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Considerations for deployment of IPTV on Passive Optical Networks

by

Shimon Hochbaum

BroadLight, Inc.

[email protected]

1 Introduction

Fiber to the Premise (FTTP) deployments have accelerated dramatically since 2003, by

both smaller and larger service providers in North America. While the Radio-Frequency

(RF) overlay has been the method of choice to deliver Video services as part of the

famed Triple-Play, most operators are now turning to IPTV as the preferred method to

deliver a low-cost, rich set of video services. This is due in part for the need to create a

homogeneous broadband access networks (DSL, Fiber, and Wireless) as well as the

goal to take advantage of richer functionalities of digital systems. This paper will

compare the various Passive Optical Networks (PON) technologies and their capacity

for providing short, medium and long-term IPTV services.

2 PON standards

There are three worldwide standards for PON :

- ITU-T generated first G.983 series of recommendation for a Broadband PON

(BPON), which supports any service encapsulated in ATM; the standard is

defined for up to 1.2Gbps downstream and 64-way split.

- ITU-T G.984 series for a Gigabit-capable Passive Optical Networks (GPON)

supports any service encapsulated in GPON Encapsulation Mode (GEM) or

ATM; line rates defined in this standard range from 622 Mbps and up to 2,488

Mbps in the downstream, and split ratio up to 128 in anticipation of advances in

optical components.

- IEEE 802.3ah (EPON) defines requirements for transport of Ethernet traffic in

point-multipoint mode at 1.2Gbps and a maximum of 64-way split.

Table 1 shows the common configurations for the PON standards. The useable

bandwidth is the bandwidth delivered to the users after the overhead for encapsulation

or point-multipoint management has been subtracted from the line rate.

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Figure 1: Unicastand Broadcastconnections on PON

3 IPTV services

TV services are offered in two modes. The basic mode is the broadcast mode, where

the same program is available to all customers at the same time. Another mode is on-demand, which encompasses a variety of offerings. The simplest one is pay-per-view

where the customer can elect to see a broadcasted program, and pay for it on a special

assessment. These programs are broadcasted simultaneously on multiple broadcast

channels and with a staggered delay, creating a near-on-demand effect to allow for

some flexibility with the viewing time. The most advanced offering is even more

personalized, allowing the user to select the start-time, and also to pause, or fast-

forward/reward the program. While all this seems transparent to the end-user during

daily use, we are going to see that the network implementations are very different.

Note: Many consumers own or subscribe to a Personal Video Recorder (PVR) offering. Thistype of service does not add anything new to the modes exposed above. The PVR device can

record broadcasted or personalized programs in the same way as a person would view the

programs.

Another consideration is the format and the encoding of the programs. While television

programs have been in standard definition (525 lines NTSC in the US, black-and-

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white, then color), a significant improvement begun during the 90s with the specification

of High-Definition TV (HDTV) at up to 1080 lines.

In order to be compatible with an IP network, TV programs must be encoded. The most

common encoding standard is MPEG-2 published in 1994 to carry video in interlaced

format (typical for TV programs) and audio in more than 2 channels over somewhat-unreliable media. A newer standard, MPEG-4, has been introduced in 1998 with many

enhancements, the most relevant for IPTV being Part 10 Advanced Video Coding (AVC)

which delivers a video quality similar to MPEG-2 at half the bandwidth (or lower). The

bandwidth requirements for the different types of programs and encoding are shown in

Table 2.

Table 2: TV Programs Encoding standards and bit-rates

Program format

Encoding

Standard Definition

(SD)

High-Definition TV

(HDTV)

MPEG-2 3.0 Mbps 18 Mbps

MEPG-4 1.7 Mbps 10 Mbps

4 IPTV and PON

As most of the TV programming is now offered, and consumed, in broadcast mode, it is

likely that IPTV on PON will take advantage of the PON broadcast functionality. This is

described in Figure 2 on the left. A large PON broadcast channel carries all TVbroadcast content over the PON. Dozens and hundreds of programs can be delivered

this way. The Set Top Boxes (STB) in the home communicate with the network on

programs requests by mean of the Internet Group Management Protocol (IGMP). The

protocol includes procedure for hosts to join and leave a group: in the IPTV application,

this means STB can request, or abandon, viewing a certain channel. The ONTs have

simply to monitor the IGMP activity to decide which programs to send into the home.

