PDF of this tutorialTo deliver these new service capabilities, the local-loop network is evolving from an analog narrowband network optimized for voice traffic to a digital broadband network engineered for voice, data, and video.
The traditional analog local-loop network transmits narrowband analog signals between subscribers and voice switches in the public switched telephone network (PSTN). This network is optimized for telephony and limited in bandwidth (see Figure 3).
Figure 3. The Analog Local Loop
With DSL technology, the copper twisted-pair lines remain the same, but new equipment is integrated, enabling a service provider to deliver voice and high-speed data service (see Figure 4).
Figure 4. The DSL Local Loop
At the subscriber, voice traffic is transmitted as standard analog telephony signals into the copper local loop. Data traffic is transmitted over the same line but via a DSL modem that transmits the data as high-frequency digital broadband signals. These signals are transmitted from the subscriber to the service-provider CO.
At the CO, the signals pass through a splitter and a local-loop management system to a digital subscriber line access multiplexer (DSLAM). The splitter filters the standard telephony signals and passes them to a voice switch. The broadband digital signals are directed to a DSLAM that terminates and consolidates traffic from multiple local loop lines. The loop management system may be in front of or behind the splitter and provides protection switching, metallic access, plain old telephone service (POTS) testing, and broadband digital testing to aid service installation, troubleshooting, and maintenance.
From the DSLAM, the traffic is sent to a router that directs traffic to the Internet.
To deliver high-speed data and multiple voice lines, additional equipment must be integrated into the network (see Figure 5).
Figure 5. The Multiservice DSL Local Loop
At the subscriber, voice and data lines are connected to an integrated access device (IAD) where voice is packetized and the packetized voice and data are multiplexed and transmitted as digital high-frequency broadband signals to the CO.
At the CO, the signals pass through a local-loop management system and terminate on a DSLAM. The loop management system provides protection switching, metallic access, POTs and broadband digital testing to aid service installation, troubleshooting, and maintenance. The DSLAM terminates and consolidates traffic from multiple local-loop lines.
From the DSLAM, the packetized traffic is transmitted to the Internet and routed to the appropriate PSTN or Internet destination.
With this new digital local-loop network, the characteristics of the traffic traveling between the subscriber and the switching office are different than for a narrowband analog network. The traffic is sent at higher frequency through a broader spectrum. The information for multiple services, possibly multiple voice lines, high-speed data, and video is transmitted as a digital signal. To manage this network effectively, service providers need new tools and new network management strategies.
