The MAC sublayer has two primary responsibilities:
� Data
encapsulation, including frame assembly before transmission, and frame
parsing/error detection during and after reception
�Media
access control, including initiation of frame transmission and recovery from
transmission failure
The Basic Ethernet Frame Format
The IEEE 802.3 standard defines a basic data frame format
that is required for all MAC implementations, plus several additional optional
formats that are used to extend the protocol's basic capability. The basic data
frame format contains the seven fields shown in Figure 7-6.
�Preamble
(PRE)�Consists of 7 bytes. The PRE is an alternating pattern of ones and
zeros that tells receiving stations that a frame is coming, and that provides a
means to synchronize the frame-reception portions of receiving physical layers
with the incoming bit stream.
�Start-of-frame
delimiter (SOF)�Consists of 1 byte. The SOF is an alternating pattern of
ones and zeros, ending with two consecutive 1-bits indicating that the next bit
is the left-most bit in the left-most byte of the destination address.
�Destination
address (DA)�Consists of 6 bytes. The DA field identifies which station(s)
should receive the frame. The left-most bit in the DA field indicates whether
the address is an individual address (indicated by a 0) or a group address
(indicated by a 1). The second bit from the left indicates whether the DA is
globally administered (indicated by a 0) or locally administered (indicated by a
1). The remaining 46 bits are a uniquely assigned value that identifies a single
station, a defined group of stations, or all stations on the network.
�Source
addresses (SA)�Consists of 6 bytes. The SA field identifies the sending
station. The SA is always an individual address and the left-most bit in the SA
field is always 0.
�Length/Type�Consists
of 2 bytes. This field indicates either the number of MAC-client data bytes that
are contained in the data field of the frame, or the frame type ID if the frame
is assembled using an optional format. If the Length/Type field value is less
than or equal to 1500, the number of LLC bytes in the Data field is equal to the
Length/Type field value. If the Length/Type field value is greater than 1536,
the frame is an optional type frame, and the Length/Type field value identifies
the particular type of frame being sent or received.
�Data�Is
a sequence of n
bytes of any value, where
n is less than or equal to 1500. If the length of the Data field is less
than 46, the Data field must be extended by adding a filler (a pad) sufficient
to bring the Data field length to 46 bytes.
�Frame
check sequence (FCS)�Consists
of 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value,
which is created by the sending MAC and is recalculated by the receiving MAC to
check for damaged frames. The FCS is generated over the DA, SA, Length/Type, and
Data fields.
Figure 7-6 The Basic IEEE 802.3 MAC Data Frame Format
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