Requires call setup and call teardown Usually provided by telephone carrier

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On-demand circuit switching is a WAN transport method in which a dedicated physical circuit is established, maintained, and terminated through a public switched telephone network (PSTN) for each communication session. Initial signaling at the setup stage determines the endpoints and the connection between the two endpoints.

Typical circuit-switched connections are:

Asynchronous modem ■ ISDN BRI and ISDN PRI

Advantages of on-demand connection types include dynamic selection of the circuit endpoint and the accumulation of charges for transport only while connections are active. Costs are directly related to connection time and distance for each plain old telephone service (POTS) line or ISDN bearer (B) channel. As traffic between endpoints increases in volume, the duration of the connection increases.

Asynchronous modem connections require minimal equipment cost and use the existing telephone network. Users can easily access a central site from any location that has a telephone connection into a telephone network.

The nature of asynchronous connections allows you to configure the connection to be enabled—only when you need the service—by using dial-on-demand routing (DDR) through the modem using an asynchronous serial interface. DDR is ideal when you need short-term access only.

You should enable DDR on your asynchronous interface when:

■ Traffic volume is low or traffic is periodic: Calls are placed and connections are established when only the router detects traffic marked as "interesting." Periodic broadcasts, such as routing protocol updates, should be prevented from triggering a call.

You need a backup connection for redundancy or load sharing: DDR can be used to provide backup load sharing and interface failure backup.

A router acts as an access server, which is a concentration point for dial-in and dial-out calls. Mobile users, for example, can call into an access server at a central site to access their e-mail messages.

Asynchronous connections are useful in these situations:

■ A backup connection required Small site

Short-term on-demand access

■ Periods of lower network traffic and fewer users

Asynchronous connections through the PSTN require modems at each end of the connection to convert digital data signals to analog signals that can be transported over the telephone network. Modem speeds typically vary from 19.2 kbps to 56 kbps, depending on line quality. The slower bandwidth speeds limit the amount of traffic you may want to send over an asynchronous line. To place or receive an asynchronous serial call, equip a Cisco router with an asynchronous serial interface. The serial standard to attach to an external modem is the EIA/TIA-232 standard. The interface to the telephone company varies by country. Within the United States, a standard RJ-11 adapter connects the modem to the telephone outlet.

ISDN Connections

This topic describes ISDN circuit-switched connections.

ISDN Connections

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ISDN connections are typically switched connections that, like asynchronous connections, provide WAN access when needed rather than through a dedicated link. ISDN offers increased bandwidth over a typical dialup connection, faster setup, and is intended to carry data, voice, and other traffic across a telephone network.

To place an ISDN BRI call, you should equip your router with a BRI interface. You may also need an ISDN terminal adapter, which is a device that is used to connect ISDN BRI connections to other interfaces, such as EIA/TIA-232. A terminal adapter is essentially an ISDN modem. You should also consult your telephone company for information specific to your connection.

Note Generally, in Europe, the service provider supplies the Network Termination 1 (NT-1). In

North America, the customer supplies the NT-1.

ISDN PRI is configured over connections such as T1 and E1 technologies. To place an ISDN call, equip your router with the proper connection. T1 is used in the United States, and E1 is common in other countries.

As with asynchronous connections, you can also configure DDR to control access for specific periods of time.

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