Understanding the network.pdf - Back to Home

• Preamble (7 bytes)—Every device on the Ethernet segment looks at all the
traffic transmitted. The preamble consists of 7 (1010101) or 8 bytes
(10101010). The preamble tells all the devices on the network to
synchronize themselves to the incoming frame.
• Start of Frame Delimiter (SFD) (1 byte)—This is a continuation of the
preamble. The first 6 bits are in the binary order (101010), the last 2 bits are
(11) so the SFD has a binary signature of (10101011). This breaks the sync
and alerts the devices that frame data is following. The preamble and the
start of frame delimiter are dropped by the Ethernet controller, and when
transmitting, it adds the correct frames for the Ethernet format in use. The
preamble and SFD are not included in the calculation of the minimum and
maximum frame size.
• Destination address (6 bytes)—This tells the network where the data is being
sent. There are slight differences between the Ethernet and IEEE 802.3
frame. The destination field also indicates if data is destined for a single
station (unicast), a group of stations (multicast), or all the stations on the
network (broadcast). The address itself can be either locally or universally
assigned.
• Source address (6 bytes)—This address tells the network where the data
was sent. Like the destination address, it can be locally or universally
assigned. Locally administered addresses are 2 bytes long. The addresses
are loaded onto flash memory on the Ethernet card. Locally administered
addresses are generally found in mainframe environments. In most
workstation environments, universal addresses are used. Universal
addresses are "burned" into the card. Because each Ethernet card must have
its own unique address, organizational unique identifiers (OUI) are assigned
to every NIC manufacturer. The NIC identifier is the first 3 bytes of the
address. The last 3 bytes of the address are uniquely assigned by the
manufacturer.
• VLAN tag (4 bytes)—This is only used on 802.3 frames that are handled by
802.1q VLAN-enabled network switches (multiport bridges). The tag is used
to identify a packet with a specific VLAN. When the packet leaves the switch,
the tag is removed.
• Type (2 bytes)—The type field is only used with the Ethernet II frame type.
It identifies the higher level protocol contained in the data field.
• Length (2 bytes)—This provides information on the size of the frame. The
minimum size of an Ethernet frame is 64 bytes.
• Data (46 to 1,500 bytes)—The PDU contains the upper layer protocol
information and the actual data. The minimum size is 46 bytes, which
consists of a LLC PDU header of 4 bytes + 42 bytes of data or padding, and
the maximum is 1,500 bytes. The PDU construction is the responsibility of
the LLC interface. 802.2 (covered at the beginning of the chapter) is the
standard LLC interface used to construct the PDU.