Although there is some minimal latency when two people speak to each other in a room due to the speed which sound waves take to travel through the air, the delay isn't noticeable, and we'll use that as the standard for "no latency" which is expected in conversation. (As depicted in Figure 1, above)

Now, to continue the "test", get two traditional landlines (no VoIP or FiOS VoIP; copper or Fiber-to-the-Curb replacements are fine) in the same room, and call from one to the other -- there will no appreciable delay, and "B" will hear "A" at the same time over the phone as by the sound waves transmitted via the air vibrations (talking) in the room. (Analog cellular was also like this, offering no appreciable delay in voice conversation.)

Figure 2, thus shows no effective difference in latency or conversational delay when two people are speaking in the same room or speaking to each other over a traditional, local landline ("POTS") call.

(NOTE: The test assumes that two people try it in a room/office which has two local landline "POTS" phones (NOT Voice-over-IP phones), and they are NOT extensions of the same line in different rooms. To accurately guage latency (or the lack thereof) over local landline service, there need to be two physical lines, one of which will call the other.)

Thus, if person "A" were to pick up one of the lines, dial to the other line, have "B" answer and say "Hello!", "A" would hear the word "Hello" simultaneously in the room/office as over the phone. There would be no noticeable delay or difference between the direct conversation in the room as compared to the one being conveyed via the analog/POTS telephone call. (In fact, if it were a large room and it took a while for the sound waves to reach from A <-> B, it may actually be faster over the phone than over the somewhat slower path via the air!)

In Figure 3, as "A" speaks to "B" in same room, "B" will hear "A" first directly, and then over the phone with a slight delay, demonstrating the latency of digital cellular communications. If "person C" were to enter the room, and "B" were to place a three-way/conference call to "C"'s (digital) cellphone, then additional latency from "A" to "C" would be present, and "C" would hear "A" say "Hello!" in the room noticeably before hearing it on his cellphone. (NOTE: This depends on how "B"'s carrier conferences two calls; the additional delay to "C" may not be a linear function as the conference call to "C" may directly connect "C" to "B"'s connection at the cellular switching center, thus not introducing any additional significant delay. This is less likely to be true for VoIP and the delays will be much more noticeable and linearly compounded with additional conferees).

By placing a series of tests like this, most cellular (and VoIp) subscribers may see for themselves the degree of latency introduced by their respective carriers. In many cases, the delay is quite noticeable, and thus quite distruptive to normal, two-way conversation.

Alternately, as noted above, you may call +1 (802) 359-9100, which will perform a "voice/echo test" which will immediately play back whatever you say as soon as our system "hears" your voice. This will allow you to try different phones and carriers to see which has the least latency when speaking. (The latency/echo test uses a digitized signal for part of the path to us, and will introduce a very slight delay, of 10 milliseconds or so, which is generally undetectable; however, it is technically not the functional equivalent of a local call via traditional landline service over copper, preferably run through an analog exchange (not that any seem to exist anymore...:( ). (Additional information is available on the Voice Telephone Latency Test page.)

In general, after performing these admittedly somewhat less-than-controlled-tests, the order of latency, from lowest to highest, would be:

1. Traditional local phone-company landline ("local loop", "POTS") telephone service. Cable TV "digital" service comes close too, but the fastest (least latency) with the same conversation-like properties as talking to another person directly remains copper or similar "phone company" local service.

(Note: The discussion below, pertaining to various "local loop" technologies, has grown beyond the scope of telephonic voice latency, and eventually will be made into its own article. For those not interested in the various nuances of local phone service and their effects on voice latency, or Verizon's manaical drive to FiOS or wireless and apparent hatred of copper, please skip to item 2.)

As noted above, Voice-over-IP based carriers, such as Vonage, ViaTalk, or Google Voice, even though they may present themselves as "Phone Company replacements", all suffer from a good deal of latency as compared to traditional local service, which causes it to be difficult to carry on normal, "back and forth" conversations without the latency causing repeated interruptions and parties to the call cutting each other off and having to say (if they're polite! :) ) "Sorry, please go ahead...".

Additionally, by "traditional" telephone or POTS service, we are not exclusively referring to copper, but to any other technology which approximates the sound quality and lack of latency as a direct copper pair of wires from a given POTS phone to the phone company's Central Office (CO). Thus, Fiber-To-The-Curb / FTTC services, such as AT&T's U-Verse, or some older Bell System techniques such as "Slick-96"/SLC-96 Subscriber Loop Carrier, eg, where the telephone company doesn't run a copper pair from the central office to each telephone, but instead has one or more T-1 voice circuits sent to a remote phone company box (sometimes with a little green or red light on it) and from that box runs copper wires to the subscriber over a much shorter distance, are all relatively latency-free and they were all designed to meet the strict Bell System or Bell Operating Company specifications for voice communication.

