QoS fundamentals and Nokia QoS architecture
What the exam tests
This domain checks that you can separate the five QoS actions - classification, marking, policing, shaping and queuing/scheduling - and place each at the correct point in a Nokia DiffServ network. You must know what a trust boundary is and why the first trusted device sets DSCP/EXP.
Core rules
- Classification inspects headers (DSCP, 802.1p, EXP/TC, IP 5-tuple) and assigns a forwarding class (FC) and a colour/profile (in or out).
- Marking writes a new DSCP, EXP or 802.1p value; it is usually done at the trust boundary and again on egress if the transport uses a different marking space.
- Policing meters a stream and drops or re-marks excess traffic; shaping buffers excess traffic and delays it.
- Queuing and scheduling decide which packet leaves next. Strict priority (SP) favours latency-sensitive traffic; weighted fair queuing distributes remaining bandwidth; a shaper caps an aggregate.
- Congestion avoidance (WRED) drops low-priority colour before a queue is full; tail drop is the last resort.
- SLAs are expressed as bandwidth, latency, jitter and loss; QoS alone cannot guarantee an SLA without admission control and adequate capacity.
Worked example
A carrier maps voice to EF and bulk data to AF11. Voice is policed at 20% of the access rate and scheduled in a strict-priority queue; data uses WRED. If voice exceeds the policer, excess packets are dropped or re-marked, not queued - that preserves latency for conforming voice. The data queue uses WRED so that TCP backs off before the queue overflows.
Traps
- Confusing policing with shaping: policing is immediate drop/remark, shaping delays.
- Assuming marking alone guarantees treatment; every downstream node must classify the marking.
- Forgetting that a trust boundary can be a SAP, a network interface or a peer.