Xdsl Vs. T1: A Real Difference ?

[Butterfly]

Ok what are the "technical" difference between a T1 and xDSL (aka TurbDSL/SDSL/ProDSL) ? are they performance difference ? what kind of technology do they use ? and why are they different ?

Someone here keeps claiming that there no "physical" (they use the same phoneline) and "performance" difference.

The T1 is just an overcharge from ISP because of their SLA (Service Level Agreement, better service and quality). Is it that simple ?

what about latency in xDSL ? what are their origins ? "physical" limitation or just "cheaper routing" ?

I am sure someone here must have the skills to answer that complex question

[Crossy]

Well, xDSL should be very comparable to a T1 in terms of performance, since xDSL is based on the same technology as DS1 or T1 (namely, 2B1Q encoding). If a dedicated T1 is considered more reliable, it is due to the fact that when businesses purchase a more expensive connection, they expect a higher quality of service.

At present, most ISPs provide very few guarantees for xDSL connectivity (or none at all), whereas, for T1 or DS1 connectivity, many ISPs and telcos will promise:

(a.) 99 percent up time

(b.) guaranteed packet delivery rates of 90 to 97 percent

(c.) no more than 120 to 150 ms latency on their routers

(d.) proactive notification within 15 minutes if they fail to ping your router

Why are ISPs offering guarantees for DS1 and no guarantees for xDSL? In short, ISPs are positioning traditional DS1 or T1 as more reliable so that they can continue to charge exorbitant rates for 1.5 Mbps connectivity. In other words, if ISPs offered the same guarantees for xDSL that they make for the traditional DS1 or T1 service, no one would purchase the older DS1 technology.

Not dealt with these technologies in Thailand, information relates to the UK, can't think that LoS will be any different though.

Edited February 3, 2006 by Crossy

[rio666uk]

You dont want much do you... ok ill see if i can help - but only brief if you want to know the in and outs of frequency modulation etc you'll have to do some research

Ok what are the "technical" difference between a T1 and xDSL (aka TurbDSL/SDSL/ProDSL) ? are they performance difference ? what kind of technology do they use ? and why are they different ?

T1 - Not so popular now but still a very reliable connection - can be run over copper but quite often is fibre run from wherever (exchange) to you... been around for a while i believe and will transmit upto 1.544 Megabit per second. You may have heard of it refered to as a leased line - or dedicated line.

ADSL - Makes use of the existing phone line and uses a splitter to sort low frequency voice and high frequency data over the same line - which is why you can talk on the phone and use the net at the same time on the same wire.... obviously using this higher and broader range of frequencies means larger data transfer - and so broad-band is born. The "A" in ADSL is asymetric - this basically means that its upstream and downstream data rates are not the same.... given that your average net surfer will be wanting to download more than they will want to upload - it makes it the sensible choice.

If on the other hand you happen to be a web designer or host other peoples websites or do a lot of ftp work upstream onto the net from your PC - you would be better off using SDSL - which splits the upstream and downstream data rates in half - meaning its just as fast to upload as download......

All the other flavours of xDSL im not really familiar with - but you can be certain of one thing - they're all just different ways of sampling more data onto the wire, faster!

Someone here keeps claiming that there no "physical" (they use the same phoneline) and "performance" difference.

See above - not true - T1 is commonly run over fibre optic cable - anyone worth their salt knows that fibre and copper are nowhere near the same... DSL is sometimes refered to as last mile connections... giving its ease and cost effectiveness using new or existing copper between the exchange and the end user.... not to mention the relatively massive differences in max distance between the two.

The T1 is just an overcharge from ISP because of their SLA (Service Level Agreement, better service and quality). Is it that simple ?

no idea at all

what about latency in xDSL ? what are their origins ? "physical" limitation or just "cheaper routing" ?

what about latency....? there will be latency in every medium you use.... copper will be affected so will fibre.... even in its purest silica form - fibre will attenuate over distance unless amplifiers or repeaters are used... its an open ended question and depends what you mean.

origins - I believe it started with a couple of "baked bean cans and a length of string between them"

Physical limitations are yet to be seen with DSL as far as I know - T1 speed i believe is about 1.5Mbps - distance over copper not sure - distance over fibre a long way

cheaper routing???? cheaper installation is copper and existing copper is even better cos its already there - so dsl i would say - although different DSL lines will use different cables... for instance an ADSL and SDSL cable are not the same.... T1 would often be piped in specifically for a purpose - hence leased line... thats pricey - or it used to be anyway!

