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Isolating Network-Related Voice Quality Issues
Communications Server 2007 R2
Topic Last Modified: 2009-03-11
Any network condition that can affect the flow of encoded audio packets between client devices or computers, between clients and Office Communications Server, or between a client and the IP/PSTN gateway can impair voice quality. The audio signal that is transmitted across a network is digitally encoded and encapsulated in RTP packets, which are not subject to the type of interference that typically affects analog audio streams.
The most common network-related issue areas are caused by the loss of audio packets or delays in the transmission of the packets (that is, jitter), which can produce unwanted audio artifacts (that is, distortions) or silence when the Office Communicator client receives and processes the audio packets. In the case of receipt of an incomplete RTP packet stream that cannot be adequately corrected (that is, healed), the audio can be poor or even unintelligible. Network issues can also be caused by high latency (that is, long round-trip times) or low bandwidth. In some cases, transmission packets that are out of order can cause network issues. The following table describes network-related voice quality issues.
Table 2. Network-related voice quality issues
Potential Symptoms and Causes
Wi-Fi networks are often a source of packet loss, generally ranging from a low level of loss (< .5% of packets) to extremely high levels (> 50% of packets). This loss can occur in public Wi-Fi hotspots, home networks, and enterprise Wi-Fi deployments. Typical sources of interference include other wireless devices and phones, competing Wi-Fi networks, low density of access points, and microwave ovens.
Wireless wide area networks (WANs) are vulnerable to connection issues caused by devices that operate on similar frequencies and use the same infrastructure.
High network traffic can cause the allocated bandwidth to be consumed, which can prevent packet transmission and cause packet loss. This can happen because of insufficient capacity of IP/PSTN gateways, edge servers, home office network components, or other system bottlenecks.
Internet Protocol security (IPSec) key negotiations can increase the time required to set up a call, which can result in dropped packets if negotiations exceed network thresholds.
Policies that are applied by software or hardware firewalls, proxy servers, routers, and gateways can affect the VoIP traffic. Such policies can result in dropping of VoIP traffic or routing of network traffic to paths or queues that might be overloaded.
Hardware failures or configuration errors can cause packet loss. These failures can be hardware issues or half-duplex configuration settings that cause premature link saturation.
High packet delays or a sudden increase in delays can cause packets to be dropped by the receiving client’s audio processing component, even if they eventually arrive at the network layers.
Wi-Fi and wireless WANs often cause high jitter among network packets.
The quality of home broadband connections can vary. The use of queuing algorithms to control the bandwidth or even the design of the transmission medium can affect the amount of jitter on the network packets.
Jitter in received RTP packets can increase one-way audio delay at the receiver.
Satellite networks have high latency because of the distances that the signal must travel.
The distance between client computers can significantly increase one-way and round-trip latencies. The distance to be travelled includes the route that network traffic takes between two Office Communicator clients and between an Office Communicator client and the Mediation Server. High latency can be the result of significant geographic distances, of inefficient deployment of Office Communications Servers, or of the underlying network infrastructure.
Public Wi-Fi hotspots that are designed for Web browsing and e-mail may not have sufficient bandwidth for audio communication. The available bandwidth at a hotspot at any given time depends on many factors, including the bandwidth usage of all users.
Wireless WAN bandwidth can vary depending on the location of the user relative to the transmission tower.
WANs and home broadband networks are often shared by multiple users, which can saturate one or more links.