Networking Concepts Quiz

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Questions and Answers

What is the time it takes for the packet to cross the link from one end to the other called?

Network delay

Serialization delay

Transmission delay

Propagation delay (correct)

Which method is considered the best solution for reducing delay in a network?

Reprioritize all packets equally

Forward less important packets first

Compress IP packet headers

Upgrade the link (correct)

What technique can customer routers use to help reduce packet delays?

TCP/RTP header compression (correct)

Random packet loss

Increasing latency

Routing loops

Which of the following is NOT a cause of packet drops?

Link upgrades Signup and view all the answers

How can an organization prevent packet loss in their network?

Upgrade the link Signup and view all the answers

What is a common issue faced during a telephone call due to packet loss?

Voice breaking up Signup and view all the answers

What does LLQ stand for, which is performed by customer routers?

Low Latency Queueing Signup and view all the answers

What is one way ISPs can manage packets effectively?

Reprioritize according to QoS policy Signup and view all the answers

What are the three major elements of a traffic class?

A case-sensitive name, a series of match commands, and an evaluation instruction Signup and view all the answers

What configuration mode should be entered to define a class map?

class-map configuration mode Signup and view all the answers

Which mode is the default for class maps when matching commands?

match all Signup and view all the answers

How many traffic classes can be associated with a single policy map?

Up to 256 Signup and view all the answers

What is the primary function of Custom Queuing?

Uses round-robin scheduling to manage queues. Signup and view all the answers

Which command is used to add a match criterion in the class-map?

match access-group Signup and view all the answers

What benefit does Weighted Fair Queuing (WFQ) provide?

Predicts transfer rates and interarrival periods. Signup and view all the answers

In a policy map, what does the QoS policy influence?

The QoS features associated with a traffic class Signup and view all the answers

Which of the following commands is used to permit or deny packets in a standard ACL?

access-list access-list-number {permit | deny | remark} source [mask] Signup and view all the answers

What happens in Weighted Random Early Detection (WRED) as congestion increases?

Packets are selectively and randomly dropped. Signup and view all the answers

What is the first step in implementing QoS?

Identify types of traffic and their requirements. Signup and view all the answers

What is one of the purposes of descriptions in class maps?

They assist in understanding large configurations Signup and view all the answers

During traffic classification, which of the following is assessed?

The relevance of each traffic type to business. Signup and view all the answers

What defines a QoS policy?

A network-wide definition of QoS levels for traffic classes. Signup and view all the answers

What characterizes the Best Effort model in QoS?

No QoS is implemented for packets. Signup and view all the answers

Which process is NOT part of Quality of Service operations?

Traffic encryption and security. Signup and view all the answers

What is the first step in converting an analog signal to a digital signal?

Sample the analog signal. Signup and view all the answers

Which codec has the lowest bit rate according to the characteristics listed?

G.729 Signup and view all the answers

What does jitter in voice packets indicate?

Voice packets arrive at varying rates. Signup and view all the answers

What is the primary function of voice gateways in VoIP networks?

Convert between analog and digital signals. Signup and view all the answers

In a mixed-mode conference, what is a requirement for the codecs used?

Different codecs can be used among participants. Signup and view all the answers

Which step involves decoding samples into voltage amplitudes to rebuild the PAM signal?

Decode the samples. Signup and view all the answers

What characterizes the behavior of voice packets in VoIP networks?

Packets can arrive in the wrong order. Signup and view all the answers

What is a disadvantage of using mixed mode for DSP in conferencing?

Fewer conferences can be supported per DSP. Signup and view all the answers

What command is used to enter the per-class policy configuration mode?

policy-map policy-map-name Signup and view all the answers

Which command can be used to define a new class map with a specific condition?

class class-name condition Signup and view all the answers

How can a service policy be attached to an interface in a router's configuration?

service-policy {input | output} policy-map-name Signup and view all the answers

In the example configuration, what is the bandwidth allocated to business-critical traffic?

1000 Signup and view all the answers

Which of the following best describes the match strategy used when defining a class map?

match-any strategy Signup and view all the answers

What is the purpose of the class-default in policy-map configuration?

To handle traffic that does not match any class Signup and view all the answers

What bandwidth is allocated to class-default traffic in the sample service policy?

6000 Signup and view all the answers

Which type of traffic is likely prioritized using QoS policies?

Voice traffic Signup and view all the answers

What is one of the main benefits of the IntServ model?

Explicit resource admission control Signup and view all the answers

Which aspect is considered a drawback of the IntServ model?

Continuous signaling requirements Signup and view all the answers

How does the Differentiated Services Model mainly classify traffic?

Through aggregates or classes Signup and view all the answers

Which method of implementing QoS requires individual configuration for each interface?

