TXC-06101AILQ Transwitch Corporation, TXC-06101AILQ Datasheet - Page 17

• Manufacturer Part Number: TXC-06101AILQ
• Manufacturer: Transwitch Corporation

Specifications of TXC-06101AILQ

• Operating Temperature (min): -40C
• Operating Temperature Classification: Industrial
• Operating Temperature (max): 85C
• Package Type: PQFP
• Rad Hardened: No
• Lead Free Status / Rohs Status: Not Compliant

POINTERS
Pointers are defined in SONET at the STS-N and VT Levels. The STS-1 Payload Pointer provides a mecha-
nism to allow the flexible and dynamic alignment of the STS-1 SPE within the SONET Transport Frame.
Dynamic alignment means that the STS-1 SPE is allowed to float within the Transport frame. Thus the pointer
is able to accommodate differences not only in the phases of the STS-1 SPE and the TOH, but in the frame
rates as well. As is shown in Figure 11, the STS-1 SPE will usually start in one frame and end in the following
frame.
The Pointer Bytes (H1 and H2) can be viewed as one 16-bit word as shown below. The last ten bits (7-16) of
the Pointer Word carry the pointer value. This value is a binary number with a range of 0 to 782 that indicates
the offset between the Pointer Bytes and the first byte of the SPE (J1). The TOH Bytes are not counted in the
offset. For example, a value of "0" indicates that the SPE starts in the byte position immediately following the
H3 Byte. The last column of this row is indicated by a pointer value of "86". A value of "87" specifies that the
SPE starts at the byte position immediately following the K2 Byte. A value of "522" would position the start of
the SPE in the byte position immediately following the C1/J0 Byte. The two S bits are not used in STS-1 Point-
ers and are set to "00".
Frequency Justification
If there is a frequency offset between the frame rate of the TOH and that of the STS-1 SPE, then the pointer
value will be incremented or decremented, as needed, accompanied by a corresponding positive or negative
stuff byte. Consecutive pointer operations must be separated by at least three frames in which the pointer
value remains constant.
If the frame rate of the SPE is too slow with respect to the TOH, then the alignment of the envelope is periodi-
cally slipped back in time and the pointer is incremented by one. This is shown in Figure 12. The operation is
indicated by inverting Bits 7, 9, 11, 13 and 15 (I Bits) of the Pointer Word. A positive stuff byte appears immedi-
ately after the H3 Byte in the frame containing the inverted I Bits. Subsequent pointers contain the new offset.
Proprietary TranSwitch Corporation Information for use Solely by its Customers
MSB
T
O
H
T
O
H
N N N N S S
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
P
O
H
Start of SPE
H1 Byte
Figure 11. STS-1 Frame
LSB
90 Columns
- 17 of 196 -
I
Previous SPE
DATA SHEET
Next SPE
D
MSB
STS-1 SPE
I
D
I
D
H2 Byte
I
D
I
D
LSB
t = 0
t = 125 S
t = 250 S
TXC-06101
Ed. 3, April 2001
TXC-06101-MB
PHAST-1