Point of presence
In telecommunications and computer networking, a point of presence (POP) is a location at which service providers can connect to one another, or where customers may connect to their service providers. An early use of the term came when long-distance telephone service was separated from local telephone service; the local telephone companies ran physical connections to the POPs of the long-distance carriers.
Concentration points
Another common use of POPs is a collection and concentration point for subscribers to a service. For example, the main servers and connection to "upstream" for a British Internet Service Provider might be in London, but they have concentrations of subscribers in Cambridge, Slough, Milton Keynes, and Birmingham. The subscribers in each of those concentrations would connect to a local POP, which would send their data to London via a high-speed connection.
The cost to send bits decreases with the speed of the communications media, but high-speed media are not cheap, and there needs to be enough usage to gain economies of scale. In this example, while it would be cost-prohibitive to give each subscriber a backup line direct to London, it could be quite reasonable to interconnect the POPs. If the link from Milton Keynes to London failed, the traffic could take a backup path from Milton Keynes to Slough, and from Slough to London. While the additional distance would slightly increase delay, economies of scale could well make it possible to have each link to London be fast enough to carry one other POP's traffic with no appreciable change in performance.
Carrier hotels
As more and more communications companies need to have underground optical fiber, constantly digging up streets is noisy, dirty, and disruptive to pedestrian and vehicular traffic. Some localities, such as San Jose, California, which run the water and sewer services, put in communications facilities while necessary digging is being done. The facility may be empty ducts through which cable can be pulled, or it can be quite cost-effective to run dark fiber through the ducts, terminating them at locations often called "carrier hotels". A carrier hotel can have backup electrical power, air conditioning, tight physical security, and other features desirable for carrier operations.
At a carrier hotel, the interconnection may simply join physical facilities of the same communications. Hypothetically, the Market Street and Santa Clara Street "local loops" to which subscribers connect could meet at a common point, and then go out a high-capacity link running along Interstate Highway 280.
Exchange points
The general concept of a POP or carrier hotel is that it is a place to either to aggregate links, or, in telephony, meet a single long-distance carrier. In the Internet, however, there are internet exchange points (IXP), to which many ISPs connect to an extremely high speed local switching fabric, over which they can route packets to multiple destination. Some IXPs also allow private high-speed connections between providers that exchange sufficient traffic that bypassing the fabric makes technical and economic sense.
IXPs often provide an efficient way to exchange traffic between different ISPs, with both the source and destination endpoints in a local area, without "backhauling" traffic to the ISPs main routing site. This is especially common in Europe; Amsterdam has several independent IXPs, where there is apt to be no more than one IXP in a U.S. metropolitan area.
While the original IXPs were ways to use specialized resources such as supercomputers, the exchange points increasingly became points of interconnection among regional service providers.
Military
Military networks, providing high-security services to fixed points (e.g., SIPRNET), will have POPs that connect to the high-speed backbone, but provide an access point for users on a base or in a municipal area.