traffic streams of the end-users are multiplexed electrically at these terminals. Therefore, there is no risk of collision of optical data streams. Whereas, the traffic multiplexing is done optically in a Passive Optical Network (PON) topology by integration of the data streams at the passive optical power combiner; to avoid collisions between individual data streams it is necessary to implement a well-designed multiplexing technique. A model of WDM PON network is shown in Fig. 8.

Fig. 8. A model of a point-to-multi-point passive optical network topology.

Several multiplexing techniques are used in PON networks, such as time division multiple access (TDMA), subcarrier multiple access (SCMA), wavelength division multiple access (WDMA), and optical code division multiple access (OCDMA). Excluding, the wavelength division multiplexing technique, these multiplexing techniques are available in wireless or wired telecommunication systems. As shown in Fig. 9, in a WDM PON, each optical network unit (ONU) uses a different wavelength channel to send its packets to an OLT in a central office. The wavelength channels can be routed from the OLT to the appropriate ONUs and vice versa by a wavelength demultiplexing/multiplexing device located at the PON splitting point. This wavelength multiplexing technique constitutes independent communication channels and the network could be able to transport different signal formats; even if the channels use different multiplexing techniques no time synchronization between the channels is needed.

Currently Fiber to the home (FTTH) access technologies provide huge bandwidth to users, but are not flexible enough to allow roaming connections. On the other hand, wireless networks offer mobility to users, but do not possess sufficient bandwidth to meet the ultimate demand for multi-channel video services with high definition quality. Therefore, seamless integration of wired and wireless services for future-proof access networks will lead to a convergence to high bandwidth provision for both fixed and mobile users in a single, low-cost transport platform, This can be accomplished by using the developed hybrid optical and wireless networks, which not only can transmit signals received wirelessly over fiber at the BS, but also simultaneously provide services received over fiber to wireless the end users.

Fig. 9. WDM over a passive optical network.
