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Mr Jörgen Olsson - Nor-Tek Teknologisenter AS (Coordinator)
Gaustadalléen 21, 0349 Oslo, Norway
Tel: +47 22 95 81 78
jorgen.olsson@nor-tek.no
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WDM PON ADVANTAGES

The present and latest updates of PONs; GPON and GEPON as well as Ethernet point to point fibre optical broadband access systems are all based on Time Division Multiplexing (TDM). In the TDM approach, one transceiver at the central office is used to communicate with the N remote nodes (ONU). A 1xN power splitter is used to divide the optical power equally between the multiple ONUs. In the upstream, each remote ONU is allocated dedicated and variable time slots where it is allowed to transmit. The OLT controls the assignment of these time slots to make sure that only a single ONU is transmitting at each time. In the downstream direction a single wavelength is used to send the signal to all ONUs. The ONUs identifies their specific data packets by address information located in the header bit streams. Currently, standardization work for next generation PON systems is targeting 10 Gbps capacity, based on TDM multiplexing.

Many operators and vendors in the industry view DWDM based access in general and in particular WDM-PON as the most interesting long-term PON technology. DWDM based access is a general transport technology where different services and networks can co-exist on the same fiber by using different wavelengths. In particular, WDM-PON offers an alternative to the GPON time-shared transmission scheme by having each ONT transmitting and receiving at a specific wavelength. Thus, the main difference between WDM-PON and the use of wavelengths on GPON (for overlaying several GPONs and/or 10G GPONs) is that WDM-PON may not use the GPON protocol but can use for example point-to-point Gigabit Ethernet. Of course, hybrid solutions with each WDM-PON wavelength carrying a GPON signal are also possible. A typical WDM-PON architecture would replace the passive power splitter with a wavelength selective filter. This filter is often implemented with an arrayed waveguide grating (AWG).

The benefits of WDM-PON include:

  • (Physical layer) un-contended bandwidth similar to point-to-point fiber, i.e. no bandwidth scheduling is needed as in GPON
  • Effective use of fiber - up to 64 subscribers/fiber (similar to GPON)
  • Longer reach is possible, using the low-loss AWG in contrast to the high-loss power splitter needed for GPON. Using a 28 dB link-budget common in GPON systems and assuming a 64-way split, a WDM-PON at 1550 nm could reach >80 km compared to around 16 km for a GPON
  • Physical separation of subscriber signals

The main obstacle to WDM-PON is the cost, since the transmitters need to emit at a specified wavelength. This is especially critical for the subscriber units (ONTs) since this cost directly affects each subscriber line. At the CO side, the cost of the multi-wavelength signal can be lowered by optical integration. The subscriber side however needs solutions with a low number of unique hardware types to keep inventory and installation costs down. This ONT wavelength adaptive feature is termed “colorless”. Several colorless architectures have been proposed and researched over the past decade. The tunable laser approach would be the most natural: the ONT wavelength would then simply be configured when taken into service, offering a high degree of flexibility and performance (reach, bitrate etc). The problem with tunables is cost-driving issues such as the potential need for wavelength locker or reference. Other more or less exotic WDM-PON techniques have also been proposed but are often found to be limited by technology (reach, capacity etc) or cost.

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