Which of the following QoS features reduces the jitter of voice packets by preventing them from being delayed
behind larger data packets in a queue?
A.
CAR
B.
LLQ
C.
cRTP
D.
LFI
Explanation:
Link fragmentation and interleaving (LFI) is a Quality of Service (QoS) feature that reduces the jitter of voice
packets by preventing them from being delayed behind larger data packets in a queue. LFI helps reduce the
latency in voice packets by fragmenting large packets into smaller packets. Once this is accomplished, voice
packets can be woven, or interleaved, between the fragmented data packets from a different flow and can pass
through the network device much quicker than if the voice packets had to wait for the large data packets to be
transmitted.
QoS enables a network to treat a specific type of traffic with a different priority than other types of traffic. For
example, QoS can ensure that voice traffic gets higher priority on a network than data traffic. QoS models
include the best-effort model, the Integrated Services (IntServ) model, and the Differentiated Services (DiffServ)
model. Each QoS model handles packet flows in a different manner. For example, IntServ requires that
applications reserve their end-to-end bandwidth requirements, and DiffServ prioritizes packets by traffic class.
Because of some inherent shortcomings in the IntServ model, Cisco recommends using DiffServ when
delivering voice traffic.
Low latency queuing (LLQ) is a queuing method that is useful for transmitting voice, video, and mission-critical
traffic. However, if a large data packet is being sent, LLQ cannot interrupt the transmission of the large data
packet in order to send a small voice packet, regardless of the size of the priority queue. Therefore, LLQ does
not reduce the jitter of voice packets by preventing them from being delayed behind larger data packets in a
queue.
Although Compressed Real-time Transport Protocol (cRTP) reduces delay, it does not reduce the jitter of voice
packets by preventing them from being delayed behind larger data packets in a queue. To reduce the size of
the IP header, cRTP associates a hash number with the 40byte IP header. After the first voice packet is sent
with the full 40byte header, subsequent packets in the same flow use only the hash number that was
associated with the header. This reduces the IP header size from 40 bytes to as low as two bytes if a checksum
is not used. If a checksum is used, the header is reduced to four bytes.
Committed Access Rate (CAR) does not reduce the jitter of voice packets by preventing them from being
delayed behind larger data packets in a queue. CAR is a traffic policing mechanism that you can use when
traffic exceeds the configured bandwidth limitations. When CAR is used, packets that exceed the bandwidth
limits are remarked with a lower priority and forwarded instead of being dropped.Cisco: Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2: Chapter: Link Efficiency
Mechanisms Overview