Overview
FPGA technology, low cost optics, and a passive architecture have made significant contributions to passive optical networks (PONs) and to the evolution of these networks. System OEMs continue to discover that FPGAs deliver both technical design and economic benefits, especially at the central office (CO) infrastructure end of the network side.
Prior to 2002, lower-performance FPGA generations served primarily as prototyping tools. Today's FPGAs are high performance, feature rich, and well suited to meet growing PON design requirements. Plus, FPGAs that offer lower design costs, flexibility, and scalability are the linchpin for turbo-charging the PON market.
A PON is a point-to-multi-point (P2MP) fiber to the premises (FTTP) network topology, which may also be defined as fiber to the curb (FTTC) and fiber to the home (FTTH). Either FTTP or CPE (customer premises equipment) is used in the PON definition.
Un-powered or passive optical splitters are used so that a single optical fiber serves multiple premises; usually 32, but as many as 64. A PON comprises an optical line termination (OLT) at the service provider's CO and a number of optical network terminals (ONTs), also known as optical network units (ONUs) going to the premises.
1. DSL and PON topologies side-by-side.
Downstream OLT signals are broadcast to each ONT sharing a fiber. Current PON standards have defined downstream data rates up to 2.5 gigabits per second (Gbps). Upstream signals are combined using time-division multiplexed (TDM) access. Compared to digital subscriber line (DSL) or cable, PONs offer an unparalleled bandwidth advantage for high-speed triple play services (voice, video, and data).
According to Infonetics, PON subscribers are expected to grow dramatically at a compound annual growth of 150 percent through 2010 in North America and Asia Pacific. Gigabit PON (GPON) is making strong headway in North America, while Ethernet PON (EPON) is mostly used in Japan. Japanese government subsidies are helping to grow the PON market substantially year to year, while EPON and GPON is being carefully weighed in China.
Broadband PON (BPON) or International Telecommunications Union (ITU-T) G.983x is the prevailing U. S. PON standard. It features a maximum downstream data rate of 622 megabits per second (Mbps) and 155 Mbps upstream. Passive optical splitters installed in the fiber allow up to 64 subscribers to use the line. This year, GPON or ITU-T G.984, an evolution of BPON, is expected to enter greater numbers of U. S. premises. It supports TDM and packet data with data rates of up to 2.5 Gbps downstream and 1.24 Gbps upstream. Key GPON advantages are support of switched digital video and native TDM voice without having to add IP.
Cost sensitive
Regardless of standard, PON systems deployed for broadband access are highly cost-sensitive. DSL is currently the most widely used technology for broadband access. DSL has set the bar for cost per port at an extremely aggressive level based on today's volumes. Consequently, DSL poses a strong challenge to PON. But PON systems have steadily continued to evolve in the last two years with low volumes and increasingly ambitious feature sets.
As the PON market develops, system OEMs and carriers are looking closely at reducing costs, particularly for OLT. On the ONT side, volume is expected to increase into millions of units due to the millions of premises PONs will serve. A number of ASIC and ASSP suppliers are focused on ONT with a variety of chip offerings. Since ONT is a high volume market segment, ASIC and ASSP suppliers can help drive costs down and enable system OEMs and carriers to offer lower prices.
On the other hand, OLT system volume is in the tens of thousands, not millions and is characterized by higher costs. For example, PON home modems costs range from $100 to $300, while PON infrastructure OLT systems cost about $10,000. In particular, OLT costs have been extremely critical for carriers and are largely centered on multi-port line cards that handle an increasing number of premises.
Volume expectations for OLT line cards remain moderate to low in the foreseeable future based on two key reasons. First, only one OLT port is required for up to 64 ONTs; secondly, each OLT line card can support four to eight OLT ports. Thus, the number of OLT line cards and components is dramatically lower than that used for high volume ONT equipments.
Design complexity compounds the cost issue. The PON OLT and ONT topology is a shared media architecture that creates challenges for system OEM designers. Interaction between OLT and individual ONTs becomes highly complex due to TDM in PON standards. TDM is used to share capacity between different premises. Early PON standards initially used static TDM so that each premise received the same capacity.
However, newer PON standards call for capacity to be dynamically assigned to different premises, according to a premise's changing requirements. This dynamic bandwidth allocation (DBA) requires signaling between ONTs and the OLT to inform the OLT of the capacity needs for each ONT. The OLT also needs to inform each ONT about the allocation of capacity. This scheme's protocol is based on request messages from the ONTs to the OLT. The OLT determines the best capacity allocation and responds with grant messages.
Also, unlike simpler Ethernet ports which are point-to-point, PON ports are more complex P2MP due to the dynamic TDM requirements. As a result, OLT ports must continuously switch between multiple ONT premises. ONTs are allocated one of 32 or one of 64 available time slots to interact with the OLT. The OLT has to rapidly and continuously lock to one ONT data stream after another, also known as burst mode. A highly specialized media access controller (MAC), serial/deserializer (SerDes), and clock and data recovery (CDR) functions are necessary to support this blazingly fast locking scheme. A PON MAC is especially critical for coordinating access to each ONT.
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