• Provides no QOS guarantees (unspecified bit rate).
• Same service as provided by UDP in the Internet, i.e. "best efforts."
• FIFO service at each switch is fine for UBR.

• Provides "better efforts" than UBR.
• Fairness (treating all connections equally).
• Low probability of cell loss.
• We promise that, of the available bit rate, you will get your fair share
• Also allows MCR = Minimum Cell Rate: When insufficient bandwidth is available for everyone, this guarantees minimum cell rate.
• ABR Traffic: Unpredictable. However, most data applications have the ability of regulating, modulating their information transfer rates as a function of the network's loading status.

• Goal: Deliver as much traffic as possible reliably and fairly, in the face of higher priority CBR and VBR traffic.

• Basic Parameters:
  • PCR (= R_p).
  • CDVT (= _p)
  • MCR (= R_m).

• Note that MCR can be enforced by
  1. Defining a new parameter _m.
  2. Applying GCRA(1/R_m, _m).
  3. Marking all non-compliant cells as low priority.

• Regulate ABR traffic between MCR and PCR to limit loss.

• The bandwidth, or equivalently the cell rate, made available to an ABR connection on a particular link by the network may vary between the MCR and the PCR because, for example,
  • Bandwidth resources are being reserved for CBR and VBR connections that are already set up.
  • Bandwidth becomes free again when CBR and VBR connections are released.
  • Non-reserved bandwidth made available to other ABR connections sharing that same link remains unused.

Simple Feedback Mechanism

• Recall PTI (Payload Type Indicator):
  • If 1^st bit = 0 then this is a data cell.
  • In this case, if 2^nd bit = 1 then there is congestion.
• This 2^nd bit is called the CI (Congestion Indication) bit.
  • It is set to 0 by the source.
  • It can be set to 1 by any congested switch.
• Destination not receiving CI = 1 sends "speed-up" message to source (in a "Resource Management (RM)" cell) saying increase rate by a fixed increment.
• Source not receiving such a speed-up message must decrease its rate by a fixed multiplicative factor.
  [Logic here = If network is already congested, don't send extra cells to decrease rate, since this will make the network even more loaded].
• Move up slowly; move down rapidly.

Implementation Parameters for ABR Flow Control

• ACR = Allowed Cell Rate (constraint imposed by the network).
• ICR = Initial Cell Rate.
  [Clearly, MCR <= ACR , ICR <= PCR]
• AIR = Additive Increase to Rate.
• RDF = Rate Decrease Factor < 1.

• On start up, ACR <-- ICR.
• If speed-up message is received from the destination
  
  then ACR <-- ACR + AIR
  else ACR <-- ACR x RDF

Concern Here:

• Consider 2 VCC's.
• VCC2 has MCR, ICR and PCR twice that of VCC1.
• Each speed-up message causes 1 AIR increase, so it takes VCC2 twice as long as VCC1 to reach PCR.

Solution:

• Make rate of speed-up cells proportional to the ACR by fixing how often they are sent in terms of number of cells.
• Use new parameter Nrm = Number of cells / resource mgmt cell.
• E.g. Nrm = 32 means we send a speed-up message after each 31 data cells.
• This algorithm was thus dubbed "PRCA" for Proportional Rate Control Algorithm.