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CHAPTER 3 FUNCTIONAL DESCRIPTION
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(2) VPI/VCI (virtual path identifier/virtual channel identifier) fields
VPI and VCI are routing bits used for identification when the virtual path (VP) level and virtual channel (VC)
level are multiplexed.
<Processing performed by
μ
PD98405>
Transmission: Inserts all the 24 bits of the user-specified VPI/VCI.
Reception:
Supports up to 16 bits of the VPI/VCI. These bits consist of the low-order bits of the VPI
and the low-order bits of the VCI. For details of the algorithm that reduces the number of
bits from 24 to 16, see
Section 5.5.4
.
The
μ
PD98405 divides the setting of one channel into a transmission VC and reception
VC.
The
μ
PD98405 can support up to 32K active VCs (virtual channels) (any combination of
reception VC and transmission VC is supported). The VC (virtual channel) identifies the
transmission and reception channels of the
μ
PD98405, and differs in meaning from the
VCI field.
(3) PTI (payload type identifier) field
This is a 3-bit field that indicates whether the payload type of a cell consists of user data or layer
management information. It contains explicit forward congestion information (EFCI) and indicates that a
cell has passed a congested network node.
The PTI field codes are assigned as follows:
PTI
Usage
000
User data cell, congestion not experienced, SDU type = 0
001
User data cell, congestion not experienced, SDU type = 1
010
User data cell, congestion experienced, SDU type = 0
011
User data cell, congestion experienced, SDU type = 1
100
OAM F5 cell (segment supported)
101
OAM F5 cell (end-end supported)
110
Resource management cell
111
Reserved for future function
SDU type = 0: All segments of an AAL-PDU except the last cell
SDU type = 1: The last cell of an AAL-PDU. This segment contains the trailer.
OAM F5 cell:
Specific OAM cell containing virtual channel connection (VCC) operation
information
<Processing performed by
μ
PD98405>
Transmission: The
μ
PD98405 sets the user-specified pattern in this field as is and transfers it. When the
AAL-5 type processing is selected (the AAL bit of the packet descriptor is 1), the
μ
PD98405 transmits the last segment after changing the least significant bit of this field to
"1."