This is one Wall Street business that big banks won't lose to upstarts and tiny rivals

Many of Wall Street's top banks just endured a difficult quarter, highlighted by earnings misses and the likelihood of lower bonuses.

But there is one business that's booming, and - even better - largely defensible from small boutiques and financial technology firms that are stealing away fees for advising on deals and trading securities.

"The debt markets have been absolutely huge," says Jeffrey Nassof, with M&A consulting firm Freeman & Co. "Last year there was a lot of refinancing, but now debt markets are being driven by acquisition financing."

So while the likes of PJT Partners and Zauoi & Co. can grab more M&A mandates, the fees from big debt deals are largely absorbed into big banks, including like Barclays, JPMorgan and Wells Fargo, which coordinate the sale of debt to support big acquisitions.

Big bond deals are being done as never before: The $203 billion raised through "jumbo" bond deals ($10 billion debt transactions or greater) recently surpassed the high bar set in 2009, when more than $180 billion was raised in big bond market transactions, according to Dealogic data.

And only more business may be coming down the pipeline for Wall Street's big banks. The monthly bond sales tally this October represents an all-time high for the month, the Wall Street Journal reported Monday.

Already, for the yet-to-be completed Anheuser-Busch InBev deal to buy SABMiller, a massive $70 billion financing package was marketed by a number of big banks to debt investors.

If Pfizer goes forward with a bid to buy pharmaceutical company Allergan, that deal will also likely include a massive debt package.

Last quarter's earnings reports showed that advisory and capital markets helped some firms that experienced signs of distress elsewhere on their balance sheets.

Of course this is a boom that will come to an end. One factor pushing all the borrowing is that the Federal Reserve is expected to start increasing interest rates - possibly as soon as December - and that could start to slow things down.