Although the SLC-96 and other similar fiber based Fiber-To-The-Curb type technologies have had their problems, they generally perform well latency-wise and offer more or less the same degree of true sounding, delay-free conversation as a copper pair being run all the way from the Central Office.

These services, when properly implemented, provide similar performance characteristics to POTS/copper lines all the way to the CO. As an example, New York Telephone invested heavily in a fiber-based SLC system in the early 1990s which was prone with problems, but eventually most were fixed and for voice service, offer nearly identical service characteristics as copper. (Other than generally not being able to support DSL; a good test to see if a line is copper through-and-through to the CO or if SLC or some fiber replacement is used is to inquire with the phone company if DSL is available - if it is, the given line is most likely copper all the way to the CO, if not, it is likely some SLC-ish service is in place. Some DSL variants can be offered via these SLC-ish type services, but they are generally slower and sparsley implemented.)

Although there should be little difference in the service characteristics between well-implemented, well-maintained SLC-96 and newer fiber variants, one can discern between the two by looking at the local distribution boxes in a given neighborhood: The newer fiber variants can be seen as little green boxes with thick black cables (the fiber line back to the Central Office) on the back of houses and buldings, while the older SLC-96 boxes are generally on telephone poles with green/red lights clearly visible at night, are much larger, and as an older pre-fiber technology, use a few copper pairs from the CO to the box on the pole, which then distributes the lines to up to 96 subscribers in the given local area.

As to Verizon's FiOS and similar all-fiber systems (eg, Fiber-To-The-Home, or FTTH), it depends which FiOS service is used. Verizon's FiOS is a trade name used to send cable-TV, phone, and internet/data over a fiber service which runs into the home, and from there the 'last mile' (or few feet) of service is from the Verizon Optical Network Unit (ONU) in the basement or utility closet, via household copper wire, to the various phones in the house. (This is also why FiOS isn't such a good idea in areas with power problems or frequent power outages - the ONU uses power provided from the customer (which also raises the customer's electric bill a bit), whereas the older SLC-96 and other Fiber-to-the-Curb technologies generally use power provided from the phone company central office, which lasts a lot longer. FiOS generally will last for 8 hours or less if the power goes out, which is not a very long time, especially at such times when landlines may be most needed.)

FiOS is generally how Verizon brands the higher-priced bundled (TV, Phone, and Internet) service, and what they try to get people to sign up for (apparently often via less than scrupulous business practices -- a colleague of ours in New York City recently had his copper service go out, and -the very next day- somehow by -extreme- coincidence there was a FiOS desk with phone company representatives and lots of glossy FiOS literature set up in the lobby, with the oh-so-ever-friendly reps saying "Oh, we're getting rid of copper, so you need to move over to FiOS". Not only was this not true, and copper was not being abandoned, but the repair times quoted if one wished to keep the copper -a month!- were outrageous. Even more outrageous was that when one called Verizon/NYTel repair, instead of the usual disinterested rep at the "611" repair office, the (800) VERIZON IVR picked up that the calling customer was a "prime candidate" (victim?) for FiOS service and transfered the call to a perky lady from the Midwest who claimed to be repair, but was really just trying to covert the caller to FiOS, and said "Oh, I'm SO sorry you are having all these problems, but of course it's due to all that bad copper you still have there. But, wait! You're in so much luck! FiOS is available for you! I'm so happy for you!" When she was politely told that no one wanted FiOS and Verizon/NYTel just needed to fix the copper, she would not stop badgering about why she is "so concerned that you want to use such a terrible, old technology as copper..." (read: she was terribly concerned she wasted 5 minutes on the call and wouldn't be getting her commission!); the call got very tense after she refused to get repair, and the NY Public Service Commission needed to be called to remedy the issue... Amazingly, the estimate of a month before completion of the copper repair (that is, what they were called about to begin with!) became 2 days and the lines were back in service then. But of course this is all just a big misunderstanding and coincidence, and no one would ever think that Verizon is using tricks to get uninformed people to abandon copper and move over to FiOS...never...who would think such a thing? ;( ).