In the example on the left, we see that ONT 2 delivers two broadcast programs to the

home, while ONT 1 delivers one broadcast program, and one personal program.

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Figure 2: IPTV on PON

As there is little content and equipment available now in MPEG-4, the near term

analysis will model the bandwidth requirements of MPEG-2 for the video services.MPEG-4 will be considered in the long-term analysis.

The model for the short and medium term with broadcast is shown in Figure 3. It is

evaluated for both EPON and GPON, using a short-term channel line-up of 100 SD and

20 HD programs, followed by a mid-term line-up of 80 SD and 70 HD programs.

Another consideration is the mix of services and their take-rate. We propose both a

conservative setup, and a more advanced one. First we show the assumptions for 3

types of users, and the different mix of services they are likely to enjoy. The customer

mix is likely to change over the next few years, with more advanced and early adopters

after a few years. Taking into account the bandwidth per service, we derive an average

bandwidth per home for the short- and mid-term. We also show the amount of

bandwidth that should be reserved on the PON for the broadcast line-up again in the

short- and mid-term. We can then subtract the amount of bandwidth to be reserved and

divide the reminder by the average bandwidth per home. The result is the number of

homes that can be supported by a PON.

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Basic

User

Advanced

User

Early

Adopter

SDTV 3 Mbps / program 1 1 On-demandchannels per homeHDTV 18 Mbps / program 1 1

Internet

20:1 oversubscription 5 15 30 Mbps for InternetAccess

Voice 100 Kbps / phone line 1.3 2.5 2.5 Phone lines / home

3.38 19.75 26.25 Bandwidth per home

[Mbps]

Customer Mix 85% 20% 5% Short-term

50% 40% 10% Mid-term

Average bandwidth

required per home

[Mbps]

8 Short-term

12 Mid-term

TV channels lineup SD HD

Now 100 20 660 PON bandwidthreserved forbroadcast [Mbps]

~ 3 years 80 70 1,500

EPON GPON

Short-term 85/20/5 25 202 Homes per PON, per modelMid-term 50/40/10 N/A 65

Figure 3: Near-term IPTV over PON model

The immediate conclusion is that an EPON will be limited in the amount of homes it can

support initially (25 is below the target split of 32:1) and is not likely to support a

broadcast model after a few years. We should also note that this model assumes that

ONTs are dedicated one per home. Some service providers are indeed planning their

network with Single Family Home (SFH) ONT. However, others are taking advantage of

the concentration of apartments in buildings to lower the cost of the network. Deploying

Multi-Dwelling Units (MDU) ONT would task EPON beyond capacity. In comparison,

GPON will continue to support up to 65 homes per PON in the near term.

Two developments are likely in the next 3 to 5 tears:

1) MPEG-4 encoding will be deployed in the network, resulting in lower bandwidth

requirements per program

2) Entertainment is going more personalized to a point where reserving bandwidth for

broadcast channels may not make sense.

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Figure 4 shows the capability of PON to support IPTV in a pure unicast model with

MPEG4 encoding for Video. As this is likely to happen somewhat in the future, the

model assumes a higher take-rate of services.

Note: the figures of 24 and 30 Mbps are modeling numbers for capacity planning, taking into

account average use, and oversubscription factors.

Basic

User

Advanced

User

Early

Adopter

SDTV 1.7 Mbps / program 3 2 3 On-demandchannels per homeHDTV 10 Mbps / program 1 2 3

Interne

t

20:1 oversubscription 20 50 100 Mbps for Internet

Access

Voice 100 Kbps / phone line 1.3 2.5 2.5 Phone lines / home

16.23 26.98 52.17 Bandwidth per home

[Mbps]

Customer Mix 50% 40% 10% Mid-term

20% 60% 20% Long-term

Average bandwidth

required per home

[Mbps]

24 Mid-term

30 Long-term

EPON GPON

Mid-term 50/40/10 36 95 Homes per PON, per modelLong-term 20/60/20 29 77

Figure 4: Long-term IPTV over PON model

As there is no reserved bandwidth on the PON, and we can now divide the full useable

bandwidth by the bandwidth per home. Here again we find that EPON will be limited,

and service providers will have to reduce the split-ratio of their EPON networks to offer

advanced entertainment options, likely to be the expected by their subscribers in a few

years. On the other hand, GPON exhibits a good margin even in the long term, with

SFH or MDU ONT, allowing up to 77 users per PON.

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