Instant Messaging | SIP Architecture

In different variations, instant messaging has been around longer than the Internet has been popular. In the 1970s, the TALK command was implemented on UNIX machines, which invoked a split screen that allowed users of the system to see the messages they typed in individual screens. In the 1980s, Bulletin Board Systems (BBSes) became popular, where people would use a modem to dial into another person’s computer to access various resources, such as message boards, games, and file downloads. On BBSes, the system operator (SYSOP) could invoke a chat feature that allowed the SYSOP to send messages back and forth with the caller on a similar split-screen. If the BBS had multiple phone lines, then the callers could Instant message with each other while they were online. As the Internet gained popularity, the ability to exchange messages with other users became a feature that was desired and expected.

Today there are a large number of IM applications that can be used to exchange text messages over the Internet and other IP networks. Although this is nowhere near a complete list, some of the more popular ones include:

• AIM, America Online Instant Messenger
• ICQ
• Yahoo Messenger
• MSN Messenger

In addition to these, there are also applications that allow communication using VoIP or other multimedia that also provide the ability to communicate using text messages. As seen in Figure 1, Skype provides a chat feature that allows two or more users to communicate in a private chat room. Each message between the participants appears on a different line, indicating who submitted which line of text and optionally the time that each message was sent. This allows participants to scroll back in the conversation to identify previously mentioned statements or topics of discussion. Although the figure depicts instant messaging in Skype, it is a common format that is used in modern IM software.

 

Figure 1: Instant Messaging through Skype

One of the important features of any IM application is the ability to keep a contact list of those with whom you routinely communicate. In many programs the contact list is also known as a Buddy List. However, even with this listing, it would be impossible to contact anyone if you didn’t know when each contact was available. If a person had a high-speed connection and was always connected to the Internet, then they might always appear online. As such, they would need a way of indicating that they were online but not available, or whether the person was available for one form of communication but not another. The ability to display each contact’s availability in a Buddy List when someone opens an IM application is called presence.