CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

Chapter 26
ISDN
- Most organizations have permanent and always-on WAN connections (eg: leased line, Frame
Relay) to their branch offices. Backup WAN connections are also important, as the primary
WAN connections might fail for some reasons. ISDN is a popular choice for backup connections.
- Integrated Services Digital Network (ISDN) is a circuit-switched and dialed digital WAN
service. Dialed circuits are cheaper than leased-lines and packet-switched WAN services, and
supports concurrent and integrated voice, data, and video transmission over existing PSTNs.
- ISDN uses digital signals, which allows faster transmission speed compared to analog lines.
ISDN bandwidth is measured in increments of 64kbps.
- ISDN was introduced after analog modems. It has phased out as a home-based Internet access
technology due to the existence of other competing remote access technologies, eg: DSL.
TA
Local Loop
(Digital BRI)
(With PCM Codec)
Telco
ISDN
Switch
@ CO
Figure 26-1: ISDN Local Loops and Equipments
- Routers often use ISDN interfaces that can directly connect to the Telco, while PCs typically
require an ISDN device called an ISDN terminal adapter (TA). Most people misinterpret it as
ISDN modem, as it was connected like an external modem. However, this is inaccurate, as
ISDN uses digital signals and there is no modulation and demodulation processing as in analog
modem communications. The functionality of TA has been incorporated into most ISDN routers.
- ISDN requires the installation of new line from Telco ISDN switch, as analog phone lines
normally connect to a voice switch (for processing analog voice signals) whereas ISDN uses
digital signals.
- ISDN has 2 types of interfaces: Basic Rate Interface (BRI) and Primary Rate Interface (PRI).
Both provide multiple digital bearer channels (B channels) for data communication. Multiple B
channels can be bundled together to increase the available bandwidth to a remote site.
- B channels are the bearer channels (bear or carry the data). It used for data transmission.
A single B channel operates at the speed of 64kbps.
- D channels are the signaling channels. Phone number for dialing to the destination site is sent
through D channel during the establishment phase of an ISDN connection. It is used for call
control functions, which are call setup, signaling, and call termination.
187
T1 Line
(Digital)
PSTN
(With PCM Codec)
Telco
ISDN
Switch
@ CO
Local Loop
(Digital PRI)
Router with
Internal
ISDN Card
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com