Frame Formats And Addressing - Motorola MC68824 User Manual

APPENDIX B
FRAME FORMATS AND ADDRESSING
B.1 FRAME FORMATS AND ADDRESSING
The frame format used in an IEEE 802.4 network is as follows:
Where:
Preamble
SO
FC
DA
SA
Data
FCS
ED
I
Preamble
I
SO
I
FC
I
DA
I
SA
I
Data
I
FCS
I
ED
I
=
Pattern used to set receiving modem's clock and level
(one or more octets)
=
Start Delimiter (one octet)
=
Frame Control (one octet)
=
Destination Address (two or six octets)
=
Source Address (two or six octets)
=
Information (zero or more octets)
=
Frame Check Sequence (four octets)
=
End Delimiter (one octet)
B.1.1 PREAMBLE
Preamble preceds every transmitted MAC frame. It is used primarily for the receiving modem to
acquire signal level and phase lock by using a known pattern. Preamble is also used to give
stations a minimum amount of time to process a frame previously received. The amount of
preamble transmitted depends on the data rate and modulation scheme. Preamble must always
be a minimum of 2 microseconds regardless of the data rate and an integral number of octets
must be sent. Therefore, on a 10 Mbitlsec network, three octets of preamble is the minimum. The
maximum preamble length is constrained by the jabber control in the physical layer.
B.1.2 START DELIMITER
The start delimiter informs the modem that a frame is coming. The signaling patterns of the start
delimiter are always distinguishable from data. The start deliminter is represented as follows:
MC68824 USER'S MANUAL
First Bit Transmitted
N
=
Non Data
o
=
Zero
o
I
0
Most Significant Bit
MOTOROLA
8-1
I