10

This question comes from a guy (from Europe) which really needs some insight here, as this topic was really confusing for me (everybody explained it differently, this was wrong, this was right, then it was wrong again...).

I didn't know where I should ask this question.

My questions:

  • whats the difference between PSTN and POTS (don't say the are the same - they are not, thats for shure) and what is it (I know there are many sources out there, but I want to hear it from an expert perspective and someone who really used it)
  • Which roles did ISDN play in the PSTN? Is ISDN any relevant today?
  • how significants is the PSTN today (internet connection? telephone?)? Doesn't DSL use it?

Please explain it in a way that a young child can get a really great picture of it. Some history would be great! (with some details, how it was used, which cables, ...). It sometimes is really good to know which older technologies are used by modern ones and how they worked, but I couldn't get a picture (for 3 years now...)

Thanks in advance!

gparyani
  • 1,845
  • 9
  • 30
  • 48
watchme
  • 261
  • 1
  • 3
  • 11

2 Answers2

12

To complement jcbermu's answer, a few historical points:

  • at first, the whole telephone network was analog. You had copper wires from a premise (office or home) to an exchange (initially manual, then automated), wires from exchange to exchange (possibly several), and again wires from the last exchange to the destination premises. Beyond some amplification and other treatments such as noise filtering or echo cancellation, it was the same original analog signal that was sent all the way through.

    Note that when you used modems, you sent a digital signal as analog, and it had to deal with the bad quality of an analog link and all the complications introduced by the systems that were meant to optimise voice, but could be a problem for digital.

  • then, in the core of the network, the links between the exchanges were replaced with digital links. The exchanges at both ends performed analog-to-digital and digital-to-analog conversion. The link between the user premises and the exchange was still analog, while in between the links were converted to digital.

  • ISDN is the extension of that digital conversion to the end-user premises. Use the same copper wires, but transmit digital signals on them, not analog. Digital/analog conversion is done on premises (depending on the exact architecture, it may be done by several different pieces of equipment).

  • the low-level technology of ISDN is DSL, by definition a "Digital Subscriber Line". BRI (T0) used very low speed DSL (144-160 kbit/s IIRC). PRI (T1/T2) higher speeds (1.5-2 Mbps). PRIs initially used special cabling (thicker wires, more shielding), but that was then replaced with SDSL which uses the old regular thin copper wiring that was original used for phone service.

    However, contrary to modern xDSL services, the core circuit-switched network of PSTN was still used. Yes, there was a little bit of packet-switched data possible on the D channel, but that remains minimal.

  • Further improvements to xDSL were introduced, including ADSL, ADSL2, ADSL2+, VDSL, VDSL2... The principle is still the same: use the existing copper wiring (often decades old) between exchanges and customer premises, but with higher and higher speeds. The difference however is that instead of carrying (mostly) voice over switched circuits (the base of PSTN), data is carried which is packet-switched (IP) or cell-switched (back in the days of long-distance ATM).

So:

PSTN = premises-to-exchange wiring + circuit-switched exchange-to-exchange network.

POTS = analog voice service over the premises-to-exchange wiring.

ISDN = digital service over the premises-to-exchange wiring. Still using circuit-switched network.

Most xDSL technologies: digital service over the premises-to-exchange wiring, packet-switched network.

Tech | End-user/Exchange | Long-distance network
-----+-------------------+--------------------------
POTS | Analog            | Analog, circuit-switched
POTS | Analog            | Digital, circuit-switched
ISDN | Digital           | Digital, circuit-switched
xDSL | Digital           | Digital, packet-switched

PSTN = End-user/Exchange wiring + circuit-switched network.

To answer your questions more specifically:

  • How significant is PSTN today? It's still used for all voice calls to/from a landline, and even mobile calls are mostly routed through the core of the PSTN

  • How significant is POTS today? Nearly all consumer landlines are POTS. Medium and large business use ISDN or VoIP. Some consumer landlines use VoIP (if you connect your phone to the back of your DSL or cable box it's VoIP).

