Motorola MPC8260 User Manual
29-26
MPC8260 PowerQUICC II UserÕs Manual
MOTOROLA
Part IV. Communications Processor Module
29.5.2.1 RM Cell Rate Representation
Rates in the RM cells are represented in a binary ßoating-point format using a 5-bit
exponent (e), a 9-bit mantissa (m), and a 1-bit nonzero ßag (nz), as shown in Figure 29-15.
exponent (e), a 9-bit mantissa (m), and a 1-bit nonzero ßag (nz), as shown in Figure 29-15.
Figure 29-15. Rate Format for RM Cells
Figure 29-16. Rate Formula for RM Cells
Initialize the trafÞc parameters (ER, MCR, PCR, or ICR) in the ABR protocol-speciÞc
connection tables using the rate formula in Figure 29-16.
connection tables using the rate formula in Figure 29-16.
Table 29-7. Fields and their Positions in RM Cells
Fields
Octet
Bits
Description
Value
Header
1Ð5
All
ATM cell header
RM-VCC
PTI=6
PTI=6
ID
6
All
Protocol ID
1
DIR
7
0
Direction of RM cell (0 = forward, 1 = backward)
BN
7
1
Backward notiÞcation (BN = 0, the cell was generated by the
source; BN=1, the cell was generated by the network or by the
destination)
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
Ñ
7
5-7
Reserved, should be cleared.
0
ER
8Ð9
All
Explicit rate; see Section 29.5.2.1
CCR
10Ð11
All
Current cell rate; see Section 29.5.2.1
MCR
12Ð13
All
Min cell rate; see Section 29.5.2.1
QL
14Ð17
All
Not used (ATM Forum ABR)
0
SN
18Ð21
All
Not used (ATM Forum ABR)
0
Ñ
22Ð51
All
Reserved, should be cleared.
0x6A for each byte
Ñ
52
0Ð5
Reserved, should be cleared.
0
CRC-10
52
6Ð7
CRC-10
53
All
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
nz
Exponent
Mantissa
Rate
2
e
1
m
512
---------
+
è
ø
æ
ö
´
nz
´
=