Frequently Asked Questions (FAQ)

1:	What is congestion, delay, and jitter?
A1:	A network experiences congestion when it is presented with more traffic than it can handle. Network congestion points are strong candidates for quality of service (QoS) mechanisms. Figure 13-1 shows an example of typical congestion points: Figure 13-1. Network Congestion Points Network congestion causes delay, caused in turn by transmission propagation and networking hardware. Both network delay and jitter must be controlled and minimized to enable real-time, interactive traffic to be transported across the network. Voice and video traffic is more sensitive than data to jitter in a network connection. Minimal jitter in a data network is often not cause for concern because the receiving devices are still able to recognize the signal incoming from the network. Jitter can result in static or clipping of voice conversations or video streams, resulting in a less-than-clear signal.
2:	What is the difference between MPLS and DiffServ?
A2:	Differentiated services (DiffServ) and Multiprotocol Label Switching (MPLS) are two separate standards, each claiming to solve the issue of quality of service (QoS) in an IP network. DiffServ works at Layer 3 only and does not deal with lower layers. DiffServ uses the IP TOS (type of service) field, renaming it the DS byte, and uses this field to carry information about IP packet service requirements. MPLS specifies which Layer 3 traffic can be mapped to connection-oriented Layer 2 transports, such as ATM and Frame Relay. MPLS adds a label containing specific routing information to each IP packet and allows routers to assign explicit paths to various classes of traffic. MPLS also offers traffic engineering capabilities whereas DiffServ does not. DiffServ relies on traffic conditioners at the edge of the network, indicating each packet's requirements. MPLS requires capital investment in a network of label-switching routers (LSRs) that can read header information and assign packets to specific paths; for example, virtual circuits on a switched network.
3:	Why is queuing discussed with QoS/CoS?
A3:	Queuing accommodates temporary congestion on a network device's interface by storing excess packets in buffers until bandwidth becomes available. Queuing is used to assign packets to a particular class of service (CoS) prior to transmission. For example, traffic can be sorted based on destination IP address, with certain addresses having first claim to network bandwidth over others. Cisco IOS routers support several queuing methods meeting bandwidth, jitter, and delay requirements of different applications. These queuing algorithms are as follows: First In/First Out (FIFO) Stores packets when the network is congested and forwards these packets in order of arrival when the network is no longer congested (Cisco IOS default). Weighted Fair Queuing (WFQ) A flow-based queuing algorithm that does two things simultaneously: - Schedules interactive traffic to the front of the queue to reduce application response time - Shares the remaining bandwidth fairly between high bandwidth flows Class-Based Weighted Fair Queuing (CBWFQ) Extends standard WFQ functionality to provide support for user-defined traffic classes. For example, traffic classes are defined based on match criteria including protocols, access control lists (ACLs), and router input interfaces. Low Latency Queuing (LLQ), which is CBWFQ with a Priority Queue (PQ) (known as PQCBWFQ) Provides strict priority queuing for CBWFQ, reducing jitter in voice conversations. Priority Queuing (PQ) Gives strict priority to important traffic ensuring that important traffic gets the fastest handling at each point where PQ is used. PQ can prioritize according to network protocol (such as IP, IPX, or AppleTalk), incoming router interface, packet size, or source/destination address. Custom Queuing (CQ) Reserves a percentage of a router interface's available bandwidth for each selected traffic type. If a particular type of traffic is not using it's reserved bandwidth, other traffic types can use the remaining reserved bandwidth.
4:	What is Cisco doing regarding QoS/CoS implementations?
A4:	QoS/CoS implementations and improvements often are made and new techniques are discovered. Cisco System's QoS Technical Assistance Center (TAC) page, found at www.cisco.com/pcgi-bin/Support/PSP/psp_view.pl?p=Internetworking:QoS_General is a good source to use to keep up with QoS changes.