Electronic Design
Wired Communiations Continue To Carry The Primary Network Load

Wired Communiations Continue To Carry The Primary Network Load

While wireless gets all the attention, hype, and coverage, wired communications systems are still the foundation of modern communications. That includes the Internet backbone of optical fiber as well as the miles and miles of cable TV coax and fiber, Ethernet local-area network (LAN) cables, and other twisted pair in the industry.


Even the cellular industry still heavily relies on wired T1 and fiber connections for backhaul. Yet for a very mature communications business, wired technology continues to grow and get better (see “Wired Communications Connect To Varying Degrees Of Market Success”).

40G/100G and Other Optical Technology

Higher speeds, lower power consumption, and smaller size—those are the perpetual goals of all communications gear. So it is with optical communications. We thought that 10 Gbits/s or 10G was the ultimate until 40G came along, and now we’re seeing 100G equipment emerge.

The IEEE has established the 40G/100G LAN and metro-area network (MAN) standards (802.3ba), which are now being translated into hardware and systems. The Optical Transport Network (OTN) standards of the International Telecommunications Union (ITU) are also expanding in the core wide-area networks (WANs). All are boosting speeds to unheard of levels to deal with the continuing growth in Internet traffic.

There are two parts to this trend. First, the new lasers, photodetectors, modulators and demodulators, multiplexers and demultiplexers, and transceiver modules mst be created to handle the higher speeds. Second, the networking protocols must be developed.

Up to now, most optical signals were of the intensity modulation/direct detection (IMDD) type where a laser beam was turned off and on or switched between intensity levels to create the bits. To achieve higher speeds and to overcome the distortions of the fiber, broadband modulation methods are being applied. Besides polarization multiplexing, phase modulation techniques like quadrature phase-shift keying (QPSK) can multiply spectral efficiency.

For long-haul fiber (greater than 1000 meters), the industry has adopted the dual-polarization QPSK (DP-QPSK) system to achieve 100 Gbits/s on a single fiber. Two parallel bit streams can be sent simultaneously over fiber if one is vertically polarized and the other horizontally polarized. The 90° of separation prevents interference with one another and makes for easy detection. It also provides a 2-bit/Hz data stream.

Now add QPSK, which uses 90° shifted sinewaves combined in a modulator to provide four phase signals producing another 2-bit/Hz stream. The combination therefore delivers a 4-bit/Hz stream. This means the data streams run at one fourth the line rate.

For example, 25-Gbit/s streams are used to get a 100-Gbit/s line rate or four bits per baud. Standard CMOS circuits can be used to build devices for this application. With such a system, it’s possible to achieve the 100-Gbit/s rate on up to 1000 to 1500 km of fiber without regeneration.

DP-QPSK uses a coherent optical receiver to recover the data. Polarization beam splitters separate the two polarized streams. These signals are mixed with a local oscillator (LO) laser to recover the in-phase (I) and quadrature (Q) components for each stream. The I and Q signals are then digitized and a DSP is used to recover the phase information.

CMOS analog-to-digital converters (ADCs) and DSPs are now fast enough not only to recover and demodulate the signals but also to make it possible to perform chromatic and polarization mode dispersion compensation.

On shorter hauls, there are many ways to get 100G. Four lanes of 25-Gbit/s or 10 lanes of 10-Gbit/s streams are common. There’s a variety of dense wavelength division multiplexing (DWDM) schemes as well. New multi-source agreement (MSA) standard modules are being developed for the various services.

Luxtera is one of the companies making 100G happen. Its single-chip 100-Gbit/s optical transceiver targets next-generation cloud computing datacenters and high-performance computing (HPC) optical connectivity (Fig. 1).

The single-chip CMOS opto-electronic transceiver includes four fully integrated 28-Gbit/s transmit and receive channels powered from a single laser for an aggregate unencoded data rate of up to 112 Gbits/s. It will work for 100-Gbit/s Ethernet, OTN, and InfiniBand applications as well as emerging Optical Internetworking Forum (OIF) Short Reach (SR) and Very Short Reach (VSR) electrical interconnect to host systems.

