ISDN



	Internetworking Technologies Instructor: Prabul, CCNA

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11. ISDN (Integrated Services Digital Network)

Top 10 Graphics:

Integrated Services Digital Network (ISDN) refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry data, voice, and other source traffic. ISDN was developed to use the existing telephone wiring system yet increase access speeds up to 128 kbps.

Remote node method - the users connect to the local LAN at the central site for the duration of the call. Aside from having a lower-speed connection, the user sees the same environment the local user sees.

Remote user appears to be a local network node
Connection to the LAN is typically through an access server

ISDN is a set of standards that define an end-to-end digital network:

Developed by the telephone companies with the intention of creating a totally digital network
End-to-end digital network for data, fax, voice, and video
Most common data link encapsulation is PPP (although default is HDLC)
Each ISDN interface allows only a single encapsulation type
All major routing protocols can use ISDN
Can carry any network-layer protocols required to multiple destinations
ISDN devices include the following:

Terminal Equipment 1 (TE1)
Designates a device that is compatible with the ISDN network
Terminal Equipment 2 (TE2)
Designates a device that is not compatible with ISDN and requires a Terminal Adapter
Terminal Adapter (TA)
Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network
Network Termination Type 1 (NT1)
Connects 4-wire ISDN subscriber wiring (S/T) to the conventional 2-wire local loop facility (U)
Network Termination Type 2 (NT2)
Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating

ISDN Services:

BRI - Basic Rate Interface (3 channels 2B+D)
- Two 64 kbps (bearer) B channels
- One 16 kbps (signaling data) D channel (for LAPD)
PRI - Primary Rate Interface (T1/E1)
- 23 (T1) or 30 (E1) B channels
- One 64 kbps (signaling) D channel

ISDN Interface Reference Points:

S/T interface - defines the interface between a TE1 and an NT. The S/T is also used to define the TA-to-NT interface.
- S - References the points that connect into the NT2, or customer switching device. It is the interface that enables calls between the various customer premises equipment.
- T - Electrically identical to the S interface, it references the outbound connection from the NT2 to the ISDN network.
- The electrical similarities between the S and T references is why some interfaces are labeled S/T interface, because although they perform totally different functions, the port is the same electrically and can be used for either function.
R interface - defines the interface between a TE2 and the TA.
- References the point (connection) that is between a non-ISDN compatible device and a terminal adapter.
U interface - defines the 2-wire interface between the NT1 and the ISDN network owned by the phone company

Customer premises equipment (CPE) - Terminating equipment, such as terminals, telephones, and modems, supplied by the telephone company, installed at customer sites, and connected to the telephone company network.

Service Profile Identifier (SPID) - Number that some service providers use to define the services to which an ISDN device subscribes. The ISDN device uses the SPID when accessing the switch that initializes the connection to a service provider. (can look like phone numbers)

Identify you to the switch at the central office (CO)
SPIDs are processed during each call setup operation

Central office (CO) - Local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs.

Dial-on-Demand Routing (DDR) - Technique whereby a Cisco router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem.

ISDN and the OSI Model

Physical layer - The ISDN Basic Rate Interface (BRI) physical layer specification is defined in International Telecommunication Union Telecommunication Standardization Sector (ITU-T) I.430. the ISDN Primary Rate Interface (PRI) physical layer specification is defined in ITU-T I.431.
Data link layer - The ISDN datalink layer specification is based on Link Access Procedure on the D channel (LAPD) and is formally specified in ITU-T Q.920 and ITU-T Q.921.
Network layer - The ISDN network layer is defined in ITU-T I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together these two standards specify user-to user, circuit-switched, and packet-switched connections.

Circuit switching - Switching system in which a dedicated physical circuit path must exist between sender and receiver for the duration of the "call." Used heavily in the telephone company network.

Packet switching - Networking method in which nodes share bandwidth with each other by sending packets.

