Overview of a PON Network

Burst Clock Data Recovery for 1.25G/2.5G PON Applications in UltraScale Devices (XAPP1277)

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1.2 English

This section is optional for experienced users. Its main purpose is to outline the operating principles of an XG-PON access network, particularly from the topology point of view.

The following figure illustrates the XG-PON architecture for downstream transmission. The OLT transmits a single optical stream at 9.95 Gb/s to the passive splitter. The splitter replicates the same data stream for each optical network terminal (ONT) connected to it. The downstream data transmission is continuous, thus all ONTs do not operate in burst mode. The data received by all ONTs is the same, but only a fraction of that can be decoded by each ONT.

Figure 1. XG-PON Architecture for the Downstream Direction

The following figure shows how each ONT recovers the clock embedded into the received data, cleans it up, and reuses it to clock the upstream transmission. The raw upstream speed is 2.488 Gb/s. Each ONT transmits data at the same frequency, as it synchronizes to the downstream link. However, data from different ONTs arrive at the OLT at a phase that is completely uncontrolled and varies significantly over time and temperature. To avoid collision, each ONT must send data only during its permitted time slot. The media time sharing across ONTs is managed by the OLT media access control (MAC) layer.

Figure 2. XG-PON Architecture for the Upstream Direction

When a new ONT is allowed to send data to the OLT, the BCDR acquires its phase and extracts the raw data in each burst. Each burst allocates adequate time to:

  • Acquire the sampling phase. This is the typical task of the BCDR.
  • Identify a start- and end-of-packet to identify the packet boundary.
  • Allow guard time for ONTs to power their laser sources on and off.
  • Allow the automatic gain equalizer in the OLT to settle, because ONTs adjacent in time can be physically far apart.
  • Allow AC coupling to charge (if present).

All these contributions negatively affect the efficiency of the upstream direction. Note that the OLT designer controls all these contributions. That is why the OLT design is critical and defines the overall efficiency of the PON line. The downstream direction is a continuous transmission and is thus much more efficient than the upstream direction. This architectural limitation fits very well with the application requirement, because users generally require more bandwidth in the downstream direction than upstream. The following figure shows the data flow in both the downstream and upstream directions. It highlights the preamble, which is only required for upstream transmission. In general, the preamble is a periodic repetition of the 10-bit pattern. This pattern allows maximizing statistical information in the preamble to optimize overall upstream efficiency. A different preamble pattern can be detected by the BCDR. The length of the pattern is set by the OLT (and provisioned to all ONTs) to a value that allows its BCDR to acquire the burst phase. The bottom part of the figure shows an example configuration of ONT phases as they appear to the BCDR.

Figure 3. Data Flow in the Upstream and Downstream Direction
Tip: Although the BCDR in this application note can detect any preamble length, it is recommended to keep the preamble to a length of at least 32 bits to provide adequate phase information during burst phase acquisition. (ITU-T G.987.2 recommends having at least 160 bits for 1.25G or 48 bits for 1.25G, thus 32 bits is adequate for both cases.