PDF of this tutorialGiven that NGDLC can effectively utilize any number of available networks, what are the important applications for this type of system? First and foremost is DLC emulation because many customers still have analog carriers, and they want to install an infrastructure that will support some of the broader-band services being developed without paying up front for that capability. They still want to be able to deploy POTS very cost effectively.
Figure 5 shows a system for universal DLC emulation in which POTS lines can be picked up off the central office switch.
Figure 5. DLC Emulation (Universal)
It does not require much modification for this system to support the old integrated TR08 with just a stand-alone terminal. It is probably not the most cost-effective way to deploy one of these systems, but those types of applications must be supported.
More important is the ability to drop and insert; that is, to fit the network topology that actually exists today instead of trying to make the network fit the architecture of the system being applied. Figure 6 again shows a central-office terminal that could use T1 interfaces or analog.
Figure 6. Drop and Insert Configuration
The essential aspect is a drop-and-insert configuration. Notice that the connection between the central office terminal and the first remote subscriber terminal happens to be T1. This customer did not have fiber between the first two terminals, only T1. Because it is very expensive to upgrade that network fiber, it is important to be able to use the media that is in place.
There is fiber between the second and third terminals. Between the third and the fourth terminals is actually T1 again. Moving deeper in the network, it is less likely that there will be fiber deployment. The key to this type of architecture is to use whatever infrastructure exists. Being able to operate in a mixed configuration such as this is what makes the system economical to deploy.
Another typical configuration being deployed by customers is a star topology, represented inFigure 7.
Figure 7. Star Configuration
In this case, there are remote subscriber terminals configured in a star shape off of the central-office terminal, and the facilities that must be used would be T1, fiber, HDSL, or coax. Finally, Figure 8 represents what most network topologies look like.
Figure 8. Tree Configuration
It is actually a tree- or mesh-type architecture, not a ring, and is a mixture of the drop-and-insert and the star topologies discussed above. There is a central-office terminal, and between terminals there could be T1, HDSL, fiber, or coax, depending on what is in place in the network. All of those services are connected back into the central-office terminal. They can be brought into the switch either via T1, analog, or maybe even routed to another location. All of that capability must be available in this type of system to make it cost-effective.
Figure 9 shows an example of an integrated video/telephony point-to-point architecture.
Figure 9. Integrated Video/Telephony Point-to-Point
The central office terminal could pick up subscribers over a TR08 or an analog link and multiplex those subscribers onto an RF carrier. This will help the RF carrier use the existing coax network that is in place. A remote subscriber terminal is installed at the end of the coax system which will demultiplex the RF carrier, extract the POTS subscribers, and put them out over the twisted pair that is there. Cable TV will be deployed uninterrupted across the existing coax network.
Another example of this type of topology is the point-to-multipoint network (see Figure 10). In this case, there is no reason to deploy a remote subscriber terminal.
Figure 10. Integrated Video/Telephony Point-to-Multipoint
There is the same RF transceiver at the central office that multiplexes all of the lines onto an RF carrier, which is used as the input to an existing cable–TV network. In this system, wall-mounted video-telephony splitters on the customer's house is used to break out the RF, demultiplex the POTS subscriber, and input it into the existing wire in the home. Inside the house is the existing coax feed.
Finally, with a mesh-type topology, customers do not typically have exclusively HFC or exclusively not HFC. They typically have both. Figure 11 shows once again the importance of network flexibility, or being able to use what already exists.
Figure 11. Network Flexibility
The bottom half shows the point-to-multipoint-type HFC configuration, while the top half is a traditional DLC–type application. Both must be served out of the same box.
This type of technology is very economical for suburban and rural applications. It is cost-effective at low line-sizes and has a low amount of common control overhead. It is able to use the existing infrastructure, which is very important. It is becoming more common for customers to ask for more advanced services and features—they need more bandwidth, and they want to invest in infrastructure by deploying fiber. The key is for a company to be able to deploy the system and upgrade it as needed, yet not pay for the upgrade until the company is ready to put it in place. The future evolution is going to be exciting—an extension of the existing technology with more bandwidth.
