Optical Technologies
Optical Access Networks – PON Concept

The PON is essentially a two-way point-to-multipoint system [1], [47]. The downstream data signal originates at a central point with a single transmitter. Passive optical couplers divide this signal among output fibers that distribute the same signal to all customers. The receiver at the customer end selects only the data directed to that terminal, discarding data directed to other users. Thus the data stream from the transmitter is divided among users. Each customer terminal has its own transmitter, all of which can return upstream signals to the central distribution point. The upstream transmission may go through the same fibers as the downstream transmission, or through.

Because all the signals go back to a single receiver, separate time slots are assigned to the transmitters at each subscriber terminal so they don’t interfere with each other. The feeder cable transmits optical signals between a central office and a splitter that allows a number of ONTs to be connected to the same feeder cable. The ONT is required for each subscriber and provides connections for various services. Because the one FTTx infrastructure usually provides a service for up to 32 subscribers, many of such networks are required for the community service provisioning. There exist different architectures for connecting of subscribers to the PON. The simplest one uses only one splitter, but in others also more splitters can be used.

Optical terminals

The central distribution terminal serves as the central controller for the passive optical network and provides an interface with the outside world. Many of these terminals can be assembled in one location, each serving its own group of subscriber terminals. The FSAN standard calls this controller and transmitter jointly as an optical line terminal OLT. Terminal at the subscriber end are called optical network terminals ONTs and they provide interfaces between the network and the subscriber’s equipment. Losses inherent in the signal splitting limit one OLT to serving no more than maximum 32 ONTs.

The downstream transmission

The standard PON works at two or three wavelengths. The OLT includes a distributed-feedback laser transmitting downstream at 1550 nm, which couples more than a miliwatt into the output fiber. Each cell or packet in the downstream signal carriers the address of its destination terminal. Passive splitters divide the light among all terminals, but each terminal only reads those packets addressed to it. The downstream data transmission also provides timing signals needed to control the upstream transmission. The OLTs can use relatively expensive 1550nm transmitters, because each PON requires only one of them. However, the PON requires many more ONTs, so they must be relatively inexpensive, and operate in the less-controllable environment of the customer site. This led to the choice of lower-cost1310 nm transmitters for the upstream channels. The FSAN standard also provides for a third wavelength channel at 1490 nm.

In a central office (a head end) for the PSTN and data networks, interfaces to the optical distribution network ODN through the optical line terminal OLT are located. The downstream 1490 nm and upstream 1310 nm wavelengths are used for the data and audio transmission. An optical transmitter of video signals converts signals of video services to the optical format at the 1550 nm wavelength. The WDM (Wavelength-Division Multiplexing) coupler and together transmitting by the downstream combine the 1550 nm and 1490 nm wavelengths. Summary, three wavelengths transmit different information signal simultaneously and in various directions over the same optical fiber. The transmission rates depend on the chosen applications, software allocates a transmission capacity to each terminal dynamically, and so it can be changed as necessary.

The upstream transmission

In the upstream transmission, the PON is the multipoint-to-point type. For avoiding of data collisions from different ONT signals incoming to a splitter in the same time, the TDMA approach is used. The TDMA can transmit data bursts from each ONT back to the OLT in a concrete, specified time. Each ONT transmission time slot is legitimate by the OLT so that packets from different ONT don't overlap with others. The upstream transmission goes through a network of fibers that are combined with passive couplers, so all transmitters send their signals to one receiver in the OLT. To keep these signals from interfering with each other, the PON uses a time-division multiple-access protocol TDMA that assigns different time slots to each ONT.

Each subscriber terminal turns on and transmits signals upstream during its assigned time slot, then switches off so the next can begin transmitting. The control software allocates these time slots and the downstream transmission provides the clock signals to synchronize the upstream transmission by all subscriber terminals.

The fiber architecture

All PONs utilize single-mode fibers with signals divided by splitters. The placement and number of splitters depend on a system design – 1x8, 1x4 and 1x8... The splitters are purely passive devices that require no power, so they can be placed in spliced cases or enclosures anywhere between the distribution center and the subscribers. The FSAN standard provides for both single- and dual-fiber systems and each have their attractions.

A single-fiber system reduces fiber costs by transmitting upstream and downstream signals through the same fibers. The trade-off is that it requires wavelength-division multiplexing (WDM) optics on both ends of the system. A dual-fiber (two-fiber) system is avoiding the added cost and complexity of a WDM optics, utilizes the ability to dedicate the first fiber to a downstream distribution of analog video signals for a cable television and the second fiber to a digital transmission of audio, data and digital video signals.

FTTx Architecture

The PON technology can be involved in all architectures of the FTTx type that offer a mechanism for allowing of sufficient network bandwidth to deliver of new services and applications. The PON network can be common for all these architectures. A question is a placement of active electronic in the outdoor environment. Only in the case of the FTTH/B configuration, all active components are apart from the outdoor environment. The FTTCab and FTTC architectures require active electronics in the outside environment to be placed in a cabinet or in a curb [2].

Equipment at the central office are connected to the PSTN network, equipped by ATM or Ethernet interfaces, and connected to a cable interface or to a satellite receiver. All these signals are then combined for inputting onto the one fiber by using of WDM elements and transmitted to end customers through a passive optical splitter. A splitter ratio can be in a range up to 32 subscribers without using active components in a network. A signal is then delivered to the house over the individual optical fiber. In equipment at the users end, an optical signal is converted into an electrical form by using of OEC converters and at the same time the OEC transforms a signal in service demanding by end users. Ideally, the OEC should have standard user interfaces without requiring special set-top boxes. Main advantages of next FTTx architectures are above all facts that they are passive networks without any active components between a central office and users, they are using only one optical fiber per end user, they have local battery backups and low power consumption, they are reliable, scalable and secure.

Various implementations of the FTTx

Multimedia and Internet services drive the need for higher bit rates (some Mbps) to the home. To ensure that the loop plant of the future will be and remain bandwidth-scalable, fiber-optic cables are replacing metallic cables wherever possible.

Depending on where the fiber is terminated, this technology is known by different names such as: