Existing Asymmetric Digital Subscriber Line (ADSL) connections have a typical downstream transmission rate in the range of 1.5 to 6 Mbps. With the advent of better coding options and additional transmission bandwidth in the new ADSL2+ standard, the maximum downstream data rates achievable in average loops ranging in length from three to nine kft. will be from 8 Mbps to 20+ Mbps, depending on the exact transmission and noise environment. This additional transport capability opens up the possibility of high-quality digital video streams delivered real time through existing copper infrastructure. However, with the demand for video service continuing to increase - both in terms of the number of concurrent channels and the quality of video needed for high-definition format - much higher speed transport architectures are being considered and deployed by telephone operators today around the world.

Most high-speed transport architectures today involve some amount of fiber optics. In the most extreme case, fiber is deployed all the way to the home or the premise, i.e., fiber-to-the-home (FTTH) or fiber-to-the-premise (FTTP). 

In this scenario (see figure 1), fiber is generally terminated by a passive optical network (PON) termination at the network interface device (NID) (external to the house), and the signal is converted back to electrical for in-house distribution.

Figure 1 - PON based Fiber-to-the-Home (FTTH) Architecture

This type of architecture has the advantage of delivering very high data rates to the customer, typically in excess of 30 Mbps, but it suffers from high deployment cost, both in terms of equipment and installation. While some operators have announced large scale plans for FTTH deployment, most operators favor a FTTH strategy only for green field deployments. These operators plan to leverage existing copper last mile infrastructure for current customers.

Figure 2 shows two typical DSL based transport network architectures. When deployed directly from the central office (CO), as is done with most ADSL deployment today, the achievable data rate is typically limited by the loop length of the twisted pair. The advantage of this architecture is complete re-use of existing copper infrastructure, resulting in minimum upfront equipment and deployment costs. However, in order to provide more bandwidth for advanced video applications, it is desirable to install fiber from the CO to cross connect boxes, or nodes, in the neighborhood and then utilize the existing copper twisted pairs from the cross connect nodes to the individual customer premises. This FTTN architecture effectively shortens the copper twisted pair for the DSL transceivers. As a result, downstream data rates in excess of 20 Mbps can be reliably achieved on point-to-point connections between the cross connect node and the majority of subscribers. 

Figure 2: DSL based Fiber-to-the-Node (FTTN) Architecture vs. 
DSL Deployed Directly from the CO

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