The right strategy depends significantly on whether you expect to use 10G EPON or GPON.
Commercial deployments of 10-Gbps passive optical networks (10G PONs) are just in their infancy. But all service providers need to plan now how they will deploy these new technologies on top of their existing networks.
Luckily, the working groups that created these standards, the ITU (FSAN) and IEEE, took into account carriers’ network migration concerns. A few simple planning steps today should eliminate a lot of headaches down the road when service providers are ready for 10G PONs.
10G PON standards
To understand the 10G PON migration strategies, it is helpful to have a basic understanding of the 10G PON standards.
10G EPON. The first completed 10G standard was IEEE 10G EPON (802.3av) in September 2009. The vendor community has completed technology development based on this standard, and products are now commercially available. IEEE 10G EPON supports both 10G downstream/10G upstream as well as a 10G downstream/1G upstream formats. In both cases, the downstream transmission uses a 1577-nm wavelength. In the upstream, the 10G version uses 1270 nm and the 1G uses 1310 nm (from 1260 nm to 1360 nm).
10G GPON (G.987). The ITU ratified the 10G GPON standard, officially known as XG-PON (“X” being Roman numeral 10), in June 2010. There are as yet no known commercial deployments. The IEEE and the ITU coordinated on their wavelength selection, so XG-PON uses 1577 nm in the downstream and 1270 nm for the upstream. Figure 1 diagrams the 10G EPON and GPON standards.
FSAN, which acts as a working group for the ITU on PON standards, continued to work on a standard beyond XG-PON. This standard is referred to as NG-PON2. A variety of technologies have been discussed as part of this standard, including several approaches that use WDM PON. At last April’s FSAN meeting in Bath, UK, service providers made significant progress toward outlining this next generation standard. They chose an approach called TWDM-PON, which supports multiple wavelengths of XG-PON in multiple windows. The details of this new standard will be worked out over the coming months, but the goal is to support the current 2.5G/1.25G GPON, the XG-PON 10G/2.5 standard on multiple wavelengths, and a symmetrical 10G/10G version.
|FIGURE 1. Comparing 10G GPON and 10G EPON.|
Based on data compiled by Broadbandtrends LLC, the vast majority of PON systems in North America are GPON (see Figure 2). This dominance manifests in both the lines and number of service providers deploying the technology. But EPON has proven more popular in other parts of the world, particularly Asia. As more cable operators begin to deploy PON, observers expect that 10G EPON will become more popular in the U.S. and Canada than was the 1G standard (802.3ah) – but GPON is still expected to dominate in North America.
Differing approaches to network migration
The IEEE and ITU took different approaches to migration to next generation PONs. In the case of 10G EPON, migration does not focus on coexistence but is accomplished by the addition of a new OLT that supports both 10G and 1G downstream. The 802.3ah 1G EPON standard used a relatively wideband Fabry-Perot laser in the upstream. This wideband laser makes coexistence of wavelengths difficult. As a result, 10G EPON requires a common receiver for 1G upstream (1310-nm) and 10G upstream (1270-nm) wavelengths.
This design enables a service provider that has already deployed 1G EPON to leave its already deployed 1G ONUs in place, assuming the ONUs are designed with blocking filters for the 1577-nm downstream wavelength. It is unclear how many deployed 1G EPON ONUs have such blocking filters. If the ONUs in a particular carrier’s network do have the filters, this approach for migration to 10G EPON is elegant – except that it inefficiently uses the capacity of the 10G EPON OLT by having to support the lower-bandwidth 1G upstream at the same time as 10G upstream.
|FIGURE 2. ONT shipments in North America by technology.|
The ITU’s FSAN approach to migration focuses on coexistence between current 2.5G downstream/1.2G upstream GPON with the new 10G GPON technology. Coexistence means that the two technologies can be deployed on the same outside plant infrastructure because they use different wavelengths in both the downstream and upstream. Both 2.5G GPON OLTs and ONTs stay in service, while 10G GPON is overlaid on the optical distribution network (ODN). This coexistence is enabled because 2.5G GPON used narrow-width DFB lasers.
|FIGURE 3. Isolation between RF signals at 1550 nm and 10G wavelengths.|
From a planning perspective, GPON ONTs deployed today must have filters that block the future 1577-nm downstream wavelength of 10G GPON. Using a well designed filter is important, especially if the service provider offers RF video. In a GPON system, RF video is carried over a 1550-nm wavelength. Meanwhile, the 10G specification is centered at 1577 nm. This 27-nm separation creates a difficult design; in reality, the design has to be tighter since both standards allow some variance around these wavelengths. In actual deployments, the separation of wavelengths could be as little as 10 nm.
To test 10G coexistence with current GPON ONTs, Calix established a test setup using a 10G laser tunable over the range of wavelengths found in 10G systems. Figure 3 shows the results of these tests, which verified that these systems can achieve very high isolation of 1550-nm RF video from wavelengths that will be seen in 10G systems. That means currently deployed systems with RF video will be able to coexist with services over future 10G systems.
Planning for network migration to 10G GPON
What other steps can service providers take to better design their GPON networks for coexistence? Today, network designers typically work with a 28-dB optical budget for 2.5G GPON networks, using the typical B+ optics, which provides for a 32-way split at 20 km. The 10G GPON standard defines a number of power levels, but the typical deployment model is expected to be 31 dB of optical budget. That means 10G will be able to be overlaid easily on top of existing ODN designs.
A wavelength-division multiplexer (WDM) will be used to combine the wavelengths from the 2.5G GPON and 10G GPON OLTs. This WDM will introduce some loss in both systems. The WDMs can be designed to have greater loss on the 10G GPON (1.2 dB) and less loss on the 2.5G GPON (0.8 dB). Therefore, to ensure future compatibility with 10G GPON, today’s 2.5G GPON networks should probably be designed with a 27-dB optical budget to easily accommodate the use of the WDM.
The same design guidelines also would apply to extended reach designs. In today’s 2.5G GPON, a C+ laser provides a 31-dB optical budget, while the extended reach version of the 10G GPON standard provides a 35-dB budget.
Technology transitions can be gut wrenching and expensive for service providers. It’s important that vendors make these transitions as painless as possible from both a capital and operational expense perspective.
The BPON-to-GPON transition was greatly simplified using technologies such as auto-detect that enabled BPON ONTs to upgrade and become GPON ONTs. While that will not be possible with 10G PON systems, due to the use of different wavelengths, the standards have provided paths for straightforward migration to next generation PON technologies.
DAVID RUSSELL is solutions marketing director at Calix.