American Fibertek SM24p-PoE Manuel d'instructions télécharger pdf (Page 32)

User Manual

Rev.A1

1-Mar-11

3-2. Media Access Control (MAC)

MAC Addressing

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 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.

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