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Wireline Telephony isn’t Flat-Lining

Wed, 08/31/2011 - 8:45pm
Rick Bye, Senior Product Manager for Wireline Products at Silicon Laboratories

FTTx deployments are driving the demand for SLICs.

Wireline telephony may have been around for more than a century, but it is still a critical enabling technology for service providers deploying the latest optical fiber-based services. Fixed-line (or landline) telephony services, including VoIP, are still important to consumers and represent a major source of revenue for service providers. In fact, many competitive broadband service providers have added wireline telephony to their broadband service bundle to drive revenue growth.

Rick ByeAs a result, some suppliers of subscriber line interface circuits (SLICs) continue to invest in new semiconductor products and innovations to continue to enable costeffective, low-power, high-performance wireline telephony services as the telecommunications world transitions from copper to optical fiber-based communications.

Let’s take a closer look at the market opportunities for fiber communications and examine the requirements for the latest generation of SLICs that will best serve these markets.

Despite a well-publicized trend toward the exclusive use of wireless telephony (primarily among single people and apartment dwellers), many families continue to subscribe to landline telephone service in their homes, and incumbent phone companies and competitive broadband service providers are eager to continue to sell this lucrative service to their customers. There are a multitude of reasons why consumers continue to demand fixed-line service, including the need for landlines for alarm systems or satellite TV set-top boxes, poor in-home mobile phone reception, lower-cost international calling, or often just a desire to have a single phone number for the entire household.

The advent of VoIP technology has made it very economical for competitive service providers to deliver landline service over broadband networks, and many cable service providers in North America and many competitive DSL providers in Europe have been very successful offering “triple-play” bundles of broadband, TV and fixed-line telephony to their customers. Surprisingly, it may actually be more expensive for incumbent phone companies to offer fixed-line telephony, because the maintenance of the legacy infrastructure required to deliver wireline phone service is very expensive. This includes the cost of maintaining the miles of copper telephone wire between the phone company’s central office (CO) and the customer’s home – and the phone company's frequently ancient equipment serving that copper plant.

As operators deploy optical fiber to deliver ultra-high-speed broadband and television services to consumers’ households, they also want to be able to use that same fiber for telephony services. By delivering voice (using VoIP) and broadband service over the same optical fiber, operators can realize the operational cost savings of abandoning the old copperbased infrastructure and the equipment associated with it.

Fiber Deployment

FTTx DEPLOYMENT TYPES
Fiber-optic technology has been used in telecommunications networks since the 1980s, initially for intercontinental, inter-city and intra-city links to connect phone company central offices, switching centers and cellular base stations. In recent years, fiber has been pushed deeper into the network, ever closer to the consumer, to enable the delivery of even faster broadband communications.

The different types of fiber deployment (see Figure 1) are differentiated by how close the fiber gets to the consumer’s home.

Fiber-to-the-node/curb (FTTN/C) brings the phone companies’ CO line cards out into a cabinet in the neighborhood. A primary purpose of these deployments is to shorten the length of the copper loops to customer homes to enable deployment of high-speed VDSL2 services (50 to 100 Mbps). Some VDSL deployments use VoIP instead of “plain old telephone service” (POTS) to improve performance by eliminating interference from the telephone.

Fiber-to-the-building (FTTB) puts the phone companies’ line cards in the basement of multi-dwelling units (MDUs). Similar to FTTN/C, FTTB deployments can be used to deploy high-speed ADSL or VDSL2 services to consumers but may often use Ethernet over Cat-5 cable to offer downstream speeds of 100 Mbps or greater into apartments. FTTB deployments served more than 20 million homes in 2010 and are expected to grow to more than 50 million homes by 2015. Initially, FTTB was deployed at a faster rate than FTTH, but future FTTH subscriber growth rates are expected to outstrip FTTB growth.

Fiber-to-the-home/premises (FTTH/P) brings flexible optical fiber right into the consumer’s home or single-family unit (SFU). Now, the phone company’s telephony line card is right inside the customer’s home as part of a residential gateway that also delivers high-speed broadband over Ethernet, Wi-Fi and other home networking technologies (such as MoCA and HPNA) and offers downstream speeds as high as 1,000 Mbps (1 Gbps).

Almost all FTTH deployments will use fiber to deliver not only broadband data, but also fixed-line telephony, allowing operators to abandon the legacy copper wiring wherever they deploy FTTH services to existing homes. According to Infonetics Research, at the end of 2010, about 40 million homes were connected to FTTH services, and this level of deployment is expected to grow to 120 million by 2015.

