IBM 3745 Series, Hardware Maintenance Reference

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IEEE 802.3 Frame
┌────────/─┬─────────/───┬─────────/─┬───────┬─┬─┬─┬───────/─┬─┬─────────/─┐
│
│ │ │ │D S C P│ │
│ HEADER │ DESTINATION │ SOURCE │ LENGTH│S S T DATA A│ FCS │
│ │ ADDRESS │ ADDRESS │ │A A R D│ │
│ │ │ │ │P P L │ │
└────────/─┴─────────/───┴─────────/─┴───────┴─┴─┴─┴───────/─┴─┴─────────/─┘
8 bytes
6 bytes 6 bytes 2 bytes 4 bytes
Table 12-2. IEEE 802.3 Frame
Field Name Byte Field Function
HEADER 0 - 7 Fixed bit sequence used for identifying the start of a
frame. The last byte is the start frame delimiter.
DESTINATION
ADDRESS 8 - D Only 48 bit addresses are supported by the 3745
Ethernet.
SOURCE ADDRESS E - 13 Address of the source Ethernet adapter for the frame.
LENGTH 14 - 15 Length from the next field to the end of the information.
This value includes the DSAP, SSAP, CONTROL, and
DATA fields. It does not include the PAD field.
DSAP 16 Destination Service Access Point. Bit 6 is the user
defined address indicator. Bit 7 is the individual/group
indicator.
SSAP 17 Source Service Access Point. Bit 6 is the user defined
address indicator. Bit 7 is the command/response indi-
cator.
CONTROL May be a one byte or two byte field. For the type 1
frames used with connectionless protocols, the field will
always be one byte. Only three types of frames are sup-
ported: Unnumbered information frame (x'03'), XID
frame (x'BF'), and Test frame (X'F3').
DATA Variable length field that contains the information portion
of the frame. The length of the information must be
within a range that allows the total size of the IEEE 802.3
frame data to less than the allowed maximum. For the
10 megabit 500 meter IEEE 802.3 standard, this value is
the same as that for Ethernet V2 1500 bytes. Note
however that the IEEE 802.2 header fields are counted
as part of the data. So that the maximum size of a
higher level protocol data unit that is embedded in an
IEEE 802.3 frame will be less than the 1500 bytes
allowed for an Ethernet V2 frame.
PAD The size of an IEEE 802.3 frame must exceed a
minimum value. For a 10 megabit 500 meter baseband
implementation, this value is 64 bytes. The entire frame
except for the PREAMBLE is included in this value.
When the 18 byte length of the DA, SA, LENGTH, and
FCS are subtracted, the 46 byte remainder is equivalent
to the Ethernet V2 minimum packet size. If necessary
the PAD field is included in an IEEE 802.3 frame to bring
the frame up to the minimum size. The length of the
PAD field is not included in the value in the LENGTH
field.
FCS A 4 byte frame check sequence is included at the end of
an IEEE 802.3 frame.
12-10
IBM 3745 Models 130, 150, 160, 170, and 17A: Hardware Maintenance Reference