Packetization is the time it takes to segment information, sample and encode any signals, process the traffic, and then encapsulate the data in packets. Serialization is the time it takes to encapsulate a packet in a frame and put the bits of a frame on a wire. Propagation is the time it takes to transmit the bits of a frame across a wire to the next networking device. Processing is the time it takes for a networking device to receive a frame, place the frame in the input queue, take the frame from the input queue, and place it in the output queue of the outbound interface. Queuing is the time a packet stays in the output queue before being forwarded out the interface to the next device.
When dealing with VoIP, packet loss should be less than 1%, one-way delay should be less than 60ms, and jitter should be less than 20ms to provide a good-quality voice connection.
802.1Q/P are used to mark Layer 2 frames with CoS information. The IP TOS field is used to carry QoS information in IP packets. This can be accomplished by using IP precedence or DiffServ.
LLQ uses a combination of PQ and CB-WFQ. The PQ has the highest priority and is processed first. All other traffic is processed using CB-WFQ.
WRRQ is the default queuing method on egress interfaces for Layer 3 Catalyst switches.
WRED is used to avoid congestion. It does this by examining CoS information and begins dropping packets when traffic for a specified CoS reaches its configured threshold. This is done to reduce the likelihood that upcoming congestion will cause problems with important applications or data.
Classification of traffic is done with the class-map commands. The policy-map commands associate your QoS parameters to your traffic classes. The service-policy command activates your QoS policies on an interface. To enable QoS, use the mls qos command.
WFQ is the default queuing method on serial interfaces of routers with speeds of E1 or less. WFQ works by breaking up traffic into high- and low-priority conversations. The default congestive discard threshold is 64.
PQ has four queues: high, medium, normal, and low. The high queue is guaranteed to be serviced. Use the priority-list commands to specify what traffic goes into what queue and activate the PQ list with the priority-group command. Use the debug priority command to troubleshoot PQ.
CQ has 16 queues that are processed in a round-robin fashion. Use the queue-list command to configure CQ. You can give preference to a queue by specifying the amount of traffic a queue can process with the byte-count parameter. Use the custom-queue-list command to activate CQ.
