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The High Speed Data Race

Wed, 03/03/2010 - 8:42am
James O. “Jim” Farmer, Enablence Systems, www.enablence.com

james farmer enablenceThis article is about speed, the speed of data communications for the home and small-and-medium-business markets. The contestants we’re discussing are the cable guys, with their DOCSIS 3.0 cable modems, and the FTTH gang with fiber all the way to the home.

The bottom line is that nothing, and I mean nothing, can beat a fiber connection, not now and not in the foreseeable future. Fiber offers the highest speeds you can achieve, not to mention its other advantages such as extremely low operational costs and versatile high quality transmission. DOCSIS over a cable TV operator’s HFC (hybrid fiber-coax) comes second, but it is a fairly distant second. The cable TV industry is aggressively promoting DOCSIS 3.0, the fourth and latest incarnation of the DOCSIS standard. It does offer some improvements over earlier versions of DOCSIS, and it may keep HFC competitive for a few years. But in the end, the greater speed of FTTH, along with its other advantages, will win.

How the networks are built

FTTH PON vs. HFC Architectures

Figure 1. FTTH PON vs. HFC Architectures

By way of background, Figure 1 illustrates an FTTH PON architecture at the top, and corresponding blocks of an HFC architecture on the bottom. (There is another FTTH architecture, known as point-to-point or active Ethernet, not shown here.) The headend (CO or central office to a phone guy) is in the left half of the figure, the distribution network just to the right of center, and one typical home to the right. We show the home with digital video, analog video (the advantage being no set top needed with older TVs), and data service. Telephone service is part of data service at this level. In FTTH, the ONT can include multiple analog telephone (POTS – plain old telephone service) interfaces. In HFC a media terminal adaptor (MTA) is used on the data side of the cable modem, or often the MTA is included with the cable modem, in which case it is called an EMTA, for embedded MTA.

For FTTH, data service is provided using an optical line terminal (OLT) at the headend. On the network side, the OLT interfaces with whatever data switching or routing facility is used, usually via one or more 1 Gb/s or 10 Gb/s Ethernet interfaces. On the subscriber side, the OLT interfaces with the last mile distribution network (a passive optical network, or PON) using optical interfaces, 1490 nm downstream and 1310 nm upstream. At the home, an optical network termination (ONT) is used to convert the optical signals back to electrical signals. The user-side of the ONT presents 10/100Base-T (and sometimes 10/100/1000Base-T) interfaces for data, and many ONTs also present standard analog telephone lines (POTS) and/or RF video interfaces just as a cable TV system would present. All data services go through the OLT and are transmitted on the optical carriers cited above. When RF video transmission is used, it is modulated onto the fiber at the headend using an analog (RF) optical transmitter in the 1550 nm range. A wave division multiplexer (WDM) combines the 1550 nm RF video transmission with the 1490/1310 nm data transmission.

Analog video is always delivered via the 1550 nm carrier. Digital video can be delivered either or both of two ways: digitally-modulated onto RF carriers using normal MPEG transport, which RF carriers modulate the 1550 nm optical carrier, as is done by cable TV operators. Alternatively, digital video can be delivered as Internet Protocol TV, IPTV, to the OLT, which then transmits it as a normal part of the data service. (Folks used to address this saying “a bit is a bit is a bit,” meaning that any digital information can traverse the same network. This is true to a degree, but not all bits are created equal. The delivery of some bits is more critical than is the delivery of others. On-time delivery of IPTV bits is very critical, because when a decoder is ready for the next information, it needs it then, not a millisecond later. This issue can be taken care of by using quality-of-service, QoS, mechanisms built into better FTTH systems. QoS mechanisms are built into DOCSIS, too, and some have proposed delivery of IPTV via DOCSIS. The case for doing so it not particularly strong, though, when compared with delivering the same video information via normal MPEG transport.)

The lower portion of Figure 1 illustrates the corresponding HFC network, as is employed by cable operators. The processing of video services is identical to that in a FTTH system using RF video, and nearly identical to FTTH using IPTV. The data interface, corresponding to FTTH’s OLT, is a cable modem termination system (CMTS). Its downstream output is digitally-modulated RF carriers that look essentially identical to those carrying digital video programming. The CMTS downstream RF signal is combined with the video RF signals and passed to a downstream optical transmitter. This transmitter may be identical to that used in the FTTH system, though some variations might be employed, depending on system design.

A fiber transports the downstream signal (in North America they usually occupy frequencies from 54 to 1,002 MHz) to an optical node (known simply as a node) in the plant, where the signals are converted back to RF. They are transported the rest of the way on coaxial cable, using amplifiers (not shown) as needed to maintain adequate signal strength. Upstream signals are modulated at the individual device (cable modem or set top) onto RF carriers occupying frequencies from 5 to 42 MHz. They are sent upstream over the same coaxial cable used for downstream transmission. At the node they are modulated onto an upstream optical carrier for transport back to the headend. At the headend, they are demodulated from light back to RF carriers, and supplied to the upstream receivers in the CMTS (and other systems as appropriate).

You can see from the above one of the reasons why FTTH has so much more capacity than does HFC: in HFC, downstream data and video must both be transported on RF carriers occupying the frequency range 54-1,002 MHz (the widest range used today – most HFC plants have less capacity). These RF carriers are all modulated onto a single optical carrier. Contrast this to the FTTH architecture, where the 1550 nm optical carrier is used exclusively for video, with the data being transported on a separate optical carrier (1490 nm). (We’ll get to the numbers regarding how much of what you can transport below.)

