Freescale Semiconductor MPC8250, Family Reference Manual

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ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
30-26 Freescale Semiconductor
30.5.2.1 RM Cell Rate Representation
Rates in the RM cells are represented in a binary floating-point format using a 5-bit exponent (e), a 9-bit
mantissa (m), and a 1-bit nonzero flag (nz), as shown in
Figure 30-15 .
The rate (in cells/second) is calculated as in Figure 30-16 .
Figure 30-16. Rate Formula for RM Cells
Initialize the traffic parameters (ER, MCR, PCR, or ICR) in the ABR protocol-specific connection tables
using the rate formula in
Figure 30-16 .
Table 30-7. Fields and their Positions in RM Cells
Fields Octet Bits Description Value
Header 1–5 All ATM cell header RM-VCC
PTI=6
ID 6 All Protocol ID 1
DIR 7 0 Direction of RM cell (0 = forward, 1 = backward)
BN 7 1 Backward notification (BN = 0, the cell was generated by the source;
BN=1, the cell was generated by the network or by the destination)
CI 7 2 Congestion indication. (1 = congestion, 0 = otherwise)
NI 7 3 No increase indication. (1 = no increase allowed, 0 = otherwise)
RA 7 4 Not used (ATM Forum ABR) 0
— 7 5-7 Reserved, should be cleared. 0
ER 8–9 All Explicit rate; see Section 30.5.2.1
CCR 10–11 All Current cell rate; see Section 30.5.2.1
MCR 12–13 All Minimum cell rate; see Section 30.5.2.1
QL 14–17 All Not used (ATM Forum ABR) 0
SN 18–21 All Not used (ATM Forum ABR) 0
— 22–51 All Reserved, should be cleared. 0x6A for each byte
— 52 0–5 Reserved, should be cleared. 0
CRC-10 52 6–7 CRC-10
53 All
0 1 2 6 7 15
0 nz Exponent Mantissa
Figure 30-15. Rate Format for RM Cells
Ra te 2
e
1
m
512
---------- +
⎝ ⎠
⎛ ⎞
×
nz
×
=