Instant messaging as seen with Google Chat, Yahoo Messenger allows subscribers to communicate in real time using short chat messages. Instant messaging (IM) works due to a feature called presence, which is a notification that the subscriber is currently online and logged into the application. Subscribers build contact lists and can see which of their contacts are currently online and available to chat.
Presence information regarding a subscriber is collected when they log on to the service via a network connection and is stored in a presence availability record that can then be distributed out to other subscribers who are currently in the contact list, these are known as watchers. Presence is not just used in IM it has many uses and one very relevant one is in VoIP.
The most common use of presence in applications is to display an icon that shows the subscribers availability or willingness to communicate. The common states are online, busy, away, idle and do not disturb. These are configurable by the subscriber who wishes to change his status from ‘online’ and free to chat to a more granular use of presence information. Furthermore, the subscriber can be invisible by switching of the presence feature. This is often used to prevent from being disturbed, for example when busy or in a meeting, as not everyone respects the do not disturb state.
When the subscriber sends a text message it travels across the internet to the relevant IM server hosting the service. The server relays the text messages to the other subscriber and it pops up on his screen in a transcript dialogue box. A complete record of the chat session is available for review and subscribers can also send files to one another. In the case of send a file the IM server initiates a direct connection between sender and receiver to make the process quicker and more efficient.
The added benefit of IM is it is cheap and a highly efficient way to communicate. IM chat messages are tiny so IM can be an efficient and cost effective way to work on even the smallest of bandwidths as well as on expensive mobile data plans