Passive optical network (PON) systems are being deployed by major service providers throughout the world. Customer acceptance of PON technology has been impressive. In the Japanese market, the number of new PON subscribers is exceeding the number of new digital subscriber line (DSL) subscribers, and Verizon's FiOS rollout is proceeding rapidly in the United States.

Current PON systems use either Ethernet PON (EPON, also known as GEPON) or broadband PON (BPON) technology, but gigabit PON (GPON) is the next generation. GPON offers significant improvements over EPON and BPON, including an increased transmission speed, better bandwidth usage efficiency, built-in security, robust management, and an increased optical power budget that doubles the number of customers who can be served from one fiber.

One area where GPON offers significant improvements is the rate and efficiency of data transmission, which enable systems that offer lower cost, higher performance, and better support for triple-play services. For example, the GPON downstream bit-rate increase is two times that of EPON and four times that of BPON. GPON's effective data rate (based on several factors, including data encoding, bit rate, the number of subscribers, and the size of the data packets), is also impressive. Using a typical model of a PON environment, BPON delivers an effective downstream data rate of 498 Mbps, EPON delivers 913 Mbps, and GPON delivers 2,340 Mbps.

GPON's higher effective data rate gives system designers several opportunities for improving PON systems, including offering more bandwidth to subscribers, increasing the number of subscribers on a PON fiber, and increasing the number of services offered. The combination of these options allows service providers to lower the overall cost of PON systems and increase their revenue per subscriber. Because GPON serves more subscribers per fiber, for example, carriers can reduce the deployment cost by using fewer optical line terminal (OLT) ports in their central office or remote terminal equipment.

Another area of potential savings is in the optical transceivers, which are among the most expensive components in a PON system. Today, almost all EPON and BPON systems offer video through a wavelength overlay in the optical module. This means that the EPON and BPON systems have two downstream lasers transmitting data. GPON has enough extra bandwidth to support video, data, and voice transmission with only one transmitting laser, so it reduces the complexity and cost of the optical transceiver.

To achieve GPON cost savings in the optical transceiver, however, the design of the serializer/deserializer (SerDes) component presents challenges. There are issues in both downstream and upstream directions.

The main technical concern with downstream transmission is that the passive optical splitters will split the amount of power that is available. This issue is handled by sending the data with enough power to account for all the splits. Figure 1 shows how the transmission of downstream data is received at each optical network terminal (ONT).

Figure 1: OLTs Transmitting Downstream Data

In upstream data transmissions, all of the ONTs need to transmit data. The system allocates time slots for each ONT to ensure that the data of each ONT is received independently at the OLT, as shown in Figure 2.

Figure 2: ONTs Transmitting Upstream Data

To keep the raw data efficiency of GPON high, spacing between the packets of the ONTs is kept to a minimum. In EPON, this spacing can be as much as 500 to 1000 ns, while in GPON, the space between packets is 26 ns and there is less than 100 ns to acquire the new data pattern.

There are two potential complications that occur when data is received at one source from multiple devices transmitting on the fiber. First, the transmitting lasers could be operating at slightly different frequencies, as shown in Figure 3.

Figure 3: Devices Transmitting Data at Slightly Different Frequencies

Second, the merging of data packets cannot be perfectly timed, so the data from different devices will be phase-shifted, as shown in Figure 4.

Figure 4: ONTs transmitting data with a different phase

PON-based networks solve the first issue by using a method called loop access timing. In this method, all the ONTs recover the clock from the downstream data transmitted by the OLT. This data pattern and recovered clock will be the same for all ONTs, and this clock reference is used by the ONTs to transmit the data (Figure 5). This ensures that all ONTs are using the same frequency to transmit data on the fiber.

Figure 5: Loop timing allowing all devices to have a matched frequency

The second issue of the data offset is solved with a burst-mode SerDes. In a typical one-laser system, the SerDes receives data patterns, and after a period of several microseconds, it will determine the exact frequency and center point of the data bits being transferred. Once the pattern is acquired, the SerDes can track the center point of the data bits. Over time, there will be slight offsets in the center of the bits, and the SerDes will track these changes.

The issue in GPON is that the OLT will see a significant change in the center of the bits as it tries to read one block of ONT data versus the next block of ONT data (as shown in Figure 4). Also, the OLT receiver does not have the luxury of taking several microseconds to find the center of the bits being transferred. To keep the data efficiency high, the OLT receiver is only allocated 100 ns to find the new center point and recover the next ONT's data.

Most existing SerDes designs do not have the 100 ns locking requirement. However, new SerDes designs are being developed that implement 100 ns locking. This advancement effectively eliminates the timing issue with multiple upstream transmissions from different ONTs and allows system designers to take full advantage of GPON's higher data rates.

GPON systems offer significant advantages over EPON and BPON systems. Suppliers are beginning to overcome the technical challenges proposed by the standard. By late 2006, we will begin to see GPON systems that achieve cost parity with BPON and EPON while offering significant bandwidth and service improvements.

Cortina Systems is driving the evolution of multiservice carrier and enterprise networks with advanced analog and digital communications ICs. Founded in 2001, Cortina Systems is funded by Morgenthaler Venture Capital, El Dorado Ventures, Kodiak Venture Partners, INVESCO Private Capital, and Redpoint Ventures and is based in Sunnyvale, California.

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