Information Technology Reference
In-Depth Information
Understanding the Enemy
Before you can deploy QoS successfully, you need to know what you are fighting against.
The following are the three enemies of your VoIP traffic:
Lack of bandwidth:
Multiple streams of voice and data traffic competing for a lim-
ited amount of bandwidth.
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Key
To p i c
Delay:
The time it takes a packet to move from the original starting point to the final
destination. Delay comes in three forms:
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Fixed delay:
Delay values that you cannot change. For example, it takes a certain
amount of time for a packet to travel specific geographical distances. This value is
considered fixed. QoS cannot impact fixed delay issues.
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Variable delay:
Delay values that you can change. For example, queuing delay
(how long a packet waits in a router's interface queue) is variable because it
depends on how many packets are currently in the queue. You can impact queu-
ing delay by selectively moving voice packets ahead of data packets.
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Jitter (delay variations):
Describes packets that have different amounts of delay
between them. For example, the first voice packet of a conversation might take
100 ms to reach a destination, whereas the second voice packet might take 110
ms. There is 10 ms of delay variation (jitter) between these packets.
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Packet loss:
Packets lost because of a congested or unreliable network connection.
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These enemies plague every network environment; however, the stakes are much higher
when you add VoIP traffic to an existing data network. Users are accustomed to a PBX-
style environment that has a separate network and dedicated bandwidth assigned just for
voice traffic. The tolerance for crackling, echoing, or dropped calls from a voice network is
very low.
QoS is designed to keep voice traffic running smoothly during temporary moments of
congestion on the network. It is not a “magic bullet” that can solve any network scenario.
For example, if there is a network environment in which the WAN link is constantly lack-
ing bandwidth, adding voice to the link and expecting QoS to take care of the situation is
similar to rearranging the deck chairs on the sinking Titanic. QoS can only do so much; ei-
ther your data applications will perform so slowly they are no longer functional or your
voice traffic will experience quality issues. This also goes the other way; if you have a net-
work environment where fiber-optic cable is the norm and gigabit speeds abound, you
might never experience network congestion. These environments will get little to no gain
by using QoS because most QoS tools only engage during times of network congestion.
Your goal with QoS is to provide consistent bandwidth to voice traffic in such a way that
there is low, steady delay from one end of the network to the other. To accomplish this,
you need to have QoS in some form at each point of the network where congestion exists.
This means doing an end-to-end audit of your network to determine the traffic types that
exist and the service levels required for those traffic types.
