The direction of PON evolution is a key issue for the telecom industry. GPON, XG-PON, and WDM-PON networks are three types of passive optical networks that provide many applications such as improved bandwidths and service support capabilities as well as the enhanced performance of access nodes and supportive equipment over their existing PON networks. This post will mainly introduce the differences between WDM-PON, GPON and XG-PON.
GPON standards for Gigabit passive optical network. It's a point to multipoint access network type. Its main feature is to use passive splitters in the fiber distribution network, enabling one single feeding fiber from the provider’s central office to serve multiple homes and small businesses.
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. For today’s network, WDM-PON is also an important technology in 5G fronthaul build.
XG-PON, known as 10G-PON, is an enhancement to GPON, which defines a mechanism of migration to acquire a signal for the 10 Gbits user and 2.5 Gbits of the user. The downstream signal for the XGPON user is defined in the range from 1575nm to 1580nm, and the upstream signal of the user from 1260nm to 1280nm. The 10G-PON has similar the point-to-multipoint (P2MP) architecture of GPON and is able to support diverse access scenarios, such as fiber to the home (FTTH), fiber to the cell (FTTCell), fiber to the building (FTTB), fiber to the curb (FTTCurb), and fiber to the cabinet (FTTCabinet).
Here we have summarized the technical performance of GPON,10G-PON and WDM-PON in the table below. And more details will be discussed in the following texts.
|Upstream line rate||1.2Gbps||2.5/10Gbps||1Gbps|
|Downstream line rate||2.5Gbps||10Gbps||1Gbps|
To improve the transmission speed and keep using the existing fiber system to reduce costs, XG-PON was introduced to realize such requirements. XG-PON has a similar network structure to GPON with different upstream and downstream wavelengths, but they can coexist with each other to protect the investment of network operators.
The differences between GPON and WDM-PON mainly lies in the capacity and fiber utilization.
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 noting that the WDM-PON has no particular advantage if the 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.
Because the 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.
The optical link budget is a big difference between WDM-PON and XG-PON. 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). Take CWDM-PON as an example, 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.
While for the XG-PON, the standardization prescribes for the XG-PON1 which defines the XG-PON, 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.
In addition, on the OLT side, by using its shared OLT port, XG-PON has advantages with regards to power consumption compared to WDM-PON which needs one dedicated OLT port per subscriber. However, as WDM-PON typically has a 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.
Users are asking for more bandwidth in the FTTx 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. 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.