Understanding the network.pdf - Back to Home

SCCP operates as an OSI-RM Layer 3.
• ISDN User Part (ISDN-UP)—ISDN-UP messages are used to control the claim
and release of CO trunk circuits that are employed for interLATA and
intraLATA transport of voice and data. Calls that are within the same CO
switch do not use ISDN-UP. The ISDN-UP is not related to the Integrated
Services Digital Network (ISDN) switched digital service. However, ISDN
and SS7 do use different messaging protocols (Q.913 and Q.700,
respectively) and these message types are not compatible. So, for an ISDN
call to be established with another ISDN terminal attached to a different
switch, the ISDN signaling information must be translated into the SS7
equivalent. This function is performed by the SSP. ISDN-UP operates at
OSI-RM Layers 3 through 7.
• Transaction Capabilities Applications Part (TACP)—Part of the fourth layer,
TACP is used to exchange AIN, non-circuit-related SS7 message exchanges.
These messages are exchanged using SCCP Class 0 delivery. TACP operates
at OSI-RM Layer 7.
• Operations, Maintenance, Administration, and Provisioning (OMAP)—Also
part of the fourth layer, OMAP messages are used to perform management
functions and send operational and maintenance messages pertaining to
common overall PSTN operations. OAMP operates at OSI-RM Layer 7.
ISDN
ISDN started to become available in 1980, around the same period that SONET was
beginning to be deployed in major population centers in the United States. It was
believed that ISDN would meet the increasing demand for reasonably affordable
small office/home office (SOHO) digital telephony, enhanced voice and data
services, and flexible digital transport for dedicated data and video services.
However, the dream of ISDN, as it turned out, was long in coming. In the United
States, starting in the 1970s, an effort had been underway by AT&T and the BOCs to
convert the original electromechanical switch-based FDM telephone system to a
TDM-based infrastructure. The TDM system provided better transport facilities for
voice and accommodated data transport requirements. TDM-based systems were
also cheaper to build, maintain, and operate. Consumers in the 1970s and 1980s
saw this effort in the form of the conversion from rotary/pulse to touch-tone dialing.
One of the downsides to TDM systems was that they require two wire pairs to
provide digital transport. Therefore, implementation of digital services over the last
mile between the consumer and the CO, which uses a single wire pair, presented
some problems. ISDN's local loop implementation, known as the Basic Rate
Interface (BRI), enabled digital transport over a single wire pair. When ISDN came
on the scene, it provided the technology needed to effectively "digitize" the
BOCs'local loop circuit. This spawned the possibility of a digital transmission path,
end to end.