Accommodating Voice Traffic on Campus Switches
QoS and voice traffic in the campus module

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.