Understanding QoS
Fundamentals
The basic overview for QoS is “Who goes 1st?” from an exit perspective
on a switch or router.
‘Evil Villains’ in the network world
1.
2.
3.
4.
Lack of bandwidth (Not really a fix, only really for temp congestion)
Packet Loss (Directly with Voice)
Delay (Directly with Voice)
Jitter (Directly with Voice)
Packet Loss = If there is not enough bandwidth available on a WAN link,
the router will queue the packets in memory, when the memory is full
the term ‘taildrop’ is used as the doors are closed on the router and it
will no longer accept packets. QoS is able to decide WHAT packets are
dropped, to ensure voice is not dropped.
Delay = Cisco best practice of no higher than 150ms. Voice is delay
sensitive.
Jitter = Variable delay. Not flat delay or consistent round trip times.
(Spikey) A Cisco router and IP phone have a ‘de-jitter’ buffer feature
where it can read ahead and dynamically adjust. If this buffer is full
then the packet is lost.
Three models for QoS
Tagging
Best Effort – Default state on a router (Post Office analogy I will
send your letter but can’t guarantee it’s delivery)
CoS for L2 traffic
Class Of Service
Integrated Services – (INT SERV) This model can reserve
bandwidth for classified traffic. This is not a scalable solution.
(Private Jet Analogy)
ToS for L3 traffic
Type Of Service
Differentiated Services – (DIFF SERV) Most popular QoS model.
Packets are marked with levels.
No reserved bandwidth.
Only for queuing packets.
QoS Toolbelt – Classifying and Marking
• Classification involves identifying and grouping different traffic
types.
• Marking tags or ‘colours’ on the packet so it can be quickly
recognised elsewhere on the network.
*Citrix traffic is extremely delay sensitive
Queueing Strategies: Weighted Fair Queuing (WFQ)
Number of queues: Per-flow
Method: Weighted Fair (Low senders are prioritised)
Delay Guarantee: No
Bandwidth Guarantee: No
Queuing Strategies: Class Based WFQ (CBWFQ)
Number of queues: up to 256 classes
Method: N/A
Delay Guarantee: No
Bandwidth Guarantee: Yes
*Not great for voice
Example: For every 50 HTTP packets send 20 FTP packets, for every
20 FTP packets sent 10 TELNET packets etc… Not ideal…
Queuing Strategies: Low Latency Queuing (LLQ)
Number of queues: 1 Priority Queue (PQ) + CBWFQ
Method: N/A
Delay Guarantee: Yes for PQ
Bandwidth Guarantee: Yes
*Preferred strategy for Voice traffic.
User Expectations
Golden QoS Rule:
“User application experience should meet or
beat what the user is accustomed to regardless
of the new applications added to the network.”