ISDN Media Types
Cisco routing devices support ISDN BRI and ISDN PRI. Both media types use B channels and D channels. The B channels carry user data. The D channel, in its role as signal carrier for the B channels, directs the CO switch to send incoming calls to particular timeslots on the Cisco access server or router. It also identifies the call as a circuit-switched digital call or an analog modem call. Circuit-switched digital calls are relayed directly to the ISDN processor in the router. Analog modem calls are decoded and then sent to the onboard modems. Figure 4-5 illustrates three sample ISDN installation options.
„_f D Channel 16 kbps (Signaling)
D Channel 64 kbps (Signaling)
ISDN E1 PR1 NET5 30 B Channels (Voice)
^ D Channel 64 kbps (Signaling)
ISDN T1 PR1 NFAS ^
Figure 4-5 ISDN Installation Options
ISDN BRI, referred to as "2 B + D," has the following characteristics:
■ Two 64-kbps B channels carry voice or data for a maximum transmission speed of 128 kbps.
■ One 16-kbps D channel carries signaling traffic—that is, instructions about how to handle each of the B channels, although it can support user data transmission under certain circumstances.
The D channel signaling protocol comprises Layers 1 through 3 of the Open Systems
Interconnection (OSI) reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps.
The BRI physical layer specification is ITU-T I.430. BRI is very common in Europe and is also available in North America. BRI allows up to two simultaneous calls.
ISDN PRI, referred to as "23 B + D" or "30 B + D," has the following characteristics:
■ 23 B channels (in North America and Japan) or 30 B channels (in the rest of the world) carry voice or data, yielding a total bit rate of 1.544 Mbps and 2.048 Mbps, respectively.
■ One 64-kbps D channel carries signaling traffic.
The PRI physical layer specification is ITU-T Standards Section I.431.
23 B Channels (Voice)
24 B Channels (Voice)
Note The PRI interface is economically preferable to BRI because an interface card supporting PRI is usually already in place on modern PBXs.
Following are worldwide standards for PRI:
■ T1-PRI: Use this interface to designate North American ISDN PRI with 23 B channels and one CCS channel.
■ E1-PRI: Use this interface to designate European ISDN PRI with 30 B channels, one CCS channel, and one framing channel.
■ ISDN-PRI Nonfacility Associated Signaling (NFAS): ISDN NFAS enables a single D channel to control multiple ISDN PRIs on a chassis. This D channel functions as the primary channel with the option of having another D channel in the group as a backup. After you have configured the channelized controllers for ISDN NFAS, you need to configure only the NFAS primary D channel. Its configuration is distributed to all the members of the associated NFAS group. The benefit of PRI NFAS is it frees the B channel by using a single D channel to control multiple PRI interfaces. One B channel on each additional interface is free to carry other traffic.
■ Fractional PRI: The term fractional PRI has different meanings in different parts of the world. One meaning indicates multiple PRI groups (bearer channels [B channel] and associated D channel) on the same T1/E1 interface. Because the NM-HDV supports only a single D channel per T1/E1, the PRI feature does not support this definition of fractional PRI. However, the other version of the term indicates the capability to define a single D channel for each interface with less than 23/31 B channels associated with it. This definition of fractional PRI is supported on Cisco voice gateways.
Continue reading here: Digital Voice Port Parameters
Was this article helpful?