How iMessage works

One of the best new features in iOS 5 is iMessage. This nifty little service subverts the traditional SMS text messaging system, allowing you to send free* text/picture messages to other iOS 5 users via the data channel (*free, presuming you don’t exceed your data cap, that is).

iMessage is very subtly implemented into the existing Messages app. You won’t even notice it until you attempt to send a text message to a contact with an iPhone, and the message suddenly goes blue. Apple automatically detects when the recipient is using iOS 5 and diverts the message via the data channel rather than your network’s SMS channel.

As PC Pro’s Paul Ockenden has argued repeatedly in his Real World Computing column, SMS messages are only a few bytes of data anyway, and it’s amazing that the mobile networks have got away with charging separately for them for so long. However, if you’re on a monthly contract, chances are you’ll have several hundred – or even thousands – of inclusive SMS messages anyway, so the cost benefit of iMessage is likely to be slim.

Perhaps the biggest benefit of iMessage is that it allows you to send text messages even when you’ve got no mobile reception. In my house, for example, there are rooms where I can’t get a reliable phone signal, but the iPhone is constantly connected to the Wi-Fi network, so I can send and receive messages (admittedly, only to/from other iPhone users) without having to dangle near an upstairs window. Alternatively, if you’re abroad, you can hook up to the free hotel lobby Wi-Fi and exchange text messages without paying extortionate roaming text tariffs.

Back to SMS

What happens, you may wonder, if the inverse is true: you’ve got mobile reception but no data, such as when you’re abroad and out of Wi-Fi range, and have turned data roaming off to avoid horrendous roaming charges. Do messages sent from iPhone contacts not get through, because there’s no data channel available?

The answer is no. If you send a message to a fellow iMessage user who has their data switched off, the phone attempts to send it via the data channel at first, but if it can’t get through after five minutes or so, the message turns green and is sent via the traditional SMS channel instead.

This could mean that you’re charged for sending an SMS message that you thought you were sending for “free” via iMessage – there’s no prompt to ask whether you want to send via SMS, it just does it automatically – but the amounts involved here are so tiny that we don’t think it’s a serious problem.

The other small advantage of sending texts via iMessage is you get an instant messaging-style speech bubble on the screen when the other person is typing a message to you, so you know when you’re about to get an incoming message.

iPad messaging

iMessage also works on the iPad, allowing you to send text messages even from devices without a 3G SIM. Synchronisation is a little patchy in our experience with a Wi-Fi only iPad using the same iTunes account as an iPhone 4S.

Messages sent from the iPad are synchronised to the phone, but not the other way round, unless you’re replying to the same thread. So you can start a conversation on the iPad and pick up where you left off on the phone, but not necessarily the other way round.

Full iMessage synchronisation between iPad and iPhone is apparently possible between 3G versions of the tablet, but we haven’t been able to test that.