3Com CoreBuilder 7000, Руководство по эксплуатации

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124 C
HAPTER
5: E
STABLISHING
ATM C
HANNELS
This capability applies only when the SETUP message contains a
Broadband bearer capability, that is, when "ABR" is indicated in the ATM
transfer capability field.
Constant Bit Rate
Service Class
CBR, or Constant Bit Rate Service, is used by connections which request a
static amount of bandwidth that is continuously available during the
connection lifetime. This QoS Class is used by both P2P and P2M
connections. It can be used for VPCs and VCCs. The amount of
bandwidth specified is referred to as the PCR or Peak Cell Rate value.
CBR states that once a connection is established, the negotiated ATM
QoS is assured to all cells when they are conforming to the user’s
parameters. In the CBR capacity, the source can emit cells at the PCR at
any time and for any duration and the QoS commitments still pertain.
CBR service is intended to support real time applications requiring tightly
constrained delay variation (e.g., voice and video), but is not restricted to
these applications. In the CBR capacity, the source may emit cells which
are at, or below the negotiated PCR. Furthermore the source may even be
silent for periods of time. Cells which are delayed beyond the value
specified by the maximum cell transfer delay (maxCTD) are assumed to be
of significantly reduced value to the application.
Real-Time Variable Bit
Rate Service Class
The rt-VBR service class is a category intended for real -time applications.
These are applications who require tightly constrained delay and delay
variation, as would be appropriate for voice and video applications.
rt-VBR connections are characterized in terms of PCR, Sustainable Cell
Rate (SCR) and Maximum Burst Size (MBS). Sources are expected to
transmit at a rate which varies with time. Thus, the source can be referred
to as ‘bursty’. Cells which are delayed beyond the value of the maxCTD
are assumed to be of significantly reduced value to the application.
rt-VBR may support statistical multiplexing of real-time sources.
Non-Real-Time
Variable Bit Rate
Service Class
The non-real-time VBR service category us intended for non-real-time
applications which have bursty traffic characteristics and are
characterized in terms of PCR,SCR, and MBS. For those cells which are
transferred within the service contract, the application expects a low cell
ratio. nrt-VBR service may support statistical multiplexing of connections.
No delay bounds are associated with this service category.