AHhaha, I enjoyed the format of your 'primer' the other day, and haven't had the focus to respond to it until now. Please allow the following to serve as a supplement to the primer, and while it maintains some relationship to some of the points you raised, it does not treat each item head to head. I thought it better to amplify on some issues, and at the same time take this opportunity to answer some PM questions I've received over the past couple of days, as well.
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Aside from the bandwidth advantages of fiber, being almost infinitely greater than coax, fiber is used as a substitute for coax in HFC because it eliminates entire strings of amplifiers which are positioned at evenly spaced intervals along long coaxial routes, and it cuts down dramatically on the overall power requirements and the ensuing noise levels to negligible proportions.
Indeed many tens of miles of fiber can be pulled from the headend to the target neighborhoods without any electrification whatsoever, while incurring only negligible amounts of noise, which is mostly small amounts of noise from the photonic action within the optical strand, itself.
Prior to utilizing fiber in the cable distribution plant, the amount of noise encountered on most cable TV systems was considerably higher due to ingress from power lines, other sources of RF, Amateur Radio Hams, machinery, lightning and other atmospherics, etc. Noise, in fact, continues to this day to be the most challenging aspect of maintaining even today's remaining shorter coax sections on HFC systems. In contrast, fiber is immune to these forms of noise impairments.
Also, the number or logistic problems caused by the need for analog amplifiers, which were placed every so many meters in the spine, were absolutely horrendous. They needed to be constantly swept and tuned, routined, and so on. Amplifiers are still required today in HFC, in the larger coaxial segments in the neighborhoods, but their presence dwindles directly with decreasing segment size, as it is reduced over time. Assuming, of course, that at some point in time they eventually do get downsized.
And some of today's newer HFCs, in fact, employ coaxial sections which are entirely devoid of amplifiers. On these, once the signal leaves the optical node it travels directly to the residence without ever being regenerated again. This is accomplished because of the shorter distances between the field node and the user's residence.
Reliability also improves, since fiber requires far less electrical power, at far fewer points, reducing the overall number of points of failure. Previously, power outages, no matter how localized they were, that affected a single amplifier in the older model (which depended on strings of amplifiers) could wind up taking down an entire, or multiple, 2,000 home segment[s].
[The one major difference between the amplifiers used today, and those which were used twenty years ago, is that today's amps are bidirectional in nature, able to amplify signals in both the upstream as well as the downstream. This point contrasts to one that you made about the directionality of coax, I believe. The point here is that coax sections are not necessarily unidirectional solely because the happen to be coax. Liewise, nor is fiber bidirectional solely becuase it happens to be fiber. I'll address these points in more detail in another post.]
In reply to some other questions:
Fiber is not just used for digital forms of data, it's also used for analog, e.g., radio frequency modulated signals; for telemetry; radiological imaging and x-ray imaging; and an assortment of other analog applications in robotics, entertainment, sensing equipment, etc. Where distances are involved, many imaging related (both static and moving) payloads are usually conveyed through FM techniques, although for many of the lower resolution situations, including CCTV (compared to x-rays, in any event) AM works out just fine, as well.
Fiber is almost universally used in the edge as the basic building block of SONET Systems between end offices and tandem switching offices (and between ISPs and peering partners) to transport purely digital information.
By purely digital, here, I mean that the baseband signals are comprised of 1s and 0s. While the basebend signal may be digital, the information carried might be analog, as in the case of voice and video images. For this to take place, the analog signal must be digitally encoded at the source, and then decoded at the receiving end. In addition to SONET, sometimes microwave is used in these situations, as well.
Very often, if the links are local to one another in a colocation office, say, the T1 and T3 pipes between providers may be linked using coaxial patch cords from one provider's mux to another's, or between digital cross connects.
Someone had asked me about the means by which ones and zeros are mapped onto fiber, between ATHM's head end and the upstream points on the ATHM netowrk. We're talking about SONET containers now, and this could apply to traditional SONET, or IP over lambda which runs at SONETized rates (OC-12, OC-48, OC-192, etc.), as well.
In such SONET systems, or OC-48 containerized bundles such as ATHM's network will contain, the most common form of optical line coding is through the use of a "non return to zero," or NRZ, scheme.
NRZ is a form of on-off keying to produce 1s and 0s with a direct detection capability at the other end. Thus, a high amplitude (or high intensity) light signal, which is sustained over an entire bit interval, is interpreted as a logical one. A lowered intensity of light for a full bit time is received as a zero. In order to invoke this (on-off) NRZ approach, a time-domain excitation technique is used to directly modulate a light source, whether it is a LED or a laser. This is a form of light intensity modulation, in other words.
This technique is a lot easier to implement and maintain than the schemes used for fiber distributed data interface, or FDDI, transceivers, and other optical layer formats, because it is straightforward, and avoids some of the more elaborate forms of secondary encoding and translation measures that are used elsewhere.
The 100 Mb/s FDDI, for example, uses a 4-bit/5-bit scheme, represented as 4B/5B, and requires lookups to translate between stages 4 and 5. Another form used is 8B/10B that does the same thing. And there are others which approximate Manchester forms of coding, as well.
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The next topic I'd like to cover, at a later time, is the transition which took place some years ago between unidirectional coaxial plant, and bidirectional coax, and then to bidirectional hybrid fiber/coax, which is today's HFC. Comments and corrections welcome.
Regards, Frank Coluccio |
