As Figure A shows, several layers of networks move messages between Research In Motion's (RIM's) BlackBerry handheld devices. The first layer consists of the wireless carriers that provide connectivity to the handheld devices. The second layer is the RIM network, which acts as a bridge between the wireless networks. The uppermost layer consists of the private networks that host Exchange Server systems and RIM's BlackBerry Enterprise Servers (BESs). Firewalls and the Internet bridge these systems to the RIM network.

As the numbered handheld devices at the bottom of Figure A show, you can use three pathways to move messages between devices. The pathway between handheld device 1 and handheld device 2 illustrates the most typical route. This pathway uses Exchange email addresses to move messages from the sender's handheld device over the RIM network to a BES, then to the sender's Outbox in Exchange. One or more Message Transfer Agents (MTAs) move the message to the recipient's Inbox in Exchange, which might be on a different server or in a different organization. The recipient's BES picks up the message and routes it back over the RIM network to the recipient's handheld device.

The pathway between handheld device 3 and handheld device 6 shows another way in which handheld devices can communicate: using a BlackBerry PIN as an address. For example, instead of addressing a message to sam.jones@company.com, you address a message to 16155432. PIN-based messages don't route through your private network or an Exchange server. They penetrate the networking layers only as deep as the RIM network, which provides an alternative communications path that isn't dependent on the availability of your systems for message delivery.

Handheld devices that operate on Global System for Mobile Communication (GSM)/General Packet Radio Service (GPRS) networks can use Short Message Service (SMS) messaging to communicate. The pathway between handheld device 4 and handheld device 5 illustrates this method. SMS messages move over wireless carriers' networks and use a handheld device's telephone number as the address.

Many organizations rely on PIN-based communications rather than SMS messages as the fallback option to Exchange email communications because not all BlackBerry handheld devices support SMS. Any handheld device that connects to the Mobitex network, for example, can't receive SMS messages.