Regardless of the speed of individual switches or links, speed
mismatches, many-to-one switching fabrics and aggregation may cause a device to
experience congestion which can results in latency. If congestion occurs and
congestion management features are not in place, then some packets will be
dropped causing retransmissions that inevitably increase overall network load.
QoS can mitigate latency caused by congestion on campus devices.
QoS is
implemented by classifying and marking traffic at one device while allowing
other devices to prioritize or to queue the traffic according to those marks
applied to individual frames or packets.
The table in
figure
lists
the campus devices involved in QoS marking or prioritizing.
Network
Availability Problem Areas
An enterprise network may experience any of
these network availability problems:

Delay – Delay (or latency) is the amount of time that it takes a
packet to reach the receiving endpoint after being transmitted from the sending
endpoint. This time period is termed the "end-to-end delay," and can
be broken into two areas: fixed network delay and variable network delay. Fixed
network delay includes encoding and decoding time (for voice and video), as
well as the amount of time required for the electrical and optical pulses to
traverse the media en route to their destination. Variable network delay
generally refers to network conditions, such as congestion, that may affect the
overall time required for transit. In data networks, for example, these types
of delay occur:
-
Packetization delay – The amount of time that it takes to segment
data (if necessary), sample and encode signals (if necessary), process data,
and turn the data into packets
-
Serialization delay – The amount of time that it takes to place the
bits of a packet, encapsulated in a frame, onto the physical media
-
Propagation delay – The amount of time that it takes to transmit the
bits of a frame across the physical wire
-
Processing delay – The amount of time that it takes for a network
device to take the frame from an input interface, place it into a receive
queue, and then place it into the output queue of the output interface
-
Queuing delay – The amount of time that a packet resides in the
output queue of an interface
Delay variation – Delay variation (or jitter) is the difference
in the end-to-end delay between packets. For example, if one packet requires
100 ms to traverse the network from the source endpoint to the destination
endpoint, and the following packet requires 125 ms to make the same trip, then
the delay variation is calculated as 25 ms.
Each end station and Cisco network device in a voice or video conversation
has a jitter buffer. Jitter buffers are used to smooth out changes in arrival
times of data packets containing voice and video. A jitter buffer is dynamic
and can adjust for changes in arrival times of packets. If you have
instantaneous changes in arrival times of packets that are outside of the
capabilities of a jitter buffer to compensate, you will have one of these
situations:
- A jitter buffer underrun, when arrival times between packets containing
voice or video increase to the point where the jitter buffer has been exhausted
and contains no packets to process the signal for the next piece of voice or
video.
- A jitter buffer overrun, when arrival times between packets containing
voice or video decrease to the point where the jitter buffer cannot dynamically
resize itself quickly enough to accommodate. When an overrun occurs, packets
are dropped and voice quality is degraded.
Packet loss – Packet loss is a measurement of packets
transmitted and received compared to the total number that were transmitted.
Loss is expressed as the percentage of packets that were dropped. Tail drops
occur when the output queue is full. These are the most common drops that can
occur when a link is congested. Other types of drops (input, ignore, overrun,
no buffer) are not as common but may require a hardware upgrade because they
are usually a result of network device congestion.