<P> The uppermost sublayer, LLC, multiplexes protocols running at the top of data link layer, and optionally provides flow control, acknowledgment, and error notification . The LLC provides addressing and control of the data link . It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines . </P> <P> MAC may refer to the sublayer that determines who is allowed to access the media at any one time (e.g. CSMA / CD). Other times it refers to a frame structure delivered based on MAC addresses inside . </P> <P> There are generally two forms of media access control: distributed and centralized . Both of these may be compared to communication between people . In a network made up of people speaking, i.e. a conversation, they will each pause a random amount of time and then attempt to speak again, effectively establishing a long and elaborate game of saying "no, you first". </P> <P> The Media Access Control sublayer also determines where one frame of data ends and the next one starts--frame synchronization . There are four means of frame synchronization: time based, character counting, byte stuffing and bit stuffing . </P>

What are two services performed by the data link layer of the osi model