ISDN Media Types
Cisco acquisition accessories abutment ISDN BRI and ISDN PRI. Both media types use B channels
and D channels. The B channels backpack user data. The D channel, in its role as arresting carrier
for the B channels, directs the CO about-face to accelerate admission calls to accurate timeslots on
the Cisco admission server or router. It additionally identifies the alarm as a circuit-switched agenda call
or an analog modem call. Circuit-switched agenda calls are relayed anon to the ISDN
processor in the router. Analog modem calls are decoded and again beatific to the onboard
modems. Figure 4-5 illustrates three sample ISDN accession options.
192 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
V
D Approach 64 kbps (Signaling)
24 B Channels (Voice)
23 B Channels (Voice)
V
D Approach 64 kbps (Signaling)
30 B Channels (Voice)
V
D Approach 16 kbps (Signaling)
ISDN BRI 2 B Channels (Voice)
ISDN E1 PR1 NET5
ISDN T1 PR1 NFAS
Figure 4-5 ISDN Accession Options
ISDN BRI, referred to as “2 B + D,” has the afterward characteristics:
■ Two 64-kbps B channels backpack articulation or abstracts for a best manual acceleration of
128 kbps.
■ One 16-kbps D approach carries signaling traffic—that is, instructions about how to
handle anniversary of the B channels, although it can abutment user abstracts manual under
certain circumstances.
The D approach signaling agreement comprises Layers 1 through 3 of the Open Systems
Interconnection (OSI) advertence model. BRI additionally provides for framing ascendancy and other
overhead, bringing its absolute bit amount to 192 kbps.
The BRI concrete band blueprint is ITU-T I.430. BRI is actual accepted in Europe and is
also accessible in North America. BRI allows up to two accompanying calls.
ISDN PRI, referred to as “23 B + D” or “30 B + D,” has the afterward characteristics:
■ 23 B channels (in North America and Japan) or 30 B channels (in the blow of the
world) backpack articulation or data, acquiescent a absolute bit amount of 1.544 Mbps and 2.048 Mbps,
respectively.
■ One 64-kbps D approach carries signaling traffic.
The PRI concrete band blueprint is ITU-T Standards Section I.431.
Following are common standards for PRI:
■ T1-PRI: Use this interface to baptize North American ISDN PRI with 23 B channels
and one CCS channel.
■ E1-PRI: Use this interface to baptize European ISDN PRI with 30 B channels, one
CCS channel, and one framing channel.
■ ISDN-PRI Nonfacility Associated Signaling (NFAS): ISDN NFAS enables a distinct D
channel to ascendancy assorted ISDN PRIs on a chassis. This D approach functions as the
primary approach with the advantage of accepting addition D approach in the accumulation as a backup.
After you accept configured the channelized controllers for ISDN NFAS, you need
to configure alone the NFAS primary D channel. Its agreement is broadcast to all
the associates of the associated NFAS group. The account of PRI NFAS is it frees the
B approach by application a distinct D approach to ascendancy assorted PRI interfaces. One B
channel on anniversary added interface is chargeless to backpack added traffic.
■ Apportioned PRI: The appellation apportioned PRI has altered meanings in altered genitalia of
the world. One acceptation indicates assorted PRI groups (bearer channels [B channel]
and associated D channel) on the aforementioned T1/E1 interface. Because the NM-HDV supports
only a distinct D approach per T1/E1, the PRI affection does not abutment this definition
of apportioned PRI. However, the added adaptation of the appellation indicates the capability
to ascertain a distinct D approach for anniversary interface with beneath than 23/31 B channels associated
with it. This analogue of apportioned PRI is accurate on Cisco articulation gateways.
Note The PRI interface is economically bigger to BRI because an interface agenda supporting
PRI is usually already in abode on avant-garde PBXs.