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I am curious about how mac automatically detects that iPhone is close without internet and suggest connecting to iphone's personal hotspot whats the tech behind it?

situation is iphone has hotspot OFF, mac has no internet but it still detects that iphone is nearby and suggest connect to personal hotspot it automatically switches the hotspot ON on iphone and connects to it.

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  • No Internet != WiFi not scanning?
    – Tom Yan
    Apr 10, 2021 at 12:45
  • Detection of other devices nearby may use different technologies such as Bluetooth.
    – Robert
    Apr 10, 2021 at 13:16
  • Bluetooth for sure it doesnt use I keep Bluetooth off on my iPhone i want to know exactly what it use
    – Madhav Chaturvedi
    Apr 10, 2021 at 13:44
  • The technology is WiFi. Your Mac is looking for networks to connect to. Your phone is broadcasting that it is available for connection.
    – benwiggy
    Apr 10, 2021 at 13:47
  • I’ll put up a general answer, these features work by several mechanisms. If you want to ask a follow on question that’s more specific, please link to that in a comment to me or my post if I miss it. Just because you have Bluetooth “off” doesn’t mean it’s not listening or that’s not part of how Apple makes the code fall back to other options.
    – bmike ♦
    Apr 10, 2021 at 13:56

2 Answers 2

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Apple devices talk to each other and listen without needing the internet or a server or a network.

Look up Rendezvous and Bonjour and mDNS as they are the marketing and technical terms of the first iteration of this capability. More recent devices have enhanced capabilities including NFC and Bluetooth range estimation - look up Bluetooth Beacons for that iteration of this feature.

Lastly, the most advanced tech that Apple ships is ultra wide band radio chipsets that augment the previous technologies with time of flight measurements to answer hypothetical situations like “Is your watch closer to the HomePod than your phone?” for instance.

The hardware and software are capable of mapping out space and time to figure out where they are in the world as well as how close to other devices they may be moment to moment. Apple then synthesizes a reaction based on multiple signals to let you know a hotspot may be near. This can be as simple as hearing a WiFi beacon indicating a device provides this service or a cryptographically signed two way handshake to validate your AppleID signed in recently on two devices and they can trust the identity and validity of a handshake data exchange.

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  • This answer is not correct. Rendezvous, Bonjour and mDNS are not involved at all. Nor is NFC or Bluetooth Range Estimation. Ultra Wide Band radio chips are not involved either. There's no mapping out of space and time performed in order to implement this.
    – jksoegaard
    Apr 12, 2021 at 12:02
  • Are you sure @jksoegaard - bluetooth brokered these connections in the past using bonjour. I’m glad you posted an answer as well - just because there is a preferred method, I don’t think all the old ones are deprecated and I can’t imagine Apple not using the new chips as well to build out a picture of what devices are near, but BTLE is surely the main actor here.
    – bmike ♦
    Apr 12, 2021 at 14:12
  • I am sure of one key fact: to communicate, 2 devices need a network, might it be as simple as a P2P connection or as complex as one going through a central server ( which is not the case here since the Mac is thought to be not connected to the Internet ).
    – dan
    Apr 12, 2021 at 14:19
  • Bingo @dan remember when WiFi wasn’t on airplanes or subway trains and people realized they could play Game Center games and AirDrop things? Seemed like magic, still is delightful when it works.
    – bmike ♦
    Apr 12, 2021 at 17:49
  • @bmike No, there was no Bonjour-brokered Instant Hotspot before. Bonjour does not make sense in this case at all. It doesn’t come into play.
    – jksoegaard
    Apr 12, 2021 at 18:03
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This feature is marketed under the name "Instant Hotspot" as part of the Continuity framework and is implemented very using Bluetooth Low Energy (BLE).

The implementation differ slightly depending on whether or not the user is part of "Family Sharing", but the end result is the same. Various forms of encryption are used to ensure security, while being able to determine that nearby devices are signed in to the same iCloud account as the current device. Then it can send a BLE message to the iPhone to request Personal Hotspot be turned on, and then it can connect via WiFi as usual.

Bluetooth LE has a limited physical range, so only devices near by the user can "hear" these messages, and only those are able to respond to them.

If you want to know how this is implemented, I can recommend the independent article "Handoff All Your Privacy - A Review of Apple's Bluetooth Low Energy Continuity Protocol".

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