VDSL2 can operate at 50 Mbps sustained/100 Mbps peak, but fiber to the home with virtually unlimited bandwidth is the end game. GPON (Gigabit Passive Optical Network) has emerged as the preferred delivery. This article examines the migration architectures.
An earlier article indicated that the combination of fiber to the node and VDSL2 running at a sustained capacity of up to 50 Mbps represents a powerful, pragmatic solution that should enable operators to expand their current 20Mbps service offers and go on to build a robust market for triple play services with multiple concurrent HDTV streams. Details are available here.
A follow-up article outlined a distributed architecture that allows the VDSL2 line cards to be deployed nearer to the various locations. So far so good.
Distributed GPON architecture
The GPON architecture, shown below, that Alcatel-Lucent is proposing is similar (see figure 2). The central DSLAM shown below (ISAM in Alcatel-Lucent (News - Alert) parlance) is replaced by an OLT (Optical Line Terminator). The distributed ISAMs are replaced by optical splitters, which can be placed underground since they are passive and require minimal maintenance.
These devices divide (split) the incoming signal into a maximum of 64 output connections. Some operators prefer two-tier architecture, whereby the OLT connects to a splitter having four outputs that connect to four splitters having eight outputs, which in turn connect to the ONTs (Optical Line Terminators). This configuration minimizes the amount of fiber that needs to be deployed.
Optical splitters are relatively cheap devices and so is fiber. More than 50 percent of the CAPEX for FTTH comes from the digging and trenching. This two-tier architecture facilitates re-use of existing infrastructures, minimizes the amount of fiber that has to be laid, but it does introduce some complexity.
In Figure 1 at right, Alcatel-Lucent parlance IP DSLAMs are knows as ISAMs (Intelligent Service Access Managers. These are the Nodes in a regular FTTN deployment, but in this architecture the node is split into smaller nodes that are located close to the end locations.
As shown below in Figure 2 below This fiber to the home architecture has a low CAPEX; it scales and the end locations can be up to 20 kms (approx. 12 miles) from the OLT. The ONTs (Optical Network Terminators) are normally deployed in the home, where they convert fiber-optic/light signals to copper/electric signals.
Passive benefits
Remote DSLAMs have to be powered and the equipment has to be deployed above ground. PON systems are passive; no power is needed in the out-side plant and the splitters can be deployed underground, although most carriers don’t employ this option.
Fiber deployment in the access loop is a costly business and the approach adopted by most service providers is to take fiber to the most economically viable point, e.g. FTTN, while planning to take it all the way to the customer premises at some future time when market conditions are right.
Anticipating, planning and preparing for a full FTTH scenario from the start can realize significant CAPEX savings. Simple measures such as deploying enough fiber in the feeder section for future migration and employing VDSL2 street cabinets that can host GPON splitters underground combine to reduce the total cost of ownership.
The technology
Alcatel-Lucent’s 7342 OLT has interface slots for the line termination and network termination cards. Each card supports four ports, which in turn support up to 256 ONTs. This indicates the ability of GPON technology to scale. Each GPON shelf currently supports up to 3584 ONTs (7168 in future) using just four glass fibers.
GPON provides an enormous amount of bandwidth — 2.5 Gbps downstream and 1.25 Gbps upstream — over a single strand of glass.
Starting at the OLT, voice and data signals are transformed from an electrical format into optical signals. This traffic is then sent over the fiber network to the appropriate ONT, where it is separated back into electrical voice and data, as shown in figure 2.
The core underlying technology is still Ethernet, with GPON Encapsulation Mode (GEM (News - Alert)) as the packaging format. Each feeder fiber can be shared by up to 64 ONTs. Although multiple users share the optical network, robust QoS and bandwidth mechanisms ensure that the traffic is correctly prioritized and that each user gets the required bandwidth.
Hybrid deployments
As indicated in the earlier article, VDSL2 running at a sustained capacity of 50 Mbps allows network operators to extend their current triple play services. VDSL2 is also a business-like way to finance the investment needed for widespread fiber deployment.
Right now the VDSL2 window is wide open and it will stay that way for some years. However, fiber is the ultimate end game, primarily because of the OPEX (News - Alert) savings and the future proof fiber capacity, but right now it’s not the additional bandwidth. Therefore in areas where there is good quality copper in the ground and where distances to the central office are not too long, mixing and matching VDSL2 with FTTH, which has a long reach, may be the preferred option. Telenor (News - Alert) in Norway and Telefonica in Spain have opted for this type of hybrid architecture.
Looking ahead
GPON meets today’s needs, but it won’t be the last word in passive optical networking. Eventually, user demand will require more capacity, and network owners will want to upgrade their networks while continuing to use their existing fiber infrastructure. The 10G GPON approach scales the time domain and provides a 10Gbps downstream and 2.5Gbps upstream capacity to each PON interface. Scaling the wavelength domain is an alternative approach. This is done using Wave Division Multiplexing (WDM). In a WDM-PON, signals would be sent over 32 different wavelengths (colors). However, this technology still needs more research and development before it becomes economically viable.
At this year’s Broadband World Forum Alcatel-Lucent demonstrated a 10G GPON backhaul solution for LTE (News - Alert). The standards are still being ratified, so see this as a proof of concept demo. At the same time the company showed how 10G PONs can co-exist with the current 2.5G solutions on the same fiber (or Optical Distribution Network). This is important for enabling cost-effective migration from current PON to 10G GPON (see image below).
This scenario below indicates that there is no forklift upgrade when moving up to 10G. This means that there is no impact on outside plant and that the current investment in PON infrastructure is protected.
In Figure 3 shown above, this scenario indicates that there is no forklift upgrade when moving up to 10G. This means that there is no impact on outside plant and that the current investment in PON infrastructure is protected.
Operators that move up to 10G PON will most likely target specific market segments, e.g., advanced business users and fiber-to-the-building backhaul with 10G PON. Solutions that employ today’s PON technology will be used for the residential market.
Initially 10G PON will happen in countries that have deployed fiber nationwide, e.g. the pioneering APAC countries such as Japan or MSOs in North America. These early movers will get bragging rights and an attractive marketing pitch. For example, consumers might switch suppliers knowing that the extra bandwidth is there whenever they need it.
Conclusions
Alcatel-Lucent has articulated a clear, compelling migration strategy. The basic premise is to leverage the legacy local loop by bringing fiber to the most economically viable point and to move up from 20 to 50 Mbps.
The next step is to deploy a GPON infrastructure, the timing of which will depend on market conditions. And a later stage this can be followed by 10G PON. Looking even further down the road we have WDM, which has an awesome bandwidth. Modern systems can handle up to 160 signals and can therefore expand a basic 10 Gbps fiber system to a theoretical total capacity of over 1.6 Tbps over a single fiber pair.
Bob Emmerson is TMC's European Editor. To stay abreast of the latest news affecting the European market, check out Bob's columnist page.
Edited by Amy Tierney
Internet Telephony Magazine
Click here to read latest issue
CUSTOMER 
Cloud Computing Magazine
Click here to read latest issue
IoT EVOLUTION MAGAZINE
