4.7 Describe basic QoS concepts

4.7.a Marking

Marking is when a packet or frame is modified in the relevant QoS field.

  • For IP packets that is usually the outdated 3 bit IPP (IP Precedence) field or the 6 bit DSCP (Differentiates Services Code Point).
  • For Ethernet frames over trunks only the 802.1Q Header has a 3 bit field called the Class of Server (CoS) or Priorty Code Point (PCP). Therefore Ethernet based QoS can only be done over trunks.

Marking is recommended by Cisco and the RFCs to be as close to the source of the packet as possible. Marking a packet can be CPU intensive and too much packet marking can reduce device performance.

4.7.b Device trust

As any user can potentially mark packets it is important to define a trust zone and a trust boundary. If the end user connects to a Cisco IP Phone the trust boundary ends after the phone as the phone will mark voice packets with the AF code point and voice signaling packets with IPP compatible CS3 code point.

4.7.c Prioritization

Due to this difference in the nature of traffic in a network, some traffic needs to be prioritized above other to keep bandwidth, jitter, delay and loss within certain ranges.
When using congestion management queues in order to keep jitter and delay within the required parameters a Low Latency Que (LLQ). This reduces the negative effect that Round Robin scheduling has on jitter and delay as the LLQ is processed before another queues.

4.7.c. [i] Voice

Cisco recommends the following for voice:

  • Delay (one way): 150ms or less
  • Jitter: 30ms or less
  • Loss: 1% or less

4.7.c. [ii] Video

Cisco recommends the following for video:

  • Bandwidth: 384Kbps to 20+ Mbps
  • Delay (one-way): 200 – 400 ms
  • Jitter: 30-50ms
  • Loss: 0.1% – 1%

4.7.c. [iii] Data

Data has much stringent requirements for bandwidth, jitter, delay and loss. However business critical applications can be assigned a minimum bandwidth requirement using a queue.

4.7.d Shaping

A traffic shaper slows traffic down by queuing traffic and then re-transmitting it at a reduced rate.  Shapers are useful when the speed of the interface is faster than the CIR bandwidth supplied by an ISP or carrier.
Keep in mind shaping can increase the jitter because the interface will only be transmitting for a reduced amount of time. For example if the shaping bandwidth is 200 Mbps and the interface speed is 1Gbps the interface will transmit for 200ms and be idle for 800ms. Giving a jitter time of up to 800ms.
To reduce this the shaper needs to be configured with a reduced Tc. The recommended Tc for voice and video is 10ms.

4.7.e Policing

Traffic policing is when traffic is measured over a period of time and the average is taken, if traffic exceeds the policing rate it is marked or discarded.
Marked traffic may be forwarded in the case that the service provider is not experiencing congestion or dropped if it is.
As the traffic measured is an average, burst in traffic are allowed.

4.7.f Congestion management

Congestion management is when traffic exceeds the line rate of the interface, this traffic is then placed into queues and sent when the interface is not busy. Queuing systems may use a single FIFO (First In, First Out) or multiple queues that utilize a LLQ.
Class-Based Weighted Fair Queuing allows the allocation of a minimum amount of bandwidth to each class and is a popular choice on routers.

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