In any case, there are essentially two FiOS voice services: FiOS Phone, which is the regulated Fiber-To-The-Home replacement for copper (or SLC-96 or FTTC-type services), and FiOS Digital Voice (or Phone), which is their unregulated (read: they can jack up your prices whenever they want and you have little if any recourse! - AVOID THIS!!!) VoIP-ish product which runs over the FiOS Internet connection.

FiOS Phone should have more or less all the voice and latency characteristics of traditional copper/POTS voice phones (NO! They don't - further tests in late 2019 and 2020 indicate there is more latency and echo with FiOS, and besides the latency and echo issues, we've had problems with modem calls, like faxing or home alarm systems from the start of FiOS service), although again, if the power goes out any FiOS service will not work after the limited batteries run out, and at all times the homeowner is paying the electric bill for FiOS whereas under copper/SLC-96/FTTC type services the phone company paid for the power. FiOS Phone is also what Verizon forces people on who don't want to opt for the higher-priced bundled/package deals with Cable-TV and Internet, and with the costs of the ONU box (which is what uses up the power and takes up space in the basement) being around $600, Verizon likely isn't too interested in having customers who are being forced off copper take this option, although in nearly all cases the customer, if they are told that Verizon is officially abandoning copper, has the right to obtain FiOS Phone service at the same rates and with the same features as their prior copper/POTS service.

(2020 Update: There is a measurable and noticable degree of latency with FiOS as compared to analog/copper or SLC-96 or similar transport systems, as well as a good deal of echo on calls made to local numbers on the same FiOS box and/or in the same telephone exchange. Touch Tones (DTMF) also don't carry well on such calls, perhaps due to the echo issues. Additionally, FiOS call progress tones (busy, fast busy, re-order, etc) seem to be very poorly generated and sound like some cheap VoIP service, and not what one would expect from the phone company. Certain features, such as the "single ring" on call fowarding to remind customers that their calls are being forwarded are also absent, and it appears that Verizon has removed (without permission?) Speed Dial (#74/#75) from customers who were forced to migrate, thus potentially violating the terms of copper retirement by not offering the feature to customers who previously had it and were forced to move to FiOS. So effectively, FiOS is not a good or comparable replacement for copper, as there is a good deal more latency, and it thus does not provide the same low/no-latency servicd which copper does.)

Additionally, in most states, new Verizon customers may also instruct Verizon to set up phone service with the regulated FiOS Phone product: Verizon, if they have abandoned copper or copper-ish products in an area or locality where new phone service is desired, may not refuse the regulated product over FiOS, although, with all the expenses they have to put in the FiOS ONU box, they often seem to be worse than used car salesmen in trying to steer new customers away from the regulated (generally cheaper) product, and instead try to "convince" them that the FiOS bundles (all unregulated) are better deals -- yes, indeed they are! -- for Verizon, which will likely never make the money back on a $600 box in the basement on a regulated service plan and has a very strong financial motivation to upsell people who only want/need good voice phone service on useless bundles of self-serving, unregulated services.

Essentially, FiOS Phone is just a (less than adequate in many cases) replacement of copper with fiber, and the rates and services must remain the same as under copper, including regulated protection from the given state's Public Service Commission/Agency. While Verizon may claim it's more reliable than copper, if Verizon made the effort to maintain their copper network there'd be many fewer problems; it's just painfully obvious that they don't want to do this and want everyone off copper and on to FiOS or wireless. And, as noted above, FiOS Phone is regulated, and afforded the same protections as copper service/POTS lines by state regulatory commissions, which is another reason why Verizon likely would prefer customers to not migrate to FiOS Phone but move to one of the unregulated bundles instead. They try REALLY hard to convince you not to take the regulated product, so beware!

FiOS Digital Voice/Phone is not the same as FiOS Phone, and is an unregulated product in most states (this may change over time as many states have realized how Verizon is pushing FiOS bundled over regular FiOS Phone and are becoming increasigly inclined to make sure that customers who are not aware of the distinction between FiOS Phone (regulated) and FiOS Digital Phone (generally unregulated) are afforded some protections). In any case, FiOS Digital Voice/Phone appears, from our experience, to be more of a VoIP-ish product (as it's name properly implies), and is not a "circuit switched" product (it doesn't meet all the requirements and specifications of traditional phone service), and appears to have increased latency. This is generally offset, though, by the admittedly very high speeds of data which FiOS can achieve, but it is still prone to more latency, or at least the potential for latency, as compared to traditional POTS service.

