ISDN Implementation

            ISDN is connection-oriented and circuit-switched, similar to the traditional PSTN. The bearer channels (B-channels) are each 64 Kbps and therefore optimized for traditional voice traffic.  Use of ISDN in this case study is optimal since students of these schools do not need access to the Internet, and only e-mail and occasional file transfer traffic may occur with this school.

            A typical WAN implementation might use BRI at the remote branch offices with  PRI at the central site(s). ISDN is not efficient or cost-effective as a WAN backbone   technology in itself. As a general rule of thumb, if the line is up for more than two or three hours per day, the costs prove prohibitive and frame relay or a leased line will   be more cost effective.

            ISDN provides periodic connections from remote sites. For example, telecommuters or small branch office that may only require periodic   communication with central site servers may be candidates for ISDN.  Such ISDN implementations are often referred to as Dial-on-Demand (DDR).  It is very important, however to correctly configure the routers at the remote sites to ensure that ISDN calls are never unnecessarily connected for any reason other than the application in   question. Application overhead traffic and dynamic routing updates are typical examples of traffic that may result in spurious calls. For this reason, static routing is often employed for ISDN. Static routes, however, do not scale well on very large networks due to the administrative overhead, thus necessitating careful design and   configuration of the dynamic routing protocol.

     ISDN also provides a cost-effective back-up for leased-line or frame relay connections, since its tariffing is mainly usage-based. ISDN's connection-oriented nature also ensures that for a single B-channel connection, packets can never arrive out of order and should experience a relatively uniform delay pattern.

     For increased throughput, it may be necessary to combine both B channels to produce 128 Kbps to the same destination. To do so efficiently requires a protocol such  as PPP Multilink that has a special sequence field to ensure that packets do not reach the destination out of sequence. Real-time applications cannot tolerate   significant variations in delay due to packets taking different paths through the public network depending on the B channel used. Out-of-sequence packets also cannot  be catered to by unreliable UDP-based applications. Even reliable TCP-based applications may incur an excessive number of retransmissions due to out-of-order   packets, thus negating the advantage of using a second channel. The deployment of ISDN will remain limited primarily due to its strongly usage-based tariff structure.

ISDN Router Commands

Sunset# config t
Sunset(config)# isdn switch-type basic-5ess
Sunset(config)# ip route 10.64.0.1 255.0.0.0 10.64.0.5
Sunset(config)# dialer-list 1 protocol IP permit
Sunset(config)# interface bri 0
Sunset(config-if)# ip address 10.64.0.5 255.255.0.0
Sunset(config-if)# no shut
Sunset(config-if)# encapsulation ppp
Sunset(config-if)# ppp authentication chap
Sunset(config-if)# dialer-group 1
Sunset(config-if)# dialer wait-for-carrier time 15
Sunset(config-if)# dialer idle-timeout 300
Sunset(config-if)# isdn spid1 4321567890
Sunset(config-if)# isdn spid2 3456789021
Sunset(config-if)# dialer map ip 10.64.0.5 name datacenter 357
Sunset(config-if)# exit
Sunset(config)# exit
Sunset# copy running-config startup-config