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WDM-PON versus GPON and XG-PON

Posted on By FS.COM

WDM-PON versus GPON and XG-PON

The success of broadband services has been overwhelming, creating huge pressure on carriers to upgrade their access and backhaul networks. But budgets are tight and the underlying question remains: How can we change the current operational model and improve end-user experience while reducing operational expense? Can we deliver more bandwidth to more endpoints using fewer active sites in the network? With PON (passive optical network), the answer is : Yes, we can.

As full services are provisioned by the massive deployment of PON networks worldwide, operators expect more from PONs. These include improved bandwidths and service support capabilities as well as enhanced performance of access nodes and supportive equipment over their existing PON networks. The direction of PON evolution is a key issue for the telecom industry. By this, two technologies must be mentioned, namely XG-PON (a.k.a. 10GPON, as a continuation of GPON and/or EPON) and WDM-PON (benefiting from the wavelength domain). Let’s start with their definition first.

Definition of WDM-PON and XG-PON

WDM-PON is an access network technology that has the potential to significantly alter carrier infrastructures. WDM-PON creates a wavelength-based logical point-to-point architecture upon a physical point-to-multipoint fiber topology. It uses WDM multiplexing/demultiplexing technology to ensure that data signals can be divided into individual outgoing signals that are connected to buildings or homes. This hardware-based traffic separation provides customers with the benefits of a secure and scalable point-to-point wavelength link but enables the carrier to retain very low fiber counts, yielding significantly lower operating costs.

XG-PON is also known as 10GPON, which defines a mechanism of migration to acquire a signal for the 10 Gbits user and 2.5 Gbits of the user. The signal for the XGPON user is defined in the range from 1575nm to 1580nm and the signal of the user from 1260nm t o 1280nm. For the coexistence of XGPON and GPON technology on the same fiber, the central needs a WDM filter that combines the signal for the user and the video signal.

 

Comparison Based on Performance in Various Aspects

Here we have summarized the technical performance of 10GPON and WDM-PON in the table below. And more details will be discussed in the following texts.

Item 10GPON WDM-PON
DS line rate 10G 1G
US line rate 2.5G 1G
Sub/feeder fiber (split) N=up to 128 N=up to 64
Reach w/o RE 20 km 50 km
Reach w RE 60 km 100 km
GPON co-existence Maybe Maybe
BW/sub DS 10G/N 1G
BW/sub US 2.5G/N 1G

Capacity: GPON versus WDM-PON

The capacity per user of a WDM-PON is easily evaluated: just one wavelength is dedicated to each end user. In general, a GbE signal is transmitted on each wavelength, assigning a capacity of just 1.25 Gbps to every end user. It's worth nothing that the WDM-PON has no particular advantage if area of the signal is constituted by pure broadcast (e.g., conventional IP-TV): the broadcast signal needs to be replicated through the OLT on every wavelength and independently sent to each user. The evaluation of the GPON capacity per user is not so simple, in that it depends critically on the bundle of services provided to the users and many elements must be taken into consideration. So as the evaluation of XGPON.

System Reach: WDM-PON versus XGPON

The system reach in the XGPON case is determined by the split. For instance, for a 32 split and a 28 dB link budget, it typically equates to about 20 km. For WDM-PON, the AWG has much lower loss than the usual power splitter (50 km looks achievable). Both XG-PON and WDM-PON can be adapted to long-reach scenarios by introducing mid-span reach extenders. For XG-PON, either Opto-Electric-Optic (OEO) or SOA extenders can be used to reach up to 60 km (limited by GPON protocol). While for WDM-PON in C/L-band, it can use Erbium-Doped Fiber Amplifier (EDFA) to reach up to 100 km.

Fiber Utilization: WDM-PON versus GPON

Because bidirectional transmission is used in the GPON case whereas in our example of WDM-PON unidirectional transmission is adopted, the fiber infrastructure is clearly better exploited through the GPON. Unidirectional transmission may be used in WDM-PON, but comes at a cost. In fact, in order to achieve a sufficient branching ratio, DWDM is needed, for example, 32 channels with a channel spacing of 100 GHz. A possible design can individuate two different bandwidths to be used upstream and downstream. They may be separated by a gap of about 800 GHz to avoid destructive interference from reflections. In this manner, a branching ratio of 16 can be achieved.

