The worldwide adoption of passive optical networks (PON) in outside plant (OSP) deployments has resulted in significant growth in access network capacity required for bandwidth-intensive applications and consumer-centric services. In fact, these Triple Play applications and services such as high-def IPTV (HD IPTV), video-on-demand (VoD), voice over IP (VoIP), online interactive gaming, video conferencing, and peer-to-peer networking are now a requirement for densely populated residences in multi-dwelling units (MDUs) and small-to-medium sized enterprise (SME) businesses. And given the current economic climate, service providers have to utilize a cost-effective solution based on standards-compliant PON technology to meet the ever-increasing demand on bandwidth.

As an example, large numbers of broadband users in Asia and Europe residing in multiple dwelling units and apartments have rapidly accepted these new services with rising demand. An estimation of bandwidth demand for an MDU site requiring bandwidth-intensive applications and services in a single PON OSP system often is comprised of 16 MDU optical network units (ONUs) providing services to 24-48 subscribers or a total of 384-768 subscribers. This results in a required PON bandwidth of approximately 7 Gb/s in PON downstream, which clearly indicates the limitations of 1 Gb/s-based PON OSP systems deployed today. In response to wide adoption and the surge for ever-increasing demand on bandwidth in high-speed optical access networks, next-generation access standard activities for 10 Gb/s-based PON systems (NG-PON) are currently under way at the IEEE P802.3av (10G-EPON) and ITU-T/FSAN (10G GPON).

The Block - Physical Layer Requirements for PON
PON systems use the shared fiber architecture where a single fiber from the central office serves several dozen users. Because the cost of deploying cables is by far the largest expense in any wireline network, the shared fiber architecture has high market penetration mainly due to the additional cost saving over point-to-point networks using the dedicated fiber plants. Figure 1 shows a typical PON system in the OSP consists of an optical line terminal (OLT), multiple ONUs, and passive optical splitters connecting these two parts. The OLT broadcasts continuous downstream packets to each of the ONUs in the wavelength band of 1480–1500 nm. The downstream packets consist of a header identifying the ONU that should receive the packet. Each ONU gets synchronized with an OLT by extracting a clock from downstream traffic. Also, by means of continuous-mode (CM) transmission, PON downstream reserves a 1550 nm-wavelength window for future extensions or additional services such as analog video broadcasting. Similarly for upstream traffic, each ONU in the wavelength band of 1260-1360 nm is using burst mode transmission for upstream channel. The given ONU transmits an optical packet only when it is allocated a time slot and it needs to transmit, and all of the ONUs share the upstream channel in the time division multiple access (TDMA) mode.

There exist various flavors of PON systems that all use the same or very similar outside fiber plants but differ significantly in the signaling rates, data formats, or protocols they employ. These PON technologies include broadband PONs (BPONs), gigabit PONs (GPONs), and Ethernet PONs (EPONs). GPON upgrades BPON with slightly higher date rates up to 2.5G in the downstream.

Figure 2 illustrates the functional building blocks of PON transceivers for both ONU and OLT in terms of the physical-layer chipsets for both continuous-mode and burst-mode operations. The ONU side consists of a downstream receiver (Rx) and an upstream transmitter (Tx), while the OLT side consists of a downstream Tx and an upstream receiver Rx. The upstream Tx contains a burst mode laser driver and a Fabry-Perot laser in Transmit Optical Sub-Assembly (TOSA) form. The upstream Rx contains a PIN or APD/TIA in Receive Optical Sub-Assembly (ROSA) format, a limiting amplifier (post-amp) and a burst-mode clock/data recovery unit. For both the ONU and the OLT, the transmitter and receiver sections are combined onto a single optical fiber through a wavelength-division-multiplexing (WDM) coupler.

NG-PON: Requirements and Challenges
Although 1G PON systems such as EPON and GPON have gained popularity with significant volume shipment, much faster PON systems are demanded for regions where dense residences and business users need large-capacity leased lines. In light of this trend, 10G PON systems (to succeed EPON and GPON), as well as WDM-PON systems (that multiplex high-speed services by means of WDM channels) are currently under intensive study.

Fundamentally, 10G PON systems (See Figure 3.)need to be fully backward-compatible with the existing 1G PON and analog video overlay infrastructures that have already been deployed in the field. Thus, it will become possible to provide a smooth migration from 1 Gb/s to 10 Gb/s systems with the same fiber plant base and allow joint operation of legacy and new PON systems. As a result, carriers can maximize the life cycle of existing fiber systems and deploy/upgrade faster services over next-generation access networks whenever needed. The technical difficulty and economic feasibility of implementing 10 Gb/s PON depends on many other requirements placed on the network, such as tight link power budget, transmission distance, burst-mode transmission, and necessity of compatibility with legacy data and video systems. These impose stringent requirements on physical layer chipsets that are perceived as enabling technologies in FTTP networks that have to operate under real-world system constraints.  

The major technological challenges in NG-PON transceivers are:

1. Optical transmitter output power as well as optical receiver sensitivity to satisfy the increasing system link budget requirements. A high sensitivity PIN or avalanche photodiode (APD) together with low-noise pre-amplifier IC and limiting amplifier IC were key elements to improve the optical sensitivity and its wide dynamic range.

2. Upstream burst mode optical transmission technologies. Burst-mode IC designs still represent one of the active areas of development, which entails the burst-mode laser driving circuit with feed-forward automatic power control (APC) and burst-mode optical receiving circuit by means of automatic threshold control (ATC) and continuous and/or cell-AGC (automatic gain control) methods that control trans-impedance gain packet-by-packet according to the amplitude of the input signal.

