QoS

Traditional data traffic characteristics:◦ Bursty data flow◦ First-come, first-served access ◦ Mostly not time-sensitive – delays OK ◦ Brief outages are survivable

Converged traffic characteristics:◦ Constant small-packet voice flow competes

with bursty data flow ◦ Critical traffic must get priority ◦ Voice and video are time-sensitive◦ Brief outages not acceptable

◦ Telephone Call: “I cannot understand you; your voice is breaking up.”

◦ Teleconferencing: “The picture is very jerky. Voice not synchronized.”

◦ Brokerage House: “I needed that information two hours ago. Where is it?”

◦ Call Center: “Please hold while my screen refreshes.”

◦Lack of bandwidth: multiple flows compete for a limited amount of bandwidth

◦End-to-end delay (fixed and variable): packets have to traverse many network devices and links that add up to the overall delay

◦Variation of delay (jitter): sometimes there is a lot of other traffic, which results in more delay

◦Packet Loss: packets may have to be dropped when a link is congested

Video Lacking Proper QoS

◦ Maximum available bandwidth equals the bandwidth of the weakest link.

◦ Multiple flows are competing for the same bandwidth, resulting in much less bandwidth being available to one single application.

Bandwidth max = min (10 Mbps, 256 kbps, 512 kbps, 100 Mbps) = 256kbpsBandwidth avail = bandwidth max / flows

Bad Voice Due to Lack of BW

• End-to-end delay equals a sum of all propagation, processing, and queuing delays in the path.

• In Best-Effort networks, propagation delay is fixed, processing and queuing delays are unpredictable.

Delay = P1 + Q1 + P2 + Q2 + P3 + Q3 + P4 = X ms

Bad Voice Due toDelay Variation

◦ Processing Delay: The time it takes for a router to take the packet from an input interface, examine it, and put it into the output queue of the output interface

◦ Queuing Delay: The time a packets resides in the output queue of a router◦ Serialization Delay: The time it takes to place the “bits on the wire”◦ Propagation Delay: The time it takes to transmit a packet

◦ Upgrade the link; the best solution but also the most expensive.

◦ Forward the important packets first.◦ Compress the payload of Layer 2 frames (it takes time).◦ Compress IP packet headers.

◦ Tail-drops occur when the output queue is full. These are common drops, which happen when a link is congested.

◦ Many other types of drops exist, usually the result of router congestion, that are uncommon and may require a hardware upgrade (input drop, ignore, overrun, frame errors).

Bad Voice Due to Packet

Loss

◦ Upgrade the link; the best solution but also the most expensive.

◦ Guarantee enough bandwidth to sensitive packets.◦ Prevent congestion by randomly dropping less important

packets before congestion occurs.

◦ Network audit Identify traffic on the

network

◦ Business audit Determine how each

type of traffic is important for business

◦ Service levels required Determine required

response time

• Latency < 150 ms*

• Jitter < 30 ms*

• Loss < 1%*

• 17-106 kbps guaranteed priority bandwidth per call

• 150 bps (+ Layer 2 overhead) guaranteed bandwidth for voice-control traffic per call

*one-way requirements

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• Latency ≤ 150 ms

• Jitter ≤ 30 ms

• Loss ≤ 1%

• Minimum priority bandwidth guarantee required is:– Video-Stream + 20%

– For example, a 384 kbps stream would require 460 kbps of priority bandwidth

*one-way requirements

• Different applications have different traffic characteristics.

• Different versions of the same application can have different traffic characteristics.

• Classify data into relative-priority model with no more than four- to five-classes:

– Mission-Critical Apps: Locally defined critical applications

– Transactional: Interactive traffic, preferred data service

– Best-Effort: Internet, e-mail, unspecified traffic

– Less-Than-Best-Effort (Scavenger): Napster, Kazaa, peer-to-peer applications

◦ Set minimum bandwidth guarantee

◦ Set maximum bandwidth limits

◦ Assign priorities to each class

◦ Manage congestion

A network-wide definition of the specific levels of quality of service assigned to different classes of network traffic

Align Network Resources with Business Priorities