User guide

EDA System Services
1/1551-HSC 901 35/3 Uen C 2005-12-02
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allowance. The Sustainable Cell Rate is an upper limit for the average cell
rate that may be transmitted in the PVC.
Video services are configured with service class VBR-nrt. This service
class is suitable for non-real-time “burst” applications requiring service
guarantee from the network. It is also characterized by PCR, SCR and
MBS.
Data services are configured with the service class UBR (Unspecified Bit
Rate). UBR is used for non-real-time, “burst” applications, which tolerates
delays and packet loss and is often referred to as a “Best Effort” service.
Segmentation is a native feature of ATM, dividing the ADSL traffic into
small cells, each containing 48 bytes payload, as opposed to Ethernet
frames of up to 1500 bytes.
Burst allowance , which is defined by the Peak Cell Rate (PCR), and
Burst Tolerance (BT) or Maximum Burst Size. The Burst Tolerance is the
maximum time for which the PVC may transmit data at Peak Cell Rate.
Limiting the Burst Tolerance to the size of a voice frame (per used POTS
port), and allowing the voice PVC to use maximum capacity, ensures
maximum QoS. The IAD maps only data coming from the POTS ports into
the voice PVC, thus ensuring that the bandwidth configured for voice is
always available.
6.3.2 Upstream Mechanisms
Upstream traffic is considered less critical from the Access network point of
view. The most critical link – the DSL link is controlled by the CPE in the
upstream direction. It is assumed that the uplink from the From the Access
Node can easily handle any traffic coming from the slower DSL line. Still,
an overload protection similar to the one in the downstream direction, but
which is based on the ATM service classes, secures the upstream traffic.
Other policing and classification mechanisms are also used. Figure 47 on
page 66 illustrates the different QoS mechanisms on the upstream
direction.