How Does GTS Work?
GTS acts to limit packet rates sent out an interface to a mean rate, while allowing
for buffering of momentary bursts.With GTS parameters configured to match
the network architecture, downstream congestion can be avoided, eliminating
bottlenecks in topologies with data-rate mismatches. GTS has the following
characteristics:
Rate enforcement on a per interface, or subinterface, basis—the mean
rate can be set to match the circuit CIR or some other value.
Traffic selection using ACLs.
GTS works on many Layer 2 interface types, including Frame Relay,
ATM, Switched Multimegabit Data Service (SMDS), and Ethernet.
It supports backwards explicit congestion notification (BECN) messages
for bandwidth throttling.
It supports WFQ per subinterface.
GTS works with the token bucket algorithm in the following way.When
packets arrive at the router, an interrupt occurs. If the queue is empty, GTS consults
the credit manager (token bucket) to see if there is enough credit to send
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254 Chapter 8 • Advanced QoS for AVVID Environments
the packet. If there is not, the packet is sent to the queue configured, in this case,
WFQ. If there is credit available, the packet is sent to the output interface, and
the associated credit value is deducted from the token bucket. Queued packets
are serviced at regular time intervals.The credit manager is checked at each time
interval to determine if there is enough credit to transmit the next packet waiting
in queue. If there is, the packet is sent to the output interface, and the VC is
charged the appropriate number of credits.