Network modernization in an optically dominated era

Tom Nolle
10 Jun 2008

In networking as in all businesses everything comes down to the spread between cost and price. Since the early 1980s optical advances have driven down the cost of capacity by driving up the number of bits that can be sent along a path. It's hard to believe it today but only 15 years ago the average corporate headquarters site had less access bandwidth available than a consumer with fiber-based Internet access could buy today for about $150 per month.

Optics has changed everything.

Despite the revolution that network optics has created network planners still sometimes fail to fully factor in the impacts of optical trends. While this rarely leads to outright project failures it often leads to deploying less-than-optimal solutions for increasingly complex problems and opportunities.

It would be fair to say that the most significant technology planning challenge today is the effective use of optical networking. The three areas of optical impact most likely to be problematic are:

  • The PON revolution in access
  • The metro bit gradient
  • The new role of the core network.

Up to now optical improvements have been felt deep in the network where traffic could be concentrated to multi-gigabit speeds. Passive optical networking (PON) has brought optical power directly to access and outside plant increasing the potential connection speed for consumers by 1 000% or more. PON has evolved from an early version based on ATM (called APON then now renamed to broadband or BPON) to today's dominant gigabit PON (GPON) and a future 10G-PON and an Ethernet or EPON. All of these technologies create an optically spliced multi-site connection path that reduces per-user cost by largely eliminating electrical devices in the access connection.

PON planning challenges

The challenge that PON poses to planners is the 'current -versus -opportunity' -cost. A PON fiber connection can support gigabits of transmission to each home or business site without being replaced where an access connection based on electrical technology like ADSL or VDSL requires regular maintenance and replacement and must be upgraded in the outside plant (the remote) to take advantage of higher-performance standards.

There are also basic physical limits on these electrical/copper hybrid access architectures in terms of maximum capacity. But PON today will cost between four and 10 times the cost of DSL to 'pass' a customer. Will the bandwidth headroom it offers in the future justify its cost in the present‾ The answer probably lies in the economic density of the area to be served. The more dollars of potential revenue are concentrated in a given area the more likely PON is the best way to serve it.

Changes in metro area traffic

The second problem area for planners is the sharp change in traffic density in the metro network relative to the core. This 'bit gradient' is exacerbated by the delivery of commercial content like IPTV which is normally served from a metro service center in each major population zone and not centrally distributed through core network connections. Three hundred HDTV channels could require as much as 2.4 Gbps of programming delivered to every central office and yet would not necessarily generate any 'core' traffic at all. Today's IPTV schemes use IP multicasting and thus require IP devices in the metro network.

The problem is that personalized or video-on-demand (VoD) services could totally change the requirements for multicasting TV channels and substitute personal video streams for broadcasting. Modeling today's TV habits a central office (CO) with a 20 000 household service area would likely require about 64 channels to be delivered which is 0.5 Gbps if they were broadcast. If the same population of users simply consumed personalized streams of HD the users would require 160 Gbps a 300-times increase.

PON deployment could have a dramatic impact as well.

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