Chap 18 - QOS

Question 1

Lists 4 QOS Characteristics

Answer 1

• bandwidth
• delay
• jitter
• loss

Question 2

What are the requirements for a Video call?

Answer 2

• Bandwidth - 384kbps to 20+Mbps
• Delay - 200-400 msec
• Jitter - 30-50 msec
• Loss - 0.1% to 1%

Question 3

What are the requirements for a Voice call?

Answer 3

• Delay - 150 msec or less
• Jitter - 30 msec or less
• Loss - 1% or less

Question 4

Where is the DSCP and IPP settings located in a TCP/IP packet?

Answer 4

• In the TOS byte of the IP header
• IPP 3 bits at start of the TOS byte
• DSCP the first 6 bits of the TOS byte

Question 5

Where is the COS marking field located?

Answer 5

COS field is located in the 3rd byte of the 4 byte 802.1q header

Question 6

On a VOIP network where is the Trust Boundary?

Answer 6

The trust boundary is at the phones

Question 7

What are the 3 Queuing Components?

Answer 7

• Classifier
• Queues
• Scheduler

Question 8

What is LLQ?

Answer 8

• Treats one or more queues as priority queues.
• Can be used in conjunction with Round Robin Queueing.

Question 9

What are 6 components of a Queueing Strategy?

Answer 9

• Use Round Robin CBWFQ for non-interactive traffic
• If too little bandwidth give business-critical apps more bandwidth
• Use Priority Queue with LLQ for interactive apps
• Voice and video in separate queues
• Provide enough bandwidth for priority queues so that built-in Policer doesn’t discard messages from the Priority Queue
• Use Call Admission Control to limit the number of Voice Calls

Question 10

Describe what Shaping does?

Answer 10

Delay the message by queueing it.

Question 11

Describe what a Policer does.

Answer 11

Discard the message or mark it differently.

Question 12

Name 4 Key Features of Policers

Answer 12

• Measures traffic rate over time
• Allows bursts of data
• Enabled on interface typically INGRESS
• Discards or re-marks messages

Question 13

Name 5 Key Features of Shaping

Answer 13

• Measures traffic rate over time
• Allows bursts of data
• Enabled on interface typically EGRESS
• Slows down packets by queueing them
• Shapers use queueing tools to create and schedule the shaping queues

Question 14

How many bits is the COS field?

Answer 14

The COS field is 3 bits.

Question 15

How many bits is the DSCP field?

Answer 15

6

Question 16

How many bits is the IPP field

Answer 16

3

Question 17

What’s another name for DSCP?

Answer 17

DiffServ

Question 18

In IPV6 where are the 6 DSCP bits located?

Answer 18

They are located in the Traffic Class Byte.

Question 19

What is another name for the COS field?

Answer 19

Priority Code Point

Question 20

What is the name and how many bits is the field used for marking Wifi traffic?

Answer 20

TID field, 3 bits

Question 21

What is the name and how many bits is the marking field of an MPLS packet?

Answer 21

EXP, 3 bits

Question 22

What are the 3 sets of DSCP values used in DiffServ?

Answer 22

• Expedited Forwarding (EF)
• Assured Forwarding (AF)
• Class Selector (CS)

Question 23

What is Expedited Forwarding (EF)?

Answer 23

A single value (46) suggested by DiffServ for use on Voice Payload packets.

Question 24

What is Assured Forwarding (AF)?

Answer 24

• a set of 12 values
• defines 4 queues each with 3 levels of drop priority
• first number - the higher the better
• second number - the lower the better

Question 25

What is Class Selector (CS) ?

Answer 25

• First 3 bits of DSCP with remaining bits set to 0
• Located where IPP bits used to be (first 3 of TOS byte)
• Provides backward compatibility with IPP
• Values are CS0 up to CS7

Question 26

What is Congestion Management?

Answer 26

• QoS toolset for managing the queues that hold packets waiting to be sent.
• Made up of 3 components:
  
  • Classifier
  • Queuing system
  • Scheduler
• Includes Round Robin queuing and LLQ

Question 27

What is Congestion Avoidance?

Answer 27

Proactively dropping TCP packets to reduce overall packet loss