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Traffic Policing

This section explains how the generic cell rate algorithm is used to police cells from a CBR source. For simplicity, the time to transmit an ATM cell is set to 1. The GCRA is specified by GCRA(Tc,t), where Tc and t denote the contracted cell interarrival times and the cell delay variation tolerance, respectively.

Suppose the CBR source violates the contract by generating cells at the fixed intervals Ti, though slightly faster than the contracted interval Tc (Ti < Tc). If t = 0, then Exhibit 4-6-1 illustrates that TAT is updated by Tc and that cells are arriving at every Ti. One extra cell appears within the increment interval of Tc. Because the CDV tolerance (t) is 0, this extra cell is policed, resulting in policing of every other cell. When policed cells are discarded, the cell loss ratio (CLR) becomes 0.5.


Exhibit 4-6-1.  Interval Updating with GCRA

For t > 0, suppose TAT is updated from the arrival time of the k-th cell. If the (k+d)-th cell is to be policed by GCRA (Tc, t), the arrival time of the (k+d)-th cell should be within the margin of CDV tolerance t of TAT for the (k+d)-th cell. An example with d = 3 is depicted in Exhibit 4-6-2. Cell k+3 is shown to be policed because its arrival time is earlier than the GCRA limit t. At an arbitrary value of d, the (k+d)-th cell is discarded when d × Ti < d × Tc - t . When policed cells are discarded, this amounts to a single cell loss in (d+1) cell arrivals. CLR changes in a stepwise fashion as Ti decreases, as shown in Exhibit 4-6-3.


Exhibit 4-6-2.  Interval Updating for a CBR Source With GCRA


Exhibit 4-6-3.  Cell Loss Rate of a CBR Source by GCRA

Policing of a VBR source is performed on the basis of the sustainable cell rate (SCR). The SCR specifies the upper bound on the average cell rate and can be significantly lower than the peak cell rate. The time intervals between two successive cells arriving at the average and the peak rates are denoted by Ts and Tp, respectively. They are inverses of sustainable and peak rates. A burst is defined as a group of cells generated at the peak cell rate. The burst tolerance determines the maximum burst size, b, that may be transmitted at the peak rate.

Assessment of Monitoring Techniques

During a connection establishment process, the ATM network has to reserve sufficient resources to meet the quality of service (QOS) demand for the connection. Once admitted, the actual incoming traffic is policed to keep the traffic entering the network in conformance to the connection traffic parameters. The usage parameter control, or UPC, procedure is a preventive control mechanism so that a potential overload from a source is not permitted beyond the preestablished limits set by the connection parameters. However, a cell stream from a source may be altered along the path as it is mixed with other cell streams. The UPC procedure has to be flexible enough to allow for some deviations from the connection parameters by including cell delay variation tolerance and burst tolerance parameters.

Because a cell stream conforming to the admission contract can be altered after a multiplexing, traffic shaping may be necessary to preserve the characteristics of the original cell stream. For example, a nonzero CDV tolerance allows the UPC to accept some cells from a CBR source, although they are spaced closer than the contracted interval.

After the UPC checks for the conformance, it may be desirable to restore cell stream into the one matching the admission contract. A need for traffic shaping is strengthened from some results, which indicate that individual cell streams become burstier after a series of GCRA control actions.

Although a usage parameter control algorithm such as the GCRA may be easily implemented, the traffic shaping is a costly operation since some history of a cell stream has to be maintained if a cell stream is to be reconstructed. Furthermore, some buffering may be needed as the cell stream is restored. The buffering per virtual channel (VC) is exactly what an ATM node wanted to avoid, however.

ABR TRAFFIC CONTROL

ABR traffic control relies on cooperative interworking among three components:

  The source end system (SES).
  The ATM switch.
  The destination end system (DES).

Briefly, their control actions are as follows: An SES keeps track of the best estimate for its cell transmission rate, or allowed cell rate (ACR), according to the perceived congestion status in the network. To obtain the congestion status in the network, the SES periodically inserts a resource management cell in its data cell stream.

The DES returns the RM cell back to the SES so that the network status is conveyed back to the SES. In the backward RM cell, a single congestion indication (CI) bit may be set for the SES to adjust its cell transmission rate. Optionally, an ATM switch may determine the best cell rate for an SES and convey it as the explicit rate in the backward resource management cell.

Source and Destination Behavior

When a source sends the first cell after connection setup, or after not sending any cells for Ttm seconds or longer, it sends a forward RM cell to obtain the network congestion status. After every Nrm-1 data cells, the source inserts a forward RM cell according to the prevailing ACR. When a backward RM cell is received with CI = 1, then ACR (i.e., the allowed cell rate) is reduced at least to ACR’RDF, where RDF is the rate reduction factor and is a constant (1/16, for example).

On the other hand, if CI = 0 in the RM cell, ACR is increased by a constant additive increase rate. After the ACR is adjusted according to the CI bit, the new ACR is taken as the smaller of the updated ACR and the explicit rate stored in the RM cell.

Destination behavior is relatively simple. Every ATM cell has an explicit forward congestion indication (EFCI) bit in its header. An ATM switch may set the EFCI bit if its buffer occupancy exceeds a certain threshold. The destination end system saves the EFCI state stored in the ATM header (i.e., the payload type field) of an incoming data cell. Upon receiving a forward RM cell, the DES retransmits the RM cell back to the SES after setting the direction of the cell from forward to backward and copying the saved EFCI state to the CI bit field in the RM cell. Namely, the CI bit from DES is set by the congestion status experienced by the most recent data cell.


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