In the upstream direction, the contrast between FTTH and HFC is even more stark: the upstream bandwidth of FTTH is more than 1 Gb/s, usually shared among 32 subscribers, whereas the upstream bandwidth of HFC is normally measured in the low tens of Mb/s shared among a few hundred subscribers.

Using the bandwidth available

Carriage of Signals in HFC and FTTH

Figure 2. Carriage of Signals in HFC and FTTH

Figure 2 illustrates how signals are carried in the two systems. For HFC, all of the downstream signals are carried in 6 MHz wide channels for historical reasons and because this still works well today. Most channels are used for video, but some must be reserved for data. The numbers work out such that any one 6 MHz wide channel can transport roughly 38 Mb/s of payload data.

This channel may be used for a number of different services:
- 1 analog TV program
- About 10 standard definition digital programs
- About 2 high definition programs
- About 38 Mb/s of DOCSIS data

Thus, the operator must make a choice of how to use his inventory of 6 MHz channels. (If one were to use the entire 54 - 1,002 MHz spectrum for data, this would represent about 6 Gb/s of data, not an insignificant figure.) The trade-off is for every 38 Mb/s of data service offered, the operator looses the ability to transmit the number of TV programs shown above. Compare this with FTTH, where separate wavelengths are available for data transmission. Because of these other wavelengths, the entire 54 – 1,002 MHz RF band may be used for TV signals. Depending on the standard, up to 2.4 Gb/s of data can be transmitted downstream to, usually, a group of 32 subscribers.

In the upstream direction, there is even more difference between FTTH and DOCSIS. FTTH can transport at least 1 Gb/s upstream from 32 subscribers. DOCSIS has a number of options for transporting upstream data, trading off bandwidth and robustness of modulation. Typically today, one 6.4 MHz wide upstream channel carries a wire rate of about 10 Mb/s, payload of about 8 Mb/s, using 16-QAM modulation. If the upstream RF path is suitable (meaning primarily that it is noise-free), higher orders of modulation may be used, resulting in higher data rates. The problem is that, in the real world, this portion of the spectrum tends to be relatively noise-prone, with noise from electrical appliances, short wave radio, and other sources invading the spectrum. Adding to the pain is the fact that, since the downstream signal is divided among many subscribers (typically in the low hundreds), then the upstream is formed of many homes, each a potential source of noise, converging on the same node. This is known as noise funneling, and exacerbates the noise problem.

DOCSIS 3.0 added the feature of channel bonding, which is directly analogous to pair bonding in DSL, resulting in a higher bandwidth data service using multiple channels. It is physically to get up to four upstream DOCSIS channels in the bandwidth available, and this is optimistically called a 120 Mb/s service (this is more of a wire speed rather than a payload speed). However, in order to get this, a node must have at least 25 MHz of very noise-free spectrum in the upstream direction. While this is certainly possible to demonstrate in the laboratory, and may occasionally obtain in the field, the more common situation is that there is enough noise in the upstream direction that some of the channels must use lower-order modulation methods, costing more in data speed.


Subscribers per channel
With FTTH, it is most common today to service 32 subscribers with one PON, which equates to given PON bandwidths (2.448 Gb/s down, 1.2 Gb/s up for GPON). Sometimes 64 subscribers are served by this bandwidth. There are some savings in PON by putting more subscribers on a PON. Much of the savings arise from the fewer number of OLT ports needed, with additional savings from deploying slightly less fiber in the network.

HFC economics tend to work slightly differently, with different numbers based on design parameters. You can get CMTSs with more than one upstream port for every downstream port, used to make the data rates in the two directions a little less asymmetrical. One downstream port on a CMTS tends to be more expensive than a PON port on an OLT, and in addition to the extra CMTS cost, the operator needs more optical transmitter and receivers, plus more (though smaller) nodes in the field, as he services fewer subscribers per CMTS port – also called a DOCSIS channel. Typically a cable TV node serves about 500 homes passed, maybe 350 homes connected, with somewhat over half of these taking data service. An operator may or may not combine several nodes in one DOCSIS channel. So the number of subscribers per DOCSIS channel is variable, but typically a few hundred subscribers, vs. 32 for a PON. The number of subscribers per DOCSIS channel is decreasing, as operators are increasing offered speeds.

If one bonds four channels downstream and upstream for DOCSIS, assuming maximum upstream speeds, and serves 200 subscribers, then the average speed per subscriber is about 800 kb/s downstream and 600 kb/s upstream (these are very generous wire speeds, not payload speeds). The reason this works for cable operators offering data speeds of several tens of Mb/s is statistics: they are an engineer’s best friend. Not all subscribers are using the data bandwidth all the time, so it can be shared very efficiently. In fact, the cable industry has over ten years of experience that says they can offer 5-10 Mb/s service with average speed per subscriber in the low 10s of kb/s, thanks to statistics.

Compare this with GPON FTTH service, with a 2.448 Gb/s downstream speed and a 1.2 Gb/s upstream speed, serving 32 subscribers typically. This yields an average downstream speed of 76.5 Mb/s and an average upstream speed of 38 Mb/s. With the same ratios used by DOCSIS engineers, think how much bandwidth you can offer.

What is this bandwidth needed for? If you’re not using IPTV, not much today. But the history of telecommunications is that there is never enough bandwidth for long. We have been trying for years to win fame and fortune based on Farmer’s law. It hasn’t worked, but we keep trying. Our law says, “No matter how much speed you offer, some clown is going to come along with an application that needs more.” Today DOCSIS can offer the speed needed, but already we are seeing applications that tax DOCSIS badly, and the bandwidth demands are going to keep going up. The way to get in front of them is FTTH, which even at today’s data rates, is using an infinitesimal portion of the bandwidth of which the fiber is capable.

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