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126
C
4: C
HAPTER
ONFIGURING
Configuring the VCM
T1/E1 Port
P
B
S330/S310 M
ATH
UILDER
ODULES
Delay and Echo Cancellation
SAR delay is caused by the amount of time it takes to fill an ATM cell (48 bytes). It
takes two T1 frames to fill a cell for T1. Since each T1 frame is 192 bits (or 24
bytes) and each T1 frame is 125 µsec long, the minimum delay associated with
filling and transporting a cell is approximately 250 µsec. This is the delay for a full
T1 payload using unstructured or structured CES.
When you design ATM WAN access, you must also consider the additional delay
added by distance. The rule of thumb is that every 100 miles adds approximately
1msec of delay to the SAR delay.
Delay decreases voice quality. Typically, up to 30 msec of delay is tolerable. In
situations where delay is greater than 30msec, however, you will probably need to
use echo cancellation.
Whether or not you use echo cancellation depends on the routes of your network
(length of the circuit) and the channel SAR time, with 30 msec being the
maximum delay you want to incur. However, since not all routes are straight, when
installing a network you should either use a test device that measures the delay on
a span or have your service provider verify the delay or mileage on the route.
To configure the VCM T1/E port:
1 From the VCM Configuration menu, shown earlier in Figure 115, select [2] Port
Configuration to open the VCM Port Configuration menu. Figure 117 shows the
VCM T1-DSX Port Configuration menu. The VCM E1 Port Configuration menu is
similar.
Figure 117 VCM Port Configuration Menu
2 Set the following port configuration parameters.
LBO (T1-DSX only) — Line Build Out. LBO refers to the signal
generation/regeneration of the distance between the PathBuilder S330/S310
switch and the next crossconnect of FOTs (Fiber Optic Terminal system), in feet:
1-110, 110-220, 220-330, 330-440, 440-550, 550-600 or 600+.
, P
,
A
ORTS
AND
PPLICATIONS
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