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Trends Redefining the Central Office: The Evolution of Access Networks

Trends Redefining the Central Office: The Evolution of Access Networks

9 October 2018 | Reading Time: 4 minutes


How is the central office and headend changing?

The central office has evolved somewhat since the days of the first telephone exchanges. Built in the 1870s, these exchanges would have seen switchboard operators sitting in a “central office”, manually connecting two copper wires to allow people to converse over the telephone.

Today, of course, fiber optics are used instead of copper wires and data switches have replaced manual operators. Nevertheless, the function of the central office as an access aggregation point remains applicable.

Although this function is likely to stay the same, we can expect even more from the central office in the coming years as networks are upgraded to multi-gigabit access speeds and ultra-low latency performance. The networks of the future will support a wide array of applications from virtual reality to connected cars, with today’s network upgrades laying the foundation for the IoT future.

There are four key trends now redefining the central office, including:

  • The evolution of access networks,
  • The convergence of wireless and wireline networks,
  • Virtualization through NFV and SDN,
  • and the move from central office to edge data center.

In this post, we’ll outline the first of these trends; the evolution of access networks, and how this is having an impact on the central office and headend.

The Evolution of Access Networks

Copper access networks are the legacy of the analog telephone networks that began in the 1870s. The majority of the world’s fixed internet services are delivered via the copper last mile, from telephone to fax to xDSL (digital subscriber line). Cable TV (CATV) systems that originated in the 1950s have since evolved to the HFC (hybrid fiber coax) networks of today. More recent is GPON and EPON, also known as fiber to the home (FTTX), which offer higher bandwidths and a faster transmission rate. In these networks, fiber-optic cabling runs all the way from the central office to the subscribers’ premises.

Today, we are on the threshold of a new generation of high-bandwidth, low-latency access technologies. These promise an advanced user experience, enabling new services and business models, and will evolve network architectures. There are three main technologies impacting the central office today:

  • G.fast. This is a copper local loop that enables a top speed of 1Gbps, with dedicated bandwidth per subscriber. This was standardized in December 2014 by ITU-T.
  • DOCSIS 3.1 Full Duplex. Standardized in October 2017 by CableLabs, DOCSIS 3.1 Full Duplex uses a Coaxial cable to achieve top speeds of 10 Gbps of shared bandwidth between subscribers in a service area.
  • XG-PON and NG-PON2. XG-PON and NG-PON2 fiber optics technologies can provide top speeds of 10 Gbps (XG-PON) and 40 Gbps (NG-PON2) of shared bandwidth between subscribers in a service area. Both technologies were standardized by ITU-T; XG-PON was standardized in June 2012 and NG-PON2 in October 2015.

These technologies have implications for the central office and headend:

The new access technologies will result in more ports and wavelengths to manage. CSPs will need to use either a new OLT (optical line terminal) in the central office or a new CMTS (cable modem termination system) in the cable headend. As space is a limited resource, higher density fiber connectivity solutions will be preferable and service providers will also leverage wavelength division multiplexing (WDM) technologies to get the most out of outside plant (OSP) fiber cables and limited resource in the fiber network. This will require high-density optical modules in the CO to multiplex and de-multiplex the wavelengths, as well as WDM units packaged for the OSP at the far end. WDM for fronthaul between baseband units and remote radio heads is a key example of how this solution is leveraged. Some PON technologies include the ability to manage multiple wavelengths, which may create an even more important role for WDM. Emphasis will be placed on the efficient use of fiber in the access network, as well as saving space in the Central Office.

Network functions virtualization (NFV) is starting to make its way to access networking equipment, with virtualized OLT and CMTS systems becoming available. In this new architecture, the access networking equipment is emulated with software that runs on commodity servers and is often combined with white-box SDN (software defined networking) switches. The CORD (central office re-architected as a data center) initiative offers a framework for the implementation of virtualization.

In distributed architectures, some network functions are moved from the central office or headend to locations closer to the consumer in the access portion of the network. Higher speeds are enabled when copper pairs are terminated at remote a remote DSLAM (Digital Subscriber Line access Terminal), closer to the subscriber. The fiber from the DSLAM to the central office then carries packetized Ethernet traffic. Similarly, distributed PON architectures exist for remote OLTs, comparable to a remote DSLAM. In an HFC network, the emerging remote PHY (physical RF layer) and remote MAC (Medium Access Control)-PHY architectures relocate to layer 1 and/or layer 2 functions from the headend to the optical node, resulting in Ethernet traffic from the optical node to the headend. Ethernet traffic will gain an increased share of access network traffic in the long term, with SDN enhancing service delivery across the entire network.

Advancing Virtualization

The world’s networks are rapidly becoming more virtualized, and an edge-based delivery model with a high-performance fiber-optic infrastructure is becoming a pre-requisite for delivering flexible, manageable network services. For service providers, they will likely have different migration strategies – perhaps even one for each geographic market, and as a result, forecasting demand remains a challenge. However, the mix of current and new technologies can help service providers deliver a flexible and adaptive infrastructure, and allow them to quickly tailor their services to customer demand.

You can learn more about considerations for modern network infrastructure here.


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