QoS for VoIP
QoS gives special treatment to certain traffic at the expense of others. Using
QoS in the network has several advantages:
■ Prioritizes access to resources, so that critical traffic can be served.
■ Allows good management of network resources.
■ Allows service to be tailored to network needs.
■ Allows mission-critical applications to share the network with other
data.
People sometimes think that there is no need for QoS strategies in a LAN.
However, switch ports can experience congestion because of port speed
mismatches, many people trying to access the switch backbone, and many
people trying to send traffic to the same switch port (such as a server port).
QoS Actions
Three QoS strategies are commonly implemented on interfaces where traffic
enters the switch:
■ Classification—Distinguishing one type of traffic from another. After
traffic is classified, other actions can be performed on it. Some classification
methods include access lists, ingress interface, and NBAR.
■ Marking—At layer 2, placing 802.1p class of service (CoS) value
within the 802.1Q tag. At layer 3, setting IP Precedence or
Differentiated Services Code Point (DSCP) values on the classified
traffic.
■ Policing—Determining whether or not a specific type of traffic is
within preset bandwidth levels. If so, it is usually allowed and might be
marked. If not, the traffic is typically marked or dropped. CAR and
class-based policing are examples of policing techniques.
Other QoS techniques are typically used on outbound interfaces:
■ Traffic shaping and conditioning—Attempts to send traffic out in a
steady stream at a specified rate. Buffers traffic that goes above that
rate and sends it when there is less traffic on the line.
■ Queuing—After traffic is classified and marked, one way it can be
given special treatment is to be put into different queues on the interface
to be sent out at different rates and times. Some examples include
priority queuing, weighted fair queuing, and custom queuing. The
default queuing method for a switch port is FIFO.
■ Dropping—Normally interface queues accept packets until they are
full and then drop everything after that. You can implement prioritized
dropping, so that less important packets are dropped before more
important ones—such as with Weighted Random Early Detection
(WRED).