dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 9

• Manufacturer Part Number: dp83261
• Description: Bmac Device Fddi Media Access Controller
• Manufacturer: National Semiconductor Corporation

4 0 FDDI MAC Facilities
4 2 1 PDU Fields
Start of Frame Sequence
The Start of Frame Sequence (SFS) consists of the Pream-
ble (PA) followed by the Starting Delimiter (SD)
The Preamble is a sequence of zero or more Idle symbols
that is used to separate the PDUs The Ring Engine Receiv-
er can process and repeat a frame or token with no pream-
ble The Ring Engine Transmitter generates frames with at
least 8 bytes of preamble The Ring Engine Transmitter also
guarantees that valid FDDI frames will never be transmitted
with more than 40 bytes of preamble
The Starting Delimiter is used to indicate the start of a new
PDU The Starting Delimiter is the JK symbol pair
The Ring Engine expects the Starting Delimiter to be con-
veyed across the PH Indication Interface as a single byte
Similarly the Ring Engine only generates Starting Delimiters
aligned to the byte boundary
Frame Control
The Frame Control (FC) field is used to discriminate PDUs
For tokens the FC field identifies Restricted and Non-re-
stricted tokens For frames the FC field identifies the frame
types and format and how the frame is to be processed
The one byte FC field is formatted as shown in Figure 4-3
The C (Class) bit specifies the MAC Service Class as Asyn-
chronous (C
The L (Length) bit specifies the length of the MAC Address
as Short (L
bit address A Long Address is a 48-bit address
The FF (Format) bits specify the PDU types as shown in
Table 4-3
The r (Reserved) bit is currently not specified and should
always be transmitted as Zero (Exception SMT NSA
Frames)
The ZZZ (Control) bits are used in conjunction with the C
and FF bits to specify the type of PDUs These bits may be
used to affect protocol processing criteria such as the Priori-
ty Protocol Class Status Handling etc
When the Frame Control Format bits (FC FF) indicate a
SMT or MAC PDU the frame type is identified as shown in
Table 4-4
0
0
1
1
FC FF
TABLE 4-3 Frame Control Format Bits
e
0
1
0
1
FIGURE 4-3 Frame Control Field
e
0) or Long (L
0) or Synchronous (C
C
SMT MAC
LLC
Reserved for Implementer
Reserved for Future Standardization
L
FF
e
1) A Short Address is a 16-
PDU Types
r
ZZZ
e
(Continued)
1)
9
Destination Address
The Destination Address (DA) field is used to specify the
station(s) that should receive and process the frame
The DA can be an Individual or Group address This is de-
termined by the Most Significant Bit of the DA (DA IG)
When DA IG is 0 the DA is an Individual Address when
DA IG is 1 the DA is a Group Address The Broadcast Uni-
versal address is a Group Address
The DA field can be a Long or Short Address This is deter-
mined by the L bit in the FC field (FC L) If FC L is 1 the DA
is a 48-bit Long Address If FC L is 0 the DA is a 16-bit
Short Address
The Ring Engine maintains both a 16-bit Individual Address
My Short Address (MSA) and a 48-bit Individual Address
My Long Address (MLA)
On the receive side if DA IG is 0 the incoming DA is com-
pared with MLA (if FC L
received DA matches MLA or MSA the frame is intended for
this station and the address recognized flag (A Flag) is set
If DA IG is 1 the DA is a Group Address and is compared
with the set of Group Addresses recognized by the Ring
Engine If a match occurs the address recognized flag
(A Flag) is set The A Flag is used by system inter-
face logic as part of the criteria (with FC L DA IG and
M Flag) to determine whether or not to copy the frame If
the A flag is set the system interface will normally attempt
to copy the frame
On the transmit side the DA is provided by the system inter-
face logic as part of the data stream The length of the
address to be transmitted is determined by the L bit of the
FC field (The FC field is also passed in the data stream )
The Destination Address can be an Individual Group or
Broadcast Address
Source Address
The Source Address (SA) field is used to specify the ad-
dress of the station that originally transmitted the frame
CLFF
1000
1100
0L00
0L00
0L00
1L00
1L00
1L00
1L00
TABLE 4-4 MAC SMT Frames Types
0001 to
0100 to
rZZZ
0000
0000
0000
1110
1111
0001
0010
0011
1111
e
1) or MSA (if FC L
Non-Restricted Token
Restricted Token
Void Frame
SMT Frame
SMT Next Station
Addressing Frame
Other MAC Frame
MAC Beacon Frame
MAC Claim Frame
Other MAC Frame
PDU Type
e
0) If the