The need for ever-faster data-transmission rates is insatiable. The good news about that is we have the ability to go faster with fiber-optic data transmission. The bad news is that fiber is expensive. With greater usage, however, that's beginning to change as newer and faster systems emerge. So while fiber has been at the core of our long-haul telephone and Internet networks, it's finally getting cheap enough to run to the home.
Fiber to the home (FTTH) has long been the ultimate goal of most telecom carriers, but cost has either slowed or stopped that desire. Passive optical networks (PONs), with their lower cost and higher speeds, are bringing the fastest speeds to the consumer.
The drive for FTTH is related to video. Telecom carriers want to offer Internet Protocol TV (IPTV) to compete with the cable companies, which are offering VoIP services and stealing wired phone customers from the telcos. The telcos also want to expand their own VoIP services.
By using fiber, the carriers can offer IPTV, video on demand (VOD), and other video services. Furthermore, with fiber, data rates for Internet access can rise to meet the growing Internet usage, where subscribers demand ever higher speeds. Especially galling to subscribers these days are the very low upload rates.
Though most subscribers seem happy with their download rates, the growing desire to upload photos and videos to sites like YouTube and FaceBook makes current upload services seem like the dial-up of old. Thus, carriers are looking to implement the triple-play (voice, video, and data) to keep subscribers healthy (see "100-Gbit Networks On The Horizon" at www.electronicdesign.com, Drill Deeper 15995).
Some experts even predict the end of plain-old telephone service (POTS) over twisted-pair cable by 2012 or so. While the likelihood of that happening seems remote, fiber will definitely play an expanding role for major telecom carriers like AT&T and Verizon.
As old POTS subscriptions decline and with DSL rates practically at their peak, the major telcos are essentially forced to go to fiber. But thanks to the improving technology of PONs, the costs are manageable—over time.
PONs are more affordable simply because they don't use the active circuits and equipment needed for high-speed, long-haul networks. They work over shorter distances up to about 20 km and use only the fiber plus some inexpensive passive optical splitters/combiners.
At one end of the network is the carrier's equipment, generally called the optical line terminal or OLT (Fig. 1). The fiber goes out to one or more splitters that divide the optical signal into four, eight, 16, 32, or up to 64 individual channels that go to the homes. At the home, an optical networking unit (ONU) or optical networking terminal (ONT) receives the signal and connects to the PC, TV, or other networking device through a router or gateway.
In some systems, the optical cable ends at an ONU in a multi-dwelling unit (MDU), where the signal is distributed to apartments, condos, or townhouses within the unit. This distribution is usually accomplished via the existing twisted-pair telephone wiring using ADSL2+, or in newer systems, the faster VDSL2. In other systems, the fiber reaches a neighborhood digital subscriber line access multiplexer (DSLAM), which then distributes the signal via the installed telephone twisted pair using ADSL2+ or VDSL2.
There are two major forms of PON. GPON, or gigabit PON, is the latest generation of PONs developed for telephone-system distribution based on the asynchronous-transfer-mode (ATM) protocol. The first version, called APON, was short-lived and replaced by BPON, which implemented 622 Mbits/s downstream and 155 Mbits/s upstream.
GPON implements 2.488 Gbits/s downstream and 1.244 Gbits/s upstream. The downstream data is carried on a 1490nm laser, while the upstream is carried on a 1310-nm laser. Sometimes, a separate 1550-nm laser also carries TV. GPON is an official ITU standard (G.984).
The other form of PON is Ethernet PON, or EPON. An IEEE standard (802.3aq), it uses the same carrier wavelengths downstream and upstream, but is symmetrical with a 1.25-Gbit/s rate. A faster 10-Gbit/s standard is being developed. EPON, which sometimes is called Gigabit Ethernet PON or GE-PON, is widely used in Japan and some other Asian countries as well as in some parts of Europe. It's not used in the U.S., where GPON is dominant.
The download and upload speeds are fast, but of course, subscribers don't get that much. Instead, the telcos parcel out speeds according to their policies and payment plans. No one gets more than about 100 Mbits/s. Most customers get lower than that. A rate of 30 Mbits/s is minimum for most HD IPTV applications. That tends to be the lowest rate available, though 50-Mbit/s plans are also available.
PONs operate throughout the world, mostly in Asia and Europe. But usage is growing significantly in the U.S. Verizon probably has the most subscribers, with just over 1 million and with as many 18 million being projected for 2011. The company's FiOS system is delivering IPTV to about a half-million customers. The rollout of this system is geared to Verizon's investment plans for broadband. FiOS brings the fiber directly to the home.
Another major player is AT&T. Its Uverse IPTV and triple-play system is just now being turned on in the U.S., with many future networks being built. AT&T's system brings fiber to the neighborhood DSLAM and delivers the final signal via VDSL2 over the installed telephone twisted pair.
