PDF of this tutorialThis section will compare ATM–PON systems with xDSL technologies and describe the issues associated with each.
ATM is an ultrahigh-speed, one-size-fits-all, cell-based data transmission protocol that may be run over many physical-layer technologies such as xDSL modems. These are attached to twisted-pair copper wiring and transmit data at speeds of 1.5 Mbps to 9 Mbps downstream to the subscriber and 64 Kbps to 1.5 Mbps upstream, depending on the condition and distance of the copper line.
Asymmetric digital subscriber line (ADSL), for instance, offers users an always-on service, but its maximum downstream and upstream speeds are ultimately limited by distance and the aging copper infrastructure; typically, only speeds of 1.5 Mbps over 12 kft are achieved. If the customer is not directly connected to a CO–based digital subscriber line access multiplexer (DSLAM), then an expensive upgrade to an existing outside-plant DLC system is usually the only solution.
Very-high-speed DSL (VDSL) extends ADSL downstream speed to a potential 52 Mbps, with a proportionately lower upstream speed, but offers a shorter distance range (1 kft to 3 kft) than ADSL. However, this too requires expensive outside plant electronics installed in a cabinet that must survive severe temperature variations.
In addition to the distance problem, xDSL technology has inherent interference problems, a liability with copper-based technology. ATM PONs cannot be interfered with by AM band radio and other radio frequency interference (RFI)/electromagnetic interference (EMI) sources. XDSL is largely considered to be a short-term broadband solution; since it can be easily installed without an expensive outside-plant infrastructure build, the existing copper plant can be used. The PON system, however, is believed to offer an ultimate, end-to-end broadband platform that is future-proofed.
