T-1 lines (etc.) - - how fast?

How fast are T-1 lines?
How about T-2 and T-3? (do these exist?)
How does these compare to a cable (TV) line?
What other types are there?

IIRC, a T1 is 1.56 Mbps (Million bits per second). It is actually a multi-channel line consisting of about 28 digital 56k lines, but some of those are used for flow control, so the throughput is actually less than 1.56.

A T3 is 28 T1’s, and so is about 44 Mbps. Again, there is some flow control going on, so the actual data throughput is a little lower.

I think the cable lines run between .4 and 1.2 Mbps, but they are generally much cheaper.

For reference, a 56k modem is .056 Mbps under optimium conditions–more frequently it’s .044 Mbps or less.

A T1 is a dedicated telephone line that supports voice and data at a rate of 1.544Mbps up and down stream.

A T3 is a dedicated telephone line that supports voice and data at a rate of 43Mbps up and down stream.

A xDSL line is a dedicated telephone line that supports voice and data at a rate of up to 32Mbps down stream and 1Mbps up stream. It is not guarenteed and is limited by distance to the DSLAM at the TELCO.

Cable is a shared line that supports data, no voice yet, at a rate of 3-10Mbps up and down stream. This is for all users on the line. So if there are two users each get 1.5-5Mbps.

There are no T2 lines.

There is fiber which can currently support 1 Gbps with no theoretical limits.

WiredGuy & satellite modems? Hughes has some, they seemt o come in at 128k up only.

Sorry, I meant down.


Posted speeds for any of thisd stuff is a theoretical maximum. You never get those speeds. Even if you had only two computers on a network directly hooked to each other you wouldn’t get the posted speeds these lines are supposedly good for (although in this scenario you’d get as close as you’re ever likely to get). When you start adding in jillions of pc’s, routers, switches and what not actual speed is usually very different from rated speed.

In the case of a modem it’s even more stupid. Although 56k modems are the norm the FCC restricts the power going through telephone lines. If you look at the box or documentation on any 56k modem you will find somewhere in there that although the modem is capable of 56k the modem is tuned down to 52k max to comply with FCC regs.


BTW - what’s the digital transmission rate for a regular ol’ phone line? Is it slower than a 56k modem?

“BTW - what’s the digital transmission rate for a regular ol’ phone line? Is it slower than a 56k modem?”

Depends. If there are no loads or coils on the line then its a DSL line with those limitations. It goes down hill from there.

Just for the sake of accuracy, fightin’ ignorance and all that anal jazz, a T1 has 24 lines.

Cable modems can actually be quite a bit faster than T1 if you don’t have a lot of people on the same router pounding away on the net. Yet T1 lines are at least 10 times more expensive, and in some areas more like 30 times more expensive.

The big difference is simply guaranteed throughput and uptime. The cable company supplying your home internet connection makes no promises that the service will be available at any given part of the day, and they also typically don’t guarantee throughput. So while you may be getting 3 mbps one day, you may find that the next day you’re only getting 128 kbps, either because they’ve added a whole bunch of new customers or because they’ve throttled you back because their infrastructure can’t handle it.

Also, some cable companies and DSL providers have monthly maximum throughput limits. They usually don’t enforce them for typical users who might go over a bit, but they are there to prevent people from using a DSL modem as a communication backbone, or to serve very large and popular web sites.

Finally, a lot of home services like @home will not let you register your domain name with them, and most won’t let you run a web server at home.

Just to add a bit of fun to the party…

T-3 (sometimes called DS3) lines, are 672 individual 64KbS channels, supporting, as WiredGuy points out, about 43 MbS.

We can venture into the wonderful world of SONET, Synchronous Optical Network, a standard for fiber-optic transmission. There are several standard OC (Optical Carrier) types connections: OC-1 (51.85 MbS, in the same neighborhood as the DS3); OC-3, 155 MbS; OC-12, 622 MbS; OC-24, 1.244 GbS; and the amazing OC-48, which trucks along at roughly 2.488 GbS.

  • Rick

If I get a T1 line at my office, the phone company doesn’t have to run a whole new special cable from their switching office to me. They just use the existing lines. So why can’t I just get a T1 modem to use the existing phone lines?