However, 100 GHz channel spacing requires cooled DFB lasers to be used both in the ONU as well as in the OLT. This fact, besides the greater cost of the MUX/DeMUX, clearly influences the cost of the system. In order to cope with this problem, the use of a WDM comb derived from the filtering of a single broadband noise source has been proposed, but it's not clear yet if real cost advantage is achieved.

Optical Link Budget: WDM-PON versus XGPON

The transmission scheme of WDM-PON is quite simple: attenuation is given by the loss of the MUX/DeMUX and fiber propagation (taking into account connectors, patch panels, along with other signal losing elements that can be present in the access infrastructure). Focalizing on CWDM-PON, standard CWDM optics can assure a transmitted power of 0 dBm, while the receiver sensitivity depends upon the used detector. Utilizing a PIN, the sensitivity at 1.25 Gbit/s (assuming that a GbE is transmitted) could be about -18 dBm. This number increases to about -28 dBm using an APD.

Standardization prescribes for the XG-PON1, a link budget sufficient to possess a reach suitable for GPON B+ and GPON C. Taking into account the slightly higher losses experimented by XG-PON1 wavelengths with respect to the GPON wavelength along with a set of other differences in the transmission line between GPON and XG-PON, a budget of 29 and 31 dB, depending on the comparison with GPON B+ or GPON C, results. This prescription does not exist for XWDM-PON, and a real power budget will be available only after that the first industrial product is produced in volumes.

WDM-PON versus XGPON

On the OLT side, by using its shared OLT port, XG-PON has an advantage with regards to power consumption compared to WDM-PON, which needs one dedicated OLT port per subscriber. However, as WDM-PON typically has lower power budget (because of much lower loss from the splitter), the power consumption per transmitter may be lower than XG-PON. And the integration (Tx, Rx arrays) as well as unused OLT ports which could be turned off can also help WDM-PON save power.

On the ONT side (the main area of the power consumption), XG-PON gains from not needing cooled lasers while WDM-PON can make use of lower speed and lower power budget components. It's expected that both a XG-PON ONT and WDM-PON ONT can be made with a power consumption within the order of 10-15 W. And a "simple" GPON ONT today has a power consumption of -9 W.

Some Technical Challenges and Cost Issues

Although PON technology is in constant development, there are still some technical challenges and cost issued faced by PON waiting to be solved, including burst mode receivers (BMR) , dynamic bandwidth llocation and clolorless ONT. The critical points have been listed as follow:

Burst Mode Receiver

1. The main technical challenge in going for higher upstream bitrate in TDM PON, while GPON requires -100 bit lock-in (phase and amplitude).

2. The most probable line coding is NRZ for XG-PON1&2, just like for GPON.

3. Work is ongoing to create commercial BMR components work at 2.5G, and both DC- and AC-coupled approaches are considered.

4. 10G BMR is still mostly experimental.

Dynamic Bandwidth Allocation (DBA)

1. XG-PON1 will have higher asymmetry vs GPON (1:4 vs 1:2).

2. More efficient DBA is necessary to efficiently use the US BW (BandWidth).

3. Approaches include finding the optimal polling cycle, techniques to find and assign the ONT present/future BW needs.

Colorless ONT

1. The important thing problem with hybrid and WDM-PON may be the need for colorless ONTs, which is 2-4 times more costly than GPON.

2. Several approaches exist: seeding RSOAs, re-modulation and tunable lasers.

3. The key is to understand which approach makes most sense for any given bitrate and distance.

4. Ultimately, tunable lasers has become the best choice if they can be cost optimized for access.

Summary

For a long time, people are asking for more bandwidth in the access with the explosion of bandwidth consumption. The need for more and more capacity both for end user access and transport networks is a reality. In this situation, PON has already become the most popular solution for FTTx deployment and been widely deployed with its point-to-multipoint architecture to provide broadband access. WDM-PON can offer higher bandwidth and reach and additional advantages with regards to its applications. By using its dedicated wavelength channel per subscriber, WDM-PON is often considered to be safer. While the advantages of XG-PON lie in standardization, maturity, cost and power consumption. The trend is that XG-PON is envisioned for residential applications, while WDM-PON is investigated for business or bandwidth intensive backhaul.


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