Figure 8-18 Effect of traffic shapping.
Figure 8-19 Traffic parameters and traffic shaping.
Figure 8-20 Shaping for the three services.
The traffic shaping function modifies the ATM traffic flow to achieve improved network efficiency, for interworking to ATM switches with small buffers and for virtual paths VPs originating at the ATM
switch. Traffic shaping can be done either at the ingress or egress side of an ATM switch.
Traffic shaping at the ingress side is used to modify the traffic flow of terminal equipment and ATM switches that are not able to keep the traffic contract, for instance, sending ATM cells with a higher peak
bit rate than allowed. Traffic shaping adapts the traffic characteristics of the ATM cell stream to the
Traffic shaping at the egress side of the ATM node is provided to protect subsequent ATM switches with small buffers from data bursts leading to buffer overflow. In addition, egress shaping is required to
ensure that VPs originating at the ATM switch conform to the traffic contract.
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by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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A CBR source shapes traffic using PCR. Thus, it schedules a cell every 1PCR unit of time. A VBR source shapes bursts MBS at the PCR rate, while ensuring that the overall SCR is maintained.
An ABR source requires dynamic shaping. In order to comply to the flow control mechanism used to support the ABR service, the ABR source traffic shaper varies according to the feedback messages it
receives from the network and the application’s requirements. Note, the source does not have to be the connection endpoint. To shorten feedback loops in networks with large delays the use of virtual sources
and virtual destinations can be set up within the ATM switches. These behave exactly like ABR sources and destinations. Figure 8-21 illustrates the various applications and the service types.
VII. Flow Control and Congestion Control
Congestion control allows the management of traffic when many users contend for finite network resources. Congestion management is one of the most important functions that an ATM switch will have
to perform, especially for ABR and UBR traffic.
The ATM CBR and VBR service categories have been designed to avoid network congestion conditions when implemented with robust connection admission controls and usage parameter controls. ABR and
UBR service categories have been defined to take advantage of the excess capacity beyond that required for CBR and VBR connections.
In the current Internet, the network components are assumed not to do any congestion control. As such, the endpoints rely on end-to-end protocols, such as TCPIP, to regulate the flow of traffic, based upon
packet loss. This model can also apply for TCPIP over an ATM transport infrastructure.
One of the effective methods of congestion control in high speed ATM networks is to control the user traffic at the user-network interface [Chao 1992, Cooper 1990, Gerla 1990, Hiramatsu 1991, Jain 1990,
Katevenis 1987, Khosrow 1991]. This type of control at the user-network interface is also called rate- based access control [Chao 1992].
Currently, the most widely known rate-based access control system is the leaky-bucket scheme in which a user is periodically allocated a certain number of tokens based on the bandwidth allocated for the user
The rate-base access control schemes have been extensively studied by the scientific community [Chao 1992, Gerla 1990, Khosrow 1991]. Additionally, Chao [Chao 1992] studied a rate control system based
on the shared buffer scheme to regulate outgoing traffic at each intermediate ATM node to prevent congestion.
The ability to function gracefully in an overload situation will be the key benchmark requirement for all ATM switches and will be the critical function most analyzed in the comparison of ATM switches from
multiple vendors. An ATM switch architecture must therefore provide robust congestion management support, multiple service classes and per VC accounting with the capability of operating on a cell time
basis.
Specifically, a congestion management implementation should include per VC accounting for maximum cells, congested cells, current cells, dropped cells and received cells all processed on a per port, per cell
time basis. And finally, an ATM switch which provides support for ABR will have to incorporate additional accounting features which of course must be handled on a cell time basis requiring processing
bandwidth and memory.
Figure 8-21 The various applications and the service types.
Table 8-7 lists various ATM switch vendors who provide a comprehensive set of congestion control mechanisms to optimize bandwidth utilization while maintaining a high level of service degree during
periods of overload.
Table 8-7 Company Switch
Newbridge 36170 Ascend CBX 500
Nortel Concorde Flow Control Features EFCI Marking Explicit
rate VSVD ABR; EFCI relative
rate, Explicit rate VS VD RM cell priority
EFCI marking Per-VC queuing
Traffic Policing Yes, Dual Leaky
Bucket GCRA Yes
Yes
CAC Yes
Yes Yes
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