Optical Networks - _11_1 Network Architecture Overview_126

Optical Networks - _11_1 Network Architecture Overview_126...

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11.1 Network Architecture Overview 631 of the two most promising access architectures—the hybrid fiber coax (HFC) network and the fiber to the curb (FTTC) approach and its variants. 11.1 Network Architecture Overview In broad terms, an access network consists of a hub, remote nodes (RNs), and network interface units (NIUs), as shown in Figure 11.1. In the case of a telephone company, the hub is a central office (also called a local exchange in many parts of the world), and in the case of a cable company, it is called a head end . Each hub serves several homes or businesses via the NIUs. An NIU either may be located in a subscriber location or may itself serve several subscribers. The hub itself may be part of a larger network, but for our purposes, we can think of the hub as being the source of data to the NIUs and the sink of data from the NIUs. In many cases, rather than running cables from the hub to each individual NIU, another hierarchical level is introduced between the hub and the NIUs. Each hub may be connected to several RNs deployed in the field, with each RN in turn serving a separate set of NIUs. The network between the hub and the RN is called the feeder network, and the network between the RN and the NIUs is called the distribution network. We saw that services could be either broadcast or switched. In the same way, the distribution network could also be either broadcast or switched. Note that in the context of services, we are using the terms broadcast and switched to denote whether or not all users get the same information. In the context of the network, we are referring to the network topology. Different combinations of services and network topologies are possible—a broadcast service may be supported by a broadcast or a switched network, and a switched service may be supported by a broadcast or a switched network. In a broadcast network, an RN broadcasts the data it receives from the feeder network to all its NIUs. In a switched network, the RN processes the data coming in and sends possibly separate data streams to different NIUs. The telephone network that we will study later is a switched network, whereas the cable television network is a broadcast network. Broadcast networks may be cheaper than switched networks, are well suited for delivering broadcast services, and have the advantage that all the NIUs are identical, making them easier to deploy. (In some switched networks that we will study, different NIUs use different wavelengths, which makes it more complicated to manage and track the inventory of NIUs in the network.) Switched networks, as their name suggests, are well suited for delivering switched services and provide more security. For example, it is not possible for one subscriber to tap into another subscriber’s data, and it is more difficult for one subscriber to
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Optical Networks - _11_1 Network Architecture Overview_126...

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