Legacy CLI Signup and view all the answers

What is a key feature of the Modular QoS CLI?

Reduces configuration steps and time Signup and view all the answers

What does PHB stand for in the context of QoS management?

Per-hop behavior Signup and view all the answers

What does the MQC method offer for QoS configuration?

Fine-tuning through configuration modules Signup and view all the answers

What is the main limitation of the flow-based approach in IntServ?

Scalability to large implementations Signup and view all the answers

Which QoS implementation method provides the fastest way to configure QoS?

Cisco AutoQoS Signup and view all the answers

What is the first step in creating QoS policy using Modular QoS CLI?

Defining the traffic of interest Signup and view all the answers

Study Notes

Basic Voice Encoding

Analog signals are sampled, quantized into binary, and compressed to reduce bandwidth.

Digital to Analog Conversion

The steps for converting digital signals to analog signals are: decompress the samples, decode samples to voltage amplitudes to rebuild the PAM signal, and then reconstruct the analog signal from the PAM signals.

Common Voice Codec Characteristics

ITU-T standards specify various codecs and their associated bit rates.
Examples include G.711 (PCM with 64 kbps), G.726 (ADPCM with 16, 24, and 32 kbps), G.728 (LDCELP with 16 kbps), G.729 (CS-ACELP with 8 kbps), and G.729A (CS-ACELP with less computation and 8 kbps).

Mean Opinion Score

The Mean Opinion Score (MOS) evaluates the quality of voice calls based on user ratings from 1 to 5 (lowest to highest).
A score of 4.0 equals "Toll Quality."
Different levels of distortion (imperceptible, just perceptible, and annoying) are associated with each speech quality rating.

DSP Used for Conferencing

Digital Signal Processors (DSPs) can support single-mode or mixed-mode conferences.
Mixed mode supports different codecs for flexibility.
Single mode requires only one codec for all participants.
Mixed mode conferences per DSP are fewer than single mode conferences.

Voice Transport in VoIP Networks

Analog phones connect to analog voice gateways.
Voice gateways convert between analog and digital signals.
After call setup, the IP network provides packet-by-packet delivery, shared bandwidth, and variable delays.

Jitter

Voice packets enter the network at a constant rate but can arrive at destinations at different rates or in the wrong order, which causes jitter.
Receiving routers need to ensure steady delivery and maintain order of packets to avoid delays or queuing problems.

VoIP Protocol Issues

IP doesn't guarantee reliability, flow control, or error correction.
VoIP uses TCP or UDP transport layer protocols to manage reliability and sequencing (order of packets).
TCP offers reliability but consumes extra bandwidth, and retransmission for lost packets isn't always needed for voice.
UDP offers no reliability, but is efficient as voice data doesn't need to be reordered.
RTP (Real-time Transport Protocol) is built on UDP and provides the required functionality for the characteristics of voice packets.

Protocols used for VoIP

Voice does not need reliability, although needs reordering, and time-stamping
UDP, with RTP, is used as an alternative to TCP, and saves network overhead

Voice Encapsulation

Digitized voice is encapsulated into RTP, UDP, and IP packets.
Typically, 20 ms of voice is packetized into a single IP packet.

Voice Encapsulation Overhead

VoIP packets are small and sent at a high rate.
IP, UDP, and RTP header overheads are enormous, especially compared to the small size of the payload.

RTP Header Compression

Compresses IP, UDP, and RTP headers, reducing bandwidth use.
Configured on a link-by-link basis.
Reduces header size substantially.
Saves considerable bandwidth.

When to Use RTP Header Compression

Use on slow links (less than 2 Mbps) if bandwidth needs to be conserved.
Consider the disadvantages of CRTP (Compound RTP), which include processing overhead and additional delays.
Tune CRTP-set the number of sessions to be compressed (default is 16).

Packetization Period Impact

Higher packetization periods result in larger IP packet sizes, which add to the payload, but lower packet rates, which reduce IP overhead.

Data-Link Overhead

Ethernet, Frame Relay, and MLP protocols have differing data-link overheads (in bytes).

Security and Tunneling Overhead

IP security (IPsec) and tunneling protocols can add overhead to VoIP packets.
Encapsulation of original frames into another protocol enlarges packets and increases bandwidth requirements.
Increased bandwidth requirements are important for voice packets because of their small packet size and high transmission rate.

Total Bandwidth Calculation Procedure

Gathering packetization information (period or size) and codec bandwidth. Gathering link information (CRTP, data-link protocol, and IPsec protocols). Calculating packetization size or period. Summing packet size, all headers, and trailers. Calculating packet rate. Calculating total bandwidth.

