Demystifying QoS

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Demystifying QoS

Peter ThompsonChief Scientist

U4EA [email protected]

© U4EA Technologies 2006 2www.u4eatech.com

What is QoS?

“Quality of Service” – many meanings

• Service availability– E.g PSTN backup for VoIP

• Service reliability– Even when subject to interference (accidental or

malicious)

• Service observability– For SLAs


Why is ‘QoS’ Necessary?

Jitter of VoIP stream mixed with TCP over a T1 link


Why is ‘QoS’ Necessary?

PESQ score of G711 subject to packet loss and jitter


QoS is Affected at Many Levels

• Layer 1: S/N ratio• Layer 2: protocol overheads• Layer 3: buffer overflow, queuing delays• Layer 4: error correction, jitter buffers• Layer 5: session management


QoS: Reliable Packet-based Services• Dynamic sharing of resources makes

packet-based services– Flexible– Efficient– Potentially unreliable!

• Making packet-based services reliable requires giving the right resources to the right amount of the right packets


Elements of QoS

Admission control

Packet classification

Packet queuing and scheduling

Allocate the right resources

Identify the right packets

Ensure the right amount


Is Standard QoS Enough?

Bursts for which loss and delay are

increased

Off

ere

d

load

Time

Average

Capacity

Can manage with standard techniques e.g. 802.1p


Is Standard QoS Enough?

Bursts where loss and/or delay become

excessive

Off

ere

d

load

Time

Capacity

Needs an advanced

multi-media QoS solution

Average


Summary: Reliable IP-based Services1. Make sure the lower layers don’t cause

trouble2. Use session control to limit the amount of

traffic needing protection3. Use standard techniques (802.1p, Diffserv

etc.) to protect/prioritize traffic where the average load is low

4. Look for advanced mechanisms to make efficient use of more loaded links

5. Manage the congestion where it occurs

Demystifying QoS

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