DP83266 datasheet(13/152 Pages) NSC

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40 FDDI MAC Facilities (Continued)

421 Frame Fields

Start of Frame Sequence (SFS)

The Start of Frame Sequence consists of the Preamble (PA)

followed by the Starting Delimiter (SD)

The Preamble is a sequence of zero or more Idle symbols

that is used to separate frames The Ring Engine Receiver

can process and repeat a frame or token with no preamble

The Ring Engine Transmitter generates frames with at least

8 bytes of preamble The Ring Engine Transmitter also

guarantees that valid FDDl frames will never be transmitted

with more than 40 bytes of preamble

The Starting Delimiter is used to indicate the start of a new

frame 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 (FC)

The Frame Control field is used to discriminate frames For

tokens the FC field identifies Restricted and Non-restricted

tokens For other frames the FC field identifies the frame

type and format and how the frame is to be processed

The one byte FC field is formatted as shown in

Figure 4-3

CLFFrZZZ

FIGURE 4-3 Frame Control Field

The C (Class) bit specifies the MAC Service Class as Asyn-

chronous (C e 0) or Synchronous (C e 1)

The L (Length) bit specifies the length of the MAC Address

as Short (L e 0) or Long (L e 1) A Short Address is a

16-bit address A Long Address is a 48-bit address

The FF (Format) bits specify the frame types as shown in

Table 4-3

The r (Reserved) bit is currently not specified and should

always be transmitted as Zero

The ZZZ (Control) bits are used in conjunction with the C

and FF bits to specify the type of frames These bits may be

used to affect protocol processing criteria such as the Priori-

ty Protocol Class Status Handling etc

TABLE 4-3 Frame Control Format Bits

FCFF

Frame Types

SMTMAC

LLC

reserved for implementer

reserved for future standardization

When the Frame Control Format bits (FCFF) indicate an

SMT or MAC frame the frame type is identified as shown in

Table 4-4

TABLE 4-4 MACSMT Frame Types

CLFF

rZZZ

Frame Type

1000

0000

Non-restricted Token

1100

0000

Restricted Token

0L00

0000

Void Frame

0L00 0001 to 1110 SMT Frame

0L00

1111

SMT Next Station Addressing Frame

1L00

0001

Other MAC Frame

1L00

0010

MAC Beacon Frame

1L00

0011

MAC Claim Frame

1L00

0100

MAC Purge Frame

1L00 0101 to 1111 Other MAC Frame

Destination Address (DA)

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 (DAlG)

When DAIG is 0 the DA is an Individual Address when

DAlG is 1 the DA is a Group Address The BroadcastUni-

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 (FCL) If FCL is 1 the DA

is a 48-bit Long Address If FCL is 0 the DA is a 16-bit

Short Address

The Ring Engine maintains a 16-bit Individual Address (My

Short Address (MSA)) and a 48-bit Individual Address (My

Long Address (MLA))

On the receive side if DAlG is 0 the incoming DA is com-

pared with MLA (if FCL e 1) or MSA (if FCL e 0) If the

received DA matches MLA or MSA the frame is intended for

this station and the address recognized flag (A

Flag) is set

If DAlG 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

Flag) is set The A

Flag is used by system interface

logic as part of the criteria (with FCL DAlG and M

Flag)

to determine whether or not to copy the frame lf the

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 (SA)

The Source Address (SA) field is used to specify the ad-

dress of the station that originally transmitted the frame

The Source Address has the same length as the Destination

Address (ie if the DA is a 16-bit Address the SA is a 16-bit

Address if the DA is a 48-bit Address the SA is a 48-bit

Address)

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