QoS

QoS refers to the ability of a network to provide better service to selected network traffic over various underlying technologies including Frame Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1 networks, SONET, and IP-routed networks.

QoS features provide better and more predictable network service by the following methods:

• Supporting dedicated bandwidth.
• Improving loss characteristics.
• Avoiding and managing network congestion.
• Shaping network traffic.
• Setting traffic priorities across the network.

 What are congestion, delay, and jitter?

 typical congestion points:

 Network congestion results in delay. A network and its devices introduce several kinds of delays.

Variation in delay is known as jitter, Both delay and jitter need to be controlled and minimized to support real-time and interactive traffic.
 What is the purpose of queueing
Queueing is designed to accommodate temporary congestion on a network device's interface by storing excess packets in buffers until bandwidth becomes available.

The default mechanism on most interfaces is First In First Out (FIFO). Some traffic types have more demanding delay/jitter requirements. Thus, one of the following alternative queueing mechanisms should be configured or is enabled by default:

• Weighted Fair Queueing (WFQ)
• Class-Based Weighted Fair Queueing (CBWFQ)
• Low Latency Queueing (LLQ), which is in fact CBWFQ with a Priority Queue (PQ) (known as PQCBWFQ)
• Priority Queueing (PQ)
• Custom Queueing (CQ)
  Queueing generally happens on outbound interfaces only. A router queues packets that are going out an interface. You can police inbound traffic, but usually you cannot queue inbound.
   difference between policing and shaping?
  
  
  
  

 Traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate. In contrast, traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate, excess traffic is dropped (or remarked).

 token bucket  vs Leak Bucket.

 

 

 With a traffic policer such as class-based policing, what do Committed Burst (BC) and Excess Burst (Be) mean and how should I select these values?
traffic policer does not buffer excess packets and transmit them later, as is the case for a shaper.he policer executes a simple send or do not send policy without buffering. During periods of congestion, since you cannot buffer, the best you can do is drop packets less aggressively by properly configuring extended burst. Therefore, it is important to understand the policer uses the normal burst and extended burst values to ensure the configured Committed Information Rate (CIR) is reached.

Burst Parameter	Purpose	Recommended Formula
normal burst	Implements a standard token bucket. Sets the maximum size of the token bucket (although tokens can be borrowed if Be is greater than BC). Determines how large the token bucket can be since newly arriving tokens are discarded and are not available to future packets if the bucket fills to capacity.	CIR [BPS] * (1 byte)/(8 bits) * 1.5 seconds Note: 1.5 seconds is the typical round trip time.
extended burst	Implements a token bucket with extended burst capability. Disabled by setting BC = Be. When BC is equal to Be, the traffic regulator cannot borrow tokens and simply drops the packet when insufficient tokens are available.	2 * normal burst