Learning by Doing: CISCO Certified Network ... - SCN Research

Stripped by the NIC:
The preamble can vary in length. The preamble basically is used to help set up the
transmission and reception of the information through synchronization. The actual
amounts of bits has varied over the years but the principle is still the same: a series of
alternating zeroes and ones encompass the preamble. Some of these can be lost during
transmission but that is ok. The incoming stream of bits “establishes” that the reception
of a packet has started. Most agree on 62 bits. (In hex: 1555555555555 In binary:
010101010101010101010101010101010101010101010101010101010101010101010101
0101010101). You will not see this with a protocol sniffer because it is stripped and
dumped.
The Start of Frame Delimiter (SOF) further helps to set up the transmission and
reception of the information and synchronization. This is only a 2-bit portion with just
two one’s. No matter how many zeros and one’s come before the SOF the NIC does
nothing until it gets to the one-one (SOF). This information is stripped by the NIC and
the NIC can “do its work” on the rest of the packet. (In hex: 3 In binary: 11) You will
not see this with a protocol sniffer because it is stripped and dumped.
Used in de-encapsulation:
The Destination Address (DA) is the physical address (MAC) of the networking device
the information is going to be sent to. This is 48 bits in hexadecimal. This will be the
first “bits” of information you will see with a protocol inspector.
The Source Address (SA) is the physical address (MAC) of the networking device
sending the information. This is 48 bits in hexadecimal.
The Length indicates how much information will follow (but not including the CRC
information).
802.2 LLC 802.2 SNAP
DSAP SSAP con Org Type
The 802.2 LLC packet is composed of three fields:
The Destination Service Access Point (DSAP) field determines what protocol this
is coming from (Novell/IP etc). The DSAP field is usually set to 0xaa for
Ethernet. This is 1 byte.
The Source Service Access Point (SSAP) field determines what protocol this is
going to (Novell/IP etc). The DSAP field is usually set to 0xaa for Ethernet. This
is 1 byte.
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