- What is Quality of Service (QoS) ?
Analogy 1: QoS is a network tool which can be implemented to effectively transport more critical traffics over IP, which gives critical traffic a priority over less critical traffic.
Analogy 2: QoS is a method of giving a priority to some specific data traffic going across our network.
- Give VoIP, Video traffic more priority than ftp file downloading traffic
- Some critical Data such as Citrix etc.
- Converged Network Quality Issues
Today’s enterprise network Characteristics:
- Benign small voice packet flows compete directly with busty data packet flows.
- Voice load and voice application data (traffic) tolerate minimal variation in delay, packet loss or jitter. The voice quality degradation is immediately felt by the users.
- Give critical traffic higher priority
- Voice and video are real-time, hence time-sensitive
- Outage/packet drops are not acceptable
Some issues from Converged Network:
- Lack of bandwidth – If more traffic is pumped through the network more than the network can handle, there will be congestions and packet loss.
- Packet Loss – If input queue pumps too much packets into an interface, output queue fills up, the packet is dropped.
- Delay –
- Processing delay – The time it takes for a router to take the packet from an input interface, examine it and put it into the output queue of the output interface
- Queuing delay – The time a packet resides in the output queue of a router
- Serialization delay – The time it takes to place the “bits on the wire”
- Propagation delay – The time it takes for the packet to cross the link from one end to the other
- Jitter –
- Packets from the source will reach the destination with different delays
- Jitter is generally caused by congestion in the IP network
- The congestion can occur either at the router interfaces or in a provider or carrier network if the circuit has not been provisioned properly
To overcome these converged network quality issues, QoS tool(s) must be used based on each network.
Some Cisco recommended QoS tool types and their characteristics:
A. Best effort
– Out of box, if you do not configure your devices, it is using best effort
– Business network with no QoS policies
– Infrastructure does not support QoS
B. Integrated Services (IntServ)
– Aims to reserve bandwidth along a specific path in the network
– Guarantees end-to-end bandwidth for mission-critical applications such as VoIP and Citrix
– End-hosts signal their QoS requirements to the network (Signalled QoS model)
– Every communication stream needs to request resources from the network.
– Edge routers use Resource Reservation Protocol (RSVP) to signal and reserve bandwidth
Some disadvantages of IntServ:
– Every device along the network must be fully RSVP aware and have ability to process QoS
– Reservations in each devices along the path need to be periodically refreshed, adds traffic and overhead along the network
– “Soft-states” or bandwidth reservation increase memory and CPU requirements on devices along the path
– Adds complexity to the network which makes network infrastructure difficult to maintain
C. Differentiated Service (DiffServ)
– Designed to overcome the limitations of Best-Effort and IntServe model, while maintaining the ability to provide an almost guaranteed QoS
– Routers and switches are configured to service multiple classes of traffic with different priorities. Bandwidth, delay and prioritization are configured on a hop-to-hop basis along the network infrastructure, making diffServ cost-effective and scalable
– For DiffServ QoS to work, network traffic must be divided into classes that are based on the company’s requirements
– Network devices identify traffic as it passes through them and enforce the configured policies, making sure that each class/service is served as instructed