
1-4
Mindspeed Technologies
28229-DSH-001-B
Introduction to IMA
CX28224/5/9 Data Sheet
1.1.1
IMA Framing
The IMA protocol employs a simple frame structure as shown in
Figure 1-3. It
consists of a single IMA overhead cell (ICP) and M – 1 ATM layer cells, where M is
the IMA frame length. Valid frame lengths are 32, 64, 128 (default), or 256. This
example shows a group composed of three links and an IMA frame length of 16. (An
invalid frame length of 16 is used for brevity, the default frame length is 128.)
Figure 1-3. IMA Frame; Length = 16; Number of links = 3
Since the ATM layer data rate is often less than the bandwidth available across the
links in the IMA group, the IMA engine generates IMA Filler cells when no ATM
layer cells are available. (The IMA Filler cells perform the same basic function as Idle
cells in a non-IMA ATM system and are discarded by the IMA receiver.)
The IMA frame rate is intentionally set slightly below the available payload
bandwidth of the IMA link. To allow for timing differences between the links in a
group, the IMA standard requires that the system insert an extra ICP cell every 2049
500027_067
ICP
3
ATM
4
ATM
10
F
ICP
3
ATM
21
ICP
3
0
15
14
13
12
11
10
987654321
F
ATM
3
ATM
2
ATM
1
F
IMA control Protocol Cell
IFSN = n
ATM payload;
cell x in the sequence
Filler cell
1 IMA frame
M = 16
ATM
x
ICP
n
F
Link 0
Link 1
Link 2
ATM
5
ATM
6
ATM
7
ATM
9
ATM
8
ATM
0
ATM
12
ATM
11
ATM
16
ATM
19
ATM
14
ATM
15
ATM
13
ATM
17
ATM
18
ATM
20
ATM
27
ATM
24
ATM
30
ATM
25
ATM
23
ATM
22
ATM
28
ATM
26
ATM
34
ATM
32
ATM
29
ATM
33
ATM
31
ATM
0
ATM
2
ATM
1
ATM
3
ATM
5
ATM
4
ATM
34
ATM
33
ATM
32
ATM
35
ATM
36
This cell will be in
the next frame
...
Incoming data from the ATM layer (which doesn't
know that the data is being split into frames)
Data output into an IMA frame
ATM
x
This cell was in the
previous frame
F
Delay between the cells arrival at
the IMA device results in Filler
cells being inserted
F
Always at least 1 ICP cell per frame
This link selected as the Timing Reference
Link, (TRL), thus first ICP starts at 0.