ISDN Overview
There are many types of WAN technologies that can be used to solve the
problems of users who need network access from remote locations.
In this chapter, you will learn about the
services, standards, components, operation, and configuration of
Integrated Services Digital Network (ISDN) communication.
ISDN has been specifically designed to solve the low bandwidth
problems that small offices or dial-in users have with traditional
telephone dial-in services.
Telephone companies developed ISDN with the intention of creating a
totally digital network whilst making use of the existing telephone
wiring system.
ISDN works very much like a telephone - When you make a data call with
ISDN, the WAN link is brought up for the duration of the call and is
taken down when the call is completed
What is ISDN?
ISDN allows digital signals to be transmitted over existing
telephone wiring.
This became possible when the telephone company switches
were upgraded to handle digital signals.
ISDN is generally viewed as an alternative to leased lines,
which can be used for telecommuting and networking small
and remote offices into LANs.
What Can ISDN Do?
ISDN's ability to bring digital connectivity to local sites has many benefits,
including the following:
• ISDN can carry a variety of user traffic signals including:
– digital video, packet-switched data, and telephone network services.
• ISDN offers much faster call setup than modem connections
because it uses out-of-band (D, or delta, channel) signaling.
– For example, some ISDN calls can be setup in less than one second.
• ISDN provides a faster data transfer rate than modems by using the
bearer channel (B channel of 64kbps).
– With multiple B channels, ISDN offers users more bandwidth on WANs
than some leased lines.
– For example, if you were to use two B channels, the bandwidth capability
is 128Kbps because each B channel handles 64Kbps.
ISDN can provide a clear data path over which to negotiate PPP links.
ISDN Components
Component Description
Terminal Equipment Type 1 (TE1) Designates a device that is compatible with an ISDN
network (A specialized device created for ISDN.). A TE1
connects to a network termination of either type 1 or type 2
(NT1 or NT2).
Terminal Equipment Type 2 (TE2) Designates a device that is not compatible with an ISDN
network and requires a terminal adapter (TA).
Terminal Adapter (TA) Converts standard electrical signals into a form used by
ISDN so that non-ISDN devices can connect to the ISDN
network.
Network Termination Type 1 (NT1) Connects four-wire ISDN subscriber wiring to the
conventional two-wire local loop facility.
Network Termination Type 2 (NT2) Connects four-wire ISDN subscriber wiring to the
conventional two-wire local loop facility. NT2 is a more
complicated device, typically found in digital Private Branch
eXchanges (PBXs), that performs Layer 2 and Layer 3
protocol services.
SPID
Before you can connect a router to an ISDN service, you must
be aware of the switch types used at the CO.
You specify this information during router configuration so the
router can place ISDN network-level calls and send data.
In addition to learning about the switch type your service
provider uses, you also need to know what Service Profile
Identifiers (SPIDs) are assigned to your connection.
The ISDN carrier provides a SPID to identify the line configuration
of the ISDN service.
SPIDs are a series of characters (that can look like phone
numbers) that identify you to the switch at the CO.
Protocols
Protocols That Are Used For These Purposes
Begin With This
Letter
E These protocols recommend telephone standards for ISDN. For
example, the E.164 protocol describes international addressing for ISDN.
I These protocols deal with concepts, terminology and general
methods. The I.100 series include general ISDN concepts and the structure
of other I series recommendations; the I.200 series deal with service aspects
of ISDN; the I.300 series describes network aspects; the I.400 series
describes how the UNI is provided.
Q These protocols cover how switching and signaling should operate. The term
signalling in this context means call set-up.
Standards
ISDN utilizes a suite of ITU-T standards spanning the
physical, data-link, and network layers of the OSI reference
model.
When you're deploying remote access solutions, several
encapsulation choices are available. The two most
common encapsulations are PPP and HDLC.
ISDN defaults to HDLC. However, PPP is much more robust
because it provides an excellent mechanism for
authentication and negotiation of compatible link and
protocol configuration.
ISDN interfaces allow only a single encapsulation type.
PPP is an open standard specified by RFC 1661.
Remote Access
• Remote access involves connecting users located at
remote locations through dialup connections.
• The remote location can be a telecommuter's home, a
mobile user's hotel room, or a small remote office.
• The dialup connection can be made via an analogue
connection using basic telephone service or via ISDN.
• Connectivity is affected by speed, cost, distance, and
availability.
• Remote access links generally represent the lowest-speed
links in the enterprise.
Remote Access cont’d
With the remote nodes method, the users connect to the local LAN
at the central site through the Public Switched Telephone
Network (PSTN) for the duration of the call.
Aside from having a lower-speed connection, the user sees the
same environment the local user sees.
The connection to the LAN is typically through an access server.
This device usually combines the functions of a modem and those
of a router.
When the remote user is logged in, he or she can access servers
at the local LAN as if they were local.
BRI
There are two ISDN services:
BRI (Basic Rate Interface)
PRI (Primary Rate Interface).
The ISDN BRI service offers two 8-bit B channels and one 2-bit D
channel.
ISDN BRI delivers a total bandwidth of a 144-kbps line into three
separate channels.
BRI B channel service operates at 64 kbps and is meant to carry user
data and voice traffic.
The third channel, the D channel, is a 16 kbps signalling channel used
to carry instructions that tell the telephone network how to handle each
of the B channels.
DDR
ISDN LAN routers provide routing between ISDN BRI and the
LAN by using dial-on-demand routing (DDR).
DDR automatically establishes and releases circuit-switched calls,
providing transparent connectivity to remote sites based on
networking traffic.
DDR also controls establishment and release of secondary B
channels based on load thresholds.
