Though Unified Communications consists of a broad portfolio of communication services, such as video conferencing, desktop sharing, and instant messaging, perhaps the most sensitive to network performance is Voice over IP (VoIP) because it is consumed in real time by the human ear. Consider the potential difference between a cell phone conversation using packetized speech and a conversation carried by a Constant Bit Rate network such as the Plain Old Telephone Service (POTS).
Quality of Experience Measurements
Mean Opinion Score (MOS) is the industry standard for measuring the quality of a voice conservation over telecommunication networks. MOS originated from listeners in a quiet room giving their opinion of voice quality on a scale of one to five, five being the best. Its purpose was to evaluate different speech encoding techniques and effects of long-distance (Toll) calls. Today, MOS can be calculated using metrics such as latency, packet loss and jitter. Variations in these metrics affect the quality of a user’s conversation and, in-turn, MOS. MOS may be different for each user in the conversation.
Latency is the average one-way delay calculated from the origination to the destination party. It includes propagation delay (affected by distance), network delay (affected by queueing in the network), and packetization delay (affected by the type of speech encoder used for the call). A network reroute may increase the distance. Network congestion from excessive traffic may increase network delay. Sophisticated speech encoding algorithms may increase packetization delay. Latency below 150 milliseconds is considered acceptable. If latency is much higher, the conversation is like using walkie-talkies where everyone has to say “over.”
Packet loss is the percentage of data packets that were not received by the end user. Since VoIP uses the User Datagram Protocol (UDP), these packets are not retransmitted. Loss can be due to bit errors caused by cabling issues (particularly in an office environment), but is more commonly due to discards caused by traffic overloads. Loss percentage greater than one percent begins to impair a call. Packet Loss Concealment (PCL) attempts to mask the effects of lost packets. This may sound like chirps or clicks to the receiver.
Jitter is the variations in delay among the arrival times of packets. A voice packet typically carries 20 to 30 milliseconds of speech. If a packet is delayed more than 20 milliseconds, there may be no speech to play for the user. PCL is used to mask the effects of late packets.
Monitoring and Managing
Monitoring and maintaining network performance is important to manage the end user’s VoIP experience. CA Unified Communications Monitor monitors the metrics that affect the user’s Quality of Experience, alerts on degradation, and helps isolate the source of problems to specific locations. CA Performance Management and CA Network Flow Analysis further isolate the problem such as bit errors, packet loss, mismarked Quality of Service traffic, or sources of congestion.
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