An Apparatus and methods for communicating in a wireless access system using a medium access control protocol data unit (MAC PDU) is disclosed. The method comprises steps of receiving, by a mobile station (MS) from a base station (BS), a dynamic service addition request (AAI_DSA-REQ) message requesting to create a service flow, wherein the AAI_DSA-REQ message comprises a first MAC header type parameter indicating a type of a MAC header included in the MAC PDU of the service flow and a flow identifier (FID) identifying a connection associated with the service flow; transmitting, by the MS to the BS, a dynamic service addition response (AAI_DSA-RSP) message in response to the AAI_DSA-REQ message, wherein the AAI_DSA-RSP message comprises a second MAC header type parameter indicating the type of a MAC header included in the MAC PDU of the service flow; and communicating with the BS using the MAC PDU comprising the MAC header indicated by the second MAC header type parameter; wherein the MAC header type parameter indicates one of a generic MAC header (GMH) for general data packet transmission and a short-packet MAC header (SPMH) for small data packet transmission and a non-ARQ connection.