ISDN Design

The Community School serviced out of the Service Center Regional Hub requires access to the WAN. The Community School is geographically located in a remote location and requires higher bandwidth that traditional dial-in telephone services.

Benefits of ISDN

Telephone companies with the intention of creating a totally digital network for increased bandwidth speeds developed ISDN.

ISDN was developed to use the existing telephone wiring system to enhance WAN usage with out incurring major networking costs.

ISDN provides access to digital video, circuit-switched data, and telephone network services by using the normal phone network that is circuit-switched.

ISDN offers much faster call setup than modem connections because it uses out-of-band (D channel) signaling. For example ISDN calls can be setup in less than one second.

ISDN provides a faster data transfer rate than modems by using the B channel. When multiple B channels are used, ISDN offer users more bandwidth on WANs.

ISDN can provide a clear data path over which to negotiate PPP links.

ISDN offers Dial on Demand Routing, which means you only pay for the time that you use the link.

Reference Points


R References the connection between a non-ISDN compatible device and a TA.
S References the points that connect into the NT2 or customer-switching device.
T References the outbound connection from the NT2 to the ISDN network or NT1.
U References the connection between the NT1 and the ISDN network owned by the telephone company.
TA Terminal Adapter
NT2 Directs traffic to and from different subscriber devices and the NT1.
NT1 Connects the four-wire ISDN subscriber wiring to the conventional two-wire local loop facility.


ISDN Services: BRI and PRI

Basic Rate Interface (BRI) Bandwidth

Primary Rate Interface (PRI) Bandwidth

ISDN Implementation in the Community School

Based on the need of the Community School we are recommending ISDN BRI services for connectivity at the school site. The BRI local loop is terminated at the customer premise at an NT1. The interface of the local loop at the NT1 is called the U reference point as diagrammed above. At the school CPE side of the NT1 is the S/T reference point. An ISDN LAN router will provide routing between ISDN BRI and the LAN by using dial-on-demand routing. DDR automatically establishes and releases circuit-switched calls, providing transparent connectivity to remote sites based on networking traffic. DDR also controls establishment and release of secondary B channels based on lad threshold. Multilink PPP will be used to provide bandwidth aggregation when using the multiple B channels.


The PC TA's connect internally by the PC bus and can provide a single PC user with direct control over ISDN session initiation and release similar to using an analog modem.

ISDN Router Configuration
- hostname of the Data Center Router connecting via ISDN to the ISP: DataCenter
- first the interface that we configure ISDN for backup have to be configured and be told that the respective bri interface should be used as backup line:

DataCenter> enable

DataCenter# configure terminal
DataCenter(config)# username ISP password student

DataCenter(config)#interface serial 0

(the serial 0 interface is configured for Frame
Relay in a separate chapter, now we setup a backup line)
DataCenter(config-if)#backup interface bri 0

DataCenter(config)# isdn switch-type basic-net3

DataCenter(config)# dialer-list 1 protocol ip permit

DataCenter(config)# interface bri 0

DataCenter(config-if)#backup delay 0 10

DataCenter(config-if)# ip address

DataCenter(config-if)# dialer-group 1

DataCenter(config-if)# dialer wait-for-carrier-time 15

DataCenter(config-if)# dialer idle-timeout 300

DataCenter(config-if)# isdn spid1 5674398703

DataCenter(config-if)# dialer map ip name ISP 445

DataCenter(config-if)# encapsulation ppp

DataCenter(config-if)# ppp authentication chap

DataCenter(config-if)# exit

DataCenter(config)# exit


If we want ISDN backup lines for the backbone routers a similar configuration should be made, except that we will configure interfaces from serial 1 to serial 8.

Home  |  WAN Requirements  |  IP Addressing  |  PPP-CHAP Configuration  |

Frame Relay Configuration  |  NAT Configuration  |  IGRP Configuration  |

ISDN ConfigurationDHCP Configuration  |  ACLs  |

Semester 3 TCS
| Main Page | Requirements | Wiring map | MDF/IDF details |
| VLANs | IP addressing scheme | Cutsheets | Router config |
| Security configuration | Details about the equipment used |