10G-PON (also known as XG-PON) is a 2010 computer networking standard for data links, capable of delivering shared Internet access rates up to 10 Gbit/s (gigabits per second) over existing dark fiber. This is the ITU-T's next generation standard following on from G-PON or Gigabit-capable PON. Optical fibre is shared by many subscribers in a network known as FTTx in a way that centralises most of the telecommunications equipment, often displacing copper phone lines that connect premises to the phone exchange. Passive optical network (PON) architecture has become a cost-effective way to meet performance demands in access networks, and sometimes also in large optical local networks for "Fibre-to-the-desk".
- User applications and demand
- ONU equipment
- OLT and access nodes
- Optical distribution network
- Field trials
- Home broadband service providers
- Commercial broadband service providers
Passive optical networks are used for the "Fibre-to-the-home" or "Fibre-to-the-premises" last mile with splitters that connect each central transmitter to many subscribers. The 10 Gbit/s shared capacity is the downstream speed broadcast to all users connected to the same PON, and the 2.5Gbit/s upstream speed uses multiplexing techniques to prevent data frames from interfering with each other. Users have a network device that converts optical signals to the signals used in building wiring, such as Ethernet and wired analogue plain old telephone service.
User applications and demand
As demand for network speed continues to grow, so new and faster technologies are spawned from the existing standards. 10G-PON is the next generation ultra-fast capability for G-PON providers, designed to coexist with installed G-PON user equipment on the same network; an example of Nielsen's law predicting demand for data downloads to double every year. The ITU-T completed parts of the standard in 2010. 10G-PON may initially find uses in connecting fibre nodes within multi-tenant units and commercial buildings.
Triple play services over IP of video, data and voice are often cited as driving user demand for heavier usage of broadband that justifies PON investment. While RF overlay has been popular in some countries and minimises congestion caused by usage of video services, the convergence of HDTV and IPTV, and the growth in internet cloud services could create demand for bandwidth that exceeds the capacity of gigabit services in future. Teleworking and video conferencing are other applications demanding such triple play capabilities.
Examples of bandwidth-intensive applications include IPTV, video-conferencing, interactive video, online interactive gaming, peer-to-peer networking, karaoke-on-demand, IP video surveillance, and cloud applications where remote storage and computing resources provide online service on demand to users with thin-client local systems. Cloud applications could take advantage of in-country content hosting, and 10GPON may encourage explosive development of innovative services that become feasible as users move to faster connections.
Business continuity systems may also take advantage of 10GPON to enable cost-effective real-time backup/recovery/replication of critical business systems across multiple sites. Other businesses may just need to connect several sites as a virtual private network, effectively a virtual office, or may have e-commerce services that require business partners to have sufficient connectivity for constant database access.
Many of these applications are already growing in both popularity and demand for bandwidth.
ITU-T G.987 is the standard for 10G-PON.
Asymmetric 10G-PON is specified as XG-PON1: 10 Gbit/s downstream and 2.5 Gbit/s upstream (nominal line rate of 9.95328 Gbit/s downstream and 2.48832 Gbit/s upstream).
Symmetric 10G-PON is also proposed as XG-PON2 with 10 Gbit/s upstream, but would require more expensive burst-mode lasers on optical network terminals (ONTs) to deliver the upstream transmission speed.
Framing is "G-PON like" but uses different wavelengths from G-PON (using a WDM to separate them) so that G-PON subscribers can be upgraded to 10G-PON incrementally while GPON users continue on the original OLT. The G-PON standard is G.984. This compares to the IEEE 802.3av standard for 10G-EPON based on Ethernet, which has standardised upstream rates of both 1Gbit/s and 10Gbit/s. The 10 Gigabit PON wavelengths (1577 nm down / 1270 nm up) differ from GPON and EPON (1490 nm down /1310 nm up), allowing it to coexist on the same fibre with either of the Gigabit PONs.
ITU-T Recommendation G.987 is a family that defines this access network standard (referred to as XG-PON). It comprises four recommendations:
There is also a companion ITU-T standard defining a management and control interface for administering optical network units, referred to by the G.987 recommendations.
The optical network unit (ONU) supplies network services from the PON to customer premises, connecting customer-premises equipment such as a home gateway or office firewall. An optical network terminal (ONT) is an ONU that functions as a demarcation point servicing a single subscriber; e.g., a dwelling or office. ONU devices supply Ethernet and possibly other services to the users, either directly or through a gateway device such as a residential gateway, firewall, and/or router, POTS, CATV signals to buildings wired for RF video, and some may even be compatible with the emerging G.hn home networking standard.
The ONU receives the downstream data from the Internet or private networks, and also uses time slots allocated by the OLT to send the upstream traffic in burst-mode. TDMA time slots prevent collisions with upstream traffic from other users sharing the same physical PON.
Fibre-to-the-cell site is another emerging application, but has extra synchronisation requirements. A specialised Cellular Backhaul Unit (CBU) can provide PON access for cellular networks.
OLT and access nodes
The OLT (Optical Line Terminal) connects the PON to aggregated backhaul uplinks, allocates time slots for ONUs and ONTs to transmit upstream data, and transmits shared downstream data in broadcast-mode over the PON to users. Since 10GPON is designed to coexist with GPON devices, migration to a 10GPON capability could be done by upgrading the OLT and then migrating individual ONUs as needed.
Normally the OLT is on a card that slots into a chassis at the Central Office (CO), which uses special uplink cards for Ethernet backhaul to the telecommunications provider's network and internet. Uplink cards on access equipment will likely use multiple Ethernet interfaces, although it remains to be seen what uplink speeds manufacturers will offer to support 10GPON access. Locating OLTs in outside plant cabinets may be an option for reach extension as a way to minimise the number of central offices covering low population density areas.
ITU and IEEE are planning for convergence of their specifications at the physical layer in 10G that would allow for the shared chips, optics and hardware platforms, thus driving cost reductions for hardware manufacturers.
Optical distribution network
PON Optical Distribution Networks use single mode optical fibre in the outside plant, optical splitters and optical distribution frames, duplexed so that both upstream and downstream share the same fibre on separate wavelengths. 10G-PON is no exception with similar reach to previous standards but supporting a higher split ratio of 128 users per PON, or more using reach extenders/amplifiers. Optical splitters creating a point to multipoint topology are also the same technology as those used by other PON systems. This means any PON network should be upgradable by changing the ONT and OLT terminals at each end, with no change to the fibre itself unless different connectors are chosen.
"An Optical Distribution Network (ODN) being installed today will likely need to support four or more generations of PON over its expected 30 – 40 year life... The fibre should enable maximum flexibility to support any potential new PON technology, be protected with proven, reliable cabling making it easy to install and reliable, and be joined by advanced, low labor and low loss connectivity. The cost of the ODN materials (fibre, cable, and connectivity) at only about 8% comprises a surprisingly small portion of the total network cost."
In an effort to extend the reach with support for 128 splits, the standard supports a range of optical budgets from 29 dB to 31 dB. A draft update to the standard is expected to further extend this to 33 dB and 35 dB budget classifications. A PON with a 35 dB optical budget could span 25 km or more and be shared/split among 128 subscribers.
Some ONTs can receive a broad range of optical spectrum from 1480 nm to 1580 nm, so making the 10G-PON downstream signal visible to G-PON receivers. As a result, ONTs must block the unwanted downstream signals with a wavelength blocking filter (WBF), a small passive optical device.