Appendix A Reference Information
Minimum AAL2 Trunk Size Requirements
A-8
255-700-714R11.0
PacketStar
®
PSAX 1-Port Channelized OC-3/STM-1 CES Module User Guide
, Issue 1
Release 11.0.0
Minimum AAL2 Trunk Size Requirements
Setting up channels with standard AAL2 multiplexing helps reduce traffic
delay, as voice samples from at two channels or more are multiplexed into
one VC, decreasing the amount of time necessary to fill one ATM cell before
it is transmitted. (Without multiplexing, only one VC is allowed per ATM cell.
The cell must wait to fill with samples from one channel before transmis-
sion). If a trunk is configured to have at least one active channel, one cell
must be sent out every 20 ms; thus, the minimum trunk size is 50 cps. At
8 Kbps, Lucent recommends two multiplexed channels. On the DSP2C mod-
ule, you can configure up to 64 AAL2 trunk groups, so that if you want to
use all 128 channels (or 224 for echo cancel mode), you must use multiplex-
ing. Enabling silence detection further enhances bandwidth savings by a fac-
tor of approximately half on the value in the cells/sec column in Table A-4.
Table A-4 summarizes bandwidth savings at different compression rates with
standard AAL2 multiplexing.
Standard AAL2 Calculation Example
The following calculation computes the compression rate for 32 Kbps (see
Table A-4). This calculation includes the AAL2 header in cells per second
(cps):
4,000 samples / 20 samples received from DSP * 23 bytes to be packed into
an AAL2 cell, where 23 bytes = 20 bytes of data in the cps packet and 3 bytes
for the cps header.
With 47 data bytes in the AAL2 cell, the cell rate = 4600/47 =~ 98
The expected cell rate with silence detection is: 98/2 = 49.
See Table A-5 for the cell rates for Lucent nonmultiplexed AAL2.
Table A-4. Standard (Multiplexed) AAL2 Bandwidth Calculation
*
Compression
Rate (in Kbps)
Data
Bytes
/sec
Data + Overhead
Calculation
Data +
Overhead
/sec
Cells/sec
calculation
Cells
/sec
Average
Expected
cells/sec with
silence
detection
enabled
64 8000
(8000/40)*43
8600
8600/47
183
92
40
5000
(5000/25)*28
5600
5600/47
120
60
32
4000
(4000/20)*23
4600
4600/47
98
49
24
3000
(3000/15)*18
3600
3600/47
77
39
16
2000
(2000/10)*13
2600
2600/47
56
28
8
1000
(1000/5/2)*13
1300
1300/47
28
14
*
If a trunk has at least one active channel, a cell must be sent out every 20 msec; therefore, the minimum
AAL2 trunk size is 50 cps.