RM Cell - Resource Management Cell

The ATM Forum Traffic Management Specification (Version 4.0) details the Resource Management Cell format. The RM Cell is used when an ABR service type has been selected. It reports on the traffic seen by a cell on the network so that the destination device can indicate the source the traffic state of the network. The RM cell reports on the bandwidth availability, state of congestion, and impending congestion on the network.

It can be generated and/or modified by the source, destination, and network elements along the way. Not all fields can be edited by any one entity. This is, for the most part, explained in the field descriptions. Table 1.0: Resource Management Cell Format (ATM Forum Version)

Field		Octet		Bit(s)		Name
Header	1-5		all			ATM Header
ID		6		all			Protocol ID
DIR		7		8			Direction
BN		7		7			BECN Cell
CI		7		6			Congestion Indication
NI		7		5			No Increase
RA		7		4			Request/Acknowledge
ER		8-9		all			Explicit Cell Rate
CCR		10-11		all			Current Cell Rate
MCR		12-13		all			Minimum Cell Rate
QL		14-17		all			Queue Length
SN		18-21		all			Sequence Number
Reserved	22-51		all			Reserved
Reserved	52		16-11			Refer to I.371
CRC-10	52-53		10-1			Refer to I.371

ATM Header - 5 Bytes RM-VPC: VCI=6 and PTI=110, RM-VCC: PTI=110 Protocol ID - 1 octet This is set to 1 for the ABR service as defined by the ITU. Message Type - 1 octet This octets is made up of bit flags with the following meanings:

- DIR: (Direction) The direction of the RM-cell with respect to the data flow of the connection. The sources set DIR = 0 for a forward motion and the destination sets DIR = 1 for a backward motion. This bit can only be modified if the network element is changing the direction of an RM-cell. - BN: (Backwards Explicit Congestion Notification - BECN) The source sets this field to BN = 0. The network or the destination may generate a BECN by setting BN=1 indicating that the RM-cell is not source generated and by setting DIR=1 to indicate a backward flow. - CI: (Congestion Indication) A destination can send a source network element a resource management cell with CI=1 to indicate congestion on the network. This will cause the source to decrease its allowable cell rate (ACR). This is typically done when the destination receives data cells with the EFCI bit on. Switches can not change this parameter from 1 to 0 and the source always uses DIR=0 for this bit - NI: (No Increase) This bit notifies the source not to increase its ACR. It does not cause the ACR to be decreased on the source. Typically this is used when a switch senses impending congestion. -RA: (Request/Acknowledge) NOT USED by the ATM Forum for ABR. If a destination end system (SES) or a virtual source (VS) generates an RM-cell it will either set this field to zero or in accordance with I.371. A destination end system (DES) or a virtual destination (VD) will set this field to zero, preserve this field. DES/VS will set this field in accordance with I.371. A switch has two options for this field to preserve the field or act in accordance with I.371.

Explicit Cell Rate (ER) - 2 octets Used to limit the source ACR to a specific value, the Explicit Rate is typically set by the source to the requested rate (PCR). It can be reduced by a network element to a value that the element can sustain. Current Cell Rate (CCR) - 2 octets Set by the source, this is the current ACR when the source generates an RM-cell. This field can not be changed by network elements. It can be used to in the calculation of ER. Minimum Cell Rate (MCR) - 2 octets Set by the source to the negotiated minimum cell rate for the connection. This field facilitates bandwidth allocation. if the value of the MCR is different in an RM-cell than what was signaled, then the switch may correct the value to the rate that was signaled. Queue Length (QL) - 4 octets Not used in ATM Forum Traffic Management. A switch will either preserve this field or Sequence Number (SN) - 4 octets Not used in ATM Forum Traffic Management. This field will either be set to zero ( 0 ) or in accordance with I.371. Reserved - 30 octets Not used by ATM Forum Traffic Management. Reserved (6 bits) plus CRC10 - 2 octets

Cell-Relay Dictionary