Luxtera’s Silicon Photonics technology utilizes the mainstream CMOS fabrication processes to deliver on-chip waveguide level modulation and photo-detection along with associated electronics, resulting in a fully integrated single-chip optical transceiver. Light from a single co-packaged laser is used to power multiple optical transmitters on a chip, eliminating the need for multiple lasers and reducing transceiver cost and power consumption.

The Luxtera transceivers will enable 25-Gbit/s single links and greater than 100-Gbit/s aggregate links in very small packages to allow large numbers of transceivers per one rack unit (1RU), achieving the high front-panel density that datacenter managers demand in space-limited server racks and switches.

“As the bandwidth demands soar and longer reaches are needed in bigger datacenters, managers are requiring more high-bandwidth optical interconnects throughout their systems on a volume scale unheard of in the optical transceiver markets,” says Brad Smith, vice president and industry analyst at LightCounting.com.

“Going forward we expect to see optics widely replacing copper interconnects, especially at reaches past 7 m. Data rates continue to increase and push well past 10 Gbits/s. As a result, copper cabling solutions are finding it increasingly difficult to stay in the game, and it is akin to hitting the sound barrier,” Smith says.

“Additionally, VCSEL-based (vertical-cavity surface-emitting laser), optical transceiver companies are having tremendous difficulty bringing to market 25-Gbit/s per channel transceivers. Silicon Photonics technology, such as from Luxtera, enables combining transistor electronics with photonics on the same chip and easily achieves greater than 25G modulation rates at reasonable costs to the end user. Lastly, Luxtera’s products enable optical interconnects from mid-circuit board in big systems with a reach over 2 km,” he says.

While Luxtera provides the basic intensity modulated laser technology with direct photodiode detection common to most optical networks, long-reach networks are relying on the latest DP-QPSK coherent optical systems that further boost the data rate.

NeoPhotonics is another company whose products are making 40G/100G systems possible. Its ITLA-30xx-C and ITLA-30xx-L compact C and L band integrable tuner laser assemblies make it possible to build flexible DWDM equipment (Fig. 2). These 1550-nm continuous-wave (CW) distributed feedback (DFB) lasers have a tuning range of more than 36 nm. Optical power output is 20 mW. They suit both Ethernet and OTN equipment. The control and electrical interfaces are multi-source agreement (MSA) module compliant.

On the network protocol side of the equation is the move to OTN. Designated G.709 by the ITU, it is the latest system for transporting optical signals at rates from roughly 1 Gbit/s to over 100 Gbits/s. Only the digital formatting is defined.

OTN does not define specific optical physical-layer (PHY) formats like on-off modulation or phase modulation methods. It does provide for switching and multiplexing as well as the needed management and supervision of the network. OTN is gradually replacing the older Sonet/SDH optical networks. The Sonet/SDH time-division multiplexed system is designed primarily for digital telephony and supports data rates to almost 40 Gbits/s.

Today, the data carried on the optical networks is video and Internet traffic rather than digital telephone calls, and higher speeds (100 Gbits/s) are necessary for the future. This calls for an Internet protocol (IP) system that OTN provides. OTN also enables simplified usage of wavelength division multiplexing (WDM). Dense WDM makes it possible to more easily boost data rates on the fiber.

Another real benefit of OTN is that it is a digital wrapper technology that lets it carry the traffic of other protocols like Sonet/SDH, Ethernet, Fibre Channel, TCP/IP, and multiprotocol label switching (MPLS). The transparent transport of any client signals makes OTN the choice for a global fiber network. The client data is formatted into OTN frames before transmission.

Another key advantage of OTN is its stronger forward error correction (FEC) coding. It uses a Reed Solomon RS (256, 239) code that is added to each OTN frame. This FEC produces a coding gain of 6.2 dB that improves the signal-to-noise ratio and boosts link reliability. Furthermore, it increases the maximum optical cable span length of the link and increases the number of potential DWDM channels. Overall, some optical component specifications and parameters can be relaxed, potentially lowering costs.

OTN conversions are already under way, but it will take time to fully convert older systems and provide for the new capital budgets required, not to mention new monitoring and management systems. Look for increased pressure to upgrade the optical backbone as video continues to dominate the traffic.