FTTx REQUIREMENTS FOR SLICs
Telephony interfaces are implemented using a specialized interface circuit called a subscriber line interface circuit. SLICs are complex mixed-signal and high-voltage circuits that include analog-to-digital and digital-to-analog conversion, as well as powerful digital signal processing capabilities. A telephone is an analog device that requires high voltages. A SLIC generates the high voltages and currents that are required by the phone. It converts the audio to and from the phone into the digital format used in the network backbone. The SLIC also amplifies the audio signal for the phone and performs impedance matching to eliminate echoes.

SLICs have three primary states of operation, and each state requires a different high-voltage supply (often referred to as a “battery”).

  • On-hook – Phones typically require about -48 V, so the SLIC needs a supply of more than -50 V.
  • Off-hook – Phones require a line feed current of typically 20 mA to 40 mA, requiring a SLIC battery of around -35 V.
  • Ringing – Typically a ringing signal of at least 40 VRMS is required to ring a phone, but this may need to be higher with longer loops or older electromechanical phones (see Figure 2); therefore, a battery supply of at least -70 V, and sometimes greater than -100 V, is needed during ringing.

FTTN, FTTB and FTTH deployments each deliver telephony services in different ways. FTTN cabinets have hundreds of channels to support all of the homes in a neighborhood. The SLICs on the telephony line cards may need to support higher ringing voltages and line feed currents to operate over copper loops that can be several hundred meters long. The SLIC must also enable a large number of channels to fit on each line card. One way to achieve this is with small packaging.

Another technique is to use a shared “bulk” power supply to deliver the highvoltage batteries required by the SLICs. Such designs typically have two or three high-voltage supplies, and the SLIC will draw power from the appropriate supply as determined by the SLIC line state. There will be a supply of about -35 V (often called VBLO) for off-hook operation, a supply of about -55 V (often called VBHI) for on-hook operation, and a supply of -100 V or more (often called VBRING) used for ringing. These bulk supplies can minimize cost and help fit more channels on a line card, but the downside to such an architecture is significantly higher power consumption.

Rotary PhoneFTTB equipment often resembles a smaller version of FTTN equipment. Instead of cabinets, the equipment may be packaged in “pizza box” style casings and will likely support only tens, not hundreds, of lines. This equipment can be located in the basement of an apartment building or on each floor of a multi-story building. Historically, these designs have also used the same kind of bulk supplies as FTTN equipment to minimize cost and size, albeit at the expense of power consumption.

FTTH equipment is located in individual homes, whether apartments or singlefamily homes, typically supporting up to two phone lines. Some FTTH deployments integrate the fiber termination, home networking (Ethernet, Wi-Fi, etc.) and wireline telephony all in one box that is deployed inside or just outside of the home.

Other deployments (such as NTT in Japan) separate fiber termination and home networking/telephony into two separate boxes. In both cases, equipment vendors are under pressure to reduce the size of their products to make them more consumer-friendly. This requires high levels of integration and very small packaging for all of the silicon components that go into these products. To minimize power consumption, FTTH gateway designs generally use tracking supplies to provide the highvoltage batteries to the SLICs, especially for gateways that require battery backup. These designs have a high-voltage supply dedicated to each SLIC line, and the output voltage changes to track the line state of the SLIC. Many tracking supply designs will even track the ringing signal during ringing to minimize power dissipation.

To enhance the energy-efficiency of all electronics, many standards organizations are attempting to codify requirements to minimize power consumption. For example, the European Commission has published guidelines in the form of codes of conduct for power consumption of most home appliances, from washing machines and refrigerators to TVs and computer equipment, and even broadband equipment, including FTTH gateways. This desire to reduce power usage is forcing designers of FTTx equipment to modify their designs to select components and system architectures that will minimize power consumption. The SLICs that provide the telephony interfaces in these products can account for a large part of the power budget, so it is essential to use SLICs with very low power consumption and the most efficient high-voltage supply architecture. This usually means that designers need to use the latest SLIC chips in conjunction with tracking high-voltage supplies.

WIRELINE ISN’T FLAT-LINING
Although wireline telephony has been around for more than a century, it is in no danger of immediate extinction, despite the rapid growth of mobile telephony. New fiber-based broadband technologies and trends to reduce power consumption are requiring a new generation of silicon solutions to enable fixed-line telephony services. Huge growth is expected in the markets for FTTx equipment, which will, in turn, drive similar growth in demand for all silicon components used in this equipment, including SLICs.

E-Mail: richard.bye@silabs.com

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