1.1 Remote Access
1.1.3 Circuit-switched connections
Circuit-switched connections are a substitute when full-time connectivity is not required, such as in environments where traffic patterns are low volume or periodic. Circuit switching is a WAN-switching method, in which a dedicated physical circuit through a carrier network is established, maintained, and terminated for each communication session. Initial signaling at the setup stage determines the endpoints and the connection between the two endpoints.

Circuit switching requires call setup and call teardown. Circuit switching is used in the telephone company networks as shown in Figure , and works like a telephone call. Typical circuit-switched connections include:

  • Asynchronous serial
  • ISDN, Basic Rate Interface (BRI), and ISDN Primary Rate Interface (PRI)

Asynchronous Serial Connections

Asynchronous serial connections, as seen in Figure , are made only when traffic dictates a need. It requires minimal cost and uses the existing telephone network. It provides users with easy access to a central site from anywhere that has a telephone connection.

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

Enable DDR on your asynchronous interface under the following circumstances:

  • Traffic patterns are low volume or periodic-Calls are placed and connections are established only when the router detects traffic marked as “interesting.” Periodic broadcasts, such as routing protocol updates, should be prevented from triggering a call. This type of connection is good for the user who logs on only long enough to do a task such as check e-mail or upload a file. An example of periodic traffic is when you dial-in to an Internet Service Provider (ISP) from home for an hour and then log off.
  • When 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 the following situations: 

  • A backup connection is required. 
  • A connection from a small remote site. 
  • Short-term on-demand access is required.

Asynchronous serial connections 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 28 to 56 Kbps. This limits the amount of traffic that can be sent over an asynchronous line. To place or receive an asynchronous serial call, a router must have an asynchronous serial interface. The EIA/TIA-232 standard specifies the interface to an external modem. The interface to the telephone company varies by country. In the United States, a standard RJ-11 adapter connects the modem to the telephone outlet. -

ISDN Connections

ISDN connections are typically circuit-switched connections that (like asynchronous connections) provide WAN access when needed, rather than requiring a dedicated link. ISDN offers increased bandwidth over a typical dialup connection; it is intended to carry data, voice, and other traffic across a telephone network.

ISDN comprises BRI and PRI, which are illustrated in Figure . With BRI, there are two B-channel services that operate at 64 Kbps per channel and one D channel that operates at 16 Kbps. The B-channel is used for voice, video, and data, whereas the D channel is used for call setup and teardown. The D channel uses Link Access Procedure on the D channel (LAPD) as the data link protocol. ISDN provides greater bandwidth than common modem alternatives of 56 Kbps. BRIs require a Network Termination 1 (NT1) device to connect to the phone service. In North America, the NT1 is not supplied by the service provider; outside North America, the NT1 is usually provided. 

With PRI, there are 23 B-channels on T1 used in North America and Japan. There are 30 B-channels on E1 used in Europe and other parts of the world. PRIs require CSUs/DSUs. The D channel for a PRI connection operates at 64 Kbps.

To place an ISDN BRI call, your router should be equipped with a BRI interface. You may also need an ISDN (TA) Terminal Adapter, which is a device 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.

ISDN PRI is generally configured over T1 and E1 connections. To place a PRI call, your router should be equipped with either connection. T1 is used in the United States, and E1 is common in Europe. As with asynchronous connections, you can also configure DDR to control access for specific periods of time.