  • Is ISDN any relevant today? For data (which was one of the primary selling points for ISDN in the consumer/SoHo market), it's probably as good as dead. For voice, it's used for medium/large businesses which haven't switched to VoIP yet.

  • How significant is the PSTN today for internet connections? Very very marginal.

  • How significant is the PSTN today for telephone? Still the backbone of the telephone system.

  • Doesn't DSL use the PSTN? Modern xDSL technologies use the "last-mile" part of the PSTN, the exchange-to-premises copper wiring, to carry data. But instead of having the line connected to a telephone exchange that does circuit-switching, it is connected to a modem in a DSLAM which is then connected (directly or indirectly) to the Internet.

jcaron
  • 1,748
  • 12
  • 19
  • 2
    PSTN is still relevant, that's how you make calls to/from landlines. And even calls between mobiles are routed via the (core of the) PSTN. POTS is still used by nearly everyone who has a landline at home (for medium to large businesses it's more often ISDN). – jcaron May 21 '18 at 15:25
  • 1
    VoIP doesn't work well for faxing unless you're using dedicated protocols, since faxing is extremely jitter-sensitive. Thus, both PSTN and POTS have business value today for customers in business lines where fax lines are essential (which for legal reasons is the case in the medical field, f/e -- one can't send PII w/o encryption over TCP/IP in the US, but *can* send it unencrypted via fax). Caveat: Last time I was in that business (and took the relevant training... and developed products with a faxing component) was mid-2000s. – Charles Duffy May 21 '18 at 18:15
  • 3
    An interesting fact is that as part of the protocol, ISDN defined services (ISUP) that delivers dialed digits to the network, answer supervision, caller ID, 3 way call handling, etc. GSM was designed to interface with these services and the ISDN network. So essentially, the (non-CDMA) cell phones we use today are evolved wireless ISDN terminals. – user71659 May 21 '18 at 18:36
  • The comparison to DSL in the fourth bullet point is inaccurate. ISDN is a digital line between the CO and the subscriber, but that doesn't make it DSL. DSL's physical layer uses frequency division to set up multiple, parallel analog channels modulated to carry the digital data where ISDN is a digital signal directly on the copper. – Blrfl May 21 '18 at 22:20
  • @Blrfl, by definition, as long as it's digital on the subscriber line, it is a form of DSL. Most xDSL variants (those known by that name, such as ADSL, VDSL...) share a number of characteristics which are quite different from those of ISDN, but that does not mean ISDN was not a form of DSL. The same technology, but used for packet-switched data was even sold as IDSL. – jcaron May 21 '18 at 22:36
  • ITU-T, which writes the definitions, disagrees. – Blrfl May 22 '18 at 12:15
  • @jcaron and how significant is a dial-up internet connection? Does this happen via ISDN? (And again, some technical background would be great! (a bit how it works..., what it uses ...)) – watchme May 22 '18 at 17:14
  • @watchme, dial-up (as it's usually understood, with modems) is probably only used for legacy systems right now. It usually happens over POTS, but it could happen over ISDN with conversion to analog (as in any case it will usually be carried over digital channels in the core of the network). There's also "pure" ISDN dialup (i.e. establishing a data B channel), but that's probably even less frequent nowadays. If you want to go into details of that, you probably want to ask another question. – jcaron May 22 '18 at 21:49
  • @jcaron When I want to access the internet via ISDN, the signal must go to the phone company first and then to the ISP which then gets you "in the internet" as ISDN is still circuit-switched, right? – watchme May 28 '18 at 16:54
  • @watchme, yes. You would establish a circuit (a B channel) to your ISP through the ISDN network (digital PSTN). The B-channel would carry the IP traffic, usually using PPP IIRC, and the ISP would do then route the IP packets to/from the rest of the internet. It's basically the same as with "regular" dial-up using modems, but without the modems at either end (it's an "adapter" rather than a modem, though the adapter would in most cases be built in a router). – jcaron May 28 '18 at 16:59
  • @jcaron when using DSL, not the same last mile is used when I would use ISDN right? I mean, when I use DSL, the cable goes to a DSLAM, which wouldn't be the case when I would use ISDN... – watchme May 28 '18 at 17:04
  • The same copper wire that you would have used for ISDN has been re-used for DSL by simply rewiring it at the exchange. In many cases (though for ISDN this was a lot less common than for POTS, and I don't remember if that worked in all situations or if there were restrictions) you could actually have both ISDN and DSL service on the same wire, with splitters at both ends. – jcaron May 28 '18 at 17:25
  • @jcaron thank you for answering all my questions, but now I read something which confused my a bit: On Wikipedia the description of a DSLAM is the following: DSLAM: a device for DSL service. The DSLAM port where the subscriber local loop is connected converts analog electrical signals to data traffic. analog electrical signals? Hasn't all last miles been "digitalised" already? – watchme May 29 '18 at 11:51
  • In the end, all electric signals are analog, and one uses modulation to encode the digital signal over that analog channel. The DSLAM is really a bunch of modems + the multiplexing/routing part. It's like your Wi-Fi: it carries digital data, but the radio signal itself is analog. – jcaron May 29 '18 at 12:04
8

