MINI-LINK BAS 7-6
Technical Description EN/LZB 111 0542 P2B
7.3 Service Categories
Two ATM service categories are supported throughout the system,
each one associated with specific QoS guarantees:
• CBR, which grants low cell transfer delay, cell delay variation
and cell loss ratio to services whose traffic profile has a well
specified maximum bandwidth (expressed in terms of peak cell
rate)
• UBR, which grants best effort handling to bursty, non-delay
sensitive services
CBR connections are intended to be used for services with stringent
transport delay constraints, such as the VCCs used in support of CE
services. In most of the cases, these services have also very stringent
requirements in terms of cell loss ratio, given the impossibility to
accommodate retransmissions.
UBR connections are intended to be used for data services, with
limited sensitivity to delay and with the capability to tolerate or
recuperate from losses. Typically, these services have a bursty nature
and the use of statistical multiplexing can facilitate the appropriate
utilization of transmission resources.
The combination of CBR and UBR connections has the potential to
enable a very effective use of transmission resources.
7.3.1 Connection Admission Control
Whenever a CBR connection is set up, the system implicitly undertake
the commitment to grant the corresponding QoS guarantees.
Therefore, prior of confirming the establishment of a CBR connection,
the system verifies whether appropriate resources are available
throughout all its involved parts. If this is not the case, the connection
establishment is rejected.
The establishment of UBR connections is or is not submitted to
verification on resources depending on how the system has been
configured.
If the operator has decided to support data services with a best effort
approach (with or without a minimum bandwidth gurantee), then no
verifications on resources are made at connection establishment, that
is, the establishment of a connection is never rejected for a lack of
resources. Conversely, if the operator has decided to support data
services with a circuit-like approach, that is by providing a constant
guarantee for a specific amount of maximum bandwidth, then the
establishment of a new connection is possible only if the
corresponding amount of bandwidth is actually available throughout
all the parts of the system.
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