Media Access Control (Mac) - Draytek VigorSwitch P1280 User Manual
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Because LAN is composed of many nodes, for the data exchanged among these nodes, each
node must have its own unique address to identify who should send the data or should receive
the data. In OSI model, each layer provides its own mean to identify the unique address in
some form, for example, IP address in network layer.
The MAC is belonged to Data Link Layer (Layer 2), the address is defined to be a 48-bit long
and locally unique address. Since this type of address is applied only to the Ethernet LAN
media access control (MAC), they are referred to as MAC addresses.
The first three bytes are Organizational Unique Identifier (OUI) code assigned by IEEE. The
last three bytes are the serial number assigned by the vendor of the network device. All these
six bytes are stored in a non-volatile memory in the device. Their format is as the following
table and normally written in the form as aa-bb-cc-dd-ee-ff, a 12 hexadecimal digits
separated by hyphens, in which the aa-bb-cc is the OUI code and the dd-ee-ff is the serial
number assigned by manufacturer.
Bit 47
st
1
byte
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.
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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
7
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
VigorSwitch P1280 User's Guide
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Type/Length
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Serial number
Data
Pad bit if any
46-1500
Bit 0
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FCS
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107
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