Nokia 7705 - 3.3 Fabric Traffic; 3.3.1 Traffic Flow Across the Fabric

Quality of Service Guide QoS and QoS Policies
Edition: 01 3HE 11014 AAAC TQZZA 93
3.3 Fabric Traffic
This section contains the following topics for traffic flow in the access ingress
direction:
• Traffic Flow Across the Fabric
3.3.1 Traffic Flow Across the Fabric
The 7705 SAR uses an Ethernet-based fabric. Each packet that is sent to the fabric
is equipped with a fabric header that contains its specific CoS requirement. Since all
of the packets switched across the fabric are already classified, queued, scheduled
and marked according to the desired QoS parameters, each of these packets has
been passed through the Traffic Management (TM) block on an adapter card, or the
Control and Switching Module (CSM) in the case of control packets. Therefore, each
packet arrives at the fabric having been already scheduled for optimal flow. The
function of the fabric is to switch each packet through to the appropriate destination
adapter card, or CSM in the case of control packets, in an efficient manner.
Since the traffic is shaped at a certain rate by the ingress adapter card (that is, bursts
are smoothened by the traffic management function), minimal buffering should be
needed on the switch fabric. However, the buffer space allocation and usage is in
accordance with the priorities at the ingress adapter card. As is the case with
schedulers at the adapter cards, there are two priorities supported on the switch
fabric. The switch fabric serves the traffic in the following priority order:
1. Expedited
2. Best Effort
Note:
• The switch fabric does not support profile scheduling.
• Since the fabric has a limited buffer space, it is possible for tail drop to occur. Tail drop
discards any packet that exceeds the maximum buffer space allocation. The shaping
that is performed on the adapter cards helps to prevent or minimize congestion.