NFAS
NFAS is a function of ISDN that is only supported in the U.S. and allows your equipment to be more efficient with channel use if more than one PRI circuit is in use. The basic premise of NFAS is to take a single D channel and configure it so that it can control multiple PRI circuits (up to 479 B channels). NFAS is limited to 479 B channels because the D channel is only 64 kbps. If you have five PRI circuits coming into the same router, you can use NFAS to reclaim a 64-kbps B channel on four of the circuits. As with any ISDN configuration, you must configure the ISDN switch type in global configuration mode first. The line coding and framing associated with PRI is not discussed in detail because they are the same formats that are used on standard T1 and E1 circuits.
Japanese PRI, is also called INS-Net 1500 by NTT, is normally deployed by using fiber optics to the customer's premises. NTT in Japan offers two main services, one with 23 B+D service and one with 24 B+D service. The 24 B+D service is offered by sending the D channel through a separate trunk to the customer's premises. Therefore, the customer has access to all 24 B channels on the other circuit. In the T1 configuration, there is a command for cable length. Remember, this is used in DSX-1 or short-haul applications for T1. HDB3 is the default line coding and does not have to be configured if that is what you are using. Although AMI is an option, it is almost never used for E1 circuits. To set up a Japanese PRI, you can use the primary-ntt switch type in IOS.
Once the controllers have been configured with the PRI-group command, you can configure the D channel. Any of the group-based commands in IOS create a virtual serial interface. In this case, it is the D channel. It is usually referred to as the controller/D channel. T1 PRI can be something along the lines of 7/0:23 and E1 can be 7/0:15. The virtual serial interfaces that are created are zero-based count.
This is specifically for a back-to-back T1 PRI on an AS5400. You can tell it is back-to-back by the isdn protocol-emulate network command. That command sets this router to act as the network side of the ISDN connection.
Command isdn incoming-voice modem specifies that incoming calls are treated as inbound analog modem calls and are connected to the proper resources within the device. This same command can map inbound voice calls to the proper voice resources for VoIP when using an AS5300. The command isdn bchan-number-order ascending specifies which direction the B channel selection occurs. The choices are ascending or descending.
