1. 2007 /
  2. October-December 07 /

Technology update: fiber and copper

In the long run, optical fiber will probably win its “battle” with copper. But copper still has great potential, and, for now, both are ready to take the next step.

December 11, 2007

In many ways copper and optical fiber complement each other. An operator might have a broadband copper network for the access and an optical fiber network for the backhaul, leading to the curb.

Jan Söderström, who is responsible for broadband development at Ericsson Research, says many things influence the operator's choice.

"It depends on the quality of their existing copper network, and on whether they are ready for a big financial investment in future technology, or prefer to update the copper. It also depends on the geography and regulations of their particular country."

Ultimately, it all boils down to a trade-off between capital (capex) and operational expenditure (opex). The copper broadband technology VDSL (Very High Speed Digital Subscriber Line) installed closer to the customers means less new fiber cable but more maintenance.

"At Ericsson Research we are now trying to bring down the capex costs for fiber to the home, and at the same time improve opex for the DSL technology," Söderström says.

So far the development of DSL has been based on using higher frequencies. VDSL2, for example, uses bandwidth of up to 30MHz, giving 50Mbps compared with ADSL2's (Asymmetric Digital Subscriber Line 2) 2.2MHz and 25Mbps downstream - both over about 500m. This race for higher frequency for DSL has for the moment come to a standstill.

"Now we are studying Dynamic Spectrum Management (DSM), a vectoring technology to cancel crosstalk between DSL phone lines that are close to each other," Söderström says. "The potential for DSM is to get two to three times the VDSL capacity: for example, 100Mbps symmetrically over 500m of copper cable, at a low price."

The fiber optical broadband technology GPON (Gigabit Passive Optical Network) will also take a big step forward. The present GPON products, e.g., the Ericsson EDA1500, work with a 2.5 Gb/s laser in the downlink towards the end users. When split into between 16-64 parts, the stream brings all the information to each user, where the receiver sorts out their information by way of a code. Upstream TDM (time-division multiplexing) technology is used for synchronization.

This step to higher capacity can be taken in two different ways:

  • One is higher bitrate GPON - 10Gbit PON, where a 10Gbit laser is used for the downlink to the end users, giving a performance increase of a factor of four.
  • The other method is WDM (wavelength-division multiplexing) technology. Lasers with different wavelengths carry all specific channels, which are then split for each user by a prism, also called a wavelength demultiplexer.

Söderström says: "Next year we will have concepts and early demos ready for 10Gb PON, WDM-PON as well as the new VDSL technologies."

Facts

  • VDSL, Very high speed DSL (Digital Subscriber Line) is the latest way to transmit high-speed (50 Mbps) data over copper wires by adding high frequency signals parallel to the ordinary voice channel which uses much lower frequencies. The range for VDSL2 at these speeds is about 500m from DSLAM (Digital Subscriber Line Access Multiplexer) unit to end user.
  • GPON (Gigabit Passive Optical Network) is a technology for even higher bitrates. With GPON the optical signal is split as near as possible to the end user. "Passive" means there is no external power used for the splitters. A point-to-multipoint (PMP) network consists of a GPON optical line termination at the central office and a number of optical network units near the end users.

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