Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
28
Bit 47 bit 0
1st byte
2nd byte
3rd byte
4th byte
5th byte
6th byte
OUI code
Serial number
Table 3-3 Ethernet MAC address
The first bit of the first byte in the Destination address (DA) determines the address to
be a Unicast (0) or Multicast frame (1), known as I/G bit indicating individual (0) or group (1).
So the 48-bit address space is divided into two portions, Unicast and Multicast. The second bit
is for global-unique (0) or locally-unique address. The former is assigned by the device
manufacturer, and the later is usually assigned by the administrator. In practice, global-unique
addresses are always applied.
A unicast address is identified with a single network interface. With this nature of MAC
address, a frame transmitted can exactly be received by the target an interface the destination
MAC points to.
A multicast address is identified with a group of network devices or network interfaces.
In Ethernet, a many-to-many connectivity in the LANs is provided. It provides a mean to send a
frame to many network devices at a time. When all bit of DA is 1s, it is a broadcast, which
means all network device except the sender itself can receive the frame and response.
Ethernet Frame Format
There are two major forms of Ethernet frame, type encapsulation and length
encapsulation, both of which are categorized as four frame formats 802.3/802.2 SNAP,
802.3/802.2, Ethernet II and Netware 802.3 RAW. We will introduce the basic Ethernet frame
format defined by the IEEE 802.3 standard required for all MAC implementations. It contains
seven fields explained below.
PRE
SFD
DA
SA
Type/Length
Data
Pad bit if any
FCS
7 7 6
6 2
46-1500
4
Fig. 3-3 Ethernet frame structure
- Preamble (PRE)
—The PRE is 7-byte long with alternating pattern of ones and
zeros used to tell the receiving node that a frame is coming, and to synchronize
the physical receiver with the incoming bit stream. The preamble pattern is:
10101010 10101010 10101010 10101010 10101010 10101010 10101010
-
Start-of-frame delimiter (SFD)
— The SFD is one-byte long with alternating
pattern of ones and zeros, ending with two consecutive 1-bits. It immediately
follows the preamble and uses the last two consecutive 1s bit to indicate that the
next bit is the start of the data packet and the left-most bit in the left-most byte of
the destination address. The SFD pattern is
10101011.