Foundation Summary

The "Foundation Summary" is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review will help solidify some key facts. For any of you doing your final prep before the exam, these tables and figures will be a convenient way to review the day before the exam.

Table 1-29 outlines some of the behaviors seen when no QoS is applied in a network.

Table 1-29 Traffic Behavior with No QoS

Table 1-29 outlines some of the behaviors seen when no QoS is applied in a network.

Table 1-29 Traffic Behavior with No QoS

Type of Traffic

Behavior Without QoS

Voice

Voice is hard to understand.

Voice breaks up, sounds choppy.

Delays make interacting difficult; callers do not know when other party has finished talking.

Calls are disconnected.

Video

Picture displays erratically; jerky movements.

Audio not in sync with video.

Movement slows down.

Data

Data arrives after it is no longer useful.

Customer waiting for customer care agent, who waits for a screen to display.

Erratic response times frustrate users, who may give up or try later.

As shown in Figure 1-36, with compression, if a ratio of 2:1 is achieved, the 80-kbps flow will only require 40 kbps in order to be sent across the link—effectively doubling the bandwidth capacity of the link.

Figure 1-36 With a 2:1 Compression Ratio Versus No Compression

Router 1

Offered Load:

80 kbps

<Compres:

FIFO Queue

FIFO Queue

<Compres:

Queue 64 kbps

40 kbps Sent

Router 1

FIFO Queue

Offered Load:

80 kbps

Queue 64 kbps

Larger Queue Due to Congestion

64 kbps Sent, Rest Queued

Figure 1-37 shows a two-queue system where the first queue gets 25 percent of the bandwidth on the link, and the second queue gets 75 percent of the bandwidth.

Figure 1-37 Bandwidth Reservation Using Queuing

Output Queue 1

4 X 1500 Byte Packets

Bandwidth

Output Queue 2

Bandwidth

The tools summarized in Table 1-30 help to improve the effects of bandwidth in a network.

Table 1-30 QoS Tools That Affect Bandwidth

Type of QoS Tool

How It Affects Bandwidth

Compression

Compresses either payload or headers, reducing overall number of bits required to transmit the data

CAC

Reduces overall load introduced into the network by rejecting new voice and video calls

Queuing

Can be used to reserve minimum amounts of bandwidth for particular types of packets

Figure 1-38 shows two contrasting examples of serialization and propagation delay.

Figure 1-38 Serialization and Propagation Delay for Selected Packet and Link Lengths

Server 1

Figure 1-38 Serialization and Propagation Delay for Selected Packet and Link Lengths

Server 1

Figure 1-39 lists the queuing, serialization, and propagation delays experienced by data, voice, and video traffic.

Figure 1-39 Delay Components: Three Components, Single Router (R1)

4 X 1500 Byte Packets

FIFQ Qutput Queue

Serialization Delay: 214 ms

1001101110101011

Propagation Delay: 4.8 ms

4th Packet 642 ms Delay

Figure 1-40 depicts LFI operation.

Figure 1-40 Link Fragmentation and Interleaving

Packet 1: 1500 Bytes, Arrives

Packet 2: 200 Bytes, Delay Sensitive, Arrives Second

Legend: Px Fy Means Packet Number x, Fragment Number y

Output Queue 1: 3 Fragments of Packet #1 Shown

P1

P1

P1

F3

F2

Output Queue 2

Output Queue 1

P1

P1

2

P1

F3

F2

F1

Although adding more bandwidth always helps, the tools summarized in Table 1-31 do help to improve the effects of delay in a network.

Table 1-31 QoS Tools That Affect Delay

Type of QoS Tool

How It Affects Delay

Queuing

Enables you to order packets so that delay-sensitive packets leave their queues more quickly than delay-insensitive packets.

Link fragmentation and interleaving

Because routers do not preempt a packet that is currently being transmitted, LFI breaks larger packets into smaller fragments before sending them. Smaller delay-sensitive packets can be sent after a single smaller fragment, instead of having to wait for the larger original packet to be serialized.

Compression

Compresses either payload or headers, reducing overall number of bits required to transmit the data. By requiring less bandwidth, queues shrink, which reduces delay. Also serialization delays shrink, because fewer bits are required. Compression also adds some processing delay.

Traffic shaping

Artificially increases delay to reduce drops inside a Frame Relay or ATM network.

