Understanding the network.pdf - Back to Home

terminals attached to the ISDN switch device. When a call is placed to the ISDN
number, the call is routed to the first available B channel. Along the same lines, if a
PBX has 100 internal switch ports and 23 external PRI channels, when an internal
ISDN terminal places a call outside the PBX, the first available B channel is used to
establish the call. To establish calls with ISDN terminals directly or indirectly
attached to the PRI, the ISDN address is used. An ISDN address is a distinct NPA
subscriber address assigned to an ISDN B channel or ISDN terminal. When this
address is assigned to a B channel, it is basically a telephone number, so when a call
is placed using an ISDN address, the call is routed to a specifically assigned B
channel. When the address is assigned to an ISDN terminal, the CO switch has a list
of assigned ISDN addresses associated with a PRI/ISDN number. Call requests
using one of the assigned ISDN addresses are routed to the PRI(s) connected to the
ISDN switch device hosting the ISDN terminals assigned these addresses. When a
call is placed using an ISDN address, it is routed to the CO switch. If a B channel is
available to the ISDN switch, the call is assigned a channel and the ISDN switch
(PBX) then routes the call to the correct ISDN switch port connected to the ISDN
terminal that has the assigned ISDN address.
These two different addressing models reflect the ISDN signaling capability of the
CPE. A CPE device using a BRI circuit depends on the LT to handle call routing and
so on, whereas a PRI terminates at the NT2 interface and is capable of performing
call routing and other ISDN signaling functions.
PSTN Packet- and Cell-Switched Networks
The operating concept of Packet-Switched Networks (PSNs) should be familiar to
you. It is the basis for the majority of LAN networking technologies. A group of
end-stations connected to the same medium share the available bandwidth to
exchange information in variable-length packets (or frames). With LANs, after the
hardware and cabling expense, the cost of operation is minimal. With WAN
networking, however, along with the cost of hardware there is a monthly PSTN
access cost. PSNs provide a usage-based data transport facility as an alternative to
dedicated bandwidth transport provided with T-carrier point-to-point links.
The PSTN, however, was not originally designed for shared bandwidth operation. Its
design is focused on providing dedicated, connection-oriented data exchanges
between logical connections. Packet-switched networks were developed to provide
shared bandwidth data transport services in line with the PSTN's operational model.
With PSNs, each network endpoint is provided a dedicated PSTN connection and its
own unique PSN address. This connection is provided over traditional PSTN
transport facilities (T-carrier/POTS), and the circuits are provisioned to provide a
dedicated network bandwidth rate.