Editing Available bit rate

{{Short description|A service used in ATM networks}}
{| class="wikitable floatright" style="width: 17em;"
|-
! rowspan="2"|[[Network architecture]]
! rowspan="2"|[[Service model]]
! colspan="4"|Guarantee ?
! rowspan="2"|[[Congestion feedback]]
|-
![[Bandwidth (computing)|Bandwidth]]
!Loss
!Order
!Timing
|-
|[[Internet]]
|best effort
|none
|no
|no
|no
|no (inferred via loss)
|-
|[[Asynchronous Transfer Mode|ATM]]
|[[Constant bit rate|CBR]]
|constant rate
|yes
|yes
|yes
|no congestion
|-
|[[Asynchronous Transfer Mode|ATM]]
|[[Traffic contract|VBR]]
|guarantee rate
|yes
|yes
|yes
|no congestion
|-
|[[Asynchronous Transfer Mode|ATM]]
|[[Available Bit Rate|ABR]]
|guarantee minimum
|no
|yes
|no
|yes
|-
|[[Asynchronous Transfer Mode|ATM]]
|[[traffic contract#Unspecified Bit Rate (UBR)|UBR]]
|none
|no
|yes
|no
|no
|}

'''Available bit rate''' ('''ABR''') is a service used in [[Asynchronous Transfer Mode|ATM]] networks when source and destination don't need to be synchronized. ABR does not guarantee against delay or [[data loss]]. ABR mechanisms allow the network to allocate the available [[Bandwidth (computing)|bandwidth]] fairly over the present ABR sources. ABR is one of five service categories defined by the ATM Forum for use in an ATM Network. 

The [[network switch]]es use locally available information to determine the explicit allowable rates or relative rate (increase/decrease) for the source. The newly calculated rates are then being sent to the sources using resource management records (RM-cells). RM-cells are generated by the source and travel along the data path to the destination and sent back. ABR sets a minimum cell rate (MCR) and a peak cell rate (PCR). When transfers exceed the PCR, cells are dropped.

Many implementers consider ABR to be overly complex, and its adoption has been modest.

==See also==
*{{section link|Traffic contract|Available Bit Rate (ABR)}}

== External links ==
*[http://www.cisco.com/en/US/tech/tk39/tk51/technologies_tech_note09186a00800fbc76.shtml Understanding the Available Bit Rate (ABR) Service Category for ATM VCs]

[[Category:Network protocols]]
[[Category:Asynchronous Transfer Mode]]


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