It would be crazy to have two transmission lines per every subscriber run from the subscriber to the ISP, do ISPs use a multiplexer then?On a cable network everyone is on the same line so multiplexing is definitely needed (since only one line is used, would denial of service attack be possible?), when a subscriber connects to the network, how does he announce himself (how will multiplexer even know about this user)?

  • 6
    "It would be crazy to have two transmission lines per every subscriber run from the subscriber to the ISP". Well that was how the telephony system was built. And the xDLS connections still use this.
    – JFL
    May 16, 2018 at 12:57
  • So every subscriber has two cables just for himself on a distance of few kilometers (or few hundred meters)?Isn't that too inefficient and wasteful (how much copper is used)?
    – JoeDough
    May 16, 2018 at 13:06
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    It seems it was efficient enough during more than a century. At the time the bulk of the telephony networks were built, there was no other alternative. And copper was much cheaper that it is now. Even now, the cost of copper is anecdotal in the total costs of an ISP infrastructure.
    – JFL
    May 16, 2018 at 13:13

3 Answers 3


The local loop is your home's or business's dedicated connection to the nearest concentrator (often within a Point of Presence (PoP)). The concentrator basically multiplexes many connections to a single or very few connections and is connected to the ISPs core network, possibly via more concentrators. This follows the general paradigm of the network idea where you connect all nodes in a hierarchical way.

Concentrators have been digital for quite a while since digital multiplexing is much easier than analog. Currently (mostly finished), the shift goes away from virtual circuits towards packet-switched networks which are far more efficient. Using packet switching, you can use the simple local loop for countless applications including telephone calls, even simultaneously.


What is Local Loop? The portion of the telephone system that connects your home or office to the nearest central office (CO) of your local telco.

The wiring used in the local loop is usually unshielded twisted-pair (UTP) cabling, the transmission method is analog transmission, and the maximum distance from the telco’s CO to the subscriber’s customer premises is about 5 kilometers.


(This answer has a UK perspective, I suspect things will be similar elsewhere but details may be different)

DSL piggybacks on existing telephone infrastructure.

When the telephone network first started phone switching was done by people with plug-boards, later it was replaced with electro-mechanical switching systems. These systems weren't something you wanted to leave in an outside street cabinet, so telephone exchanges are generally big buildings serving a large area. The UK for example only has about 5600 telephone exchanges which is less than one per ten thousand people!

The "local loop" is the connection from the customer to the telephone exchange. With the introduction of modern solid-state electronics there have been attempts at various times to put active telephone infrastructure closer to the customer, but at least in the UK none of these attempts caught on and most customers still have a dedicated copper pair to the telephone exchange. In suburbia this is typically a kilometer or two, in rural areas it can be much longer. For obvious practical reasons this won't normally be one continuous set of wires, but will go through patching locations. A BT phone line typically goes through two patching locations, a "primary connection point" which is normally a green cabinet at the roadside and a "distribution point" which may be either a box attached to a telephone pole, a wall or even in a manhole depending on local conditions.

With "traditional ADSL" the DSL service piggybacks on the entire local loop from the telephone exchange to the customer. At the phone exchange the broadband and phone signals are split and dealt with separately. More recently "fiber to the cabinet" services have become common, in this system a cabinet containing VDSL equipment is installed close to the PCP cabinet serving the customer and wiring is re-patched to divert the phone line via this new equipment. So while the full "local loop" continues to carry the phone service, only the much shorter "distribution subloop" carries the broadband service. Phone pairs are lossy at high-frequencies, so a shorter pair means higher available speeds.

The largest ISPs often have a presence at the telephone exchange, and take the traffic onto their network there. Smaller ISPs will typically have their traffic carried from the exchange to one of the ISPs points of presence over a back end network run by BT (the incumbent telco).

When you turn on your DSL modem it will first characterize the line and negotiate the low-level connection parameters. Once this is done it will then attempt to connect to and log into the ISP. Typically some variant of PPP (PPPoE, PPPoA, L2TP) is used to carry the traffic over the broadband back-end network to the ISP, often a connection may involve more than one variant of PPP (IIRC on the modern BT system, the customer-side is PPPoA, but the ISP side is L2TP).

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