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6.0 Traffic Management
6.1 Overview
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
In order to supply these services, the xBR Traffic Manager directs the seg-
mentation on each active VCC by controlling the segmentation coprocessor. By
intelligently selecting when each VCC transmits a cell, the Traffic Manager guar-
antees that the output of the RS8234 conforms to the negotiated traffic contract.
This selection process (called Scheduling) executes dynamically, based upon per-
VCC parameters.
The xBR Traffic Manager consists of two primary components. The first is
the Dynamic Cell Scheduler which provides for CBR, VBR, UBR, and GFR traf-
fic. Second, for ABR service classes, is the ABR Flow Control Manager, an addi-
tional state machine which works in conjunction with the xBR Cell Scheduler.
Due to the complexities of ABR, a dedicated state machine is required to
achieve full rate performance — one which reacts to feedback from the network
and adjusts cell transmission accordingly. This specification models ABR to align
with the TM4.0 ABR specification. The RS8234 supports the rate based flow
control and service models specified for ABR in TM4.0.
The RS8234 also provides Generic Flow Control. This XON/XOFF protocol
complements the GFC algorithm by allowing switches to significantly overallo-
cate port bandwidth.
6.1.1 xBR Cell Scheduler
The Cell Scheduler maintains the QoS guarantees for each service category. It
rate-shapes all segmentation traffic according to per-channel parameters. This
provides each channel with appropriate transmission opportunities and guarantees
conformance with network traffic contract policing algorithms applied to the out-
puts.
The Cell Scheduler selects individual channels based upon the Dynamic
Schedule Table. Schedule Slots in this table may be pre-reserved for CBR ser-
vices. The VPs and VCs with CBR service reserve cell slots for transmission, and
max Cell Transfer Delay (CTD)
supportedd
not supportedd
Cell Loss Ratio (CLR)
supportedd
not
supportedd
not
supportedd
OTHER ATTRIBUTES:
Feedbacke
supported
a. The rt-VBR service category is intended for real-time applications; i.e., those requiring tightly constrained delay and delay variation,
as would be appropriate for voice and video applications.
b. The nrt-VBR service category is intended for non-real-time applications which have bursty traffic characteristics.
c. The UBR service category is intended for non-real-time applications which do not require tightly constrained delay and delay varia-
tion. Examples of such applications are traditional computer communications applications, such as file transfer and e-mail.
d. This is a network parameter. The RS8234 provides counters which monitor this parameter.
e. Feedback refers to the several types of control cells called Resource Management Cells (RM cells) which are conveyed back to the
source in order to control the source transmission rate in response to changing ATM layer transfer characteristics.
Table 6-1. ATM Service Category Parameters and Attributes (2 of 2)
Attribute
ATM Layer Service Category
CBR
rt-VBRa
nrt-VBRb
UBRc
ABR
GFR