A Fiber Optic System for the Subscriber Loop: Pilot Project, Further Developments, Economics

Summary

An economically viable fiber optic system for the subscriber loop is presented, which is the basis for the Leipzig field trial in 1991.

The main application of fiber optic systems is the connection of residential and small business subscribers. Therefore, today only existing telecommunication services such as POTS and ISDN as well as TV distribution are carried over the fiber optic system (see Fig. 1).

Besides the digging costs for the cable plant the main cost elements in a fiber optic system are the fiber with the optoelectronics. To reduce the costs per subscriber the feeding fiber is split into several distribution fibers using passive optical couplers (laser/fiber sharing). Close to the subscriber premises each distribution fiber is terminated by the Distant Unit (DU), which serves several subscribers over copper wires. Thus, the initial solution is not a genuine FTTH system, but a hybrid optical/copper network (FTTC). At the Central Office the feeding fiber is terminated by the Central Unit (CU), which interfaces to the exchange and to the TV headend. High system reliability is achieved by using a passive optical network (PON), i.e. there is no active electronics between CU and DU. The optical network structure is based on the existing structure of the local loop plant (logical and physical multi-star configuration). The interactive services and TV distribution services use the same topology but preferably different fibers for decoupling both networks technologically and operationally. The fiber optic system planned for use in Leipzig serves 288 subscribers located in a residential area and a business area. For each area 6 DUs will provide POTS services for 24 subscribers each. TV distribution service is only included in the 6 DUs in the residential area (Fig. 2).

Access to the switch can be performed over single a/b wires (channel individual access) or over a proprietary 2 Mbit/s multiplex interface (Fig. 3). For Leipzig the system independent, although expensive, channel individual approach has been chosen. For each line card In the DU a corresponding card in the CU as well as in the switch front-end have to be provided to emulate, for instance, an analog telephone line.

For overall cost reduction reasons as well as increased reliability a wavelength multiplexing (WDM) using optical wavelengths of 1300 nm in one direction and 1550 nm in the other direction on one fiber is used for the transmission of signals for interactive services. To avoid any need for changing the already available coaxial home cabling system at the subscriber’s premises or for adding either additional top converters to existing TV sets or placing such converters in the DUs the common VSB-AM TV signal is used for TV distribution.

The downstream signal from the CU to the DUs is simply a TDM signal broadcasted simultaneously to all DUs. The time division multiple access (TDMA) protocol is used in the upstream direction between the DU and CU to ensure that no collision occurs when bit streams of DUs are combined at the distribution point.

Special care has to be given to the problem of power feeding, because the optical fiber cannot transport the electrical energy required for the DUs. Remote powering from the Central Office can be done reasonably for distances between CU and DUs of up to about 2 km. For larger distances, local or pseudo-local power supply to the DUs is the only viable approach. For Leipzig all DUs are locally powered with battery backup for local power failures.

A detailed cost analysis based on a typical FTTC network model (see Fig. 4) has been made to investigate the economics of the proposed FTTC system. In case of common transmission of interactive services and TV distribution services the average FTTC investment costs per subscriber are only 80% of the average investment costs for a conventional copper access if only multi-family homes are considered (Fig 5).