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Washington School District WAN
ISDN Learning Objectives and Network Design
Learning Objectives
In Semester 4, chapter 5 we learned about Integrated Services Digital Network (ISDN) and how it can be implemented in a network. ISDN service runs over the public switched telephone network, and is an alternative to leased lines. It has several benefits, including the ability to carry several different types of user traffic, faster call setup than regular dialup modems, higher data transfer rates than modems, and providing a clear path for negotiating PPP links. It also has two chief design issues: security, and cost containment.
There are five types of ISDN components:
- Terminal Equipment Type 1(TE1) – Any end-user device that is compatible with ISDN
- Terminal Equipment Type 2 (TE2) – Any end-user device that is not compatible with ISDN and requires a terminal adapter (TA)
- Terminal Adapter (TA) – Converts ISDN signals into non-ISDN signals, and vice versa, so that TE2 equipment can connect to an ISDN network
- Network Termination Type 1 (NT1) – Connects four-wire ISDN wiring to the conventional two-wire public network
- Network Termination Type 2 (NT2) – Performs switching and concentrating on the ISDN network
ISDN also defines four reference points on the customer side of the ISDN connection:
- R – References the connection between a TE2 device and a TA
- S – References the connection between an NT2 and the customer side of the ISDN network
- T – References the outbound connection between an NT2 and the NT1 device
- U – References the connection between an NT1 device and the service provider side of the ISDN network
Two important pieces of information needed when configuring a router to access the ISDN network are the switch type, and the Service Profile Identifiers (SPIDs). The switch type defines what type of switch is being used by the service provider. In the US, the two most common are AT&T's 5ESS and Nortel's DMS-100. The SPIDs, which resemble telephone numbers, are used to identify your network to the service provider's switch so that it knows what services to provide.
We also learned in this chapter about the different types of protocols that define the standards for ISDN. E protocols define the telephone network standards for ISDN; I protocols deal with concepts, terminology, and general methods; and Q protocols define switching and signaling, and general quality of service.
The chapter defines three scenarios where one would implement ISDN:
- Remote Access – Users at a remote location connect through a dialup connection on analog telephone lines, generally dialing into a modem bank at the central site.
- Remote Nodes – Users at a remote location connect through the PSTN into the central network, generally through an access server, which combines the functions of a router and a modem. Other than a slower connection, the remote users see the same network environment as local users.
- SOHO – A remote office with a few users initiates a connection to the central office via a single router, so that it appears as a single IP to the outside world.
There are two types of ISDN service: BRI and PRI. BRI uses two 8-bit B channels and one 2-bit D channel, and delivers a total of 144kbps (128kbps usable for data). PRI uses 23 8-bit B channels and one 8-bit D channel plus a framing bit to deliver 1.544Mbps. ISDN service often uses a LAN router, which uses dial-on-demand routing (DDR) to automatically establish and release the call. The router must be configured to select a switch type and specify the triggers that initiate the DDR call, as well as configuring the interface with ISDN addressing, DDR dialer information, and other optional features. If necessary, SPIDs are also configured on the interface.
When setting up DDR, it is important to consider how the connection will be initiated, maintained, and disconnected. If done correctly, DDR can allow the remote network to act as if it had full-time connectivity to the central network without running up costs by constantly bringing up the ISDN connection. This is accomplished by defining "interesting traffic," to restrict the kind of traffic that will bring up the link.
Application of Concepts in the Network Design
On the District WAN, ISDN is being used for the connection between the Community School and the Sunnyslope Core router. For security, PPP is being used for encapsulation, with CHAP used to provide authentication. The Sunnyslope router itself is an NT1 device, and has a U interface to connect to the service provider side of the ISDN network. It has been configured to use an AT&T 5ESS switch. The Sunnyslope router will use SPIDs 5555555555 and 5555555556, while the Community School will use SPIDs 5555555557 and 5555555558 to initiate its connection.
The Community School will act as a remote node, with users able to access the District network as if they were local users, albeit at a lower speed since they are using an BRI connection rather than a T1. Both the Community School router and the Sunnyslope router will be using DDR to initiate the connection. Interesting traffic will be defined as any IP traffic that is routed to the BRI interface and destined for the opposite side. Configuration on the Sunnyslope router can be seen here.