Cisco QoS 642-642 Study Notes

Chapter 1 “QoS Overview”

QoS defined: “The ability of the network to provide better or “special” service to a set of user/applications to the detriment of the other users/applications”

Four characteristics that QoS tools can affect:
Jitter (Delay Variation)
Packet Loss

Clock rate command vs bandwidth command:
clock rate – defines actual layer 1 bit rate
bandwidth – tells IOS the assumed bw available on the interface, i.e. EIGRP, QoS

QoS tools that affect bandwidth:
The best QoS tool for bw is MORE BW!
Some link-efficieny QoS tools improve bw by reducing the number of bits required to transmit data, i.e. compression (before or after queuing process)
CAC (call admission control) CAC tools decide whether the network can accept new voice and video calls
Queuing tools can affect amount of bw that types of traffic receive

QoS Tool : How it affect BW
Compression : Compress either payload or headers, reduce number of bits to transmit
CAC : Reduce overall load by rejecting new voice and video calls
Queuing : reserve minimum amounts of bw for packet types

All packets experience delay

Types of Delay
Serialization delay (fixed)
Propagation delay (fixed)
Queuing delay (variable)
Forwarding/processing delay (variable)
Shaping delay (variable)
Netowrk delay (variable)
Codec delay (fixed)
Compression delay (variable)

Serialization Delay
forumla to calculate serialization delay #bits sent/link speed, (125-byte packet over fastethernet, 125bytes= 1000bits, 1000bits/100,000,000bps=.01ms / 56kbps 1000bits/56000bps=17.85ms)

Propagation Delay
Defines the time it takes a single bit to get from one end of the link to the other, the only variable that affects propagation delay is the lenght of the length, NOT link speed or clock rate…
forumla Length of Link(meters)/2.1 (or 3.0 speed of light in a vacuum) x 10^8 meters/second

Queuing Delay
Packets experience queing delay when they have to wait for other packets to be sent

Forwarding Delay
time between fully-received frame and when packet is placed in an output queue, this is the time required for the router to process the route or forward the packet. Forwarding delay is typically small enough to ignore in overall delay budgets calculations…

Shaping Delay
Traffic shaping causes delays by serving queues more slowly, this helps to match the forwarding rate of traffic when a carrier might discard traffic if the rates exceed the CIR.
EXAMPLE: 64kbps CIR, AR (acess rate) 128kbps, shaping to send only half the time (router forwards bits at phyiscal link speed or AR, so this would shape or average to be about 64kbps…
Shaping attempts to enhance drops, but must sacrifice another characterisitc (DELAY) to do so…

Network Delay
The serialization delay to send the packet into the cloud, The serialization delay at the egress Frame Relay switch, sending the packet back to your router.
EXAMPLE: Propagation + Serialization (ingress) + Serialization (egress) = Network Delay (of course the delay will vary, dependent on the provider, and queuing delay adds more variables)

QoS Tool : How it affects Delay
Queing : reorder packets so that delay-sensitive packets leave before delay-insensitive packets
LFI : breaks lager packets into smaller framgments before sending them
Compression : either payload or headers, also adds some processing delay
Traffic Shaping : artificially increases delay to reduce drops inside a FR or ATM network

variation in the arrival rate of packets

QoS Tools that affect Jitter

Packet Loss
Loss that is due to QoS mechinisms, not other reason such as failed FCS discards

QoS Tools that affect Loss
Random Early Detection : drops packets randomly in filling queues slowing TCP, reduces overall load.
Think of RED tools as managing the end of a queue, while queuing tool manages the front of the queue!

Planning and Implementing QoS Policies

Step 1 – Identify traffic and its requirements
Step 2 – Divide traffic into classes
Step 3 – Define QoS policies for each class

Example service classes for Step 2

Voice and Video three classes:
One for voice payload, One for video payload, One for both voice and video signaling traffic
Data Cisco recommended classes:
Mission Critical – interactive, with importance to business
Transactional – interactive, with importance to business
Best-Effort – General web browsing, email, etc…
Scavenger(less than best effort) – insignificant, Napster, Kazaa, chat, etc…


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