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A PRI also contains a single 64K signaling channel, called the D channel. The D channel is responsible for synchronization between the user and the ISDN switch as well as for call setup and teardown. Figure 3−6: ISDN PRI Unlike a BRI, each of the B channels on a PRI usually shares the same directory number. This means that all calls coming into the PRI will be hunted onto the first available B channel. Each B channel can be provisioned for data only, voice only, or both voice and data. An ISDN PRI circuit does not have a SPID associated with it.
Each B channel can be provisioned for data only, voice only, or both voice and data. Figure 3−4: ISDN BRI circuit Each B channel of an ISDN BRI is also assigned a Service Profile Identifier (SPID). The SPID is used when the ISDN end−user device initially synchronizes to the ISDN switch. The analyzer traces later in this chapter will show how the SPID gets sent to the ISDN switch. As an example, a BRI circuit ordered with 2 B channels and data−only capability might be assigned the following parameters: B channel # Directory Number (DN) SPID Capability 1 9148313510 91483135100101 Data 2 9148313511 91483135110101 Data An ISDN BRI circuit also includes a 16K signaling channel, called the D channel.
In Europe and other countries, an ISDN circuit is delivered on two twisted−pair wires and is referred to as an ST interface ISDN circuit. Many of the Cisco routers with built−in ISDN interfaces have an ST interface. In order to convert the U interface circuit from the carrier to an ST interface circuit that the router can handle, an external NT1 (network terminating unit) is needed. This is depicted in Figure 3−5. An ISDN circuit is usually delivered by the Telco on an 8−pin RJ−45 jack. 16 Figure 3−5: ISDN BRI U and ST interfaces ISDN PRI An ISDN PRI (Primary Rate Interface) is delivered on a T1 circuit.
All in One Cisco CCIE Lab Study Guide Second Edition