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6.0 Traffic Management
6.4 GFC Flow Control Manager
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
6.4 GFC Flow Control Manager
6.4.1 A Brief Overview of GFC
Generic Flow Control (GFC) is a one-way control mechanism which allows the
network equipment to control the input from an end station, for the class(es) of
traffic defined as controlled. This mechanism does not allow the end station to
exert any control on traffic from the network.
The usefulness of GFC is that it allows overbooking of the bandwidth on the
input side of the network switch buffers. This allows a much higher degree of
multiplexing than is otherwise possible, and allows the network-side costs of con-
nections to be significantly reduced. By overbooking the input bandwidth, a high
degree of sharing is possible and the buffer system can be utilized by more end
nodes than full bandwidth input would allow. GFC is used to coordinate access to
that bandwidth when temporary conflicts occur.
GFC provides a link-level, short-term, XON/XOFF-type flow control mecha-
nism that only works on the link from the end station to the first piece of network
equipment. The GFC protocols are defined and described in ITU Recommenda-
tion I.361.
6.4.2 The RS8234’s Implementation of GFC
The RS8234 implements the GFC one-queue mode. The reassembly coprocessor
provides Auto Configure and Command Detection. The segmentation coproces-
sor provides Halt Processing and Per-transmit Queue SET_A control. It does not
implement the optional Queue B.
Once the link has been configured for GFC operation (as described in the sub-
section below), a received HALT indication will cause the segmentation copro-
cessor to halt processing of all channels, both controlled and uncontrolled. This
halt condition will continue until a cell is received without the HALT indication.
A received SET_A indication will increment the GFC credit counter by one. A
GFC controlled cell can only be sent when the GFC credit counter is equal to one.
Transmission of a GFC controlled cell decrements the credit counter by one. Each
of the eight transmit priority queues can be configured for GFC control by setting
the appropriate GFCn bit(s) in the Scheduling Priority (SCH_PRI) register. In this
way the SAR can segment both GFC controlled and GFC uncontrolled traffic
simultaneously. GFC controlled queues will be active only when the GFC credit
counter is equal to one.
Note that CBR traffic is not affected by the SET_A command since it is not
mapped into a transmit priority queue. In addition, the segmentation coprocessor
implements a credit borrow algorithm that provides better utilization of the line
when receive and transmit cell streams are not synchronized. Up to one credit can
be borrowed.
The user must control the transmitted GFC field via the HEADER_MOD and
GFC_DATA fields in the buffer descriptor entries. For GFC controlled channels,
GFC_DATA = 0101; and for non-GFC controlled channels, GFC_DATA = 0001.