Why Do I Need GTS on My Network?
Many times a situation exists in which a carrier provides a circuit with a CIR less
than the access rate of the physical interface. For example, a Frame Relay service
may be provisioned with a 1544 Kbps CIR, but the circuit is delivered on an E1
(2048 Kbps) interface. In the absence of traffic shaping, the router will send up to
a rate of 2048 Kbps.This may cause problems, since the traffic in excess of the
CIR could be dropped in the Frame Relay network. In this situation, you may
get considerably more throughput than the CIR at times, but you are at the
mercy of the Frame Relay network. During times when the network is not busy,
you may get all your traffic through, but during congested times, many of your
packets may be dropped.You may think that any amount of bandwidth over the
CIR is a bonus, but when packets like TCP are dropped in large quantities, the
retransmission can cause not only increased congestion, but global synchronization
as well. Additionally, if you are transmitting real-time data, any dropped packets
will immediately degrade performance. Depending on your network applications,
it may be better to take the more conservative approach by using traffic shaping
and sleep soundly knowing you have a reliable service.
Although GTS is available on a variety of interfaces, it may not be that useful
in light of other QoS mechanisms and modern technologies. For example, you
would rarely want to limit traffic rates on a shared, private medium such as
Ethernet, especially if it was switched Ethernet. Also, in the case of ATM, if a
variable bit rate (VBR) service was ordered, the carrier would most likely tell
you the sustainable cell rate (SCR), peak cell rate (PCR), and maximum burst
size (MBS). By configuring an ATM VBR service on the router with these
parameters, you have already enabled traffic shaping. Adding GTS on top of this
would be redundant. Finally, for Frame Relay circuits, FRTS, not surprisingly, has
features that are more suited to this medium.