Microsoft ActiveSync Explained: Syncing Windows Mobile Devices With a PC

For most of the 2000s, if you owned a Windows-powered handheld — a Pocket PC, a Windows Mobile smartphone, or an industrial Windows CE device — the way you got information onto it and off it was Microsoft ActiveSync. It was the small desktop program that sat between your PC and your device and kept the two in step. This guide explains what ActiveSync did, how a "sync partnership" worked, where the 4.5 update fits in, and why you will not find it on a modern PC.

What was Microsoft ActiveSync?

ActiveSync was a free Windows desktop application whose job was to connect a mobile device to a PC and synchronise data between them. When you plugged in a supported device, ActiveSync detected it, established a connection, and then reconciled the information on the handheld with the information on the desktop so that both sides held the same, up-to-date copy.

It served the Windows-based mobile platforms of its era: Windows Mobile (the smartphone and Pocket PC operating system), the older Pocket PC releases, and the broader Windows CE family of embedded and handheld devices. On the desktop it was designed for the consumer and business versions of Windows in use at the time — Windows 2000, Windows Server 2003 and Windows XP — rather than for later versions of Windows.

What ActiveSync could keep in sync

The point of a sync program is to move the things you care about between two devices without you copying them by hand. ActiveSync handled the usual personal-information categories plus files and software:

• Contacts — your address book, kept in step with Microsoft Outlook on the PC.
• Calendar — appointments and reminders, so a meeting added on the phone appeared on the desktop and vice versa.
• Email — messages could be synced with Outlook, letting you read and compose on the device and send from the desktop connection.
• Tasks and notes — to-do items and jotted notes carried across with the rest of your Outlook data.
• Files — documents and other files could be copied between the device's storage and a folder on the PC.
• Applications — installing software onto a Windows Mobile or Pocket PC device was typically done from the desktop, through the ActiveSync connection, rather than downloaded directly onto the handheld.

Because so much of this data lived in Microsoft Outlook, ActiveSync leaned heavily on Outlook as the desktop counterpart. That relationship matters for the version history below: some Outlook releases were supported and some were not.

How a sync partnership worked

ActiveSync's central idea was the partnership — an ongoing relationship between one device and one PC. Setting it up worked broadly like this.

Connecting the device

By the mid-2000s the normal connection was a USB cable between the device's cradle or sync port and the PC. Earlier handhelds had also connected over a serial cable or by infrared, and some setups could sync over a network or Bluetooth connection depending on the device. Whatever the physical link, ActiveSync's job was to notice the device and open a channel to it.

Creating the partnership

The first time you connected a device, ActiveSync offered to create a partnership. During setup you chose which categories of information to synchronise — contacts, calendar, email, tasks, files and so on — and how to handle items that already existed on both sides. Once established, the partnership remembered those choices, so future connections synced automatically without asking again.

Guest connections

You did not always have to commit to a partnership. ActiveSync also supported connecting a device as a guest, which gave you a temporary connection for copying files or installing an application without setting up an ongoing sync relationship — useful on a PC that was not the device's regular home.

Reconciling changes

Once a partnership existed, each sync compared the two sides and applied changes in both directions: a contact edited on the phone updated the desktop copy, and an appointment added in Outlook appeared on the device. This two-way reconciliation is what made a handheld feel like a genuine extension of your PC rather than a separate island of data.

Where ActiveSync 4.5 fits in

ActiveSync went through several major versions over its life. The 4.x line was the generation aimed at Windows Mobile 5.0-era devices and their successors, and ActiveSync 4.5 was an update release within that line rather than a ground-up new program. The specific build, 4.5.5096, was released on 13 February 2007, and the original windows-now.com announcement that this page expands simply noted that 4.5 had been posted to Microsoft's download site. We will not invent a detailed 4.5 changelog here, because the specific per-build change entries are not something we can verify precisely — . What we can state from the original announcement and from well-established platform facts are the following concrete points.

Supported desktop systems

The 4.5 release listed compatibility with the mainstream business and consumer Windows platforms of the period — the various editions of Windows XP (including Home, Professional, the x64, Tablet PC and Media Center editions), Windows 2000 with its latest service pack, and the Windows Server 2003 family — along with the Outlook and Office versions of the day (Outlook XP and Outlook 2003, Office XP and Office 2003) and a current Internet Explorer. The presence of Windows XP but the pointed absence of Windows Vista is the key takeaway: ActiveSync belonged to the XP generation.

Supported devices

On the device side, ActiveSync 4.5 supported the Windows-powered handhelds spanning several generations — from Windows Mobile 6 back through Windows Mobile 5.0, the Windows Mobile 2003 releases, and the earlier Pocket PC 2002 and Smartphone 2002 devices built on Windows CE, as well as Windows Embedded CE 6.0. In other words, one desktop program covered a wide range of Windows Mobile and Windows CE hardware that people were still using at the time.

What replaced ActiveSync: the Windows Mobile Device Center

The reason ActiveSync 4.5 warned against installing it on Vista is that Microsoft split the tool going forward. On Windows Vista and later, the desktop side of Windows Mobile syncing was handled by the Windows Mobile Device Center instead of ActiveSync. It did the same fundamental job — connect the device, manage the partnership, and keep Outlook data and files in step — but it was a separate program built for the newer Windows platform. If you were on Windows XP you used ActiveSync; if you moved to Vista, you moved to the Windows Mobile Device Center.

Where this leaves you today

ActiveSync is now firmly a piece of history. The devices it was built for — Windows Mobile, Pocket PC and Windows CE handhelds — have long been discontinued, and the whole model of tethering a phone to a desktop program to synchronise it has been replaced by cloud accounts. On a modern phone, your contacts, calendar and mail stay in step because they live in an online account (a Microsoft, Google or Apple account, for example) that every device signs into, rather than because a cable-connected PC reconciles them. The word "ActiveSync" does still survive in one narrow place: the Exchange ActiveSync protocol, a related but distinct mail-and-calendar sync protocol that modern mail clients still use to talk to Exchange servers. That protocol is a separate topic from the desktop ActiveSync program described here.

If you are researching the Windows Mobile era more broadly, our Windows Mobile section collects related material, and you can browse more posts by this author on the Dena archive page or dip into the full windows-now.com archive.