ISDN On The Washington Elementary School District Network

ISDN Implementation

The above drawing shows the implementation of ISDN on the network. Reference point U shows the connection between each router, acting as an NT1, and the ISDN network owned by the phone company. Reference points R, S, and T, which refer to the connections between NT1, NT2, and terminal adapter devices, are all internal to the Sunnyslope and Community School routers.

Bandwidth and Data Communications

Total bandwidth available for on the ISDN link is 144Kbps, with 128Kbps for data communication and 16Kbps for signaling on the D channel. For cost reasons, the link will be a dial-on-demand (DDR) link, with each router only bringing up the link for interesting traffic. Interesting traffic will be defined as IP traffic originating from or addressed to the Community School network. The route between the Community School and the Sunnyslope Central Office will be put into each router's routing table as a static route, preventing routing table updates from bringing up the connection every 90 seconds, with the Sunnyslope router will be configured to redistribute static routes.

Equipment Needed

Both the Sunnyslope router and the Community School router require a BRI interface. The Sunnyslope router was ordered with a 4-port BRI port adapter with a U interface, which means that no additional equipment will be required to implement ISDN on the WAN side.

Benefits of ISDN for Washington Elementary School District Network
Unlike the other schools in the district, the Community School will not be connected to the district network via a T1 line. The Community School has lower bandwidth requirements, and fewer users, and therefore does not require a dedicated connection on a leased-line. ISDN is an attractive alternative for use on the connection from the WAN core to the Community School, because it provides a relatively high-speed connection at low cost. Rather than a dedicated leased-line connection, ISDN uses public telephone lines, and the link is only brought up as needed. Analog dial-up modems also run over public telephone lines, but can only carry data at 56Kbps, while BRI ISDN can carry data at 128Kbps.