XRT75VL00D datasheet(66/92 Pages) EXAR

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XRT75VL00D

E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER

REV. 1.0.3

consecutive STS-1 frames.

Figure 41 presents an illustration of an STS-1 SPE straddling across two

consecutive STS-1 frames.

A PTE that is receiving and terminating an STS-1 data-stream will perform the following tasks.

• It will acquire and maintain STS-1 frame synchronization with the incoming STS-1 data-stream.

• Once the PTE has acquired STS-1 frame synchronization, then it will locate the J1 byte (e.g., the very byte

within the very next STS-1 SPE) within the Envelope Capacity by reading out the contents of the H1 and H2

bytes.

The H1 and H2 bytes are referred to (in the SONET standards) as the SPE Pointer Bytes. When these two

bytes are concatenated together in order to form a 16-bit word (with the H1 byte functioning as the "Most

Significant Byte") then the contents of the "lower" 10 bit-fields (within this 16-bit word) reflects the location of

the J1 byte within the Envelope Capacity of the incoming STS-1 data-stream.

Figure 42 presents an

illustration of the bit format of the H1 and H2 bytes, and indicates which bit-fields are used to reflect the

location of the J1 byte.

FIGURE 41. AN ILLUSTRATION OF AN STS-1 SPE STRADDLING ACROSS TWO CONSECUTIVE STS-1 FRAMES

TOH

STS-1 FRAME N

STS-1 FRAME N + 1

J1 Byte (1st byte of SPE)

J1 Byte (1st byte of next SPE)

H1, H2

Bytes

SPE can straddle across two STS-1 frames

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