ATM & ISDN difference

Unanswered Question
May 17th, 2009

Hi GS,

Can any one tell me shortly how ATM & ISDN technolgy works.

Possibly with a short description and a useful link.

Appreciate your efforts.


Akber Mirza.

I have this problem too.
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Giuseppe Larosa Sun, 05/17/2009 - 05:34

Hello Akber,

it is difficult to be shortly.

I try in the following:


Integrated Services Digital Network.

ISDN is a circuit-based technology with some level of abstraction and signalling protocols.

An ISDN call is like a phone call on PSTN: at least one B channel (64 kbps) is used.

ISDN moves further from PSTN by allowing voice and data services.

One basic characteristic is that a call can use more B channels as needed for example for a videoconference.

To make the call one device has to use an E.164 address that is similar to a phone number including all possible prefixes.

Typical interfaces are BRI = 2 channel B bearer channels + 1 D channel (for signalling 16 kbps)

PRI = primary rate interface

E1 PRI = 30 B channels one channel used for signalling, one B channel for framing

T1 PRI = 23 B channels + 1 one used for signalling


b) ATM

Asynchronous Tranfer Mode.

note: one of the other names of ATM is broadband ISDN.

ATM tries to overcome the limitations of the 64 kbps channels of ISDN that are a problem for data networks (also without considering billing).

ATM uses the FR concepts of VCs (logical circuits):

a single access link can be used to reach different destinations using different PVCs or SVCs: each one has a different VPI/VCI value that is unique on the link between the ATM DTE (router atm interface) and the ATM DCE (ATM switch port).

ATM uses small fixed size frames called cells (53 bytes in size with a 5 byte header) and concentrate on switching these cells inside the ATM cloud along pre-established paths (Permanent VCs) or via on demand paths (SVCs that need to be setup before traffic can flow).

ATM provides extensive QoS support and needs to provide AAL services= capacity to take an IP packet for example to encapsulate it inside an AAL PDU, then to segment it on cells that are sent to the other side where the AAL PDU is reassambled and the IP packet can be reassembled and de-encapsulated to be fowarded towards destination as needed.

Actually ATM is most used with PVCs then with SVCs but it has the potential for a call and ATM NSAP addresses can be seen as similar to E.164 phone numbers.


IP/MPLS forum

Hope to help



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