Bandwidth Calculation Example

Formula for calculating bandwidth in kbps, using total packet size, bytes per packet, IP overhead, packetization period, and packet rate.
Example values are included.

Quick Bandwidth Calculation

Formula for calculating total packet size (bits).
Provides parameters and values for G.711 and G.729 codecs, and examples for Frame Relay.

Enterprise Voice Implementations

Gateways, gatekeepers, Cisco Unified Call Manager, and IP phones are constituents of enterprise voice networks.

Deploying CAC

Call Admission Control (CAC) helps limit concurrent voice calls within WAN resources.
Needed due to QoS problems in setting up too many voice calls.

Cisco Unified CallManager Functions

Headquarters manage call processing, dial plan administration, signaling, phone features, directories, and XML services; they also provide an interface to external applications.

Example: Signaling and Call Processing

Cisco Unified CallManager cluster handles signaling for call setup and call processing.

Enterprise IP Telephony Deployment Models

Provides different deployment models (single site, multisite, distributed, and clustering over WAN) with different characteristics for each model.

Single Site

CallManager, applications, and DSP resources are located together.
IP WAN is not used for internal voice traffic, and the PSTN is used for external calls.

Multisite with Centralized Call Processing

CallManager servers and applications are centralized, but DSP resources are distributed among other sites.
Voice signaling over IP WAN and data traffic over IP WAN.

Multisite with Distributed Call Processing

All components (CallManager, applications, and DSPs) are located at each site.
All inter-site calls use the WAN (signaling and media) and use PSTN when WAN is down.

Clustering over WAN

All CallManager and DSP resources for a deployment are deployed over multiple sites.
All calls use the WAN (signaling and media) with a 40 ms or less delay.

VoIP QoS

Introduction of QoS, its requirements, and problems to solve.
QoS is needed to help interactive voice, video conferencing, and other applications receive high quality of service.

Converged Network Realities

Constant small-packet voice flow competes with bursty data flows.
Critical traffic must have priority.
Voice and video are time-sensitive.
Network outages need to be minimized

Converged Network Quality Issues

Lack of bandwidth, end-to-end delay (fixed and variable), and variation from delay cause some of the problems affecting quality issues in a VoIP network.

Increasing Available Bandwidth

Upgrading the link (best solution but expensive).
Improving QoS for critical traffic.
Compressing Layer 2 frames and headers.

Using Available Bandwidth Efficiently

Using advanced queuing mechanisms, and header compression improve bandwidth usage for voice and interactive traffic.

Types of Delay

Processing, queuing, serialization, and propagation delays need to be considered.

Ways to Reduce Delay

Upgrading the link, forwarding important packets first, reprioritization and frame/header compression.

Reducing Delay in a Network

Customer and ISP (Internet Service Provider) routers perform TCP/RTP header compression, LLQ, and reprioritization (according to the QoS policy)

Impacts of Packet Loss

Problems like voice breaking up, jerky video, corrupted files, and hold messages occur when packet loss is experienced in a VoIP network.

Types of Packet Drops

Output queues filling up and congestion on a link are common reasons for packet loss or drops in a VoIP network.

Ways to Prevent Packet Loss

Upgrading the link or increasing bandwidth usage to meet traffic demands.
Preventing congestion by dropping less-important packets.

What is Quality of Service?

Network managers need to control delay, jitter, and packet loss to manage bandwidth allocations and to provide desired network application performance.

Different Types of Traffic Have Different Needs

Real-time applications (voice) are especially sensitive to QoS metrics such as delay, jitter, and packet loss.
Management of bandwidth allocations deliver desired quality of experience in application performance.

Cisco IOS QoS Tools

Queue management (PQ, CQ, WFQ, and CBWFQ)
Congestion management (WRED)
Link efficiency (fragmentation & interleave)
Traffic shaping and policing (RTP & CRTP)

Priority Queuing

PQ puts traffic into queues based on priority (high, medium, normal, and low).
Classification determines queue length for priority.
Output hardware interface hardware manages priority scheduling.

Custom Queuing

Traffic is classified, interface buffer resources are managed, and link bandwidth is allocated in proportion to traffic demands.

Weighted Fair Queuing

Uses weighted fair scheduling to share network resources among different flows based on weights set.
Weights are determined by QoS (requested by the applications or flows) and the throughput of each flow.

Weighted Random Early Detection

WRED stochastically discards packets at random when congestion begins to increase on a link.
Provides a method to avoid packet loss if the queue occupancy increases.

Implementing QoS

Steps include identifying traffic types, dividing traffic into classes, defining QoS policies for each class.

