When an Ethernet frame is sent out an interface the destination MAC address indicates

Functions of the MAC Sub-layer

The MAC sub-layer has the following functions:

• Provides access to physical links.

  The MAC sub-layer is associated with the physical layer. That is, different MAC sub-layers provide access to different physical layers.

  Ethernet has two types of MAC sub-layers:

  • Half-duplex MAC: provides access to the physical layer in half-duplex mode.

  • Full-duplex MAC: provides access to the physical layer in full-duplex mode.

  The two types of MAC sub-layers are integrated in a network interface card. After the network interface card is initialized, auto-negotiation is performed to choose an operation mode, and then a MAC sub-layer is chosen according to the operation mode.

• Identifies stations at the data link layer.

  The MAC sub-layer reserves a unique MAC address for each station.

  The MAC sub-layer uses a MAC address to uniquely identify a station.

  MAC addresses are managed by Institute of Electrical and Electronics Engineers (IEEE) and allocated in blocks. An organization, generally a device manufacturer, obtains a unique address block from IEEE. The address block is called an Organizationally Unique Identifier (OUI). Using the OUI, the organization can allocate MAC addresses to 16777216 devices.

  A MAC address has 48 bits, which are generally expressed in 12-digit hexadecimal notation. For example, the 48-bit MAC address 000000001110000011111100001110011000000000110100 is represented by 00e0:fc39:8034.

  The first 6 digits in hexadecimal notation stand for the OUI, and the last 6 digits are allocated by the vendor. For example, in 00e0:fc39:8034, 00e0:fc is the OUI allocated by IEEE to Huawei, and 39:8034 is the address number allocated by Huawei.

  The second bit of a MAC address indicates whether the address is globally unique or locally unique. Ethernet uses globally unique MAC addresses.

  MAC addresses are divided into the following types:

  • Physical MAC address

    A physical MAC address is burned into hardware (such as a network interface card) and uniquely identifies a terminal on the Ethernet.

  • Broadcast MAC address

    A broadcast MAC address indicates all the terminals on a network.

    The 48 bits of a broadcast MAC address are all 1s, such as ffff:ffff:ffff.

  • Multicast MAC address

    A multicast MAC address indicates a group of terminals on a network.

    The eighth bit of a multicast MAC address is 1, such as 000000011011101100111010101110101011111010101000.

• Transmits data over the data link layer. After receiving data from the LLC sub-layer, the MAC sub-layer adds the MAC address and control information to the data, and then transmits the data to the physical link. In the process, the MAC sub-layer provides other functions such as the check function.

  Data is transmitted at the data link layer as follows:

  1. The upper layer delivers data to the MAC sub-layer.

  2. The MAC sub-layer stores the data in the buffer.

  3. The MAC sub-layer adds the destination MAC address and source MAC address to the data, calculates the length of the data frame, and forms an Ethernet frame.

  4. The Ethernet frame is sent to the peer according to the destination MAC address.

  5. The peer compares the destination MAC address with entries in the MAC address table.

     • If a matching entry is found, the frame is accepted.

     • If no matching entry is found, the frame is discarded.

  The preceding describes frame transmission in unicast mode. After an upper-layer application is added to a multicast group, the data link layer generates a multicast MAC address according to the application, and then adds the multicast MAC address to the MAC address table. The MAC sub-layer receives frames with the multicast MAC address and transmits the frames to the upper layer.

Ethernet Frame Structure

• Format of an Ethernet_II frame

  Figure 1-4 Format of an Ethernet_II frame

  Table 1-5 describes the fields in an Ethernet_II frame.

