What is a T1 in modern terms?

I often hear this term used "We have a T1"... used on SF and other sites. I googled it and it seems like an ancient technology possibly related to frame relay but I'm not sure.

Maybe things have changed and the term means different things now.

What speed is a T1, do users get all 24 channels at 1.5Mbit? How does it relate to Frame Relay? something we used in my company over 15 years ago before ADSL became competitive.

T1 is not something offered in my part of the world, that's why I'm asking really.

It's still a thing, yes. In fact, some places, like where I work, even still use some fractional T1's (which would be a T1 with a bandwidth cap on it).

In terms of data, a T1 is a [specific type of] 1.5 Mbit connection. Nothing more, nothing less, at least as it relates to modern networking. Since your question relates to "modern" networking, I should point out that if you see T1s today, you will most often see a "bundle" of T1's, which are multiple T1 lines aggregated together to increase capacity, and you get 1.5 Mbits of bandwidth for every T1 in the bundle. To your question about end users, in terms of data, you can hook your T1 up to a switch (as we do at our locations with T1's), and theoretically have as many endpoints as you want sharing the connection... but they all have to share the 1.5 Mbits of bandwidth (per T1 in the bundle).

In terms of voice, if you use a T1 (or bundle of T1's), you get the same data rate, but more importantly, the ability to digitize 24 channels of voice communications simultaneously... so a T1 for voice (which is the same technology as for data), means that you have the ability to have 24 simultaneous land-line phone calls in and/or out of the PBX it's connected to.

As to why they're still used... well, faxes are still used, and they're even older, and technically speaking, easily replaced by far superior technologies. Infrastructure has a lot of inertia, especially given the high cost of replacing it with something better. And that's to say nothing of other sources of inertia, like the fact that my bosses still actually believe that T1's are more reliable than fiber or whatever else, or prior business relationships only add to the weight behind sticking with the status quo. The fact that you can "bundle" multiple T1's together allows you to get... tolerable... data rates out of just T1's, and if you've got an ISP that is offering deep discounts on their T1 lines to squeeze some extra money out of their old infrastructure, then you can even run into situations where you can make a compelling business case for going with T1's over a newer technology.

In our specific case, we also have remote sites that are in rural areas, where the best available connections are the T1 lines that were run many years ago, so there's just no other options for a few of our sites.

A T1 is an old telecom technology that provides 1.5M of bandwidth between two sites. A T1 can be used for many purposes: phone calls, point-to-point data, internet connections, ISDN, etc. Today they are generally used for providing 1.5M/sec of bandwidth between you and your ISP, or for providing a connection to your company's phone system (in which case it provides the ability to make 24 phone calls at a time).

The actual details of a T1 are very complex. It was designed in the 1970s, when any kind of communication hardware was very expensive and difficult to design. Consider this: The moon landings had just happened with about as much technology as what goes into a desk calculator today. The phone company didn't have access to technology that was any more advanced. The T1 achieved 1.5M/sec and that was considered amazing for its day. In reality, it was really 24 individual 64Kb channels. They only had to build hardware that worked at 64Kb, yet they got 1.5M/sec (I'm exaggerating... it was quite a feat).

The bandwidth is actually provided as 24 individual channels, plus a control channel. When the T1 was invented it was used for phone calls only. The control channel was used to communicate to a PBX (phone system) to indicate phone calls incoming/outgoing and what channel they arrive on.

When using a T1 for data, the 24-individual channels are "bonded" to act like one, but deep down it is still 24 individual channels sending data. You can split the T1... use some channels for data and some for phone calls. In fact, I think Cisco PBXs let you assign certain channels for use as phone calls, but will use them for data when there is no phone call active.

T1 is rather inefficient compared to modern technologies. However in the 1990s the phone company controlled "the last mile" to an office. They wouldn't run anything other than a standard phone line or a T1. It was kind of a "hack" to get around the phone companies for ISPs to get really good at sending internet data over a T1, which really wasn't designed for data. However since the phone companies wouldn't run fiber, that was the only choice.

A few addendums:

T1 has the same bandwidth up the pipe as down.

Cable and DSL services, by contrast usually have MUCH faster download rates than upload rates. Today, T1 will outpace older/cheaper cable services for serving files, to say nothing of home DSL. But with T1's fixed speed, it will continue to look worse as time goes by. In a few years we may hear startups bragging about their dedicated T3's/OC1s instead.

The T1 has a place of note in Internet history.

The first sites on the Internet were connected with dedicated T1s. As the Internet grew beyond being the closed, military "ARPANET", becoming the Internet we know today, with colleges and companies tying in, the T1 was the de facto standard to connect an institution with.

A sense of historical scale.

Around 1990, ftp.wustl.edu was the biggest site on the Internet, getting 1/3 of all traffic, worldwide, by some estimates. If you had a file to share with the world, you got it archived at ftp.wustl.edu. Wustl.edu was connected to the Internet with a pair of T3s: That is, 60 T1s. In other words, about 25 years back, the entire Internet could have been piped through 180 T1s. The entirety of Internet traffic in 1990 added up to about 4,320 simultaneous phone calls.

It means you have a site with the internet connection (or point-to-point) carried over a 1.544 Mbit link. Unless you're using a multiplexer that does something very silly and weird (handing each DS0 to a different station), it probably just means that the router has a 1.5 Mbit connection to the internet, so yes, the full bandwidth should be available to any endpoint in the office.