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Explicit Rate Control Algorithm (ERCA)

An explicit rate control algorithm (ERCA) was proposed to address the problems of previous binary feedback schemes. In binary feedback schemes, a single-bit is used only to tell the source whether it should go up or down. It was designed in 1986 for connectionless networks in which the intermediate nodes had no knowledge of flows or their demands.

In connection-oriented ATM, however, the switches know exactly who is using the resources and flow paths. The binary feedback schemes were also designed for window-based controls and are too slow for rate-based controls in high-speed networks. In window-based control, a slight difference between the current window and optimal window will show up as a slight increase in queue length. In rate-based control, a slight difference in current rate and optimal rate will show up as continuously increasing queue length. The reaction times should be fast. ERCA can ensure the source gets to the optimal operating point within a few round trips. ERCA uses a positive feedback approach, and it is based on counter-based approach as in PRCA.

In ERCA, each source periodically sends an RM cell containing its current cell rate (CCR), desired rate (DR), and a reduced (R) bit. When a switch receives an RM cell from the source, it monitors the VC’s rate and computes a fair share using an iterative RM format procedure. The fair share is computed as follows:

      Fair share = Link Bandwidth

                     - S Bandwidth of Underloading VCs/Number of VCs

           - Number of Underloading VCs

If a VC’s DR is more than the fair share, the switch will reduce the DR field and set a reduced bit in the RM cell. However, any VC can grant the DR if its DR is less than the fair share. Upon receiving an RM cell from the source, the destination returns the RM cell to the source. A source then adjusts its rate to that indicated in the RM cell. If the reduced bit is clear, the source could demand a higher desired rate in the next RM cell. If the bit is set, the source uses the current rate as the desired rate in the next RM cell.

The data and control traffic of ERCA in a congested network are illustrated in Exhibit 4-7-10. The VC1’s DR is more than the fair share at switch 2. In this scenario, ERCA has several advantages:

1.  Policing is straightforward because the entry switches can monitor the returning RM cells and use the rate directly in their policing algorithm.
2.  The system reaches the optimal operating point quickly because of fast convergence time—the initial rate has less impact.
3.  ERCA is robust against errors or loss of RM cells because the next correct RM cell will bring the system to the correct operating point. However, ERCA still employs per-VC accounting, which is considered very expensive with current hardware technology.


Exhibit 4-7-10.  ERCA with Per-VC Buffering Switches

Enhanced Proportional Rate Control Algorithm (EPRCA)

An enhanced proportional rate control algorithm (EPRCA) is intended to solve the ACR beat down problem and to combine the previous separated rate-based schemes with two enhancements: intelligent marking and explicit rate setting

This scheme is a merger of PRCA with ERCA. The scheme adapts intelligent marking to achieve better fairness among connections without a need of per-VC queueing or accounting because per-VC accounting requires an additional control complexity at the switch, even though the fairness could be achieved if each connection is maintained separately at the switch. It also adapts explicit rate setting, which reduces the rate of each connection explicitly. Exhibit 4-7-11 illustrates EPRCA with output buffer switches.


Exhibit 4-7-11.  ERCA with Output Buffer Switches

Therefore, the switch can have a responsibility for determining the cell transmission rate of a selected connection. In EPRCA, a source sends a data cell with EFCI set to zero. After every n data cells, it sends an RM cell that consists of its current cell rate (CCR), explicit rate (ER), and a congestion indication (CI) bit. The source initializes the ER field to its peak cell rate (PCR) and sets the CI bit to zero.


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