Information Technology Reference
In-Depth Information
DID
DID allows calls from the PSTN to be routed directly to extensions on PBXs or other systems, such as CUCM. When using DID,
there is no need to use an operator or call attendant to forward calls to internal extensions. Instead, a certain number of dialed digits
are forwarded to a PBX or voice gateway, and these digits allow the calls to be automatically routed to internal extensions.
CAMA
The Enhanced 911 (E911) telephone system is, to a great extent, separate from the regular PSTN in the United States, and CAMA is
a protocol used to ensure that a call from a particular number is routed to the public service access point (PSAP) that is designated to
handle calls from that number. The PSAP is responsible for dispatching emergency services.
CAMA is used for this purpose because it has the capability to send both calling and called party in-band, and the calling party
number is used for call routing in the E911 telephone system.
ISDN (BRI/PRI)
ISDN is a digital transport service that can support both telephony and data applications over existing telephone wires.
There are two ISDN access methods:
■
Basic Rate ISDN:
Provides two 64-Kbps bearer channels and a 16-Kbps signaling channel (2B+D).
■
Primary Rate ISDN:
PRI provides 23 or 30 64-Kbps bearer channels, depending on whether it is delivered as T1 or E1,
together with a single 64-Kbps signaling channel (23B+D or 30B+D).
At Layer 2, ISDN signaling is provided by the Link Access Procedure on the D channel (LAPD). LAPD is specified in ITU-T
Q.920/921.
At Layer 3, ISDN call control signaling and access to services is specified by ITU-T Q.931/932. As previously mentioned, Q.931
allows call setup, maintenance, and teardown.
