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Chapter 38 — QoS, Voice, Video, and Performance Engineering

A complete networking lesson based on the course chapter menu.

30 topicsPractical examplesTroubleshooting checksReview Q&A

Estimated reading time0% read

Chapter 38: QoS, Voice, Video, and Performance Engineering

This chapter follows the topics shown in the Networking chapter menu. Work through each section in order, then use the review questions to check recall and troubleshooting reasoning.

38.1 Quality of Service

Quality of Service is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Quality of Service in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Quality of Service without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.2 Voice Traffic

Voice Traffic is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Voice Traffic in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Voice Traffic without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.3 Video Traffic

Video Traffic is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Video Traffic in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Video Traffic without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.4 Bandwidth

Bandwidth is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Bandwidth in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Bandwidth without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.5 Congestion

Congestion is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Congestion in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Congestion without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.6 Queues

Queues is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Queues in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Queues without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.7 FIFO

FIFO is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place FIFO in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain FIFO without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.8 Priority Queuing

Priority Queuing is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Priority Queuing in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Priority Queuing without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.9 Traffic Classification

Traffic Classification is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Traffic Classification in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Traffic Classification without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.10 Traffic Marking

Traffic Marking is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Traffic Marking in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Traffic Marking without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.11 DSCP

DSCP is a field in the IP header used to mark packets for differentiated treatment by QoS policies along the path.

Example: place DSCP in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain DSCP without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.12 802.1p/CoS

802.1p/CoS is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place 802.1p/CoS in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain 802.1p/CoS without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.13 QoS Trust Boundary

Quality of Service classifies and treats traffic differently so delay-sensitive or business-critical applications can receive suitable queueing, scheduling, marking, and congestion behavior.

Example: place QoS Trust Boundary in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain QoS Trust Boundary without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.14 Policing

Policing is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Policing in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Policing without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.15 Shaping

Ping is a basic reachability and round-trip-time test. A failed ping does not always prove the destination is down because policy may block ICMP.

Example: ping the local loopback, local interface, default gateway, remote IP, and finally a hostname. The first failed step helps narrow the fault domain, but remember that ICMP filtering can produce false negatives.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Command or data example

ping 192.0.2.1

Practice: Practice: explain Shaping without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.16 Latency

Latency is the time required for traffic to travel between endpoints. Propagation, serialization, queueing, processing, and path choice can all contribute.

Example: place Latency in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain Latency without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.17 Jitter

Jitter is variation in packet delay. Real-time voice and video are especially sensitive to excessive delay variation.

Example: place Jitter in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain Jitter without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.18 Packet Loss

Packet loss means frames or packets fail to arrive at the destination. Causes include congestion, physical errors, faulty interfaces, wireless interference, policy, or overloaded devices.

Example: place Packet Loss in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain Packet Loss without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.19 Jitter Buffers

Jitter is variation in packet delay. Real-time voice and video are especially sensitive to excessive delay variation.

Example: place Jitter Buffers in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain Jitter Buffers without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.20 TCP vs UDP for Real-Time Traffic

TCP is connection-oriented and provides sequencing, acknowledgments, retransmission, flow control, and congestion-control behavior for reliable byte streams.

Example: place TCP vs UDP for Real-Time Traffic in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain TCP vs UDP for Real-Time Traffic without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.21 RTP

RTP is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place RTP in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain RTP without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.22 SIP

SIP is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place SIP in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain SIP without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.23 VoIP

VoIP is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place VoIP in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain VoIP without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.24 Voice VLAN

A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Voice VLAN without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.25 PoE for Voice Devices

PoE for Voice Devices is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place PoE for Voice Devices in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain PoE for Voice Devices without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.26 Wireless QoS

Quality of Service classifies and treats traffic differently so delay-sensitive or business-critical applications can receive suitable queueing, scheduling, marking, and congestion behavior.

Example: two clients see the same SSID, but one has poor performance at the edge of coverage. Compare signal strength, noise, SNR, channel utilization, roaming behavior, and retry rate before assuming the internet circuit is slow.

What to check

Measure RF conditions instead of relying only on the number of signal bars.
Check client capability, channel width, interference, retries, roaming, and access-point load.
Validate the design in the actual physical space and during expected busy periods.

Practice: Practice: explain Wireless QoS without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.27 WMM

WMM is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place WMM in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain WMM without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.28 Bufferbloat

Bufferbloat is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Bufferbloat in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Bufferbloat without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.29 Oversubscription

Oversubscription is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Oversubscription in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Oversubscription without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

38.30 Capacity Planning

Capacity Planning is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Capacity Planning in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Capacity Planning without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

Chapter 38 Review Questions

1. What should you remember about Quality of Service?

Answer: Quality of Service is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

2. What should you remember about Bandwidth?

Answer: Bandwidth is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

3. What should you remember about FIFO?

Answer: FIFO is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

4. What should you remember about DSCP?

Answer: DSCP is a field in the IP header used to mark packets for differentiated treatment by QoS policies along the path.

5. What should you remember about Policing?

Answer: Policing is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

6. What should you remember about Jitter?

Answer: Jitter is variation in packet delay. Real-time voice and video are especially sensitive to excessive delay variation.

7. What should you remember about TCP vs UDP for Real-Time Traffic?

Answer: TCP is connection-oriented and provides sequencing, acknowledgments, retransmission, flow control, and congestion-control behavior for reliable byte streams.

8. What should you remember about Voice VLAN?

Answer: A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

9. What should you remember about WMM?

Answer: WMM is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

10. What should you remember about Capacity Planning?

Answer: Capacity Planning is one of the core topics in QoS, Voice, Video, and Performance Engineering. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.