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ABR mechanisms allow the network to allocate the available bandwidth fairly and efficiently among the ABR sources (fig. 1). ABR sources limit their data transmission to rates allowed by the network.
Figure 1 Available ABR bandwidth
The network consists of switches, which use locally available information to calculate the explicit, allowable rates or relative rate indications (e.g. to decrease rate) for the source. This feedback information is sent to the sources using Resource Management cells (RM-cells). RM-cells are generated by the source and travel along the data path to the destination. After toggling the direction bit of the RM-cells, the destination returns the RM-cells to the source. The switches might modify the passing RM-cells (fig. 2).
For very large networks or networks using satellite links, the round trip delay of the RM-cells potentially cause instabilities. Therefore, ATM Forum’s Traffic Management Specifications describe a partitioning of the network into smaller loops by using several virtual sources/destinations (VS/VD) implemented in the switches. VS/VD implementation exceeds the scope of this introduction and is therefore not further treated here.
The RM-cell can be detected with the help of the Payload Type Identifier (PTI), which equals 110b for all RM-cells. The other fields of the ATM cell header contain the same values than the regular data cells of the same connection.
The payload of the RM-cells is partially filled with information to provide Available Bit Rate (ABR) service.
| Field | Description | Octet | Bit | Value |
| ID | Protocol Identifier | 6 | all | 1 |
| DIR | Direction | 7 | 8 | 0: forward (source
generated) 1: backward (destination generated) |
| BN | BECN Cell | 7 | 7 | 0: source generated |
| CI | Congestion Indication | 7 | 6 | 0: not congested 1: congestion indication |
| NI | Non Increase | 7 | 5 | 0: not congested 1: non increase |
| RA | Request / Acknowledge | 7 | 4 | 0 or according to ITU-T, I.371 |
| Reserved | 7 | 3 - 1 | 0 | |
| ER | Explicit Cell Rate | 8-9 | all | |
| CCR | Current Cell Rate | 10 – 11 | all | Allowed Cell Rate (Source) |
| MCR | Minimum Cell Rate | 12 – 13 | all | MCR parameter |
| QL | Queue Length | 14 – 17 | all | 0 or according to ITU-T, I.371 |
| SN | Sequence Number | 18 – 21 | all | 0 or according to ITU-T, I.371 |
| Reserved | 22 –51 | all | 6A (hex) for each octet | |
| Reserved | 52 | 8 – 3 | 0 | |
| CRC-10 | Cyclic Redundancy Check | 53 | all | According to ATM Forum, Traffic Management Specitfications, Version 4.0 |
Once a source has received permission, it begins transmitting cells at the Allowed Cell Rate (ACR). Initially the ACR is set to the Initial Cell Rate (ICR). The ACR is always bounded by the Minimum Cell Rate (MCR) and the Peak Cell Rate (PCR), both determined at connection setup. Before the first data cell is sent, a RM-cell cell is transmitted. The source sends a RM-cell after every Nrm data cells. The source places the ACR value in the Current Cell Rate (CCR) field of the RM-cell. The ER field is set to the rate at which the source wishes to transmit cells (usually the PCR value).
The RM-cells traverse forward through the network. The destination returns the RM-cells back towards the source. If the destination suffers from internal congestion, it is allowed to alter the contents of the RM-cells. While traveling through the network, the switches modify the RM-cells according to one of the fair rate allocation algorithm, described in the next chapter. The alternated field may be the Congestion Indication (CI) or Non Increase (NI) bit in the RM-cell. More sophisticated algorithms use also the Explicit Rate (ER) field.
Upon return of the RM-cell, the source should adapt its ACR to the information carried in the RM-cell. If CI is not set, the source may linearly increase its ACR by a fixed increment (RIF*PCR), where the Rate Increase Factor (RIF) is specified at connection setup. If CI is set, the source must exponentially decrease its ACR by an amount greater than or equal to a proportion of its current ACR, (RDF*ACR), where the Rate Decrease Factor (RDF) is also specified at connection setup. If NI is set, the source should observe the CI field, but is not allowed to increase its ACR. If the value in the ER field is smaller than the ACR calculated according to the CI / NI bit settings, the new Allowed Cell Rate (ACR) will equal the Explicit Rate (ER), but in any case be larger than the Minimum Cell Rate (MCR).
The Allowable Cell Rate (ACR) is the highest rate at which the source is permitted to transmit cells. In case of internal congestion, the source may not be able to send at this rate. The remaining bandwidth will then be allocated to other sources.
Introduction to ATM|Introduction to ABR|The Virtual Output Queue|Simulation Results|ABR Publications|ABR Links|Back to iPOINT
Last updated 04/09/98 by Matthias Bossardt