Passive Optical Networks

Unlocking the Bandwidth Potential of Fiber

The need for telcos to deliver the Triple Play of voice, video and data in order to compete with services offered by cable companies has reignited focus on broadband optical access systems based on Passive Optical Networking (PON). While the economic downturn stalled implementations of new technologies as the communications industry focused on maximizing revenue from existing infrastructure (such as DSL over copper wires), the Triple Play has returned PON to the frontline of emerging technologies. PON components are key to unlocking the bandwidth potential of fiber.

Fiber to the Pedestal (FTTP) has two sub-categories designed to deliver broadband services to homes and businesses: Fiber to the Home (FTTH) and Fiber to the Curb (FTTC). Passive fiber splitters are used to split the fiber to allow users access to the total available bandwidth for the application.

PON technology has three application-specific technologies:

* APON: ATM-Asynchronous Transfer Mode, Passive Optical Networking

* EPON: Ethernet Passive Optical Networking

* BPON, WDM: Wave Division Multiplexing, Passive Optical Networking

This Article specifically addresses APON plc fiber optic splitter applications and the associated synchronization requirements required for the smooth transmission of real-time services over broadband networks. It should be noted, however, the same concepts apply to all passive optical network technologies. As traffic passes across network boundaries (between the access network and the supporting transport and switching networks) synchronization to a common, accurate clock is needed to minimize slips and reduce buffering. Slips typically result in either complete packet loss or the need for retransmission, and buffering introduces additional latency and jitter. APONs must support a high performance quality for all real-time traffic carried over broadband access topologies.

Relevant Aspects of APON

Line rate: Symmetrical 155 mb/s downstream/upstream

. Asymmetrical, (Optional), 622 mb/s downstream, 155mb/s

. upstream

Maximum fiber distance: 20 KM

Number of fiber splits, typically: 32 (Optional 64)

Precision synchronization improves network reliability, efficiency, and Performance

The APON access solution typically allows a maximum of 32 (64 optional) users to share the available bandwidth of the split fiber. This creates issues such as cell collisions if several users attempt to send traffic in the upstream direction at the same time. APON solves this problem by incorporating Time Division Multiple Access (TDMA) with a grant mechanism for upstream traffic. Synchronizing the OLT and the ONT to a common reference is a requirement for maintaining frame alignment in order to achieve a Constant Bit Rate (CBR) for upstream traffic. A ranging technique is also used to support collision avoidance.

The notion of ranging is simple. The intent is to place all ONT devices at the same virtual distance from the OLT.The intent is to delay the onset of transmission from the closer nodes such that, in principle, “simultaneous” transmissions from any two nodes will arrive at the OLT at the same time.

Time Division Multiple Access (TDMA) is used in a variety of applications, including cellular telephony and cable. The key to TDMA is the establishment of time-slots or transmission windows created and maintained by the OLT. The OLT provides a grant that allows an ONT access to a time slot. Precision synchronization is needed to avoid collisions and ensure low cell-delay-variations.


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