Thus, in terms of local service:

• POTS Service: Copper lines to the Central Office, SLICK-96/SLC-96 Subscriber Line Carrier, local neighborhood fiber distribution via SLC-like service, Fiber-To-The-Curb, etc, all should have no noticable difference in terms of latency. While the audio quality can vary from copper to SLC to fiber (Greenwich CT, which is served by NYTel/Verizon and not Southern New England Telephone/SNET or whatever they call themselves today, is a good example of poorly implemented SLC-96 service and has had problems for years), or between other technologies, the actual latency of calls should be the same regardless of which of the above technologies/local loop delivery mechanisms are used.
• FiOS Phone: FiOS Phone is the regulated, replacement for copper in Verizon territory (not to be confused with FiOS Digital Voice, which is un-regulated and can offer lower standards than the regulated copper replacement product). It offers fiber "lines" (as a transport conduit instead of copper) from the Central Office all the way to the home. While there are concerns with respect to power outages and the ability to use fax and alarm monitoring systems (like ADT, et al), as well as those pertaining to no service during power outages as well as increased electric bills due to the ONU placed in the basement or utility room which the phone subscriber now has to assume, the FiOS Phone service is generally similar to traditional POTS service for voice calls. While there is more latency over FiOS (we've compared FiOS calls to copper and the delay via FiOS is noticeable when compared to delay-free copper), and there is more echo and distortion when calling from FiOS line to FiOS line one the same FiOS box/ONT, to most customers the latency and echo will not be too much of a problem (compared to pure Voice-over-IP carriers for example). Thus, with all technologies (POTS-type, above, and FiOS Phone) being equally well-implemented and maintained (which, granted, is wishfull thinking seeing how Verizon is doing it's best to degrade copper!), the end customer should experience some, but not extreme, difference in terms of latency and audio quality between the (no-latency/no-echo) copper lines and the somewhat more latent and echo-prone FiOS service.
• FiOS Digital Voice/Phone: Fiber "lines"/conduit from the Central Office all the way to the home. Same issues as "FiOS Phone" above, with the additional problem of potentially increased latency due to the VoIP-ish service being offered. While in effect, due to the high speed of FiOS and it's cable company equivalents, VoIP should work relatively well over such circuits, they generally can take multiple, circuitous paths to reach the VoIP interconnection with the telephone network, and thus will likely still suffer a greater degree of latency.

2. Traditional "circuit switched" long distance service (such as using AT&T or a Bell/LEC carrier's long-distance service); there should be no noticeable difference in latency between landline local calls and landline long-distance calls; if there are, the carrier may be using VoIP or some other poor-quality long-distance ("interexchange") carrier. Some long-distance carriers may use VoIP-ish or slower, more latent connections for calls outside of North America, which will cause delays in conversation and slow down (or make inoperable) services like faxing. AT&T Long Distance still seems to offer the best and most reliable international service with near local-sounding calls to many foreign destinations (but they have most of their customer service offshore and are, from our experience, difficult to deal with).

3. Analog Cellular Service, which is no longer available in the US.

4. GSM-based cellular service providers (the two primary carriers in the US being T-Mobile and AT&T Wireless).

5. iDEN-based cellular service; the now defunct Nextel used to offer iDEN service in the US; it is still available via SouthernLinc and AirPeak in the United States and Telus and AirTel in Canada; slightly more latency than well-implemented GSM but not as much as CDMA.

6. CDMA-based cellular service providers (the two primary carriers in the US being Verizon and Sprint). As noted above, CDMA also distorts and masks background noises and music much more so than other cellular transmission protocols.

7. VoIP-based services, such as Vonage, which tend to add the most latency and make immediate conversations awkward and unnatural.

In the near future (2015 onwards), cellular carriers plan to introduce packet-based, IP-like voice service so that effectively voice calls will be carried over the same air interface/protocol/link to the cell tower as data. We haven't had a chance to try either a GSM or CDMA based phone which packetizes all calls (voice and data), and despite the promised improved sound quality, if steps are not taken to limit and control latency, may suffer from the same infirmities as the current GSM (to a lesser extent) or CDMA (to a greater extent) protocols.

Overall, then, customers who enjoy the sound quality and immediacy of "landline"-type calls would be best to find a suitable GSM carrier instead of CDMA. Not only do GSM carriers appear to have less latency, but they also offer a more "lifelike"-sounding call experience as compared to the often overly-digitized sound quality of CDMA. (On the other hand, CDMA does mask background noise, which, although again detracting from its "lifelike", normal conversational qualities, is very useful in noisy places where the caller does not want any background noise to be conveyed to the person he is calling.

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