XRT75R03D_06 Datasheet, PDF (100/135 Pages) Exar Corporation – EXAR DATA SHEET FORMAT TEMPLATES

XRT75R03D
THREE CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCRONIZER
REV. 1.0.4
FIGURE 35. THE BYTE-FORMAT OF THE TOH WITHIN AN STS-1 FRAME
9 Rows
3 Byte Columns
87 Byte Columns
AA11 AA22 CC11
BB11 EE11 FF11
DD11 DD22 DD33
HH11 HH22 HH33
BB22 KK11 KK22
DD44 DD55 DD66
DD77 DD88 DD99
DD1100 DD1111 DD1122
SS11 MM00 EE22
EEnnvveellooppeeCCaappaacciittyy
BByytteess
The TOH Bytes
Although the role of the H1, H2 and H3 bytes will be discussed in much greater detail in “Section 10.3, Jitter/
Wander due to Pointer Adjustments” on page 103. 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.
10.2.1.1.2
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 34 and Figure 35 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.
10.2.1.1.3
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 36.
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