Quality of Service

Presented By:

S.M Raza Sajjad

Quality of Service

• QoS refer to both CoS( Class of Service) and ToS (Type of Service).

• The basic goal of CoS & ToS is to achieve the bandwidth and latency needed for a particular application.

• The CoS enables a network administrator to group different packet flows each having distinct latency and bandwidth requirement.

QoS

• VoIP comes with its own set of problems (Delay, Jitter).

• QoS can help to solve some of these problems like Packet Losses, Jitter and Handling delay.

• Some of the Problems can’t solve by QoS are Propagation Delay, digitization delay, Sampling delay, Codec Delay.

QoS Network Tool kit

• Compressed Real-Time Transport Protocol(CRTP).

• Queuing– Weighted Fair Queuing (WFQ)– Custom Queuing(CQ)– Priority Queuing (PQ)– Class-Based Weighted Fair Queuing(CB-WFQ)

• Packet Classification– IP Precedence– Policy Routing– Resource Reservation Protocol

• Fragmentation

CRTP

• To reduce the large percentage of bandwidth consumed by a G.729 voice call you can use cRTP.

• CRTP enables you to compress the 40-byte IP/RTP/UDP header to 2 to 4 bytes most of the time.

• With CRTP amount of traffic per VoIP call is reduced from 24kbps to 11.2 kbps.

• This is major improvement for low bandwidth links.

• For Example a 56 kbps link carry four G.729 VoIP Calls at 11.kbps each, without CRTP it can carry only two VoIP Calls.

CRTP

• Should not use CRTP on high speed interfaces.

• As with any compression, the CPU incurs extra processing duties to compress the packet.

• It should first analyzed that the advantage of lower bandwidth against higher CPU utilization.

Queuing

• Queuing in and of itself is a fairly simple concept.

• The easiest way to think about queue is to think about highway system.

• In Queue the concept of FIFO System is used.

• Today’s network with their variety of applications, protocols and users, require a way to classify different traffic.

Queuing

• Out of different Queuing techniques the most popular technique is WFQ.

• It is the default for any router interface less than 2mbps of bandwidth.

Weighted Fair Queuing

• WFQ on the other hand uses multiple queues to separate flows and gives equal amounts of bandwidth to each flow.

• WFQ ensures that queues do not starve for bandwidth and that traffic gets predictable service.

Weighted Fair Queuing

• WFQ is superior to TDM, simply because when a stream is no longer present, WFQ dynamically adjusts to use the free bandwidth for the flows that are still transmitting.

• The algorithm enables bandwidth to be shared fairly.

Custom Queuing

• Custom Queuing enables users to specify a percentage of available bandwidth to a particular protocol.

• Each queue is served sequentially in a round robin fashion.

• Transmitting a percentage of traffic on each queue before moving the next queue.

Priority Queuing

• PQ enables a NE to configure four Traffic Policies– High – Normal– Medium– Low

• Inbound traffic is assigned to one of the four output queues.

• Traffic in high priority queue is served until the queue is empty

• Then packets of next priority queue is transmitted.

CB-WFQ

• CB-WFQ has all the benefits of WFQ, with the additional functionality of providing support for network administrator-defined classes of traffic.

• Using CB-WFQ, you can create a specific class for voice traffic.

• Network Administrator define these classes through Access Lists.

Packet Classification

• To achieve intended packet delivery, you must know how to properly weight WFQ.

• Packet classification have the different weighting techniques and way you can use them in various networks to achieve the amount of QoS.– IP Precedence– Policy Routing– RSVP

IP Precedence

• IP Precedence refer to the three bits in the ToS field in the IP Header.

• These three bits are used for eight different CoS

• Enables a routing to group traffic flows based on the eight precedence settings.

• There is no extra signaling involved nor does additional packet header overhead exist.

• Due to these factors IP Precedence is the QoS mechanism that large scale networks used.

ToS (IP Precedence)Service Type Purpose

Routine [Lowest]

Set routine precedence (0)

Priority Set priority precedence (1)

Immediate Set immediate precedence (2)

Flash Set Flash precedence (3)

Flash-Override Set Flash-Override precedence (4)

Critical Set Critical precedence (5)

Internet* Set Internetwork control precedence (6)

Network* [Highest]

Set network control precedence (7)

IP precedence 6 & 7 are reserved for network information. This leaves 6 remaining precedence settings for normal IP traffic flows.

Policy Routing

• Policy based routing can configure a defined policy for traffic flows and not have to rely completely on routing protocols to determine traffic forwarding and routing.

• Policy routing also enables you to set the IP Precedence filed so that the network can utilize different classes of service.

• Policies can base on IP Address, Port Numbers, Protocols and the size of packets.

Policy Routing

• You can used one of these descriptors to create a simple policy , or can use all of them to create a complicated policy.

• All packets receive on an interface with policy based routing enabled are passed through enhanced packet filters known as route maps.

• The route maps decides where to forward the packets.

RSVP

• RSVP is an out of band, end to end signaling protocol that requests a certain amount of bandwidth and latency with each network hop that supports RSVP.

• If a network hop (Router) does not support RSVP , it moves onto the next hop.

• A network node has the option to approve or deny the reservation based upon the load of the interface to which the service is requested.

Traffic Policing

• Use to regulate or limit the amount of traffic an application is allowed to send across various interfaces or networks.

• CAR(Committed Access Rate) and traffic shaping tools are similar in that they both identify when traffic exceeds the thresholds set by the network administrator.

Traffic Policing

• Often, these two tools are used together .

• Traffic shaping is used at the edge of the network(customer premises) to make sure the customer is utilizing the bandwidth for business needs.

• CAR is often used in service networks to ensure that a subscriber does not exceed the amount of bandwidth set by contract with the service provider.

Fragmentation

• Large packets(1500-byte MTUs)take a long time to move across low-bandwidth links(768 kbps and less).

• Fragmentation breaks larger packets into smaller packets.

Link Fragmentation and Interleaving (LFI)

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VoicePacket

Jumbogram

64 kbps

1500 bytes 190ms

For links < 128kbps

Link Fragmentation and Interleaving (LFI)

Supported interfaces:• Multilink PPP• Frame Relay DLCI• ATM VC

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64 kbps

Conclusion

• QoS is not an exotic feature any more• QoS allows specific applications (VoIP,

VC) to share network infrastructure with best-effort traffic

• QoS in IP networks simplifies their functionality avoiding Frame Relay and ATM usage

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Questions???