Quality of Service. Traditional Nonconverged Network. Traditional data traffic characteristics:

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1 Quality of Service 1 Traditional Nonconverged Network Traditional data traffic characteristics: Bursty data flow FIFO access Not overly time-sensitive; delays OK Brief outages are survivable 2 1

2 Converged Network Realities Converged network realities: Constant small-packet voice flow competes with bursty data flow. Critical traffic must have priority. Voice and video are time-sensitive. Brief outages are not acceptable. 3 Converged Network Quality Issues Lack of bandwidth: Multiple flows compete for a limited amount of bandwidth. End-to-end delay (fixed and variable): Packets have to traverse many network devices and links; this travel adds up to the overall delay. Variation of delay (jitter): Sometimes there is a lot of other traffic, which results in varied and increased delay. Packet loss: Packets may have to be dropped when a link is congested. 4 2

3 Campus Network Design Requirements for Deploying VoIP QoS Requirements for Voice Voice packets are small, typically between 60 bytes and 120 bytes in size. VoIP cannot tolerate drop or delay because it can lead to poor voice quality. VoIP uses UDP because TCP retransmit capabilities are useless for voice. For optimal voice quality, delay should be less than 150 ms one way. Acceptable packet loss is 1 percent. 5 Campus Network Design Requirements for Deploying VoIP Comparing Voice and Data Traffic 6 3

4 Purpose of Voice in the Campus Network More efficient use of bandwidth and equipment Lower costs for telephony network transmission Consolidation of voice and data network expense Increased revenue from new service Capability to leverage access to new communications devices Flexible pricing structure Emphasis on greater innovation in service 7 Purpose of Video Deployments in the Campus Network Collaboration: Video conferencing technologies such as TelePresence and the video support in WebEx support enhanced collaboration. Cost-savings: Video technologies reduce travel costs by enabling remote users to attend meetings, trainings, and so on without being physically present. 8 4

5 Unified Communications 9 Understanding QoS 10 5

6 What Is Quality of Service? Two Perspectives The user perspective Users perceive that their applications are performing properly Voice, video, and data The network manager perspective Need to manage bandwidth allocations to deliver the desired application performance Control delay, jitter, and packet loss 11 Three QoS Models Model Best effort Characteristics No QoS is applied to packets. If it is not important when or how packets arrive, the best-effort model is appropriate. Integrated Services (IntServ) Applications signal to the network that the applications require certain QoS parameters. Differentiated Services (DiffServ) The network recognizes classes that require QoS. 12 6

7 Implement the DiffServ QoS Model Introducing Queuing Implementations SWITCH v6 13 Cisco QoS Model Traffic classification and marking Traffic shaping and policing Congestion management Congestion avoidance 14 7

8 Classification Classification is the process of identifying and categorizing traffic into classes, typically based upon: Incoming interface IP precedence DSCP Source or destination address Application Without classification, all packets are treated the same. Classification should take place as close to the source as possible. 15 Marking Marking is the QoS feature component that colors a packet (frame) so it can be identified and distinguished from other packets (frames) in QoS treatment. Commonly used markers: Link layer: CoS (ISL, 802.1p) MPLS EXP bits Frame Relay Network layer: DSCP IP precedence 16 8

9 Classification and Marking in the LAN with IEEE 802.1Q IEEE 802.1p user priority field is also called CoS. IEEE 802.1p supports up to eight CoSs. IEEE 802.1p focuses on support for QoS over LANs and 802.1Q ports. IEEE 802.1p is preserved through the LAN, not end to end. 17 Classification Tools IP Precedence and DiffServ Code Points Version Length ToS Byte Len ID Offset TTL Proto FCS IP SA IP DA Data IPv4 Packet IP Precedence Unused DiffServ Code Point (DSCP) IP ECN Standard IPv4 DiffServ Extensions IPv4: three most significant bits of ToS byte are called IP Precedence (IPP) other bits unused DiffServ: six most significant bits of ToS byte are called DiffServ Code Point (DSCP) remaining two bits used for flow control DSCP is backward-compatible with IP precedence 18 9

10 The Impacts of Packet Loss Telephone call: I cannot understand you. Your voice is breaking up. Teleconferencing: The picture is very jerky. Voice is not synchronized. Publishing company: This file is corrupted. Call center: Please hold while my screen refreshes. 19 Types of Packet Drops Tail drops occur when the output queue is full. Tail drops are common and happen when a link is congested

11 Ways to Prevent Packet Loss Upgrade the link (the best solution but also the most expensive). Guarantee enough bandwidth for sensitive packets. Prevent congestion by randomly dropping less important packets before congestion occurs. 21 Congestion and Queuing Congestion can occur at any point in the network where there are points of speed mismatches or aggregation. Queuing manages congestion to provide bandwidth and delay guarantees

12 Speed Mismatch Speed mismatches are the most typical cause of congestion. Possibly persistent when going from LAN to WAN. Usually transient when going from LAN to LAN. 23 Aggregation 24 12

13 What is Queuing? Queuing is a congestion-management mechanism that allows you to control congestion on interfaces. Queuing is designed to accommodate temporary congestion on an interface of a network device by storing excess packets in buffers until bandwidth becomes available. 25 Queuing Algorithms First-in, first-out (FIFO) Priority queuing (PQ) Round robin Weighted round robin (WRR) 26 13

14 FIFO First packet in is first packet out Simplest of all One queue All individual queues are FIFO 27 Priority Queuing Uses multiple queues Allows prioritization Always empties first queue before going to the next queue: Empty queue number 1. If queue number 1 is empty, then dispatch one packet from queue number 2. If both queue number 1 and queue number 2 are empty, then dispatch one packet from queue number 3. Queues number 2 and number 3 may starve 28 14

15 Round Robin Queuing Uses multiple queues No prioritization Dispatches one packet from each queue in each round: One packet from queue number 1 One packet from queue number 2 One packet from queue number 3 Then repeat 29 Weighted Round Robin Queuing Allows prioritization Assign a weight to each queue Dispatches packets from each queue proportionately to an assigned weight: Dispatch up to four from queue number 1. Dispatch up to two from queue number 2. Dispatch 1 from queue number 3. Go back to queue number

16 Router Queuing Components Each physical interface has a hardware and a software queuing system. 31 Traffic Policing and Shaping Overview These mechanisms must classify packets before policing or shaping the traffic rate. Traffic policing typically drops or marks excess traffic to stay within a traffic rate limit. Traffic shaping queues excess packets to stay within the desired traffic rate

17 Policing Versus Shaping Incoming and outgoing directions. Out-of-profile packets are dropped. Dropping causes TCP retransmits. Policing supports packet marking or re-marking. Outgoing direction only. Out-of-profile packets are queued until a buffer gets full. Buffering minimizes TCP retransmits. Marking or re-marking not supported. Shaping supports interaction with Frame Relay congestion indication. 33 Congestion Avoidance Congestion-avoidance techniques monitor network traffic loads in an effort to anticipate and avoid congestion at common network bottleneck points. The two congestion avoidance algorithms used by Cisco switches are: Tail Drop this is the default algorithm Weighted Random Early Detection (WRED) 34 17