System information

Troubleshooting Ethernet 4-59
Troubleshooting Ethernet
Figure 4-2 Ethernet and IEEE 802.3 Frame Formats
Both Ethernet and IEEE 802.3 frames begin with an alternating pattern of ones and zeros called a
preamble. The preamble tells receiving stations that a frame is coming.
The byte before the destination address in both an Ethernet and an IEEE 802.3 frame is a
start-of-frame (SOF) delimiter. This byte ends with two consecutive one bits, which serve to
synchronize the frame reception portions of all stations on the LAN.
Immediately following the preamble in both Ethernet and IEEE 802.3 LANs are the destination and
source address fields. Both Ethernet and IEEE 802.3 addresses are 6 bytes long. Addresses are
contained in hardware on the Ethernet and IEEE 802.3 interface cards. The first three bytes of the
addresses are specified by the IEEE on a vendor-dependent basis, and the last three bytes are
specified by the Ethernet or IEEE 802.3 vendor. The source address is always a unicast (single node)
address, whereas the destination address may be unicast, multicast (group), or broadcast (all nodes).
In Ethernet frames, the 2-byte field following the source address is a type field. This field specifies
the upper-layer protocol to receive the data after Ethernet processing is complete.
In IEEE 802.3 frames, the 2-byte field following the source address is a length field, which indicates
the number of bytes of data that follow this field and precede the frame check sequence (FCS) field.
Following the type/length field is the actual data contained in the frame. After physical-layer and
link-layer processing are complete, this data will eventually be sent to an upper-layer protocol. In
the case of Ethernet, the upper-layer protocol is identified in the type field. In the case of IEEE 802.3,
the upper-layer protocol must be defined within the data portion of the frame, if at all. If data in the
frame is insufficient to fill the frame to its minimum 64-byte size, padding bytes are inserted to
ensure at least a 64-byte frame.
After the data field is a 4-byte FCS field containing a cyclic redundancy check (CRC) value. The
CRC is created by the sending device and recalculated by the receiving device to check for damage
that might have occurred to the frame in transit.
Troubleshooting Ethernet
Table 4-2 provides troubleshooting procedures for common Ethernet media problems.
Table 4-2 Troubleshooting Procedures for Common Ethernet Media Problems
Preamble
71 6 6 2 446-1500
Ethernet
Type FCSData
Destination
address
Source
address
S
O
F
Preamble
71 6 6 2 446-1500
IEEE 802.3
Length FCS
Destination
address
SOF= start-of-frame delimiter
FCS = frame check sequence
Source
address
802.2 header
and data
S
O
F
Field Length,
in bytes
Field Length,
in bytes