The following applies to North American Hierarchy:

T-1 (DS-1) = 1.544 Mbps (24 voice channels)
T-1c (Internal to Telco these days) = 3.152Mbps (48 voice channels)
T-2 (DS-2) = 6.312 Mbps (96 voice channels)
T-3 (DS-3) = 44.736 Mbps (672 voice channels)
T-4 (DS-4) = 274.176 Mbps (4032 voice channels)

(T-2 and T-4 are used almost exclusively by the telcos, with T-4s used in carrier backbone networks. They are generally not available for end user consumption.)

T stands for either Trunk or Terrestrial, depending on who you talk to and usually referrs to the actual physical line. DS stands for Digital Signal and refers specifically to the signal before it enters the channel bank in the CO. It is a nit-pickity difference and many people use the term interchangeably.

Glad you asked.

OC-1 = 51.84 Mbps
OC-3 = A sonet channel equal to three DS-3s, or 155.52 Mbps.
OC-12 = 622.08 Mbps
OC-48 = 2.488 Gbps
OC-192 = 9.953 Gbps
OC-256 = 13.271 Gbps

We can stop there. OC stands for Optical Carrier; an OC-256 will accommodate 172,032 voice circuits, which is equivalent to 7,168 DS-1s or 256 DS-3s.

I sometimes sit here in front of my Trunk ports & Gigaswitches and admin ports down just for funzies.

Thanks to my slow typing, Rick stole my thunder.

Howard: We don’t have to run a special cable from the CO to you, but we may have to run one from a network interface device somewhere on the street or up on a pole, to you. Regular raggedy old telephone wire won’t cut it. I spend a large part of my day weeding out troubles on T-1s & T-3s because some contractor extended our demarc with lamp cord, or something equally ridiculous.

You don’t use a modem for T1s either. We’ll need to install a special transceiver unit (affectionatly called a “smart jack”) to which you plug your company’s router interface. Then you give me a call and we spend the better part of the morning arguing over your router settings versus my switch settings, and the options on all of the office cards in between.

If you want a DS3, we’ll need to install a mux (multiplexer/demultiplexer), so you can tap into the gusher of bandwidth and channel it off into your network accordingly & efficiently. The purpose of the router or the mux is to apportion the available bandwidth to your end users. DS1 is way more than a single user requires, and a DS3 would be a downright waste unless your business requires an end-to-end video feed. More than one video feed, and you might- just might be able to get your boss to shell out the thousands of dollars each month that an OC-3 costs.

Heh. Check out this company, which claims to be delivering bandwidth at OC 768.

Thanks Opus for the info.
Is there a difference in the wire used for running T1 into my building as opposed to POTS?
Is there a one to one ratio of wires going into the local switching station for every house it serves?
And since so many homes now have DSL (Mine is being hooked up as soon as I troubel shoot a problem) does it have teh bandwidth for real videophones (not those snapshot thingies but real moving images)? If yes, do you know of any?


Well now you’re talking about what the guys in cable & construction do, which is different from what I do (provisioning & troubleshooting), so I’m doing a little but of educated guessing here. In my conversations with outside techs (they frequently call in to my center to see if whatever circuit they’re working on has come up & is running error-free), I get the impression that the cable is much like cat-5 ethernet, but with tougher (weather-resistant) sheathing. At least two pairs of wires are brought in from the service area interface box, and probably more in case your hi-cap T1 goes down and your business absolutely can not afford to be without a data line during the repair process. If you are one such business owner who can’t tolerate a single moment of service interruption & you don’t have the common sense that God gave gravel to have had a backup ISDN line installed, we may advise you that your line can be thrown to a “hot spare” to restore your service while we work on the primary line. We’ll also suggest this if the repair looks like it’s going to take longer than 24 hours.

If I understand your question, certainly not. There are a lot more lines coming out of the CO than are actually wired to customers. This is so we have extras (e.g. the hot spares) in case people want additional lines or what not. If the only lines coming out of our CO were those already connected to consumers, we’d have to dig up the ground every time somebody wanted a new line.

Regarding your DSL question, your POTS line can be used for that and all you require is a DSL modem in your PC thingy. I don’t have DSL myself (no employee discount), because I don’t have the need for speed. There have been several DSL threads here and your question as to the quality of streaming video via DSL is probably already answered in one of them.