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As telcos turn to triple-play services (video, voice and high-speed internet access) to reduce churn and increase revenue, they need to find a way of delivering TV and video to the end-user as cost-effectively as possible. Each of the major point-to multi point PON variants – BPON, GPON and EPON – offers two different methods of offering TV/video service: RF (radio frequency) overlay and IPTV. However, point-to-multipoint PON operators could use both methods simultaneously for TV and video service delivery if they were seeking, say, a gradual migration to IPTV.

RF overlay
In a point-to-multipoint PON architecture, one wavelength frequency is assigned for downstream data and telephony (1490 nm) and another is assigned for upstream data (1310 nm). With the RF overlay approach, through the use of wave-division multiplexing (WDM), a third downstream frequency is assigned – 1550 nm – which is dedicated to video broadcast. RF overlay is capable of delivering all types of video service: analogue broadcast; digital broadcast and HDTV; and video-on-demand.

Advantages
■ As TV broadcasting is assigned to a dedicated frequency it will not eat into the data throughput of the PON.
■ It offers a comparatively short entry-to-market time to deliver TV services (no set-top boxes are needed for the delivery of analogue broadcast over coaxial cable in the home).
■ Through the use RF overlay, operators can gradually migrate to IPTV (by using both delivery systems simultaneously) without the need for external set-top conversion apparatus (assuming they have shipped hybrid IPTV/RF set-top boxes to their customers). This could be an important consideration for operators since IPTV, unlike RF overlay, is not yet a proven technology for delivering video services on a large scale3.

Disadvantages
■ RF overlay can only create a ‘me too’ service. The only things to differentiate the telco’s TV offering from that of the cable operator would be price and content.
■ It becomes complex to offer VoD services without hybrid IPTV/RF set-top boxes. As the RF signal is one way, an RF adaptor must be installed at the customer premise to send ‘requests’ to the central office via the 1310 nm upstream frequency.

IPTV
In the IPTV environment, the video signal (collected at the IP headend and converted into IP data streams) is transmitted over the same IP link as data for high-speed internet access. The IP set-top box converts the IP data stream back into a video signal.

Advantages
■ The telco can differentiate itself with high-quality video and sound, as well as provide a wide range of interactive and personalised services via IPTV unicasting (where a dedicated channel is delivered to the customer). This is not possible with RF overlay broadcasting. Unicasting provides an unlimited number of channels; the broadcast capacity of RF overlay is restricted by a finite amount of spectrum.
■ For telcos serving both copper-based and fibre-based customers, IPTV removes the need for two management systems (RF overlay is not suitable for xDSL customers but IPTV is).

Disadvantages
■ There is a risk that video will absorb too much of the operator’s data capacity on the PON, which limits its ability to offer other services. (The video and data services share the same downstream frequency in an IPTV environment.) Through channel replication, however, it is possible to reduce the impact on video data consumption when ONTs attached to the PON want to receive the same broadcast information. In this instance, each ONT would receive the same signal managed by the OLT. Channel replication would not, of course, be an option with video-on-demand.
■ IPTV set-top boxes are more expensive than standard digital set-top boxes, although increased volumes should narrow the cost gap.

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