Figure 1-41 shows the jitter experienced by three packets as part of a voice call between phones at extension 301 and 201.

Figure 1-41 Jitter Example

Server 1

Figure 1-41 Jitter Example

201

"1

RTP RTP RTP

"1

301

RTP

RTP

Table 1-32

The same set of tools that affect delay also affect jitter; Table 1-32 lists some of these QoS tools.

QoS Tools That Affect Jitter

Type of QoS Tool

How It Affects Jitter

Queuing

Enables you to order packets so that delay-sensitive packets leave their queues more quickly than delay-insensitive packets.

Link fragmentation and interleaving

Because routers do not preempt a packet that is currently being transmitted, LFI breaks larger packets into smaller fragments before sending them Smaller delay-sensitive packets can be sent after a single smaller fragmenl instead of having to wait for the larger original packet to be serialized.

Compression

Compresses either payload or headers, reducing overall number of bits required to transmit the data. By requiring less bandwidth, queues shrink which reduces delay. Also serialization delays shrink, because fewer bits are required. Compression also adds some processing delay.

Traffic shaping

Artificially increases delay to reduce drops inside a Frame Relay or ATM

30 20

With a longer maximum queue size, likelihood of loss decreases. However, queuing delay increases, as shown in Figure 1-42.

Figure 1-42 Queuing Effects on Packet Loss

Tail Drop

Tail Drop

Queue1—Size 50— Less Loss, More Latency

Queue1—Size 50— Less Loss, More Latency

Table 1-33 summarizes the points concerning the two types of QoS tools for affecting loss.

Table 1-33 QoS Tools That Affect Loss

Table 1-33 summarizes the points concerning the two types of QoS tools for affecting loss.

Table 1-33 QoS Tools That Affect Loss

Type of

QoS Tool

Brief Description

Queuing

Implementing longer queues increases delay, but avoids loss.

RED

Implementing RED drops packets randomly as queues approach the point of being full, slowing some TCP connections. This reduces overall load, shortening the congested queue, while affecting only some users' response times.

Figure 1-43 outlines the format of an IP packet using RTP.

Figure 1-43 IP Packet for Voice Call—RTP

20 Bytes 8 Bytes 12 Bytes Variable

IP

UDP

RTP

Voice Payload

1

Port Ranges: Popular Values:

16384 - 32767 G.711: 160 Bytes

(Even Ports) G729a: 20 Bytes

Port Ranges: Popular Values:

16384 - 32767 G.711: 160 Bytes

(Even Ports) G729a: 20 Bytes

Table 1-34 contrasts the QoS requirements of voice payload and signaling flows.

Table 1-34 Comparing Voice Payload to Voice Signaling: QoS Requirements

Table 1-34 contrasts the QoS requirements of voice payload and signaling flows.

Table 1-34 Comparing Voice Payload to Voice Signaling: QoS Requirements

Bandwidth

Delay

Jitter

Loss

Voice Payload

Low

Low

Low

Low

Video Signaling

Low

Low

Medium

Medium

Table 1-35 lists the bandwidth requirements for various types of voice calls, as listed in the DQOS course.

Table 1-35 Bandwidth Requirements with Various Data-Link Types

Table 1-35 lists the bandwidth requirements for various types of voice calls, as listed in the DQOS course.

Table 1-35 Bandwidth Requirements with Various Data-Link Types

L2 Header Type

Header Size

IP/UDP/RTP Header Size

Codec

Bandwidth

Ethernet

14

40 bytes

G.711

64 kbps

85.6

MLPPP/FR

6

40 bytes

G.711

64 kbps

82.4

Ethernet

14

40 bytes

G.729

8 kbps

29.6

MLPPP/FR

6

40 bytes

G.729

8 kbps

26.4

The delay components that affect all types of traffic are listed in Table 1-36.

Table 1-36 Components of Delay Not Specific to One Type of Traffic

Delay Component

Definition

Where It Occurs

Serialization delay

Time taken to place all bits of a frame onto the physical medium. Function of frame size and physical link speed.

Outbound on every physical interface; typically negligible on T3 and faster links.

Propagation delay

Time taken for a single bit to traverse the physical medium from one end to the other. Based on the speed of light over that medium, and the length of the link.

Every physical link. Typically negligible on LAN links and shorter WAN links.

Queuing delay

Time spent in a queue awaiting the opportunity to be forwarded (output queuing), or awaiting a chance to cross the switch fabric (input queuing).