Other carriers are rolling out PONs, too. Quest, another big carrier, sees fiber not only as the future, but also as its salvation. Some experts estimate that over 50% of PONs are really installed by those rarely heard-of small local carriers. In any case, PONs are on the rise and represent the future of broadband. While cable TV will hold the lead in broadband for the time being, the telcos will continue to catch up. Eventually, they could give cable some real competition.
There's one way to tell that a segment of industry is on the rise: Look at the number of new ICs directed at that sector. A slew of new products is helping to push the PON movement forward.
Leading the pack seems to be Broadlight, which has a chip solution for almost every part of a PON system. On the carrier or OLT side, the BL3238 GPON OLT media-access-control (MAC) chip supports Ethernet packets and time-division-multiplexed (TDM) data over the GPON encapsulation mode (GEM).
The standard downstream rate is 2.488 Gbits/s and either 1.244 Gbits/s or 622 Mbits/s upstream. The chip, which features configurable upstream and downstream FEC and AES encryption, supports up to 128 ONTs. Broadlight PONmaker software speeds OLT development.
One of Broadlight's newest products, the BL3458 quad GPON optical line terminal controller ASIC, provides some of the industry's highest integration for packet and TDM transport over GPON. It can dramatically reduce power, space, and costs for multiport OLTs. At the heart of the BL3458 lie four BL3238s.
Also integrated on chip are four serializer/deserializers (SERDES) and burst mode CDRs. An embedded MIPS32 4KEc processor makes device provisioning and dynamic bandwidth allocation (DBA) easy. The PONjacket software simplifies OLT development and speeds time-to-market.
Broadlight's BL2347 is a GPON ONT chip that performs bridging and modem functions and specifically targets cost reduction. This system-on-a-chip (SoC) integrates a VoIP DSP and a four-port Ethernet switch with 1-Gbit/s performance. The GPON MAC fully complies with ITU-T G.984.
The BL2348 is a full GPON residential gateway ONT on a chip (Fig. 2). It features dual PONRunner network processors to handle the traffic, a GPON MAC, and a MIPS 24k processor. In addition, it has the four-port VoIP switch and DSP. Also on chip are standard interfaces for home-networking technologies such as 1 Gigabit Ethernet, 100-Mbit/s Fast Ethernet, 802.11a/b/g/n, MoCA, HomePNA and HomePlug, and USB.
The BL2348 gives you most of an ONT and residential gateway in one chip, greatly reducing size, cost, and power consumption. The accompanying GateMaker software lets you program gateway functions like firewall, security, SIP, management, quality of service (QoS), and NAT. Broadlight will begin sampling at the end of 2007.
Conexant's PON SoCs target ONU client products. The CX95292 XenonIIIG is for GPON systems and fully complies with ITU-T G.984, while the CX95203 Xenon-IIIE is for IEEE 802.3ah EPON (GE-PON). Both chips have full 2.488-Gbit/s downstream and 1.25-Gbit/s upstream capability with physical-layer encryption, security, bridging, and routing to ensure QoS in IPTV applications.
Also included are four concurrent VoIP channels. Each brings enhanced routing capabilities combined with deep classification and filtering support to enable the best QoS. Interfaces are additionally provided for Gigabit Ethernet, MoCA, HomePlug, and HomePNA.
Cortina Systems has just introduced its second-generation Ethernet PON chip sets. The CS8021 is a highly integrated OLT on the central-office side of the PON. It supports four EPON ports in a single chip, so equipment manufacturers can build high-port-density chassis or stackable rack-mount systems. An extensible SDRAM frame buffer facilitates QoS. On the CPE side, the CS8016 ONU is a single-chip solution with embedded frame buffer memory, RAM, and both PON and Ethernet SERDES. A built-in ARM9 processor is used for management and control.
Network chip supplier PMC-Sierra also recently made several PON announcements. These are based on the PON chip designs from Passave, which PMC-Sierra recently acquired. Using the company's GigaPASS architecture, the PAS65311 GPON ONT reference design includes a fully featured ITU-T G.984 GPON MAC, an advanced classification engine, robust QoS queuing, advanced VLAN bridging and manipulation, and IPTV filtering, all performed at line-rate speeds for all packet sizes.
The GigaPASS architecture integrates multiple hardware-based, packet-processing engine subsystems to deliver wire-speed packet processing at all packet sizes. It combines them with a control processor supporting Linux and VxWorks operating systems and extensible middleware platforms. Available now, the reference design features the PAS6201-G0 and PAS6211 (Fig. 3).
Key to any PON system is the splitter/combiner. Ignis Photonyx AS's IXA70 line includes models in 1xN configurations with two, four, eight, 16, 32, and 64 lines that can be used to combine or split the optical signals. Also, they're made with silica-on-silica planar light-wave circuit technology. With low polarization dependent loss, they have an insertion loss from 7 dB on the 1x2 unit to 16.5 dB on the 1x32 device.