universal bit. A binary value of 0 implies that a burned-in or Universally Administered
Address (UAA) is being used; a binary 1 implies that a Locally Administered Address
(LAA) is being used.
contents of the headers and trailers for each LAN type--in particular, the addresses and their
location in the headers. Also, the name of the field that identifies the type of header that follows
the LAN headers is important. Finally, the fact that a frame check sequence (FCS) is in the
trailer for each protocol is also vital. Figure 4-5 summarizes the various header formats.
The data was designed to hold some Layer 3 packets. The term maximum transmission unit
(MTU) is used to define the maximum Layer 3 packet that can be sent over a medium; hence,
with 802.3 Ethernet, 1500 is the largest MTU allowed.
4-5) is covered rather extensively in Chapter 3, "OSI Reference Model & Layered
Communication." Any computer receiving a LAN frame needs to know what is in the data
portion of the frame. Table 4-2 summarizes the fields that are used for identifying the types of
data contained in a frame.
lists the values. Xerox owns the
assignment process.
IEEE Token
Ring, ANSI
FDDI
controls the assignment of valid
values. The source SAP (SSAP) and
destination SAP (DSAP) do not have
to be equal, so 802.2 calls for the
sender's protocol type (SSAP) and the
destination's type (DSAP).
IEEE Token
Ring, ANSI
FDDI
when DSAP is hex AA. It is needed
because the DSAP and SSAP fields
are only 1 byte in length.