Brother MFC-9840CDW Network User's Manual page 12

Introduction
Due to these factors, wireless may not be the best method of connection for all types of documents and
applications. If you are printing large files, such as multi-page documents with mixed text and large graphics,
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you may want to consider choosing wired Ethernet for a faster data transfer, or USB for the fastest throughput
speed.
Although Brother MFC-9840CDW can be used in a both wired and wireless network, only either one of the
connection methods can be used at a time.
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