As brand names go, Sony's Walkman is the ultimate survivor.

The brand that virtually defined the portable music market back in the days of the cassette has since endured multiple format changes through the years?from the CD, to the short-lived MiniDisc, to mobile phones and MP3 players. The company has emerged bloodied, bruised but still standing.

But for how long? It now faces its greatest challenge since the iPod replaced it as the portable music brand of choice: The music market is transitioning from downloading to streaming. In a world dominated by locker services like Apple's iCloud and subscription music services like Spotify and Rhapsody, the brand that will matter most to music fans is that of the app and the service, not of the device. And in that regard, the Walkman has some serious catching up to do.

Today's Walkman MP3 players support only one streaming music service: Napster to Go. Users can download subscription tracks from Napster and transfer them to their Walkman devices, but similar tracks from Rhapsody, MOG and others aren't yet supported.

According to Sony director of mobile music Mike Kahn, the company's next step is to integrate Sony's own Music Unlimited subscription service into Walkman devices, something he expects will happen "in the near future." But even if other services are eventually added, Kahn says the Walkman strategy for the foreseeable future is based on transferring downloaded files?otherwise known as side loading?rather than streaming directly from the source.

"Side loading is going to be around for quite a while," he says. "It's still relevant. But we're absolutely looking at how to integrate [streaming] with the Walkman brand."

That's where the next challenge for the Walkman comes into play. Walkman-branded MP3 players hold barely a single-digit share of the overall MP3 player market, which itself is shrinking in the face of smartphones taking over as music devices. Even Apple revealed that sales of the market-leading iPod dropped 20% last quarter compared with the same quarter a year ago in the face of increasing iPhone sales, and that's with the addition of the Wi-Fi-capable iPod Touch.

Currently, cloud streaming is something left to mobile smartphones, with their associated apps that allow for such direct access. To date only one Walkman-branded Android smartphone has been announced, the W8. Other Walkman-branded mobile phones available today aren't smartphones and don't have the ability to download the apps required to access these services.

The first stab at adding streaming features is evident in the new A and S series Walkman MP3 players. These devices focus on outgoing, not ingoing streaming. Using Bluetooth, the devices will allow users to stream music stored on the devices to wireless headphones, speakers or car stereo units with Bluetooth capabilities as well. Streaming directly from the cloud isn't possible on these devices, since they don't have Wi-Fi capabilities or cellular network connections.

Instead, Sony is banking on other features. For instance, using technology gained from its acquisition of Gracenote, the SenseMe feature builds playlists from music stored on Walkman devices based on mood or situation, and adds a lyrics feature with karaoke mode. But there are plenty of apps available that can offer the same features. Sony late last year finally laid to rest its original Walkman cassette player, ceasing production in all markets. But the Walkman brand, at least for now, lives on.

"I don't think you'll see us drop the Walkman brand anytime in the near future," Kahn says. "It's still very relevant to our portable devices, especially internationally." But the Walkman is an analog brand trying to remain relevant in a digital world. Unless its streaming strategy becomes aligned with the remainder of the market, even the lingering brand may soon find itself out of next steps.