  Table 1-5 Fields in an Ethernet_II frame

  Field	Description
  DMAC	It indicates the destination MAC address. DMAC specifies the receiver of the frame.
  SMAC	It indicates the source MAC address. SMAC specifies the station that sends the frame.
  Type	The 2-byte Type field identifies the upper layer protocol of the Data field. The receiver can know the meaning of the Data field according to the Type field. Ethernet allows multiple protocols to coexist on a LAN. The hexadecimal values in the Type field of an Ethernet_II frame stand for different protocols. Frames with the Type field value 0800 are IP frames. Frames with the Type field value 0806 are Address Resolution Protocol (ARP) frames. Frames with the Type field value 8035 are Reverse Address Resolution Protocol (RARP) frames. Frames with the Type field value 8137 are Internetwork Packet Exchange (IPx) and Sequenced Packet Exchange (SPx) frames.
  Data	The minimum length of the Data field is 46 bytes, which ensures that the frame is at least 64 bytes in length. The 46-byte Data field is required even if only 1-byte information needs to be transmitted. If the payload of the Data field is less than 46 bytes, the Data field must be padded to 46 bytes. The maximum length of the Data field is 1500 bytes.
  CRC	The Cyclic Redundancy Check (CRC) field provides an error detection mechanism. Each sending device calculates a CRC code containing the DMAC, SMAC, Type, and Data fields. Then the CRC code is filled into the 4-byte CRC field.

  The fields of an Ethernet_II frame are described as follows:

  • DMAC

    It indicates the destination MAC address. DMAC specifies the receiver of the frame.

  • SMAC

    It indicates the source MAC address. SMAC specifies the station that sends the frame.

  • Type

    The 2-byte Type field identifies the upper layer protocol of the Data field. The receiver can know the meaning of the Data field according to the Type field.

    Ethernet allows multiple protocols to coexist on a LAN. The hexadecimal values in the Type field of an Ethernet_II frame stand for different protocols.

    • Frames with the Type field value 0800 are IP frames.

    • Frames with the Type field value 0806 are Address Resolution Protocol (ARP) frames.

    • Frames with the Type field value 8035 are Reverse Address Resolution Protocol (RARP) frames.

    • Frames with the Type field value 8137 are Internetwork Packet Exchange (IPx) and Sequenced Packet Exchange (SPx) frames.

  • Data

    The minimum length of the Data field is 46 bytes, which ensures that the frame is at least 64 bytes in length. The 46-byte Data field is required even if only 1-byte information needs to be transmitted.

    If the payload of the Data field is less than 46 bytes, the Data field must be padded to 46 bytes.

    The maximum length of the Data field is 1500 bytes.

  • CRC

    The Cyclic Redundancy Check (CRC) field provides an error detection mechanism.

    Each sending device calculates a CRC code containing the DMAC, SMAC, Type, and Data fields. Then the CRC code is filled into the 4-byte CRC field.

• Format of an IEEE 802.3 frame

  Figure 1-5 Format of an IEEE 802.3 frame

  As shown in Figure 1-5, the format of an IEEE 802.3 frame is similar to that of an Ethernet_II frame except that the Type field is changed to the Length field in an IEEE 802.3 frame, and the LLC field and the Sub-Network Access Protocol (SNAP) field occupy 8 bytes of the Data field.

  Table 1-6 Format of an IEEE 802.3 frame

  Field	Description
  Length	The Length field specifies the number of bytes in the Data field.
  LLC	The LLC field consists of three sub-fields: Destination Service Access Point (DSAP), Source Service Access Point (SSAP), and Control.
  SNAP	The SNAP field consists of the Org Code field and the Type field. Three bytes in the Org Code field are all 0s. The Type field functions the same as the Type field in Ethernet_II frames.

  For description about other fields, see the description of Ethernet_II frames.

  Based on the values of DSAP and SSAP, IEEE 802.3 frames can be divided into the following types:

  • If DSAP and SSAP are both 0xff, the IEEE 802.3 frame changes to a Netware-Ethernet frame that carries NetWare data.

  • If DSAP and SSAP are both 0xaa, the IEEE 802.3 frame changes to an Ethernet_SNAP frame.

    Ethernet_SNAP frames can be encapsulated with data of multiple protocols. The SNAP can be considered as an extension of the Ethernet protocol. SNAP allows vendors to define their own Ethernet transmission protocols.

    The Ethernet_SNAP standard is defined by IEEE 802.1 to guarantee interoperability between IEEE 802.3 LANs and Ethernet networks.

  • Other values of DSAP and SSAP indicate IEEE 802.3 frames.

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