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ATM (Asynchronous Transfer Mode) Services-CBR,rt-VBR,nrt-VBR,ABR,UBR,GFR

This page of ATM (Asynchronous Transfer Mode) tutorial covers various ATM services with applications. The ATM services include CBR, rt-VBR, nrt-VBR, ABR, UBR, GFR etc.


Refer Following links to subtopics on this ATM tutorial:
ATM Network Architecture & Interfaces   ATM Protocol Stack  ATM services  ATM Switch  ATM Call Flow  ATM vs STM  ATM vs TDM  ATM versus Frame Relay  ATM AAL Layer Types 



As we know ATM network is developed to take care of transportation of different kinds of traffic such as voice, video and data. These different forms of information is transmitted in the form of ATM cells. The ATM cells are handled based on characteristics of traffic flow needed and applications of use in the network. Broadly ATM services are categorized into real time services and non-real time services. Real time services include CBR and rt-VBR. Non real time services include nrt-VBR, ABR and UBR. Figure depicts percent of line capacity with respect to time for all the ATM services.

ATM services

ATM CBR-Constant Bit Rate

• Used for applications requiring fixed data rate available always during the lifetime of connection.
• Used for applications requiring stringent transfer delay uppper limit.
• Used for uncompressed video and audio
• APPLICATIONS:Telephony, Videoconferencing, Audio and Video distribution and retrieval etc.

ATM rt-VBR, Real time Variable Bit Rate

• Used for applications requiring tight limit on delay i.e. used for time sensitive applications
• It allows more flexibility to the network compare to CBR.
• APPLICATIONS: LIVE TV Transmission having video information

ATM nrt-VBR, Non Real Time Variable Bit Rate

• Used for applications which do not have tight limit on delay
• Used for bursty nature of traffic
• It is used for applications requiring critical time response limits during data transfer.
• APPLICATIONS: bank transactions, airline reservation, railway reservations etc.

ATM UBR-Unspecified Bit Rate

• At any moment of time, some amount of capacity in ATM network is utilized by CBR and VBR type of traffics. The left over unused capacity is made available for UBR applications.
• Used for applications which can tolerate ATM cell losses as well as variable delays for example TCP based traffic.
• EXAMPLES: 1. remote terminal access applications
• 2. Used for text/image transfer, messaging, retrieval and distribution applications

ATM ABR-Available Bit Rate

• It is used to improve services of bursty sources serviced by UBR.
• Applications utilizing ABR services will specify PCR (Peak Cell Rate) and MCR (Minimum Cell Rate) for the applications.
• The remaining part of capacity not utilized by ABR is available for UBR type of traffic.
• EXAMPLE: LAN to LAN connection for information transfer

ATM GFR-Guaranteed Frame Rate

•  used for non-real-time applications which guarantee a minimum rate guarantee • MCR is guaranteed by network, the rest upto MCR is delivered on a best effort basis.

Networking related links

What is Bridge
What is Router
What is Gateway
Firewall basics
TCP-IP Packet format
Difference between OSI and TCPIP layers
Difference between TCP UDP
Packet Switching vs Message Switching

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