ISDN protocols are organized according to the following topic areas:

Protocols that begin with "E" recommend telephone network standards for ISDN. For example, the E.164 protocol describes international addressing for ISDN.
Protocols that begin with "I" deal with concepts, terminology, and general methods. The I.100 series includes general ISDN concepts and the structure of other I-series recommendations; I.200 deals with service aspects of ISDN; I.300 describes network aspects; I.400 describes how the User-Network Interface (UNI) is provided.
Protocols beginning with "Q" cover how switching and signaling should operate. The term 'signaling' in this context means the process of call set used. Q.921 describes the ISDN data-link processes of LAPD, which functions like Layer 2 processes in the OSI reference model. Q.931 specifies ISO/OSI reference model Layer 3 functions.

ISDN Configuration tasks:

Global Configuration -
- Select switch type (must matches the ISDN provider's switch)
- Specify traffic to trigger DDR call
Interface Configuration -
- Select interface specifications
- Configure ISDN addressing
Optional Feature Configuration


Router(config)# isdn switch-type 'switch-type'

specifies the type of ISDN switch with which the router communicates
- basic-5ess - AT&T basic rate switches (USA)
- basic-dms100 - NT DMS-100
- basic-ni1 - National ISDN-1 (North America)
- basic-1tr6 - German 1TR6 ISDN switches
- basic-ts013 - Australian TS013 switches
- basic-net3 - Switch type for Net3 in United Kingdom and Europe
- ntt - NTT ISDN switch Japan
- none - No specific switch specified
Other line configuration requirements vary for specific providers


Router(config)# interface bri interface-number

Selects the interface for ISDN BRI operation


Router(config-if)# encapsulation [ppp|hdlc]

Selects framing for ISDN BRI


Router(config-if)# isdn spid1 spid-number [ldn]

Sets a B channel Service Profile Identifier (SPID) required by many service providers
ldn - local dial number (optional)


Router(config-if)# isdn spid2 spid-number [ldn]

Seta a SPID for the second B channel

ISDN BRI Simple Configuration Example


! Set up switch type, static route and dialer for ISDN 
! on Cisco A isdn switch-type basic-5ess
! Selects the AT&T switch as the CO ISDN switch type 
! for this router.
!
ip route 172.16.29.0.255.255.255.0 172.16.126.2
dialer-list 1 protocol ip permit 
! Associates permitted IP traffic with the dialer group 1. 
! The router will not start an ISDN call for any other packet 
! traffic with dialer group 1.
!
! configure BRI interface for PPP; 
! set address and mask interface bri 0 
! Selects interface with TA and other ISDN functions 
! on the router.
!
encapsulation ppp 
! Use PPP encapsulation on the selected interface.
!
ip address 172.16.126.1 255.255.255.0
!
! refer to protocols in dialer-list to identify interesting 
! packets dialer-group 1 
! Associates the BRI 0 interface with dialing access group 1.
!
! select call start, stop, and other ISDN provider details
!
dialer wait-for-carrier-time 15
! Specifies a 15-second maximum time for the provider to 
! respond once the call initiates.
!
dialer idle-timeout 1000
! Number of seconds of idle time before the router drops 
! the ISDN call. Note that a long duration is configured to 
! delay termination.
!
isdn spid1 0145678912
! call setup details for router
!
dialer map ip 172.16.126.2 name cisco-b 445
! Command       Description 
! ip            Name of protocol. 
! 172.16.126.2  Destination address.
! name          An identification for the remote side router. 
!               Refers to called router. 
! 445           ISDN connection number used to reach this 
!               DDR destination.

Verifying ISDN operation


Router# ping or telnet

Triggers a link (assuming it is a part on interesting traffic)


Router# show dialer

Displays current status of link, including amount of time link is connected


Router# show isdn active

When using ISDN, displays call status while call is in progress


Router# show isdn status

Displays the status of an ISDN connection


Router# show ip route

Displays all routes, including static routes


Router# debud q921

Verifies that you have a connection to the ISDN switch


Router# debug dialer

Shows call setup and teardown activity


Router# clear interface

Clears currently established connections

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			`aprabul@yahoo.com` `Copyright © 2003`