Chapter 3 System Features
AC 60 Software Features
3-40
255-700-025
PacketStar
™
Access Concentrator 60 User Guide
, Issue 1
Release 6.3.0
Traffic Shaping
The traffic shaping feature is a method for controlling the flow of data traffic.
It is implemented in firmware on modules that are offered with traffic-
shaping variations. Those modules are the OC-3c Multi-Mode, OC-3c Single-
Mode, STM-1 Multi-Mode, and STM-1 Single-Mode modules.
Traffic shaping ensures that variable bit-rate (VBR) traffic entering the Access
Concentrator system (via the OC-3c and the STM-1 modules) complies with
the parameters of established service contracts. If bursty VBR traffic exceeds
the parameters of the output connection, the rate of the traffic flow is
controlled to comply with the specified output rate by means of an input cell-
selection algorithm before the traffic flow reaches the Access Concentrator
backplane. If traffic exceeds the buffer capacity of the OC-3c or the STM-1
module (that is, rises above the maximum-capacity level), cells are discarded.
Traffic shaping allows the network side of the Access Concentrator system to
multiplex more efficiently the traffic-shaped virtual channel connections
(VCCs) with other customer premises equipment (CPE) traffic (voice, video,
and so on) for transport across the ATM network link. Constant bit-rate
(CBR) traffic is unaffected by traffic shaping.
The only application of the traffic-shaping module is shown in Figure 3-10.
An end user has an ATM DS3 network connection and has subscribed to a
VBR VCC connection contract from a carrier (service provider) with the
following traffic parameters: 1) sustained cell rate (SCR) is 40,000
cells/second; 2) peak cell rate (PCR) is 80,000 cells/second; and 3) maximum
burst size (MBS) is 250 cells. Even though LAN switches usually maintain a
sustained cell-transport rate of 40,000 cells/second, they allow LAN traffic to
burst in violation of carrier traffic contracts, causing clusters of cells to exceed
the MBS parameter. Because carriers monitor traffic at the edge of a network
and enforce adherence to traffic contracts by discarding cells that exceed the
Figure 3-10. Traffic-Shaping Application
ATM
Network
LAN
ATM
Switch
LAN
ATM
Switch
Bursty LAN ATM Traffic
Bursty LAN ATM Traffic
ATM
ATM
OC-3c
OC-3c
DS3
DS3
Shaped Traffic
to Comply with
ATM Service Contract
PacketStar
Access Concentrator
PacketStar
Access Concentrator
Содержание PacketStar AC 60
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