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3. Configuration
Some numeric values on the ATM Traffic Profiles screen represent cells per
second. An ATM cell is 53 bytes (48 bytes of data plus 5 bytes of overhead), or
424 bits. Conversion guidelines are listed in
Table 3-2.
A voice circuit typically requires 80
are 53 bytes in length (a 5-byte header and 48 bytes for payload). After allowing
4 bytes for the AAL2 overhead, you have approximately 44 bytes available for
data. Therefore, the total overhead is 53/44. For a 64 kbps stream, this results in
77.1 kbps (64 x 53/44). The 80
signaling.
The ATM line cards implement Usage Parameter Control (otherwise known as
policing) as described in the ATM Forum's Traffic Management Specification 4.1
(AF-TM-0121.000). According to this specification, traffic contracts for the two
VBR ATM service categories (rt-VBR and nrt-VBR) include traffic parameters for
Peak Cell Rate (PCR) and Sustainable Cell Rate (SCR). Policing is implemented
using the recommended dual Generic Cell Rate Algorithm (GCRA) functions.
3-34
VOICE (Models 8335, 8365, and 8385 only): This profile allocates enough
bandwidth for one voice call (approximately 80 kbps). If more than one voice
call is placed on the connection using this profile, then the cells will be tagged
(CLP bit set). This profile is policed to 188 cells/sec (enough for one phone
call).
Conversion Guidelines
For Standard Speeds
Kilobits per
Equals cells per
second . . .
second . . .
2320
1552
1168
784
528
400
272
144
February 2003
Table 3-2, Conversion
For Nx128k Speeds
Kilobits per
second . . .
5471
1536
3660
1024
2755
768
1849
512
1245
384
943
256
642
144
340
, which is 188 cells per second. ATM cells
kbps
allows extra bandwidth for additional inband
kbps
Guidelines.
Equals cells per
second . . .
3623
2415
1811
1208
906
604
340
8335-A2-GB20-70
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