People often groan when they hear that their network needs to be upgraded for faster speeds or more users. Exactly what do they have to upgrade when change occurs—electronics, cabling or both?

For premises cabling based on structured cabling standards, people may have already been through half a dozen versions of UTP category-rated copper cables and four or five multimode fiber types in upgrades, as ethernet speeds jumped from 10 megabits per second (Mbps) to 10 gigabits per second (Gbps or G).

Data center techs started at 1G and are now at 100G, testing faster speeds up to 1 terabits per second. Outside plant networks, metro or long-haul are also testing links in the terabit speed range.

There is often a disconnect between the cable makers and electronics equipment suppliers. Although they often work closely together to develop the next-generation networks, sometimes they get totally out of sync. For example, UTP Cat 4 lasted a few months before Cat 5 was needed for 100-Mb ethernet. Cat 6 was supposed to support gigabit speeds but was inadequate, so Cat 6A was required. OM5 fiber was intended for multimode wavelength division multiplexing (WDM), which never caught on.

New cables needed

Electronic equipment manufacturers are always developing new technology. While early gigabit ethernet needed special Cat 5e cable, within a few years electronics were developed that worked on older Cat 5. Likewise, manufacturers developed 5 Gbps ethernet for wireless access points that ran on older Cat 5e.

Fiber optics has always been capable of higher speeds than UTP cable, but as networks topped 10G, multimode ran out of speed. Parallel optical networks were required that used multiple channels of 10 Gbps over separate fiber pairs, e.g., using 20 fibers for a 100G link.

OM5 fiber and multimode WDM never caught on because the total system cost was higher than the cost of single-mode systems, and single-mode offered easy upgrades to higher speeds. However, one equipment manufacturer came up with another solution using the multimode fiber already in place. Instead of WDM, they multiplex signals on different modes in a multimode fiber. Unfortunately, like OM5, this has not yet been a big market success.

The gigabit passive optical network (PON) technology used in fiber to the home has been more successful in premises cabling applications. The hundreds of millions of PON connections have driven prices down, giving PONs an even greater cost advantage over traditional structured cabling. PONs use only single-mode fiber, of course, and new versions of PONs allow easy upgrading to 10G, even allowing 1G and 10G networks to operate simultaneously on the same fiber.

Coherent technology

The single-mode fiber we use today is practically identical to what was installed in 1985 and can support terabit speeds and multiple wavelength channels. A big step forward has been the development of coherent transmission. Regular fiber links are limited by the inherent speed limitation of lasers, about 25–50G. Beyond that, transmitters use multiple wavelength channels on the same fiber to get higher speeds.

Coherent transmission uses a completely different technique for links over 100G. Lasers are run at constant power and modulated externally by some electro-optical tricks. The coherent transmitter can encode optical data in the amplitude of the signal pulse and in the polarization in the fiber, putting multiple bits in each signal bit, making it possible to encode very high data rates.

Coherent receivers also use a technique that can correct for most signal distortion. The result is that a coherent circuit is much faster and can span much longer distances, i.e., hundreds of gigabits per second over thousands of kilometers.

Coherent transceivers are complicated, and early ones were very expensive. However, electro-optical hybrid integrated circuits and high volume have already brought the cost down to be competitive for applications in data centers and metro networks. Soon they will be the norm for single-mode links and offer easy upgrades for current fiber networks.

Coherent technology is even being developed into PONs, a powerful combination. While today’s PONs typically max out at 10G and 64 users per port, coherent PONs are expected to operate at 100G with over 1,000 users per port—all on the same single-mode fiber used today.

Coherent transmission and WDM can often allow easy upgrades over current single-mode fiber cable plants without construction, and that generally means big cost and time savings.

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