Distributed Access Architecture
What is Distributed Access Architecture?
Distributed Access Architecture (DAA) is a method of decentralizing cable networks by relocating functions that have typically resided in the headend or hub to intelligent fiber nodes, closer to the subscriber. Moving these operations away from the hub helps to relieve the space, hardware and cooling constraints of the headend as node counts continue to grow exponentially. A 10 Gb Ethernet fiber link is used to connect the remaining components to the intelligent node replacing the previous analog optical link.
As distributed access architecture has taken shape, variations have evolved that strategically move select portions of the architecture downstream. Remote physical layer architecture (R-PHY) moves the modulation and demodulation to the fiber node and leaves other functionality at the headend. Remote MAC-PHY, or R-MACPHY, also positions the processing MAC layer at the node. This leaves only servers, switches and routers at the headend.
A third approach known as Split-MAC moves the PHY layer but only a portion of the MAC layer functions to the fiber node. The implementation of R-PHY and R-MACPHY architecture can also be a staged approach, depending on the operator’s existing equipment and preferences.
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What is Distributed CCAP Architecture?
Distributed CCAP is a subset of distributed access architecture that is based on the converged cable access platform introduced in 2011. CCAP technology was designed to upgrade and replace the cable modem termination systems (CMTS) at the headend by unifying all switching, routing and QAM functions and transmitting data and video from the same device.
All channels became digital and all traffic became IP based. This eliminated the need for combiner functionality at the headend. Replacing the CMTS with CCAP produced incremental efficiencies with decreased energy consumption and improved overall QoS, but no substantial space savings were evident since the physical equipment size did not significantly change.
CCAP takes a sizable leap forward through the implementation of distributed CCAP architecture. Much like DAA, the PHY layer, DOCSIS MAC layer, or both can be migrated to the intelligent node. The timing issues caused by the separation of the PHY and MAC layers can be eliminated by combining these functions at the node.
Deploying Distributed Access Architecture
The deployment of conventional CCAP architecture, as it ultimately dovetailed with the onset of distributed access architecture, provides a case in point for measured and far-sighted approaches to deployment as the sands of technology rapidly shift. Staged approaches to deployment can be the most cost effective because Opex logistics can become increasingly complex as more functions are migrated from the headend to the nodes. Operators may also choose to bypass levels of deployment altogether as new innovations like virtual DAA loom on the horizon.
To support these deployment efforts, CableLabs has moved to introduce new specifications for DAA and distributed CCAP that standardize interoperability between the DAA devices of different manufacturers. R-PHY is a sound transitional deployment step for many with immediate ROI potential.
Although the equipment change may be considered incremental, the common ingredient for any distributed access architecture deployment is the extensive retrofitting of digital nodes and the deeper fiber runs that connect to them. Although deployment strategies may differ substantially, the need for ongoing distributed architecture deployment to buttress demand is no longer in question.
Benefits of Distributed Access Architecture
The most obvious and important benefits of distributed access architecture are the space, power and HVAC demand reductions at the headend (hub) location. This becomes increasingly valuable as subscriber rates and bandwidths multiply and improvements in traditional headend equipment efficiency and density struggle to keep pace.
The digital optical link ultimately leads to lower operation and maintenance costs and improved network visibility. Relocating the PHY layer closer to the end user also produces quality and performance benefits with improvements in speed, noise reduction, and modulation all directly resulting from the shift.
With the scope of headend operations reduced, the equipment can be consolidated with more service groups potentially originating from the same physical space. This helps set the stage for gigabit-plus broadband, producing a scalable architecture that aligns with the FTTx build-out model.
The transition to digital fiber allows for the multiplexing of more optical wavelengths and higher order QAM for improved spectral efficiency which boosts overall network capacity as the fiber migrates deeper into the network. The digital cable is also able to transverse greater distances (80km) with higher throughput than its analog predecessor.
Challenges of Distributed Access Architecture
As with any significant advancement, distributed access architecture also creates inherent challenges that must be overcome. The crowded headend of conventional architecture can still be beneficial for some aspects of maintenance logistics since equipment and connections are centralized. DAA increases the importance and functionality of the fiber nodes such that troubleshooting and upgrades that were once performed at a single location are now geographically dispersed.
These node positions will now include various outdoor facilities with inherently harsher environmental conditions that can sometimes be less conducive to servicing. Environmental testing of equipment becomes paramount to mitigate these new constraints. The decentralization inherent to DAA also produces potential opportunities for vandalism, theft or other damage in the field with greater QoS and financial consequences.
Other distributed access architecture challenges include the interoperability and synchronization of hardware elements that were previously co-located but might now be miles or hundreds of miles apart. These potential challenges are vastly outweighed by the benefits of DAA, although new modes of planning and expertise are essential for successful implementation.
Fiber Deep and Distributed Access Architecture
The process of driving digital fiber optic technology closer to the customer to improve efficiency and performance is known as fiber deep. Node+0 is the most extreme form of fiber deep, meaning the optical fiber extends all the way from the headend to the last-mile fiber node which provides service to the end user, and all amplifiers are removed from the lines except for those in the nodes themselves. Very few operators have significant plans for node+0 today, most are reducing amplifier cascades down to node+3 with plans to evaluate the economics of n+0 vs fiber to the home (FTTH) as the next future step.
Fiber deep in concert with distributed access architecture produces a viable alternative to FTTH with a less intrusive overhaul of existing infrastructure required for implementation. While many cable operators have chosen to commit themselves to extensive FTTH transition. DOCSIS 3.1, DAA and fiber deep have empowered the existing hybrid fiber-coaxial (HFC) infrastructure to offer gigabit services and keep pace with speed and bandwidth demands.
Centralized vs. Distributed Access Architecture
Despite the numerous advantages of distributed access architecture, some operators have taken a measured approach to adoption. The sunken investment in existing centralized architecture infrastructure can influence transition planning from the ROI or cost perspectives. Security is another important factor leading some to favor a centralized approach. The central headend location can be securely locked, and access can be closely monitored to prevent equipment or data theft and sabotage. Maintenance and service responsibilities associated with DAA can also be an influential factor considering the distributed access architecture approach could potentially necessitate travelling to a wide geographic area for recovery tasks that could once be performed in a controlled, central location.
The ongoing upswing in demand for high-speed data services is certain to expose bottlenecks that only become more insurmountable as the usage spikes. For cable operators, the burden on headend architecture became one of the most challenging obstacles to overcome.
Downsizing and integration of electronics can go a long way towards addressing these concerns, but pressures on headend capacity accelerated so quickly that a complete paradigm shift was necessary. Distributed access architecture, distributed CCAP architecture and fiber deep not only remove the bottleneck by shifting essential functionality downstream, they also improve QoS and introduce scalability factors to future proof this architecture as the demand inevitable grows.
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