XRT75VL00D datasheet(58/92 Pages) EXAR

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XRT75VL00D

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

REV. 1.0.3

Although the role of the H1, H2 and H3 bytes will be discussed in much greater detail in “Section 9.3, Jitter/

Wander due to Pointer Adjustments” on page 60. For now, we will simply state that the role of these bytes is

two-fold.

• To permit a given PTE (Path Terminating Equipment) that is receiving an STS-1 data to be able to locate the

STS-1 SPE (Synchronous Payload Envelope) within the Envelope Capacity of this incoming STS-1 data

stream and,

• To inform a given PTE whenever Pointer Adjustment and NDF (New Data Flag) events occur within the

incoming STS-1 data-stream.

9.2.1.1.2

The Envelope Capacity Bytes within an STS-1 Frame

In general, the Envelope Capacity Bytes are any bytes (within an STS-1 frame) that exist outside of the TOH

bytes. In short, the Envelope Capacity contains the STS-1 SPE (Synchronous Payload Envelope). In fact,

every single byte that exists within the Envelope Capacity also exists within the STS-1 SPE.

The only

difference that exists between the "Envelope Capacity" as defined in Figure 33 and Figure 34 above and the

STS-1 SPE is that the Envelope Capacity is aligned with the STS-1 framing boundaries and the TOH bytes;

whereas the STS-1 SPE is NOT aligned with the STS-1 framing boundaries, nor the TOH bytes.

The STS-1 SPE is an "87 byte column x 9 row" data-structure (which is the exact same size as is the Envelope

Capacity) that is permitted to "float" within the "Envelope Capacity". As a consequence, the STS-1 SPE (within

an STS-1 data-stream) will typically straddle across an STS-1 frame boundary.

9.2.1.1.3

The Byte Structure of the STS-1 SPE

As mentioned above, the STS-1 SPE is an 87 byte column x 9 row structure. The very first column within the

STS-1 SPE consists of some overhead bytes which are known as the "Path Overhead" (or POH) bytes. The

remaining portions of the STS-1 SPE is available for "user" data. The Byte Structure of the STS-1 SPE is

presented below in Figure 35.

FIGURE 34. THE BYTE-FORMAT OF THE TOH WITHIN AN STS-1 FRAME

D10

D12

D11

Envelope Capacity

Bytes

Envelope Capacity

Bytes

3 Byte Columns

87 Byte Columns

9 Rows

The TOH Bytes

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