Step 1: Identify Types of Traffic and Their Requirements

Network audit: Identify traffic types and network bandwidth requirements.
Business audit: Determine the importance of each traffic type for business operations.
Setting service levels: Determine the required response time for different traffic types.

Step 2: Define Traffic Classes

Classifying traffic based on characteristics (voice, video, data) and providing predefined priorities, which are used in QoS policy and configurations.
The important issues in determining important traffic factors are low latency, bandwidth requirements (guaranteed), guaranteed delivery, and/or no guaranteed delivery.

Step 3: Define QoS Policy

Implementing policy for specific features, service levels, and prioritizing each class (voice, best-effort).
Provides wide-ranging definition for specific levels of QoS for different classes of network traffic.

Quality of Service Operations

Classify traffic, establish queues, and selectively drop or prioritize traffic by using QoS features and tools.

Three QoS Models

Best-effort: No QoS is applied to packets.
Integrated Services (IntServ): Applications signal QoS requirements.
Differentiated Services (DiffServ): Network recognizes and classifies traffic classes that require QoS.

Best-Effort Model

No QoS (Quality of Service).
Simple, highly scalable.
No service guarantees.

Integrated Services (IntServ) Model Operation

Guaranteed and predictable network behavior for applications with multiple service levels.
Signaling protocol (RSVP) to reserve resources based on specified QoS parameters by application.
QoS parameters are linked to packet streams.

IntServ Functions

Control plane: Routing selection, admission control, and reservation setup.
Data plane: Flow and packet scheduling.

Benefits and Drawbacks of the IntServ Model

Explicit admission control.
Per-request policy admission control.
Continuous signaling.
Large, complex implementations aren't scalable.

The Differentiated Services Model

Overcomes IntServ's limitations.
Soft QoS (rather than hard QoS).
Classifies flows into aggregates.
Minimizes signaling and state maintenance requirements.

Methods for Implementing QoS Policy

Legacy CLI (Time consuming, individual interface configuration)
MQC (Uses configuration modules, good for QoS fine-tuning)
Cisco AutoQoS (Applies a possible QoS configuration to interfaces)
Cisco SDM QoS wizard (Good for simplified QoS configurations)

Modular QoS CLI

Command syntax reduces configuration steps and time.
Configures policy, not individual commands.
Uniform CLI for different QoS platforms.
Separates classification engine from policy.

Modular QoS CLI Components

Define traffic classes by using a class map.
Define QoS policies by using a policy map to define actions based on traffic classes.
Apply QoS policy to network interfaces using a service policy.

Step 1: Creating Class Maps

A class map defines traffic classes based on match conditions.

Configuring Class Maps

Configure class map mode & matching strategy with commands.

Classifying Traffic with ACLs

Standard ACLs and extended ACLs allow classifying traffic by the source, destination, port.

Step 2: Policy Maps

Policy maps define QoS policies for specific classes of network traffic.
Provide commands to define case-sensitive policy names.
Defines QoS policies for each class.

Configuring Policy Maps

Configure policy maps by entering the per-class policy configuration mode.

Step 3: Attaching Service Policies

Attach QoS policy maps to network interfaces (input or output).

Modular QoS CLI Configuration Example

Example shows how to configure traffic classes, QoS policies, and apply policies to interfaces. This example uses MQC Configuration model.

MQC Example

Example shows a voice traffic needing priority versus interactive data.

Basic Verification Commands

Commands such as show class-map, show policy-map, and show policy-map interface type number to display QoS configuration.

Implement the DiffServ QoS Model

Introduces the DiffServ QoS model which overcomes many limitations of best-effort and IntServ Models.

Classification

Classifies traffic based on incoming interface, IP precedence (DSCP), source/destination address, or application.

Marking

Marking is the component used to differentiate traffic into classes by tagging with certain values.
Protocols used for QoS marking are CoS (802.1p), MPLS EXP bits, Frame Relay, DSCP and IP precedence for the network layer.

DiffServ Model

Describes services associated with traffic classes, unlike IntServ, which defines per-flow state.
Complex traffic conditioning is performed at the edge.
No protocol state is required inside the core network infrastructure.

Classification Tools

IP Precedence and DiffServ Code Points (DSCP) are classification tools used to differentiate traffic.

IP TOS Byte and DS Field Inside the IP Header

Shows the position of the Type of Service (ToS) fields in IPv4 packet header.

IP Precedence and DSCP compatibility

Shows how IP precedence in version 4 IP packets is compatible with DiffServ code point (DSCP) used in version 4 packets.

Queuing

Introduction to queue structures in QoS.

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Description

Test your knowledge on networking delays, packet loss, and traffic management techniques. This quiz covers essential concepts related to how data packets interact in a network. Gauge your understanding of various methods to optimize network performance and reduce latency.