3Com 3C63400-3AC-C - PathBuilder S700 Switch, Справочное руководство

Страница: 93 / 272

ATM Module Overview 79
Priority Queuing
The PathBuilder S700 with STX provides separate cell buffer queues per virtual
interface. You can assign multiple VCs to each virtual interface and map them to
the desired queue. The STX services the queues in order: queue 0, then queue 1,
then queue 2 and queue 3. Figure 48 illustrates this priority queuing.
In Figure 48, all 24 VIs on a triple-slot bus have been allocated to a single slot. You
can also distribute VIs among the slots supported by a bus. For details, see
“Configuring Virtual Interfaces” in Chapter 5.
Figure 48 PathBuilder S700 with STX Priority Queuing
If the STX finds data in a higher queue while it is servicing another queue, it
services the higher queue, thereby maintaining the priority. For example, if the STX
finds data in queue 0 while it is servicing queue 2, the STX services queue 0. By
queuing the VCs in this way, the STX ensures that the Class of Service is
maintained.
The ensuing traffic from the queues is then shaped according to the traffic
shaping parameter that you set (64Kbps to 155 Mbps) in cells per second. In
addition, each circuit can be policed before reaching its queue. Tagged cells may
be dropped if a queue is found to be congested based on the threshold you have
set. Therefore the combined cell rate of all circuits mapped to a VI can be greater
than the shaper rate of the VI.
ATM Module
Overview
The PathBuilder S700 supports the following application modules with ATM
interfaces:
n
DS3 UNI
n
E3 UNI
n
OC3/STM-1 UNI
n
DS1 UNI with IMA
n
E1 UNI with IMA
Q0 = 500 cell buffer
Q1 = 1500 cell buffer
Q2 = 3000 cell buffer
Q3 = 3000 cell buffer
Traffic
Physical
Port
Cell Bus
"Virtual
1
2
3
Queue Controller
Cell buffers
CBR
VBR
Interfaces"
Shaper
24