PSTN : Public Switched Telephone Network

POTS: Plain Old Telephone Service

PSTN is the infrastructure, it means the cabling from home to operator buildings, the switching equipment in those buildings, the links from site to site using E3 connections, microwaves,etc.

POTS is the traditional voice service offered using the PSTN infrastructure. A service to make and receive calls using numbers.

ISDN is part of PSTN, with ISDN is possible to provide voice /data services using a single connection.

ISDN has lost relevance in home / soho business because xDSL technology provides better speed (In the order of 2-5 Mbps) than BRI connections (128Kbps), however ISDN PRI connections (2 Mbps / 30 digital voice channels) are still used to give voice services in medium - big size companies.

xDSL is used to provide data services to home and soho business, but it's being slowly replaced by fiber optic connections.

jcbermu
  • 17,278
  • 2
  • 52
  • 60
  • What had to be changed in order to allow ISDN? (I think ISDN replaced POTS, right...) – watchme May 21 '18 at 12:29
  • And xDSL uses the PSTN, right? As with DSL, higher frequencies get modulated above the ISDN-frequencies (where the splitter is needed) – watchme May 21 '18 at 12:33
  • @watchme ISDN was designed to replace POTS but never got enough traction and when xSDL appeared it became obsolete. With xSDL the external infrastructure (mainly cabling) can be shared with POTS using frequency splitters. – jcbermu May 21 '18 at 12:51
  • So xDSL hasn't changed the infrastructure of PSTN (ok, cablings got renewed), but just used/uses the already existing one. – watchme May 21 '18 at 12:57
  • @watchme, ISDN was the conversion of the exchange-to-premises wiring from analog to digital, but still using the PSTN circuit-switched architecture (which was already digital at the core at the time). xDSL does not use the PSTN, but just the wiring between exchange and premises, but uses a separate usually packet-switched network rather than the traditional circuit-switched network of PSTN. In most cases, xDSL didn't even require cabling to be renewed, that was the whole point (re-use the wiring between the exchanges and the premises as is). – jcaron May 21 '18 at 13:49
  • @jcaron OH! So DSL doesn't use the telephone-network, but the old cabling of it. After it gets to the exchange it then doesn't go to the PSTN (which is circuit switched), but to the network of the ISP (which is packet-switched). If this is true everything makes sense. – watchme May 21 '18 at 13:53
  • 1
    Yes, the old "last mile" cabling, from the exchange to the user premises. If you want to go into details, more recent technologies (such as VDSL, VDSL2) actually use only a part of that cabling, between cabinets (that used to be passive) and premises, moving the DSLAMs (DSL modems) there, to shorten the length of copper to go through. – jcaron May 21 '18 at 13:59