Possible on every output interface. Input queuing unlikely in routers, more likely in LAN switches.

Forwarding or Processing Delay

Time required from receipt of the incoming frame, until the frame/ packet has been enqueued for transmission.

On every piece of switching equipment, including routers, LAN switches, Frame Relay switches, and ATM switches.

continues continues

Table 1-36 Components of Delay Not Specific to One Type of Traffic (Continued)

Delay Component

Definition

Where It Occurs

Shaping delay

Shaping (if configured) delays transmission of packets to avoid packet loss in the middle of a Frame Relay or ATM network.

Anywhere that shaping is configured, which is most likely on a router, when sending packets to a Frame Relay or ATM network.

Network delay

Delays created by the components of the carrier's network when using a service. For instance, the delay of a Frame Relay frame as it traverses the Frame Relay network.

Inside the service provider's network.

Figure 1-44 shows an example of delay concepts, with sample delay values shown. When the delay is negligible, the delay is just listed as zero.

Figure 1-44 Example Network with Various Delay Components shown: Left-to-Right Direction ->•

Delays for Packets Flowing Left-to-Right: Total Delay: 94 ms

Hannah

Forwarding: 0 Queuing: 0 Serialization: 0

Forwarding: 0 Queuing: 15 Serialization: 4 Propagation: .5

Forwarding: 0 Queuing: 0 Serialization: 0 Propagation: 0

Server 1

Forwarding: 0

Queuing: 15 Serialization: 9 Propagation: .5

Forwarding: 0 Queuing: 0 Serialization: 0 Propagation: 0

Server 1

Network: 50

(Note: Do Not Count R2 Serialization Here and at R2!)

Network: 50

(Note: Do Not Count R2 Serialization Here and at R2!)

Table 1-37 outlines the suggested delay budgets.

Table 1-37 One-Way Delay Budget Guidelines

1-Way Delay (in ms)

Description

0-150

ITU G.114 recommended acceptable range

0-200

Cisco's recommended acceptable range

150-400

ITU G.114's recommended range for degraded service

400+

ITU G.114's range of unacceptable delay in all cases

All the delay components for a voice call are summarized in the example in Figure 1-45.

Figure 1-45 Complete End-to-End Voice Delay Example

Hannah

Delays for Packets Flowing Left-to-Right: Total Delay: 164 ms

Forwarding: 0 Queuing: 0 Serialization: 0

Codec: 10-

Packetization: 20

Forwarding: 0 Queuing: 15 Serialization: 4 Propagation: .5

s0 s0

Forwarding: 0 Queuing: 15 Serialization: 9 Propagation: .5

Forwarding: 0 De-jitter: 40 ms

Queuing: 0 I

Serialization: 0 I Server 1 Propagation: 0

Forwarding: 0 De-jitter: 40 ms

Queuing: 0 I

Serialization: 0 I Server 1 Propagation: 0

s0 s0

Network: 50 (Note: Do Not Count R2 Serialization Here and at R2!)

De-jitter: 40 ms

Network: 50 (Note: Do Not Count R2 Serialization Here and at R2!)

De-jitter: 40 ms

Table 1-38 lists the different delay components and whether they are variable.

Table 1-38 Delay Components, Variable and Fixed

Table 1-38 lists the different delay components and whether they are variable.

Table 1-38 Delay Components, Variable and Fixed

Delay

Component

Fixed or Variable

Comments

QoS Tools That Can Help

Codec

Fixed

Varies slightly based on codec and processing load; considered fixed in course books (and probably on exams). Typically around 10 ms.

None.

Table 1-38 Delay Components, Variable and Fixed (Continued)

Delay

Component

Fixed or Variable

Comments

QoS Tools That Can Help

Packetization

Fixed

Some codecs require a 30-ms payload, but packetization delay does not vary for a single codec. Typically 20 ms, including when using G.711 and G.729.

None.

Propagation

Variable

Varies based on length of circuit. About 5 ms/ 100 km.

Move your facilities to the same town.

Queuing

Variable

This is the most controllable delay component for packet voice.

Queuing features, particularly those with a priority-queuing feature.

Serialization

Fixed

It is fixed for voice packets, because all voice packets are of equal length. It is variable based on packet size for all packets.

Fragmentation and compression.

Network

Variable

Least controllable variable component.

