ISDN Digital Subscriber Line (IDSL)

A. IDSL is a cross between ISDN and xDSL. As with ISDN, it uses a single wire pair to transmit full-duplex data at 128 Kbps and at distances of up to the Revised Resistance Distance range of 15,000 to 18,000 feet. IDSL also uses a 2B1Q line code to enable transparent operation through the ISDN "U" interface. IDSL is essentially a leased line ISDN Basic Rate Interface (BRI), or an ISDN BRI that is not switched and does not contain signaling (a D channel). IDSL and ISDN BRI use the same 2B1Q line modulation. On the router, this equates to putting the BRI interface in a leased line configuration. The line can be configured for a speed of 64 Kbps, 128 Kbps, or 144 Kbps.

The frames that are going across the wire are standard High-Level Data Link Control (HDLC) frames. IDSL can be configured with Point-to-Point Protocol (PPP) or Frame Relay encapsulation for the leased line BRI interface. The easiest way to think about it is as if the BRI interface was a slow speed synchronous serial port. Also, existing Customer Premises Equipment (CPE) (ISDN BRI terminal adapters, bridges, and routers) can be used to connect to the central office.

A. No. The Cisco 2500 series does not support IDSL because its BRI hardware does not support channel aggregation.

A. Yes. IDSL is currently supported with the ISDN WAN Interface Cards (WICs) and network modules when they are configured in leased line mode.

A. No. PPP over Frame Relay is supported only for serial interfaces.

A. No. Cisco IDSL does not need a Service Profile Identifier (SPID) or phone number for leased-line ISDN. Cisco uses clear channel 64K or 128K; ISDN uses two 64K channels.

A. No. You do not need the ISDN switch type command.

A. Yes. However, if you use Frame Relay encapsulation, turn off the Local Management Interface (LMI) on the Cisco CPE.

A. No. The Cisco 804 does not currently support Point-to-Point Protocol (PPP) over Frame Relay.

A. Yes. The Cisco 6160 has an eight-port IDSL ITU-C Line Card (ITUC-8-2B1Q-DIR-1). Cisco IOS® Software Release 12.1(2)DA and later is required. Also, the NI1 DSL Access Multiplier (DSLAM) platforms do not support IDSL.

A. IDSL can operate up to 18,000 feet, more if repeaters are used.

A. No. IDSL carries only data. IDSL is connected to a DSLAM, unlike an ISDN circuit which is connected to a voice switch.

A. Following are two examples of a basic configuration for IDSL.

PPP configuration

isdn leased-line bri 0 128
username <username> password <password>
interface BRI0
ip address 3.3.3.3 255.255.255.0
encapsulation PPP
ppp authentication chap

Frame Relay Configuration

int BRI0
ip address 10.1.1.1 255.255.255.0
encapsulation frame-relay IETF
fair-queue 64 16 0
frame-relay map ip 10.1.1.2 300 broadcast IETF
frame-relay limi-type ansi