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6.0 Traffic Management
6.2 xBR Cell Scheduler Functional Description
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
6.2.4 VBR Traffic
The RS8234 Cell Scheduler also supports multiple priority levels for VBR traffic.
The VBR service class takes advantage of the asynchronous nature of ATM by
reserving bandwidth for VBR channels at average cell transmission rates without
hardcoding time slots, as with CBR traffic. This dynamic scheduling allows VBR
traffic to be statistically multiplexed onto the ATM line, resulting in better utiliza-
tion of the shared bandwidth resources.
6.2.4.1 Rate Shaping vs. Policing
The Cell Scheduler rate shapes the segmentation traffic for up to 64 k connec-
tions. The outgoing cell stream for each VCC is scheduled according to the
GCRA algorithm. This guarantees compliance to policing algorithms applied at
the network ingress point. Channels can be rate shaped as VCs or VPs, according
to one of three leaky bucket paradigms, set by the SCH_MODE bit in the chan-
nel’s Seg VCC Table entry.
6.2.4.2 Single Leaky Bucket
The first and simplest bucket scheme is single leaky bucket. The user defines a
single set of GCRA parameters — I (Interval) and L (Limit). I is used to control
the per-VCC Peak Cell Rate (PCR) and L is used to control the Cell Delay Varia-
tion Tolerance (CDVT) of the outgoing cell stream. The user enables this scheme
by setting the SCH_MODE bits to “100” (VBR1).
6.2.4.3 Dual Leaky Bucket
The user can also select, on a per-VCC basis, to apply two leaky buckets to a sin-
gle connection. The user enables this scheme by setting the SCH_MODE bits to
“101”(VBR2).
When using VBR2 SCH_MODE, you are limited to 256 values for the I2 and
L2 parameters. These parameters are stored as bucket table entries. (See
Table 6-9 for the definition of a bucket table entry.) There is complete flexibility with
APPLICATION EXAMPLE: Tunnels
Figure 6-5, previously, shows priorities six and seven used as CBR tunnels for UBR traffic. Note that
the two tunnels are assigned the two highest priorities. The Host assigns a fixed number of Schedule Table
slots to the tunnel to reserve a fixed rate. Each time an assigned slot is encountered by the Cell
Scheduler, it selects a VCC from a round-robin queue of active VCCs assigned to that priority.
For example, 100 UBR VCCs with PRI = 7 might currently be segmenting data. Each will get
1/100th of the CBR bandwidth assigned to the tunnel. These channels will take priority over all other
traffic classes. Tunneling enables system level end users to purchase CBR services from a WAN
service provider. The purchaser may then dynamically manage the traffic within this leased CBR
tunnel as either CBR or UBR services.
In this example, the user has configured the RS8234 to manage two independent tunnels. One,
priority six, is through a private ATM network, perhaps a corporate ATM campus backbone. The other,
priority seven, carries traffic through a public network. This topology allows the end user to lease reserved
CBR bandwidth from an administrative domain but manage the usage of the tunnel in an arbitrary
fashion.