DP83261 datasheet(8/132 Pages) NSC

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40 FDDI MAC Facilities

41 SYMBOL SET

The Ring Engine recognizes and generates a set of sym-

bols These symbols are used to convey Line States (such

as the Idle Line State) Control Sequences (such as the

Starting and Ending Delimiters) and Data

Additional information regarding the symbol set can be

found in the FDDI PHY Standard

The Ring Engine expects that the Starting Delimiter will al-

ways be conveyed on an even symbol pair boundary Fol-

lowing the starting delimiter data symbols should always

come in matched pairs

Similarly the Ending Delimiter

should always come in one or more matched symbol pairs

The symbol pairs conveyed at the PHY Interface are shown

in Table 4-1

42 PROTOCOL DATA UNITS

The Ring Engine recognizes and generates two types of

Protocol Data Units (PDUs) Tokens and Frames

The Token is used to control access to the ring Only the

station that has captured the token has the right to transmit

new information The format of a token is shown in Figure

4-1

SFS

EFS

FIGURE 4-1 Token Format

Frames are used to pass information between stations The

format of a frame is shown in Figure 4-2 with the field defini-

tions in Table 4-2

SFS

Protected by PCS

EFS

INFO

FCS

FIGURE 4-2 Frame Format

TABLE 4-1 Symbol Pair Set

Type

Symbols

Starting Delimiter

Ending Delimiter

TT or TR or TS or nT

Frame Status

RR or RS or SR or SS

Idle

ll or nl

Data Pair

Note

n represents any data symbol (0–F)

Symbol pairs others than the defined symbols are treated as code violations

Section 72 has additional information on the symbol pairs generated and interpreted by the Ring Engine

TABLE 4-2 PDU Fields

Name

Description

Size

SFS

Start of Frame Sequence

Preamble

8 or More Idle Symbol Pairs

Starting Delimiter

JK Symbol Pair

Frame Control Field

1 Data Symbol Pair

Destination Address

2 or 6 Symbol Pairs

Source Address

2 or 6 Symbol Pairs

INFO

Information Field

FCS

Frame Check Sequence

4 Symbol Pairs

EFS

End of Frame Sequence

Ending Delimiter

At Least 1T Symbol for Frames

At Least 2T Symbols for Tokens

Frame Status

3 or More R or S Symbols

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