Shaping, fragmentation, designs mindful of reducing delay.

De-jitter buffer (initial playout delay)

Variable

This component is variable because it can be configured for a different value. However, that value, once configured, remains fixed for all calls until another value is configured. In other words, the initial playout delay does not dynamically vary.

Configurable playout delay in IOS gateways; not configurable in IP Phones.

Figure 1-46 shows an example of jitter for packets 3 and 4.

Figure 1-46 De-Jitter Buffer Underrun Due to Jitter

T=X - Instant That First Packet Has Been Received

De-Jitter Buffer

De-Jitter Buffer

T=X - Instant That First Packet Has Been Received

20 ms Voice Payload - Packet 1

No Playout of Voice ->

No Playout of Voice ->

T=X+20 - Instant That Second Packet Has Been Received, NO JITTER

T=X+40 - 3rd Packet Not Received Yet

De-Jitter Buffer

20 ms Voice Payload

20 ms Voice Payload

- Packet 2

- Packet 1

De-Jitter Buffer

20 ms Voice Payload - Packet 2

T=X+60 - 3rd Packet Received; Had +20 Jitter

De-Jitter Buffer

De-Jitter Buffer

T=X+60 - 3rd Packet Received; Had +20 Jitter

20 ms Voice Payload - Packet 3

T=X+80 - 4th Packet Still Has Not Arrived

T=X+100 - 4th Packet Still Has Not Arrived; More Than 20 ms Jitter

De-Jitter Buffer

De-Jitter Buffer

Nothing Left to Play out; Listener Hears Nothing w

Playing out Packet 3; De-Jitter Buffer Empty_

Nothing Left to Play out; Listener Hears Nothing w

Table 1-39 summarizes some of the key bandwidth differences between voice and video traffic.

Table 1-39 Voice and Video Bandwidth Contrasted

Feature

Voice

Video

Number of flows in each direction

1

2 (1 audio, 1 video)

Packet sizes

Static, based on codec

Variable

Packet rate

Constant (isochronous)

Variable

Table 1-40 summarizes some of the key bandwidth differences between all three types of traffic.

Table 1-40 Voice, Video, and Data Bandwidth Contrasted

Feature

Voice

2-Way Video

Data

Number of flows

2 (1 in each direction)

4 (1 audio and 1 video in each direction)

1 bidirectional flow

Packet sizes

Fixed, based on codec

Variable

Varies greatly

Packet rate

Constant (isochronous)

Variable

Varies greatly

Two factors affect the delay requirements of a data application. Table 1-41 lists these requirements.

Table 1-41 Factors to Consider for Data Delay

Factor

Mission Critical

Not Mission Critical

Interactive

Should get the lowest delay of all data applications. Most shops strive for 1-2-second application response time—per-packet delay must be shorter.

Applications could benefit from lower delay. Also differentiating between mission critical and not mission critical can be difficult.

Not interactive

Although mission critical, noninteractive applications typically need particular bandwidth requirements met, delay can vary greatly as long as bandwidth is supplied.

Best candidate for getting any leftover bandwidth, with all other voice, video, and data applications getting better QoS treatment.

Table 1-42 summarizes the QoS requirements of data, in comparison to voice and video.

Table 1-42 Comparing Voice, Video, and Data QoS Requirements

Table 1-42 summarizes the QoS requirements of data, in comparison to voice and video.

Table 1-42 Comparing Voice, Video, and Data QoS Requirements

Bandwidth

Delay

Jitter

Loss

Voice Payload

Low

Low

Low

Interactive

(2-Way)

High

Low

Low

Streaming

(1-Way)

High

High

High

Low

Video Signaling

Low

Low

Medium

Medium

Voice Signaling

Low

Low

Medium

Interactive, Mission Critical

Variable, typical medium

Medium

Medium

Medium

Data: Not Interactive, Mission Critical

Variable, typically high

High

High

Interactive, Not Critical

Variable, typical medium

High

High

Medium

Data: Not Interactive, Not Critical

Variable, typically high

High

High

High

Advance SEO Techniques

Advance SEO Techniques

Turbocharge Your Traffic And Profits On Auto-Pilot. Would you like to watch visitors flood into your websites by the 1,000s, without expensive advertising or promotions? The fact is, there ARE people with websites doing exactly that right now. How is that possible, you ask? The answer is Advanced SEO Techniques.

Get My Free Ebook


Post a comment