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Rate adaptation allows
the ISDN channel to adjust to a lower speed, if requested in the
call setup by the access router. You must, therefore, use the
rate-adaptation feature for cases in which the destination does not
use the default DS0 of 64 Kbps, but rather uses 56 Kbps. The Figure
shows a connection with a reduced speed, which is set on the D
channel during the call setup process. The manual assignment of
calling speed is done on a per-destination basis. If not configured
for the lower speed of 56 Kbps, the interface uses the default speed
of 64 Kbps. The data rate can be adjusted to 56 Kbps if call setup
requests it.
ISDN: 56 Kbps Speed Limit
The 56-Kbps data rate limitation comes from the restricted digital
information (RDI) technique. This method stipulates that octets
cannot have all zeroes. To meet the RDI requirement of having at
least one bit turned on in each octet (called one-density), every
eighth bit of each octet is set to on. If one bit per octet is
borrowed, the data rate drops to 56 Kbps for an original 64-Kbps
channel.
For ISDN interfaces only, you can specify an optional speed parameter for dialer map commands, as shown
below. This option informs the ISDN software whether it should place a call at 56
Kbps or 64 Kbps. If you omit the ISDN speed parameter, the default is 64
Kbps, as follows:
Router(config-if)#dialer map
protocol next-hop-address [name name] [speed
speed] [broadcast] dial-string
Symptoms of ISDN Speed Mismatch
A connection toward a specific destination being dropped following a connect ack might be a symptom that a rate adaptation is needed.
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