Routing - not really an issue untill you get down to the nuts and bolts of it - extra costs will arise from the need to convert incoming/outgoing optical digital data back to copper digital data and onto your network... if you see what i mean

hope this has helped

[autonomous_unit]

While they all are modulating data over the copper wire between your location and the central office equipment, the DSL variations are using only the high frequency band above the normal audible frequencies and also tolerating the normal ring signal and DC voltage bias. This is what allows a simple filter/splitter to have both a plain old telephone and DSL operating on the same pair. A T1 line isn't really all that fast, but it probably has a lower error rate if it is using the full copper bandwidth without plain old telephone interference.

However, the major difference is really the business practice and quality-of-service arrangements on the other side of the modem at the central office. Getting your data to and from the central office is only the first obstacle, and you also need prioritization and "backbone" capacity to get your data from the central office to wherever else it is going, e.g. to another central office and another T1 line for private leased lines, or to an ISP's router for a general Internet hookup.

So, I wouldn't jump to conclusions that the other services are a rip off or not, but the modulation scheme from the central office to the customer site is probably not a major factor in the apparent service quality, except when the copper wire to the customer is really long or poor quality.

As to your QoS questions: latency mostly comes from: the signalling bitrate (how long does it take to push a packet worth of bits into the wire) and packet queueing delays (affected by prioritization in the router configuration). There is also the actual wave propogation delay, which is why a trans-continental or trans-oceanic link has noticeable delay based on the geographic distance. However, for metropolitan area links, the delay is mostly because of the equipment on the endpoints and not the wire/fiber itself.

Finally, it is not possible for a service provider to guarantee Internet QoS to arbitrary destinations, but it is quite tractable to manage their own backbone to provide guarantees between two sites in their network (a typical leased-line scenario).

[MattFS218]

Butterfly, posted this thread in response to my answers to his T1/internet issues in Bangkok in response to a business related question at

http://www.thaivisa.com/forum/index.php?sh...77entry632377

your comments are also welcome there.

[solo siam]

Isnt a T1 uncontended, whilst most DSL lines are contended and work on a packeted switch technology?

Basically a T1 operates consistently and always at the exact speed (give or take a couple of %) whilst DSL has to be kind of turned on or requested, which introduces latency.

DSL is the popular choice because many users can use it simultaneously with very little drop in performance, because of how the internet works your computer will not constantly be streaming the full bandwidth available up and downstream constantly. It works in short little bursts, so many people can use the same connection hopefully at different times so they all have access to the full available bandwidth.

T1 really comes into its own in regards to reliability and also the fact that the bandwidth is uncontended. Say for example you had to urgently send importnat large files on a daily/hourly basis then with a T1 line you know that X megabytes is going to get through. Whereas with DSL, if someone else comes online, or everybody at once wants to download or send files then the available bandwidth gets divided between all the users.

[Butterfly]

Basically a T1 operates consistently and always at the exact speed (give or take a couple of %) whilst DSL has to be kind of turned on or requested, which introduces latency.

...

T1 really comes into its own in regards to reliability and also the fact that the bandwidth is uncontended. Say for example you had to urgently send importnat large files on a daily/hourly basis then with a T1 line you know that X megabytes is going to get through. Whereas with DSL, if someone else comes online, or everybody at once wants to download or send files then the available bandwidth gets divided between all the users.

This is exactly my argument. But are the DSL latency a product of the technology (signal, line quality equipment, whatever) ? or is it just because ISP choose to give less priority to DSL packets ? is DSL latency "inherent" ? or is it just a tuning issue to become equivalent to the T1 ?

Edited February 4, 2006 by Butterfly

[Butterfly]

However, the major difference is really the business practice and quality-of-service arrangements on the other side of the modem at the central office. Getting your data to and from the central office is only the first obstacle, and you also need prioritization and "backbone" capacity to get your data from the central office to wherever else it is going, e.g. to another central office and another T1 line for private leased lines, or to an ISP's router for a general Internet hookup.

I find this hard to believe since I have both xDSL and T1 with the exact same SLA, and I see performance and latency issues with the xDSL line. Definitely technically not the same performance from what I witnessed. That's what I am trying to find out. What are the causes of those performance difference ?

As to your QoS questions: latency mostly comes from: the signalling bitrate (how long does it take to push a packet worth of bits into the wire) and packet queueing delays (affected by prioritization in the router configuration).

...

