Especially for small to medium-size businesses (SMBs), planning a network to be VoIP-compatible may seem like a daunting task. In order to help de-mystify the process, FacetCorp wrote a whitepaper titled “Planning Networks for VoIP.” The whitepaper is available in its entirety for download here, and is summarized below.
Voice Quality Requirements
To start off, FacetCorp (News - Alert) notes that “Users have certain quality expectations for telephone communication.” In most cases, this means the delivery of “toll-quality voice,” or high-quality signal comparable to that achieved using traditional telephone networks.
Achieving this level of quality on a VoIP network means measuring call quality using metrics such as the Mean Opinion Score (MOS) system, and making necessary corrections anywhere the performance is sub-par.
“MOS score is an indication of what users would think about the call,” FacetCorp explains in the whitepaper. “It was developed using surveys of users of different technologies, but today it is calculated through the use of engineering formulae.”
The MOS scale is shown below.
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MOS
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Listening Quality
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Listening Effort
|
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5
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Excellent
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Complete Relaxation
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4
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Good
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Attention Necessary
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3
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Fair
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Moderate Effort
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2
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Poor
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Considerable Effort
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1
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Bad
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No Meaning Understood
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When applying the MOS score to call quality of a VoIP network, the goal is for the score to never drop below 3, and to be above 3.7 in most cases.
Codecs
VoIP devices (e.g. telephones, gateways, PBXs, key systems, adapter cards) make it possible to send voice traffic over an IP network by encoding the voice into packet format at the sending end, and then decoding it back into voice on the receiving end. This coding-decoding process is called codec for short.
“There are different codecs available to use for connecting voice calls from one VoIP endpoint to another,” FacetCorp said in the whitepaper.
The differences between these codecs lie primarily with call quality. Codecs that use less bandwidth result in lower call quality (and a worse MOS score).
The codec that results in highest call quality is G.711. This codec addresses one of the reasons why call quality degrades, namely when it is encoded and decoded several times between the sending and receiving ends.
“Since it has one-to-one mapping with the TDM PCM in the phone network, there is no loss of quality due to encoding and decoding,” FacetCorp explains.
Network Layout
Typically, companies choose a network design that includes high-speed LAN switches and routers at each location, with lower-speed connections between sites.
“Often, each office has a single connection to an ISP and that ISP ha s a high-speed backbone network that connects each of its locations to one another,” FacetCorp said in the whitepaper.
The image below shows a typical layout for a company with three locations. Each site has a WAN router that connects to the ISP through a lower-speed link. At the Dayton and Cleveland offices, the link is a T1, and at the Akron office it is a DSL line.
“The most important thing to note about this network is that the customer has installed a
QoS capable router at each location,” FacetCorp notes.
There are two main requirements to keep in mind when selecting a QoS (quality of service) router:
Planning Capacity
Determining how much bandwidth you will need to carry voice and data traffic is a five-step process. Each step is outlined here.
a. Estimating Voice Bandwidth
Voice traffic and its associated call control packets are marked and handled separately from data traffic. To calculate the bandwidth needed, multiply the number of calls with the amount of bandwidth per call.
To be on the safe side, any calculation should be increased by ten percent to cover call control traffic that also will be marked for Low Latency Queuing.
b. Estimating Data Bandwidth
According to FacetCorp, the topic of data bandwidth for WAN links requires its own discussion, since it involves understanding the number of users at each location, relation of applications to users, bandwidth needed for applications, and other factors.
“If the routers are configured properly, then data will be limited so that it does not take
bandwidth away from voice,” FacetCorp said. “However, when all of the voice lines are in use, there is some bandwidth that is required to maintain your data applications.”
c. Under Subscription
Typically, when engineers plan a data network, they incorporate a self-imposed limit of 75 percent of the bandwidth specified by the provider—this allows extra room to accommodate data usage, which tends to come in bursts (e.g. users download files at random or unpredictable times).
“If the network is primarily used for data, then we add 30 percent to the estimated total
bandwidth usage to account for this phenomenon,” FacetCorp notes.
d. Calculating Total Required Bandwidth
When configuring routers for a VoIP network, voice traffic and call control traffic should be classified as Low Latency for LLQ treatment. This ensures that voice packets always take priority over other packets.
FacetCorp advises that you should assign no more than a third of your WAN bandwidth to LLQ.
“For most installations, you can calculate the total voice bandwidth and multiply by three
to find your total required link bandwidth,” FacetCorp says. “This assumes that your required data bandwidth is less than twice the necessary voice bandwidth.”
e. Compromises
It may be that your company can’t afford optimal bandwidth configurations. If this is true, you must sacrifice some voice quality.
The company warns, however, that “you should never try to force more VoIP voice traffic through the link without LLQ protection. This may work in some limited tests, but it is likely to fail unpredictably on the busiest day of the year.”
Router Selection and Configuration
a. Router Requirements
When selecting and configuring routers to connect your VoIP equipment with the WAN, you should first and foremost keep voice quality in mind. This means choosing routers that support priority queuing for voice packets. Any router or switch that lies on the path between two VoIP devices should include this feature.
“Look for switches that have ‘Quality of Service’ features in their description, and routers that support ‘Low Latency Queuing’,” FacetCorp advises. “Replace any Ethernet hubs in the path that voice traffic uses with switches.”
b. Router Configuration
Among the factors involved with configuring routers are:
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Marking all voice and call control packets as Low Latency
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Setting the Low Latency class to go into the Low Latency Queue
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Avoiding the use of router features such as Weighted Random Early detection for voice packets
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Determining a method of fragmenting large data packets
Because configuring routers is a complex process, FacetCorp recommends hiring an an engineer who educated in VoIP Quality of Service (QoS) issues.
c. Traffic Shaping Appliances
Your company may choose to install a traffic shaping device—located between the LAN switches and the WAN router—instead of configuring a sophisticated router. This type of device queues and reorders packets based on the LLQ strategy, and may take some of the burden off the WAN router while reducing latency.
Summary
When you get right down to it, planning a successful VoIP network means building one that supports voice conversations considered by users to be toll-quality. Of course, you also need to ensure that data traffic is delivered in an acceptably timely manner.
“Designing a converged voice-data network requires you to meet both of these objectives,” FacetCorp notes.
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To learn more about planning VoIP networks, visit INTERNET TELEPHONY Conference & Expo, WEST, October 10-13, 2006 in San Diego.
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Mae Kowalke previously wrote for Cleveland Magazine in Ohio and The Burlington Free Press in Vermont. To see more of her articles, please visit Mae Kowalke’s columnist page.
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