Protocol Data Units

You are probably already familiar with the term ‘packet’. It is often used to generically describe data that will be passed between systems over a network. As data flows down the layers of the OSI model, the various layers encapsulate it, usually by adding header (and possibly trailer) information. At each layer where this happens, the term used to describe the data is different, and it should be properly referred to by its Protocol Data Unit (PDU) name. The reference below outlines the names used to describe Protocol Data Units at different OSI layers.

Application = Data

Presentation = Data

Session = Data

Transport = Segment

Network = Packet

Data Link = Frame

Physical = Bits

Notice that once passed to the Transport layer, data is properly referred to by its protocol data unit (PDU) name, a “segment”.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.