II.3.4 LATENCY
In general, the period of time that one component in a system
is spinning its wheels waiting for another component. Latency, therefore, is
wasted time. For example, in accessing data on a disk, latency is defined as
the time it takes to position the proper sector under the read/write head.
In networking, the amount of time it takes a packet to travel
from source to destination. Together, latency and bandwidth define the speed
and capacity ofa network.
In VoIP terminology, latency refers to a delay in packet
delivery. VoIP latency is a service issue that is usually based on physical
distance, hops, or voice to data conversion. 15
13
http://en.wikipedia.org/wiki/Jitter_buffer#jitter_buffers,
September 24, 2006
14
http://en.wikipedia.org/wiki/Queueing_delay,
September 24,2006
15
http://serverwatch.webopedia.com/TERM/L/latency.html,January
24,2007
II.4 QUALITY of SERVICE II.4.1 INTRODUCTION
Quality of Service (QoS)
refers to the capability of a network to provide better service to
selected network traffic over various technologies, including Frame Relay,
Asynchronous Transfer Mode (ATM), Ethernet, Synchronous Optical Network
(SONET), and IP-routed networks that may use any or all ofthese underlying
technologies.
The primary goal of QoS is to provide priority including
dedicated bandwidth, controlled jitter and latency (required by some real-time
and interactive traffic), and improved loss characteristics.
Also important is making sure that providing priority for one
or more flows does not make other flows fail. QoS technologies provide the
elemental building blocks that will be used for future business applications in
campus, WAN and service provider networks.
II.4.2 QoS CONCEPTS
Fundamentally, QoS enabled to provide better service to
certain flows. This is done by either raising the priority of a flow or
limiting the priority of another flow. When using congestion- management tools,
it tryies to raise the priority of a flow by queuing and servicing queues in
different ways.
The queue management tool used for congestion avoidance
raises priority by dropping lowerpriority flows before higher-priority flows.
Policing and shaping provide priority to a flow by limiting the throughput of
other flows. Link efficiency tools limit large flows to show a preference for
small flows.
QoS tools can help alleviate most congestion problems.
However, many times there is just too much traffic for the bandwidth supplied.
In such cases, QoS is merely a bandage. A simple analogy comes from pouring
syrup into a bottle.
Syrup can be poured from one container into another container
at or below the size ofthe spout. If the amount poured is greater than the size
of the spout, syrup is wasted. However, it can use a funnel to catch syrup
pouring at a rate greater than the size of the spout. This allows to pour more
than what the spout can take, while still not wasting the syrup. However,
consistent over pouring will eventually fill and overflow the funnel.
| 
