DP83222 datasheet(3/12 Pages) NSC

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10 Functional Description (Continued)

13 STREAM CIPHER (DATA DESCRAMBLING)

The analysis of the Stream Cipher descrambler design is

somewhat more complex than the scrambler circuit shown

below The concept of the stream cipher is based upon hy-

pothetical comparison Because FDDI signalling includes

known line states comprised of unique 5-bit patterns it is

possible to utilize these patterns as comparison information

when analyzing the incoming scrambled datastream

Four FDDI line state patterns are commonly transmitted

within an FDDI ring during ring initialization and sustained

ring operation Three of these line state patterns HALT

MASTER and QUIET are used during PCM initialization and

ring fault indication The fourth pattern the IDLE is em-

ployed during PCM initialization and normal ring operation

The stream cipher operates in two modes Sample mode

and Hold mode While it is possible to become synchronized

in the Sample mode during the reception of HALT

MASTER or QUIET patterns synchronization is lost as soon

as anything other than these patterns are received It is only

the IDLE line state that allows synchronization and asser-

tion of the Hold mode The Hold mode allows fully synchro-

nized descrambling of all incoming data regardless of sym-

bol type The IDLE line state is both sufficient and required

for assertion of the Hold mode of synchronization due to

frequent interspersion among normal FDDI traffic such as

Tokens and Frames

The IDLE line state pattern will be analyzed for this exam-

ple The IDLE symbol pair in the NRZ 5B format becomes

ten ones or 11111 11111 The analysis begins by examining

the logic design provided in

Figure 3 The NRZ scrambled

data SD is routed to Register A which is an 11-bit serial

shift register with taps at registers 9 and 11 Taps 9 and 11

are input to the XOR-C gate which outputs the SD data-

stream SD is connected to one input of XOR-D The other

input to XOR-D is the original SD datastream

TLF11885 – 3

FIGURE 3 Simplified Stream Cipher Descrambler Logic

DP83222 Datasheet(PDF) 3 Page - National Semiconductor (TI)