Packet Discard Schemes BANDWIDTH ENFORCEMENT

REACTIVE CONGESTION CONTROL 109 In the explicit rate mode, a switch computes a local fair share for the connection and marks the rate at which the source is allowed to transmit in the ER field of the backward RM cell. The switch does that only if the bandwidth it can offer to the connection is lower than what it is already marked in the backwards RM cell. The source, upon receipt of the backward RM cell, extracts the ER field and sets its transmission rate to the ER value. When detecting congestion, a switch can generate a backwards RM cell in order to convey the congestion status, without having to wait for a backwards RM cell to arrive. Source behavior The source is responsible for inserting an RM cell every Nrm-1 data cells. These RM cells are part of the source’s allowed cell rate ACR. If the source does not have enough data cells to send, an RM cell is generated after a timer has expired and Mrm data cells have been transmitted. Mrm is fixed to 2. The data cells are sent with EFCN = 0. The source adjusts its ACR according to the information received in an RM cell. ACR is greater than or equal to MCR and less than or equal to PCR. The ACR is adjusted as follows: a. If CI = 1, then the ACR is reduced by at least ACR × RDF , where RDF is a prespec- ified rate decrease factor. If the reduction results to a value below the MCR, then the ACR is set equal to the MCR. b. If the backward RM cell has both CI = 0 and NI = 0, then the ACR can be increased by no more than RIF × PCR, where RIF is a prespecified rate increase factor. The resulting ACR should not exceed the source’s PCR. c. If the backward RM cell has NI = 1, then the ACR is not increased. After ACR has been adjusted as above, it is set to at most the minimum of ACR as computed above and to the ER field, but no lower than MCR. Destination behavior When a data cell is received, its EFCN is saved in the EFCN status of the connection. Upon receiving a forward RM cell, the destination turns around the cell and transmits it back to the source. The DIR bit is changed from forward to backward; the BN = 0; and the fields CCR, MCR, ER, CI, and NI in the RM cell remain unchanged, except in the following cases: a. If the saved EFCN status of the connection is set, then the destination sets CI = 1 in the RM cell, and resets the EFCN state. b. If the destination is experiencing internal congestion, then it can reduce the ER to whatever rate it can support and set it to either CI = 1 or NI = 1. The destination can also generate a new backward RM cell, with CI = 1 or NI = 1, DIR = 1, and BN = 1. This permits the destination to send feedback information to the source without having to wait for a source-generated RM cell to come by. The rate of these backwards RM cells is limited to 10 cellssec. 110 CONGESTION CONTROL IN ATM NETWORKS Switch behavior At least one of the following methods is implemented in a switch: a. EFCN marking: The switch can set the EFCN bit in the header of the data cells. b. Relative rate marking: The switch can set CI = 1 or NI = 1 in forward andor backward RM cells. c. Explicit rate marking: The switch can reduce the ER field of forward andor backward RM-cells. The first two marking methods are part of the binary mode, whereas the third one is for the explicit rate mode. The term binary is used because the switch provides information of the type: congestionno congestion. To send feedback information to the source without having to wait for a source- generated RM cell, a switch can generate backwards RM cells. The rate of these backwards RM cells is limited to 10 cellssec. Its fields are marked as follows: CI = 1 or NI = 1, BN = 1, DIR = 1. A switch can also segment the ABR closed loop using a virtual source and destination. This can be useful in cases where the loop between the source and destination involves many hops, or long haul links with a large propagation delay. In such cases, the time it takes for the RM cells to return to the source can be significant. This might impact the time required for the source to react to an RM cell. The calculation of the ER has to be done in such a way so that the available bandwidth in the switch has to be shared fairly among all of the competing ABR connections. A number of different algorithms for calculating the ER have been proposed in the ATM Forum standards. In the binary mode operation, the switch has to decide when to raise the alarm that congestion is pending. If we consider a non-blocking switch with output buffering, then if congestion occurs at an output port, the number of cells in its associated output buffer will increase dramatically. Typically, there are two thresholds associated with this buffer: a low threshold T low and a high threshold T high . When the number of cells goes over T high , the switch can start marking the EFCN bit of the data cells or turn on the CI or NI bit in a forward or backward RM cell. As the sources begin to react to the feedback information, the number of cells in the buffer will go down below T high . However, the switch continues marking until the number of cells in the buffer goes below T low . At that moment, the switch stops the binary marking. Simulation studies have shown that in a binary feedback scheme as the one presented above, some connections might receive more than their fair share of the bandwidth. Let us consider the case where source A is very close to a switch, and source B very far away. Then A will react to the feedback information from the switch much faster than B. For instance, if switch congestion occurs, then A will decrease its transmission rate more quickly than B. Similarly, when the congestion is lifted, A will increase its transmission faster than B. As a result, source A can put through more traffic than B. PROBLEMS 1. Consider a 64-Kbps voice connection transmitted at constant bit rate silence periods are also transmitted. a. What is its PCR?