Cisco BPX 8650, Installation And Configuration Manual

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Cisco BPX 8600 Series Installation and Configuration
Release 9.3.30, Part Number 78-12907-01 Rev. E0, May 2005
Chapter 21 Configuring ATM Connections
ATM Connection Flow
The cells received by a port are not automatically transmitted by that port out to the network trunks at
the port access rate. Each VC is assigned its own ingress queue that buffers the connection at the entry
to the network. With ABR with VS/VD or with Optimized Bandwidth Management (ForeSight), the
service rate can be adjusted up and down depending on network congestion.
Network queues buffer the data at the trunk interfaces throughout the network according to the
connection’s class of service. Service Classes are defined by standards-based QoS. Service Classes can
consist of the five service classes defined in the ATM standards as well as multiple subclasses to each of
these classes. Service Classes can range from constant bit rate services with minimal cell delay variation
to variable bit rates with less stringent cell delay.
When cells are received from the network for transmission out a port, egress queues at that port provide
additional buffering based on the Service Class of the connection.
CoS Management provides an effective means of managing the quality of service defined for various
types of traffic. It permits network operators to segregate traffic to provide more control over the way
that network capacity is divided among users. This is especially important when there are multiple user
services on one network.
Rather than limiting the user to the five broad classes of service defined by the ATM standards
committees, CoS management can provide up to 16 classes of service (service subclasses) that can be
further defined by the user and assigned to connections. Some of the CoS parameters that may be
assigned include:
• Minimum bandwidth guarantee per subclass to assure that one type of traffic will not be preempted
by another
•
Maximum bandwidth ceiling to limit the percentage of the total network bandwidth that any one
class can utilize
• Queue depths to limit the delay
•
Discard threshold per subclass
These class of service parameters are based on the standards-based Quality of Service parameters and
are software programmable by the user. The BPX switch provides separate queues for each traffic class.
Connection Flow Example
The example shown in Figure 21-3 shows the general ATM connection flow through BXM cards in BPX
switches. The
cnfport, cnfportq, cnfln, cnftrk, and cnftrkparm commands are used to configure
resources affecting the traffic flow of a connection. For examples, see the Traffic Shaping for CBR,
rt-VBR, nrt-VBR, and UBR section.
Ingress from CPE 1 to BXM 3
ATM cells from CPE 1 that are applied to BXM 3, Figure 21-3, are processed at the physical level,
policed per individual VC based on ATM header payload type, and routed to the applicable one of 15
per card slot servers, each of which contains 16 CoS service queues, including ATM service types CBR,
rt-VBR, nrt-VBR, ABR, and UBR.
ATM cells undergoing traffic shaping, for example, ABR cells are applied to traffic shaping queues
before going to one of the 15 per card slot servers. ATM cells applied to the traffic shaping queues
receive additional processing, including congestion control by means of VS/VD or ForeSight and virtual
connection queuing.