However, for metropolitan area links, the delay is mostly because of the equipment on the endpoints and not the wire/fiber itself.

Let me see if I got this straight, the latency is not only due to equipments but also the signal difference between a T1 and xDSL ? which would you qualify as being "inherent" to the latency problem of xDSL ? are they really difference on performance (latency) between equipments running xDSL and T1 ?

Edited February 4, 2006 by Butterfly

[Butterfly]

Someone here keeps claiming that there no "physical" (they use the same phoneline) and "performance" difference.

See above - not true - T1 is commonly run over fibre optic cable - anyone worth their salt knows that fibre and copper are nowhere near the same... DSL is sometimes refered to as last mile connections... giving its ease and cost effectiveness using new or existing copper between the exchange and the end user.... not to mention the relatively massive differences in max distance between the two.

Ok, so would the wire of the last mile could cause more latency if it was for SDSL ?

Routing - not really an issue untill you get down to the nuts and bolts of it - extra costs will arise from the need to convert incoming/outgoing optical digital data back to copper digital data and onto your network... if you see what i mean

hope this has helped

Assuming same bandwith for T1 and SDSL, would that cause more lentency or performance issue for SDSL vs. T1 ?

[Tywais]

Might want to read this wiki

[MattFS218]

An SLA or Service Legal Agreement is a a legal agreeement and not related to technology.

Butterfly,

You are v.good at asking questions to "make your argument". Any latency that is at the last-mile is unnoticed compared to the latency of your ISP's backbone provider.

You can simply admit you are wrong. There's also the placebo effect ...I pay more for T1, so it MUST be faster, but more than likely your ISP set the priority of your expensive T1 data to be higher than your inexpensive xDSL data. It has nothing to do with the line quality.

I've worked in IT for several years, its okay .

Matt

DSL is the popular choice because many users can use it simultaneously with very little drop in performance, because of how the internet works your computer will not constantly be streaming the full bandwidth available up and downstream constantly. It works in short little bursts, so many people can use the same connection hopefully at different times so they all have access to the full available bandwidth.

T1 really comes into its own in regards to reliability and also the fact that the bandwidth is uncontended. Say for example you had to urgently send importnat large files on a daily/hourly basis then with a T1 line you know that X megabytes is going to get through. Whereas with DSL, if someone else comes online, or everybody at once wants to download or send files then the available bandwidth gets divided between all the users.

Ben,

Do you know if it's possible these days to buy a high quality xDSL connection that will give the same "priority" and "unshared" service that T1's once provided? I know it's possible in the states, but not sure in Thailand. I'm not referring to the cheap 900 baht/mo home based aDSL, but the expensive business grade stuff. Thanks

Matt

[Tywais]

Do you know if it's possible these days to buy a high quality xDSL connection that will give the same "priority" and "unshared" service that T1's once provided? I know it's possible in the states, but not sure in Thailand. I'm not referring to the cheap 900 baht/mo home based aDSL, but the expensive business grade stuff. Thanks

Matt

To get comparable capabilites as T1 you will need SDSL. Here is a company that offers it but I suspect we are talking serious money.

http://www.pacific.net.th/en/product/product_detail.php?ID=3

"Pacific Internet (Thailand) Limited is a member of Pacific Internet Limited (NASDAQ: PCNTF) - an Asias largest Internet Service Provider (ISP). Pacific Internet Thailand began providing commercial Internet access services under Pacific Internet brand in June 2000.

Like its parent, Pacific Internet Thailand has been voted as the Best ISP in Thailand by readers of Bangkok Post, a leading English newspaper in Thailand. As a group, Pacific Internet has also won recognition as an industry leader through a variety of awards in the region.

The company has been voted the Best Asian ISP for three consecutive years, 1999, 2000 and 2001 by readers of TelecomAsia Magazine, the regions leading telecoms publication out of Hong Kong. In addition, Pacific Internet has also been recognized as the Best ISP in Singapore by computer World Magazine in 1997, 1998 and 1999.

Pacific Internet (Thailand) offers a range of quality Internet services to consumer and corporate customers inclusive of dial-up, ISDN, ADSL, SDSL and leased lines access as well as a host of value added services. All services are backed up by technical expertise, engineering excellence and friendly 365 days customer support.

Whether it is dial-up, leased lines, ISDN, ADSL, SDSL access, web hosting or Internet systems integration of other value-added services, Pacific Internet offers a full spectrum of end-to-end solutions that are backed by technical expertise, engineering excellence and friendly 365 days customer support. This make Pacific Internet the partner of choice for both consumer and corporate customers across the region."

