Apple Inc. (Nasdaq:AAPL) has outdone itself in iPhone sales for years and it's expected to bring more of the same in its most recent quarter, in part driven by the release of the iPhone 6S in September. But the positive outlook is overshadowed by concerns about what the future holds for its bread-and-butter device.

For the three months ended Sept. 30, iPhone sales are expected to fall in the 48 million-49 million-unit range compared with 39.2 million units sold in the same period last year, according to consensus estimates from analysts polled by Fortune. While Wall Street wants to see positive numbers on that end, market watchers are keeping an eye on Apple’s December quarter revenue projections, the period when it typically sells the most iPhones. But with a record 74.5 million units sold during the December period last year, there’s concern Apple won't be able to clear the bar this time around.

“Sentiment [on Apple] has gone from glass half-full to glass half-empty in the last three months,” said FBR Capital Markets senior analyst Daniel Ives. “The street wants to see a good quarter, but the big focus is on December guidance.”

For the fourth quarter, Apple is anticipated to pull in profits of $10.7 billion on $51.1 billion in revenue, compared with $8.5 billion in profit on $42.1 billion in revenue during the prior year, according to consensus estimates from analysts polled by Thomson Reuters. Earnings per share are expected to come in at $1.88, up 46 cents from the same period in the year prior.

Driving investor concerns around the iPhone is China, the second-largest revenue segment for Apple, but also a market where smartphone growth has slowed amid a weakened economy. The iPhone 6 and 6 Plus were still top sellers in the region during a three-month period ended in August, according to Kantar Worldpanel ComTech. However, with the iPhone 6S now on the market, analysts are looking to see if it can outdo its predecessor.

While iPhone accounts for over half of Apple’s revenue, Wall Street also is keeping an eye on its other revenue streams. Since the launch of the Apple Music subscription service in July, nearly 15 million users are on the service. Of that total, 6.5 million are paid users, while 8.3 million users are still on a three-month free trial, Apple CEO Tim Cook told an audience at the Wall Street Journal Digital conference on Oct. 19.

That’s only the beginning for the service. As previously announced, Apple Music is expected to roll out to Android users sometime this fall, according to Engadget. Subscription prices for the U.S. start at $9.99 a month. But it can go low as 10 yuan ($1.57) a month in China and similar prices in markets such as India.

The story isn’t expected to change much for the iPad, which has seen unit sales fall for six straight quarters on a year-on-year basis. Sales for the period are expected to come in at around 10.3 million units, down from 12.3 million in the year prior. In the December quarter, Apple isn’t expected to turn the unit shortfall around. However, sales of its higher-priced iPad Pro may help offset some of the revenue loss, according to an investment note issued by KGI Securities analyst Ming-Chi Kuo.

Mac sales may come in slightly higher in the period, with shipments of 5.6 million units, up from 5.5 million last year, according to Gartner. That’s despite the PC market as a whole seeing its shipments shrink 7.7 percent in the quarter to 73.7 million units.

As for the Apple Pay mobile wallet, the company claims it has seen double-digit growth in transactions since its October 2014 launch. Apple hasn’t revealed specific revenues from the service. However, a survey from Mercator Advisory Group found that Apple Pay users paid for purchases with it more often than mobile payment users as a whole.

One thing that hasn’t changed is Apple’s stance on Apple Watch sales numbers, which it will not disclose in its earnings release or conference call. “This is competitive information,” said Cook during the WSJD Live conference. “I don’t want to help the competition. We shipped a lot [of watches] in the first quarter, then the last quarter we shipped even more.”

Apple is expected to report its results after market close on Tuesday, followed by a conference call to discuss the results, starting at 2 p.m. PDT/5 p.m. EDT.