Generic flow control
The four-bit generic flow control (GFC) field is used only across the UNI; information encoded in this field is not carried end-to-end. The ATM Forum UNI specification defines two modes of operation regarding the GFC field. Uncontrolled access is defined for the current ATM environment, where no GFC mechanism is yet in place. In this mode, the GFC bits are set to 0000. In controlled access, allowed by the ATM Forum but not yet implemented, ATM CPE will modify their transmissions based on this field. Common wisdom suggests that the GFC field is inadequate for the needs of ATM and that other schemes will be employed for flow control and congestion management.
However, ITU-T Recommendation I.361 defines GFC use for controlled access. Rarely implemented, most ATM networks following ITU-T specifications still opt for the allowed uncontrolled access where the GFC bits are set to 0000. But the ITU-T additionally allows for the establishment of up to two groups of controlled connections: group A and group B.
When the network needs to control the flow of user cells into the network, the first GFC bit (and the first bit of the cell header itself) is set to 1. This forms the HALT command, which cuts off the flow of cells into the network. What the remote device on the customer premises does with the halted cells is not defined by the GFC specification. Even uncontrolled cells are halted until the network resets the bit to allow cell entry again.
The general network-to-user structure of the GFC to halt user cell flow is the bit pattern “1ABX.” The 1 bit is the halt itself. The second bit (A) halts only group A connection cell flows, and the third bit (B) halts only group B connection cell flows. The last bit indicates the presence (1) or absence (0) of controlled groups. The ITU-T scheme has been criticized as “XON/XOFF for ATM” and inadequate for realistic networks. In any case, GFC information is not transported across the network. Other flow control mechanisms could be used between nodes within the network, such as internal OAM procedures.