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CHAPTER 3 FUNCTIONAL DESCRIPTION
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3.2 AAL-5 SAR SUBLAYER FUNCTION
The transmission function of the
μ
PD98405 pads user data of different lengths (0 to 65,535 bytes) prepared
by the host system, to the nearest multiple of 48 bytes, adds the trailer shown in Figure 3-3 to the data to create
CPCS PDU, then disassembles the CPCS PDU of AAL-5 into 48-byte segments.
The reception function assembles a CPCS PDU from the received cells and stores it into the receive buffer.
The
μ
PD98405 checks the trailer of the packet, checks for errors, and reports those errors, if any, to the host.
The receive data is stored in CPCS PDU format consisting not only of (a) user data field but also fields (b)
through (f).
Figure 3-3. PDU Format of AAL-5
(d)
CPI
(c)
C
(a) User data
(b)
Padding
(f)
CRC-32
(e)
Length
0 to 64K bytes
1 byte
2 bytes
4 bytes
1 byte
48 bytes
48 bytes
48 bytes
Trailer
Segment
0 to 47 bytes
Segment
Segment
(a)
User data field:
Data of up to 65,535 bytes.
(b)
Padding field:
A field of 0 to 47 bytes, inserted between the user data and trailer, to increase the length of the
CPCS PDU to the nearest multiple of 48 bytes. The
μ
PD98405 automatically inserts all-zero data
into this field.
(c)
CPCS user-user information (CPCS-UU) field:
Used to transfer CPCS user-user information. The
μ
PD98405 can set and transfer any data.
(d)
Common part identification (CPI) field:
This field, containing all-zero data, is used to set the length of the CPCS PDU trailer to 8 bits. No
other usage or set value is defined for this field. This field of a packet that is transmitted by the
μ
PD98405 can be set in any way by the user.
(e)
Packet length (Length) field:
Indicates the user data length (0 to 65,535) of the CPCS PDU, in binary. This field is calculated and
inserted by the transmitter. The receiver is provided with a function to verify this field against the size
of the packet actually received.