3. Higher date rates. The sensitivity of optical receivers decrease with the increase of signal’s rate. To compensate for loss of optical sensitivity in 10G receivers, 10G PON specification has to employ strong forward error correction (FEC) and high-power transmitters to meet the high power budget requirements. Despite the circuit complexity, FEC operation actually becomes mandatory for all 10G-EPON links operating at 10 Gb/s rate for both symmetric and asymmetric configurations.

Furthermore, with an increased photonic integration level and more compact optical front-end assembly in optical transceivers, the increase in electrical and optical crosstalk between the transmitting and receiving channels becomes another significant challenge. The crosstalk issue is particularly critical for the video channel while considering video overlay.

Figure 4 shows the 10G EPON wavelength plan, which is allocated in a specific way allowing for coexistence (backward compatibility) with 1G EPON and RF video. This enables support of symmetric 10G ONUs, asymmetric 10G/1G ONUs, and symmetric 1G ONUs, as well as RF video overlay on the same ODN.

The more challenging aspect in a PON system is the upstream traffic where one OLT collects all upstream traffic from multiple ONUs in the 1310-nm window using a point to multiple point (P2MP) configuration. To avoid data collisions between ONUs, upstream traffic is managed by a time-division-multiple-access (TDMA) scheme. The OLT allocates transmission time slots, (i.e., gates) to each ONU. When the ONU receives the gate frame, the ONU will transmit a MAC frame at gigabit speed during the allocated time slot. Because of time slots assigned by the OLT, the whole system is synchronized so that the data from the ONUs will not interfere with each other. Therefore, the OLT always receives a bursty packet separated by the guard time in which no signal is transmitted by any ONU.

The multiple upstream access and burst mode behavior require that an ONU transmitter must generate a time gated bursty signal within the allocated time slot assigned by MAC. The standards require that the laser power levels and the extinction ratio must be stabilized within a short time period, and then remain unchanged before the time slot completes.

In the meantime, the ONU transmitter should remain shut-off and must not send upstream light or even low-level noise during the time slot allocated to other ONUs. Otherwise, this would create cross talk and disturb the upstream operational services. This requires fast ONU switching that should normally fall within a few nanoseconds rise and fall time, after a power up or power down or a first connection to the network.

To summarize, there are basically two requirements on the ONU transmitter. First, the fast switching and initialization is very important when developing high split ratios, particularly for the quick recovery of the traffic after network failure which, in turn, affects the number of ONUs. Second, the fast tracking of slow power drift is also needed which is important for long packet transmission.

As well, standards require the OLT receiver to show low sensitivity, wide dynamic range, and a fast response for flexibility in network deployment. Every ONU to OLT could have a large difference in transmission loss, depending on how far it is located down the link. The OLT could receive the bursty signal in fast succession with a largely varying signal level and phase from packet to packet. The worse case performance scenario occurs when a weak package follows a strong one. The OLT receiver must have a fast response to amplitude variation and a short settling time. Due to coexistence of 1G and 10G signaling in the upstream, the OLT needs to be dual-rate burst mode receiver to recover the bursty signal for both 1 Gb/s and 10 Gb/s rates. Furthermore, the minimum transmitter extinction ratio of the ONU is reduced such as 6dB for cost reduction reasons thereby making recovery of the bursty signal even more stringent.

Chip, Chip, and Away!
It is worthwhile to mention that several network operators have strong interest in WDM-PON for the longer term as it provides virtual point-to-point connectivity through a dedicated wavelength. This feature brings about many inherent advantages: protocol transparency, unlimited bandwidth, security, and simplicity in electronics. What's more, the splitting ratio is not limited by the splitting loss of the remote node. Currently WDM-PON is limited in field deployment mainly due to cost reasons. In the long run, the hybrid WDM/TDMA-PON could be the most favorable for increase of the split ratio (thus lower cost), since it can combine multiple TDMA-PONs without the significant increase of optical loss.

For a smooth evolution from TDMA-PON to the hybrid WDM/TDMAPON, it is required to have enough power loss budget such that it can accommodate the optical distribution network (ODN) of TDMA-PON. Another requirement for the hybrid WDM/TDMA-PON is the colorless transmitter for the upstream transmission, since it is very difficult for network operators to manage ONUs with different wavelengths for each subscriber.

In summary, 1Gb/s PON systems have been rapidly deployed to reach several tens of millions of subscribers across the globe. The ever-increasing bandwidth demand in fiber access networks will potentially further accelerate the adoption of next-generation access technologies where using an evolutionary overbuild based on existing PON systems rather than revolutionary rebuild upgrade approach is favorable. Physical-layer chipsets play an important role in overall system building blocks to overcome NG-PON design challenges while achieving system cost targets and quality of service.  

About the Author
Frank Chang is a principal systems engineer in Vitesse Semiconductor Corp. who actively drives next-generation PON standards. He can be reached at ychang@vitesse.com.

About the Author
Angus Lai is a product marketing manager with Vitesse Semiconductor Corp. who is responsible for the PMD (physical media dependent) device product line including PON/FTTx physical-layer chipsets for existing and NG-PON. He can be reached at alai@vitesse.com.

Vitesse Semiconductor Corp. designs, develops, and sells semiconductor solutions for carrier and enterprise networks worldwide. For more information, please visit www.vitesse.com.

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