5.6 Optional Configuration
5.6.6 ISDN rate adaptation

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.