XRT75L00D Datasheet, PDF (66/92 Pages) Exar Corporation – E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER

XRT75L00D
E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER
REV. 1.0.2
consecutive STS-1 frames. Figure 41 presents an illustration of an STS-1 SPE straddling across two
consecutive STS-1 frames.
FIGURE 41. AN ILLUSTRATION OF AN STS-1 SPE STRADDLING ACROSS TWO CONSECUTIVE STS-1 FRAMES
TOH
H1, H2
Bytes
STS-1 FRAME N
STS-1 FRAME N + 1
J1 Byte (1^st byte of SPE)
J1 Byte (1^st byte of next SPE)
SPE can straddle across two 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.
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