[autonomous_unit]

I'm not really interested in being a referee for some strange pissing contest, but I reply in case there is still some technical inquiry here... It may be helpful to know a little more about how these pieces fit together. Here is a typical ADSL picture in a nutshell:

customer A device <-> router <-> customer modem <-> local loop wire <-> telco modem <-> ... <-> ISP backbone

ADSL actually only refers to the two modems and the modulation scheme over the local loop wire! The rest is just a typical deployment but has nothing intrinsically to do with ADSL. You could replace that with some other technology such as a T1 (or an OC48) and the picture still makes sense for connecting you to the Internet. The ellipses is because there are many different ways to connect the telco modem to the ISP. The ISP could have a co-located router in the telco central office, immediately injecting customer packets into their own network. Or, there could be more steps to get to the remote ISP; these steps could be packet based, flow based, or an actual dedicated physical circuit in a really obscure case.

Note, the frequent discussions about True ADSL service are mostly complaining about congestion in the telco modem to ISP link (neighborhood oversubscription), in the ISP backbone, and in the peering between the ISP (True) and the rest of the world's ISPs (CAT international link). The only real relationship this has to ADSL is that ADSL has allowed True to introduce cheaper local loops and therefore grow their customer base very rapidly, exacerbating the oversubscription problem.

Note, when people buy a "leased line" between two sites, e.g. between two company offices, there are different solutions depending on price and local infrastructure. A true leased line is the fastest, but only works when a dedicated wire (or fiber) can directly connect the two sites:

customer A <-> router <-> customer A modem <-> dedicated circuit <-> customer B modem <-> router <-> customer B

In this case there are no extraneous "hops" or buffering and re-modulation steps to slow data. However, this requires actually leasing a real line and a more typical "fractional" cases would interpose telco equipment between two separate "local loops" for each customer location. For high end customers, it can be cheaper to lease a whole fiber and operate their own optical modems without telco devices being involved. Even then, the bleeding-edge "modems" to drive a fiber optic cable at high speed might cost as much as an exotic car!

The fractional cases are not true dedicated physical circuits, but are becoming more popular because they can be more cost-effective when the telco can use a more fungible infrastructure to satisfy SLAs. The extra telco equipment could, again, use a variety of mechanisms where additional QoS details are configured; at one extreme, a permanent virtual circuit assigns enough capacity and priority to deterministically behave as a fractional circuit; at the other end, a best-effort packet-switched backbone could mix the "psuedo-leased line" traffic with other unrelated customer traffic. The absolute worst case would be where you go back to the first scenario and have each site connect through telco and ISP, exchanging all of their traffic over the general Internet path between them.

Every extra step of demodulating, buffering, and remodulating a data stream will add a little latency due to the physics involved in the electronics and electro-optics. A store-and-forward packet switching scheme will have more latency still, compared to a bit-stream multiplexer. This is because the last bit for a packet will have to arrive in the buffering device before the first bit is sent back out. It is analogous to a local commuter train (store and forward packet) stopping at every station while the intercity express train (bit stream) flies through the station without pause. The previously mentioned queue priorities have to do with which "sidings" a train gets put onto in the station and how trains might be allowed to leave the station in different orders from which they arrived.

[solo siam]

I had an uncontended VDSL line here in Phuket two years ago, It was an uncontended 384kbps up and down, but cost a whopping 15,000 Baht + VAT a month. I was very pleased with it though and I rarely saw it drop below 330kbps or so, and although it did have occasional problems (say once or twice a month) these where remedied immediately with a quick telephone call.

I now pay 3,500 Baht a month for a shared VDSL connection, which is 256kbps - This as well as my last VDSL line descibed above DOESNT run on the telephone cable, the company route copper last mile to my home. Its faster than most 256kbps ADSL lines I have seen, and actually faster than our offices 512kbps line but not as good as my 384 kbps from before.

[Butterfly]

Actually a good question, what should you expect to pay for a good T1 in Bangkok ? And if anyone has one here, how much are you paying ? would be interesting to investigate

[Simmo]

Actually a good question, what should you expect to pay for a good T1 in Bangkok ? And if anyone has one here, how much are you paying ? would be interesting to investigate

When I was looking into a dedicated leased line connection last year (can't remember the speed), I was quoted in the ballpark of 40,000 - 60,000k p /month from a number of providers. Ended up going with a small business ADSL package.