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 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. 

Sync Ensures Performance Quality

Precise synchronization in APON broadband access deployments enhances quality of service parameters in three ways: 

1. Synchronizing the PON network to the Public Switched Telephone Network (PSTN) and the ATM/data network to a common clock reference that is highly accurate (typically Stratum 1) reduces or eliminates slips at the network boundaries that degrade quality of service. The quality of real time services is affected by retransmission of packets, which is a result of slips. 

2. Synchronizing the components of the PON architecture allows the upstream cell traffic to be mapped into the TDMA stream in the proper time slots. 

3. Several legacy services, such as ISDN (BRI as well as PRI), are essentially synchronous and the terminal equipment relies on the network to provide a signal from which a quality timing reference can be extracted. This mandates the availability of such a timing reference at the ONT and/or ONU. 

APON Frame Synchronization 

A burst mode synchronization scheme is used to align the bit rate of the ONT to the OLT during transmission of upstream TDMA traffic. Burst mode synchronization is an adaptive clock recovery method that enables the ONT to recover clock from the first few overhead bits of a frame. 

APON Optional 1 kHz Synchronization Field 

There is a field available in a frame that allows the option to insert a 1 kHz timing marker for transmission from OLT to ONT with the purpose of aligning counters in the OLT to counters in the ONT so frame alignment is maintained. With an accurate and stable synchronization reference available to the OLT, this method of maintaining frame alignment should be even more reliable and stable than the burst mode synchronization method. 

The International Telecommunication Union (ITU) recommendation G.983.1 for APON in subsection 8.2.3.1 related to synchronization, states the following: 

When the OLT and end office are in the normal operating state the nominal bit rate of the OLT to ONU/ONT is traceable to a Stratum 1 clock accuracy of 1x10-11 or better. When the end office is in the free running mode, the rate of the downstream signal is traceable to a Stratum 3 clock accuracy of 4.6x10-6 or better. When the OLT is in the free running mode the accuracy of the downstream signal is that of a Stratum 4 clock, 3.2x10-5 or better. Degradation in the synchronization quality of the PON architecture from Stratum 1 quality to a less accurate Stratum level such as Stratum 3 or 4 will affect the ability of the PON to pass traffic error free into other networks that still maintain Stratum 1 accuracy. The ONU/ONT shall transmit a signal upstream equal to the accuracy of the downstream signal in order to maintain synchronization of the PON. 

The optical access architecture will be required to support legacy digital services such as ISDN BRI and ISDN PRI for an unspecified time in the future. The requirements for meeting the slip rate objectives of all digital networks enhances the need for placement of a holdover clock with access to a Stratum 1 clock source at the OLT location. 

Summary 

ATM Passive Optical Networks must be engineered to meet the most stringent timing requirements so customers consistently get the best performance quality – no matter which application is being used. Precise timing at the ONU/ONT will ensure that upstream transmissions from multiple ONUs/ONTs do not collide, as well as minimize guard bands to improve network efficiency. A solid synchronization plan will achieve a timing accuracy at the ONU/ONT that meets the sync requirements of the services provided via the ONU/ONT. 

APON Glossary 

* OLT: Optical line termination. A broadband multi-service device that controls the flow of information to and from all subscribers through the optical port.

* ONU: Optical Network Unit. Used in Fiber to the Curb (FTTC) applications where it is not practical to extend the fiber reach all the way to the customer premise.

* ONT: Optical Network Termination. Used in FTTH to terminate the optical signal at the customer premise and provide the electrical interface to the customer equipment.

* Passive fiber splitters: devices that are approximately the size of a pen that allow the fiber to be split without a reduction in the bit rate.

Source: http://cablequest.org/articles/broadband/item/1273-passive-optical-networks.htmlSource: http://cablequest.org/articles/broadband/item/1273-passive-optical-networks.html