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2.0 Architecture Overview
2.5 Advanced xBR Traffic Management
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
2.5.4 UBR Traffic
The Unspecified Bit Rate (UBR) service category is intended for non-real-time
applications which do not require tightly constrained delay and delay variation,
such as traditional computer communications applications like file transfer and e-
mail.
Those VCCs which have not been assigned to one of the other service catego-
ries as covered above are scheduled as UBR traffic. All UBR channels within a
priority are scheduled on a round-robin basis. To limit the bandwidth that a UBR
priority consumes, the system designer should use a CBR tunnel in that priority
level.
2.5.5 xBR Cell Scheduler
The xBR Scheduler slates traffic for transmission according to a Dynamic Sched-
ule Table maintained in SAR shared memory. The table contains a user-program-
mable number of schedule slots. The duration of a single slot is a user-
programmable number of system clock cycles. The xBR Scheduler sequences
through this table in a circular fashion to schedule traffic. By configuring the
number of slots in the table and the duration of each slot, the system designer
chooses a range of available rates. A specific rate for any channel is determined
by how many slots in the table that channel is assigned. Schedule slots not
reserved for CBR during table setup are used for the rest of the service categories.
The xBR Scheduler implements Rockwell’s proprietary per-VCC rate shaping
algorithms. The predecessor to the RS8234, the Bt8230 SAR, proved the core
algorithms. The RS8234 extends their use to other service classes. The RS8234
xBR Scheduler shapes all traffic classes, including CBR, single leaky bucket
VBR, dual leaky bucket VBR, ABR, and UBR. The Host configures the Dynamic
Schedule Table during system initialization, defining the table size in number of
schedule slots and the length of each schedule slot in clock cycles. After setup,
the RS8234 dynamically manages the entire table.
The key features of the xBR Scheduler are:
1
Per-VCC rate control guarantees conformance to GCRA UPC/policing
2
Dynamic reallocation of link bandwidth to active channels
3
Dynamic, fair sharing of bandwidth on oversubscribed lines
4
Multiple scheduling priorities
5
Fine grained rate control
6
Rate based on a user supplied reference clock
Dynamic management provides on-the-fly reallocation of link bandwidth
without host intervention. The RS8234 fairly distributes the link bandwidth to
channels based upon their QoS parameters and assigned transmission priority. As
covered previously, the RS8234 supports eight priorities in addition to preallo-
cated CBR time slots.
The xBR Scheduler facilitates advanced network traffic management topolo-
gies. The RS8234 rate shapes VCs or VPs. Additionally, CBR tunneling allows
UBR and/or VBR traffic management schemes to operate under a preallocated
CBR limit.