Processing Muscle

When you’re sending and receiving data at 40G or 100G, you really need some circuits to process it. That job up until recently fell to FPGAs. Now we’re beginning to see chips that will handle that kind of processing.

Cortina Systems recently introduced its 100G application specific standard product (ASSP) for the converged OTN and Ethernet markets. The CS605x is a breakthrough family of highly integrated processors supporting the transport and aggregation of 10G, 40G, and 100G signals.

The continual growth in bandwidth demand propelled by services such as video streaming, mobile data coupled with emerging cloud computing, and hosted application services is driving service providers to upgrade to higher-capacity, faster, and more efficient networks deploying 40G and 100G wavelengths.

Building upon its 10G and 40G leadership, the CS605x provides advanced analog integration, proven optical transport networking, and FEC flexibility. It meets the requirements of both OTN and Ethernet networks, permitting new applications for the 100G converged market, including datacenters, carrier Ethernet, core and edge router applications, DWDM, and packet optical transport. Also, it provides direct interfacing to the latest 100G, 40G, and 10G optical modules, enabling leading OEMs to deploy highly efficient line cards.

The CS605x supports the latest OTN and Ethernet standards with features that enable designers to create one line card for 10G, 40G, and 100G signals (Fig. 3). It handles 100G (OTU4 and 100GE) and 3x40G (OTU3 and 40GE) interfaces and can aggregate and map multiple protocols and client applications at the 10-, 40-, and 100-Gbit/s rates.

Key features include OTN 10G/40G and 100G framing and mapping, 100G Ethernet and 3x40G Ethernet PCS/MAC termination with performance monitoring, and support for IEEE 1588v2 Transparent Clock mode, as defined in IEEE P1588D2.2. Integrated jitter attenuation and clean-up phase-locked loops (PLLs) eliminate the cost and board space of separate clean-up PLLs.

The CS605x family offers multiple FEC options and overhead selection including standard FECs as well as proprietary FECs for flexibility in designing the optimum system. Direct interfaces to 100G and 40G optical modules supporting OTL4.10, CAUI, OTL3.4, and XLAUI are provided in addition to a 120G-capable Interlaken packet interface to network processing units (NPUs), traffic managers, and FPGAs. The CS605x devices are slated for production volume for the first half of 2012.

The core optical networks are undergoing a major transformation from the older 10G systems to new 40G and 100G systems. OTN is the basic carrier, as it can multiplex almost any other previous optical protocol including Sonet/SDH and carrier Ethernet. While current prices for 100G equipment are high, they will eventually decline as carriers build out their systems to support the necessary faster rates.

As for what the future holds, think 400 Gbits/s and 1 Tbit/s. Already the IEEE and the ITU-T working groups are examining ways to implement next-generation optical systems that can blaze away at 400 Gbits/s or an unheard of 1-Tbit/s rate. Some high-volume Internet providers could use faster systems now, but a 400G standard may emerge by 2014 with a 1T standard by 2017. For example, 10 lanes of 100G would produce the 1T rate.

There are many issues to resolve before these standards come forth. For instance, multiple-lane solutions, new modulation methods, line side interfaces, and even MIMO are under consideration. Special attention will be paid to the ultra-long-haul systems for underwater deployment. As these long-haul developments occur, they will drive the 40G and 100G systems down to the metro network realm—a good thing for all.


While OTN fulfills the long-haul backbone network needs, Ethernet continues to serve the LAN and MAN/metro networks. Ethernet standard 802.3ba covers the 40G and 100G equipment standards. This broad standard includes variations from 1 m over a backplane and 7 m over copper cable to versions that hit 40G on 100 m of multimode fiber and 100G on 40 km of single-mode fiber.

The Ethernet 40G and 100G rates are fully compatible with equivalent rates, making the two standards interoperable. The OIF has defined multi-source modules in 10x10G and 4x25G formats to ensure equipment compatibility.

While optical connections advance, the LANs and datacenters are still primarily cable with versions of unshielded twisted-pair (UTP) CAT5 and CAT6. As data rates have increased to the 10-Gbit/s level, more switches and servers have upgraded to 10GBaseT interfaces.

