Cisco Cisco ATM Port Adapter 데이터 시트
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ADVANCED TRAFFIC MANAGEMENT
Advanced traffic management mechanisms in the ATM PA-A3 architecture allow for the support of bursty, client/server traffic, while
supporting applications that require guaranteed or best-effort service.
The ATM PA-A3 supports multiple ATM service classes, including Available Bit Rate (ABR), nrt-VBR and UBR, including UBR+ extensions.
Table 3.
Supported Service Categories
ATM PA-A3 Service Classes
Typical Uses
Non-real time Variable Bit Rate (nrt-
VBR)
VBR)
Used for all applications that require a level-of-service guarantee through the ATM network
Unspecified Bit Rate (UBR and
UBR+)
UBR+)
Used mostly for data applications requiring only best-effort service with little configuration required
Available Bit Rate (ABR)
Used to maximize bandwidth utilization of the ATM link through the use of congestion feedback
notification (with ability to define minimum bandwidth). (Not available on OC-12c/STM-4)
notification (with ability to define minimum bandwidth). (Not available on OC-12c/STM-4)
PER-VC AND PER-VP TRAFFIC SHAPING
Traffic shaping is a function typically provided on ATM edge devices to ensure that bursty traffic conforms to a predetermined “contract.”
More specifically, traffic shaping ensures that traffic from one VC does not adversely impact another, resulting in data loss. This function is
very important when connecting to an ATM WAN or public ATM network—especially when the ATM switches enable traffic policing that
will discard all traffic that exceeds the predetermined contract at the ingress of the ATM Network.
The ATM PA-A3 supports traffic shaping on a per-VC basis and per-VP basis. Supporting traffic shaping in hardware means that there is no
performance degradation when shaping is enabled. Providing traffic shaping on a per-VC and per-VP basis allows flexibility and control over
every VC and VP configured.
Depending on the selected ATM service class, the ATM PA-A3 supports highly configurable parameters such as: peak cell rate (PCR),
sustainable cell rate (SCR), maximum burst size (MBS), and minimum cell rate (MCR). These parameters can be defined based on the specific
bandwidth requirements of an individual VC, as needed for a specific application.
The ATM PA-A3 hardware “shapes” the VC to the specific parameters using a wheel-based scheduling algorithm to ensure fairness across the
ATM interface. In the event that two cells compete for the same time slot, the VCs, by default, are prioritized in the following order (starting
with highest priority): 1) OAM cells and signaling; 2) nrt-VBR; and 3) ABR; 4) UBR. Prioritizing the VCs in this manner ensures that the high
priority and guaranteed traffic have precedence over the best-effort traffic. It is also possible to configure a custom prioritization scheme on a
per-VC basis.
To provide further flexibility, the ATM PA-A3 allows each of these parameters to be set over a wide range of small increments.
Table 4.
Supported Traffic Shaping Granularity—OC-3, DS3 and E3 Versions
Parameter
Range
Increments
Peak Cell Rate
(PCR)
(PCR)
56 kbps to line rate
4.57-kbps increments for OC-3c/STM-1, 1.33-kbps increments for DS3, and 1.03-kbps
increments for E3
increments for E3
Sustainable Cell
Rate (SCR)
Rate (SCR)
56 kbps to line rate
4.57-kbps increments for OC-3c/STM-1, 1.33-kbps increments for DS3, and 1.03-kbps
increments for E3
increments for E3
Maximum Burst Size
(MBS)
(MBS)
1 to 64,000 cells
One cell