The most common kind of online reminder service sends messages in text format to users. It might send messages through e-mail, an instant message client or to your cell phone via short messaging service (SMS).
Depending on the service, users might have to visit a particular Web site to submit information and set up reminders. Most services have a pretty simple user interface (UI). There's usually a section in which the user types in the reminder message. Most have another section that allows the user to tell the service on what date and at what time it should send the reminder. Several also give the user the option to set up multiple reminders for the same message -- like an alarm clock on snooze, the service will send the same message multiple times. Other services require the user to set up each reminder instance separately.
With some services, users can send an e-mail message to a specific address and set up a reminder for later. Each service has its own format users must follow to create a reminder. Many send the user a confirmation message letting him or her know that the service received the request.
Online reminder services use simple sets of instructions called algorithms. The service sets a time trigger for each reminder as directed by the user. When the time arrives, the service follows a simple set of steps and sends out the reminder. In the case of an e-mail service, this involves sending an automatically generated e-mail to the user. The service takes the message typed in by the user and plugs it into the body of the e-mail before sending it to a pre-designated e-mail address.
An SMS reminder service is similar but has a few restrictions. An SMS message is better known as a text message -- it's the kind of text-based message many cell phones can send and receive. These messages have a strict 160 character size limit, so reminders have to be short. Anything over 160 characters won't show up in the reminder message.
Just as with e-mail reminder services, an SMS reminder service activates on a time-based trigger. Instead of sending an e-mail, the service sends the user's message via SMS to the cell phone number specified by the user in the registration process. The message travels through mobile switching centers (MSCs) over a cell phone network's control channel in a packet of data. The message goes to a centralized short message center (SMSC), which then relays the message to the recipient's cell phone.
For online service reminders with instant message (IM) support, users have yet another option. Several services support multiple IM clients like AOL Instant Messenger or ICQ. These online reminder services also follow an algorithm to send reminders to users. The service uses the IM client the same way anyone else does -- it just has a particularly large contact list. Once the time trigger activates, the service copies the reminder message created by the user into the IM client's send field and sends it to the appropriate person.
If you don't feel like typing in your reminder messages, you've got another choice. Find out more in the next section.