Up to now, they have been expensive and power hungry. As a result, that standard has not been as widely adopted as many predicted years ago. As the semiconductor companies continue to ride the Moore’s law curve to ever smaller nodes, 10GBaseT is finally coming into its own. Some even say that 2012 will see breakthrough adoption of this standard.

One company helping to make that happen is PLX Technologies. Its TN80xx 10GBaseT PHY chips offer lower power consumption and numerous advanced features thanks to their 40-nm design. The B0 versions will allow new network interface card (NIC) and LAN on motherboard (LOM) designs that should significantly increase the use of the standard. With many 10-Gbit/s switches already in place, the time is right for a PHY with the right characteristics to match.

The B0 series boasts low power consumption. The base figures are 3.5 W at the 100-m specification, 2.5 W at 30 m, and 2 W at 7 m. All the usual interfaces are available, but these PHYs include a KR interface for backplanes, something missing in other solutions. They offer an enhanced version of Energy Efficient Ethernet (802.3az) and a Wake on LAN addition as well.

The chips also support the IEEE 802.1ae standard MAC Security. This encryption takes place within the LAN, modifying the Ethernet packets to protect against invasion inside the LAN (Fig. 4). It allows secure private networks to be established while IPSec or other security takes place outside the firewall.

The TN8020 is a single-port device while the TN8022 and TN8044 are dual- and quad-port devices, respectively. Samples will be available in February.

Power-Line Communications

Progress continues to be made in power-line communications (PLC), both in home networks and the Smart Grid. While wireless dominates home networking technology, PLC is increasingly being used where wireless reliability issues limit whole home coverage. After years of multiple standards, the PLC movement has settled on only a few.

Today, the most widely adopted standard is HomePlug from the HomePlug Alliance. Its OFDM-based (orthogonal frequency-division multiplexing) system is available in a variety of formats. A basic low-speed version for home energy monitoring and control called HomePlug Green PHY (GP) has a raw date rate of 3.8 Mbits/s. A high-speed version called HomePlug AV2 can deliver a PHY data rate up to 1 Gbit/s for home HD video distribution.

The HomePlug technology recently became the basis for the IEEE’s 1901 PLC standard, making it a choice of IC vendors like Broadcom, Qualcomm, and STMicroelectronics. HomePlug also supports the IEEE’s 1905.1 converged digital home networking standard.

The HomePlug Green PHY version targets Smart Grid applications like HVAC/thermostats, smart meters, home appliances, and plug-in hybrid vehicle charging. HomePlug GP will also support the IEEE 1901.2 PLC standard. This low-rate narrowband standard is designed for longer range, outside-the-home applications such as utility backhaul in Smart Grid applications.

Finally, the HomePlug Alliance recently joined the Wi-Fi Alliance and the ZigBee Alliance in an agreement that defines an interoperable arrangement with and between these different wireless standards. Called the Smart Energy Profile 2 (SEP 2), it lets homeowners use various energy management devices together.

The National Institute of Standards and Technology (NIST) chose SEP2 as an approved standard for home energy management devices. While other competing PLC standards such as HomePNA and Multimedia over Coax Alliance (MoCA) have found niches, HomePlug seems to have emerged as the leader in this space.

Another wired technology for home networking making its way forward is G.hn. It too is a HomePlug competitor. G.hn is the ITU-T’s standard that provides a common protocol that can use the power line as well as home installed cable TV coax or twisted-pair telephone wiring as the transmission medium.

While G.hn has not been widely adopted, chipsets from Lantiq, Marvell, and Sigma Designs are paving the way to G.hn products. Marvell’s G.hn chipset comprising the 88LX3142 CPU and the 88LX2718 analog front-end interface can serve any of the basic wiring media (Fig. 4). It also introduces MIMO that boosts speed and builds in added link reliability (see “1-Gbit/s Transceiver Chipset Expands G.hn’s Potential” at electronicdesign.com).

The jury is out on the future of G.hn, but look for it in coming Internet protocol television (IPTV) set-top boxes, over-the-top (OTT) boxes, broadband routers, and gateways in the coming years.

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