Introduction: GPON, XG-PON, XGS-PON
GPON is the access technology of passive optical network (PON) based on ITU-T G.984.x standard. It’s considered as the ideal solution to FTTx (especially FTTH) with its high bandwidth, great interoperability and manageability, high efficiency, etc, which gains more and more ISPs’ favor. However, with the flourishing of PON and demand for full services, people expect a higher performance of the bandwidth, service supporting capability, access network devices and so on while protecting the existing investment. Therefore, it comes to GPON’s evolution to the next generation, which is 10G GPON including XG-PON and XGS-PON.
The major technology adopted at present is GPON and XG-PON. Both GPON and XG-PON are asymmetric PON. The bandwidth of XG-PON is 2.5G of upstream and 10G of downstream. As for GPON, the upstream is 1.25G and the downstream is 2.5G. Moreover, the upstream rate of asymmetric PON is relatively low and the cost of sending components of ONU such as lasers is also low so the total price of the equipment is low. But with the rise of services such as live broadcast and video surveillance that require higher performance of upstream bandwidth and symmetrical upstream/downstream circuits, XGS-PON is put forward. XGS-PON a symmetrical PON with 10G of upstream and downstream speed rates. It’s the evolution of GPON and XG-PON, supporting the access of GPON ONU, XG-PON ONU, and XGS-PON ONU.
Coexistence of XGS-PON and XG-PON
Like XG-PON, the downlink of XGS-PON adopts broadcast mode, and the uplink adopts TDMA mode.
Since the downlink wavelength and downlink rate of XGS-PON and XG-PON are the same, the downlink of XGS-PON does not distinguish between XGS-PON ONU and XG-PON ONU, and the optical splitter broadcasts the downlink optical signal to the same ODN link For each XG(S)-PON (XG-PON and XGS-PON) ONU, each ONU chooses to receive its own signal and discard other signals.
The upstream of the XGS-PON transmits data according to the time slot, and the ONU sends the data in the time slot permitted by the OLT. The OLT dynamically allocates time slots according to the traffic requirements of different ONUs and the types of ONUs (is it XG-PON or XGS-PON?). In the time slot allocated to the XG-PON ONU, the data transmission rate is 2.5Gbps; in the time slot allocated to the XGS-PON ONU, the data transmission rate is 10Gbps.
It can be seen that XGS-PON naturally supports hybrid access with two ONUs, XG-PON and XGS-PON.
Coexistence of XGS-PON and GPON
Since the uplink/downlink wavelengths are different from GPON, XGS-PON adopts the Combo solution to share ODN with GPON. The Combo optical module of XGS-PON integrates GPON optical module, XGS-PON optical module and WDM combiner.
In the upstream direction, after the optical signal enters the XGS-PON Combo port, the WDM filters the GPON signal and the XGS-PON signal according to the wavelength, and then sends the signal to different channels.
In the downstream direction, the signals from the GPON channel and the XGS-PON channel are multiplexed through WDM, and the mixed signal is downstream to the ONU through the ODN. Due to the different wavelengths, different types of ONUs select the required wavelength through the internal filter to receive the signal.
Since XGS-PON naturally supports coexistence with XG-PON, the Combo solution of XGS-PON supports the mixed access of three types of ONUs, namely GPON, XG-PON and XGS-PON. The Combo optical module of XGS-PON is also called three Mode Combo optical module (the XG-PON Combo optical module is called a two-mode Combo optical module because it supports the mixed access of GPON and XG-PON ONUs.
