1. ITEC 275 Computer Networks – Switching, Routing, and WANs
Week 11
Robert D’Andrea
Fall 2016

2. Agenda Learning Activities TDM and FDM differences Industry Tests
Build and Test a Prototype
Write and Implement a Test Plan
Tools for Testing a Network Design
Multicasting
QoS
Queuing and Traffic Shaping

3. ATM Video Frame Relay videos:
Stop at LMI
ATM Video:
Start at 1:45
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

4. TDM and FDM
TDM (Time Division Multiplexing) and FDM (Frequency Division Multiplexing) are two methods of multiplexing multiple signals into a single carrier. Multiplexing is the process of combining multiple signals into one, in such a manner that each individual signal can be retrieved at the destination. Since multiple signals are occupying the channel, they need to share the resource in some manner.
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

5. TDM and FDM
The primary difference between FDM and TDM is how they divide the channels. FDM divides the channel into two or more frequency ranges that do not overlap. TDM divides and allocates certain time periods to each channel in an alternating manner. Because of this fact, we can say that TDM, each signal uses all of the bandwidth some of the time, while for FDM, each signal uses a small portion of the bandwidth all of the time.
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

6. TDM and FDM
1. FDM divides the channel into multiple, but smaller frequency ranges to accommodate more users, while TDM divides a channel by allocating a time period for each channel.
2. TDM provides much better flexibility compared to FDM.
3. FDM proves much better latency compared to TDM.
4. TDM and FDM can be used in tandem.
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

7. FDM
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

8. TDM and FDM
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

9. Reasons for Network Testing
Verify that the design meets key business and technical goals
Validate LAN and WAN technology and device selections
Verify that a service provider provides the agreed- up service
Identify bottlenecks or connectivity problems
Determine optimization techniques that will be necessary
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

10. Reasons for Network Testing
Proving that your network design is better than a competing design
Passing an “acceptance test” that gives you approval to go forward with the network implementation
Reassure mangers and co-workers that your design is effective
Identifying any risks that might impede implementation and planning for contingencies
Determine how much additional testing might be required. Will the new system be deployed as a pilot and undergo additional testing before being implemented
This is a short list of the reasons that testing a new network design is important. There are many other reasons also, many of which are listed in Top-Down Network Design, the book.

11. Testing Your Network Design
Use industry testing services
Build and test a prototype system
Use third-party and Cisco tools

12. Respected Independent Test Labs
The Interoperability Lab at the University of New Hampshire (IOL)
ICSA Labs
Miercom Labs
AppLabs
The Tolly Group
Penetration Testing test your network and applications before the bad guys do.

13. Simple verses Complex Network Designs
Simple network designs can rely on test results from vendors, independent labs, or trade journals to prove to your customer that your design will perform as intended.
Complex network designs require more considerations.
Testing should be implemented in-house
Testing will require more than component testing. There will be a need for unit, integration, and system testing.

14. Scope of a Prototype System
Normally, it is impractical to implement a full-scale network system.
A prototype should verify important capabilities and functions that might not perform adequately.
Risky functions include complex, intricate functions and functions that were influenced by the need to make tradeoffs with other network components.

15. Live Production Network
Perform initial testing during off-hours to minimize issues with user community, performance, and existing traffic flow.
Perform final testing during normal hours and benchmark the performance.
Perform final testing at various times to exercise the network during typical loads and benchmark the performance.

16. Test Plan What is a Test Plan?
A test plan is primarily comprised of test cases and test items. Think of a test case as a scenario or a finite state in which your network might find itself. 
In each test case, you'll have a list of test items or functions or features that you want to evaluate. Each test item should include not just an action, but the success criteria, and if you want to get more sophisticated, the testing too must be more critical. For example, you might want to make sure a business-critical application still work after a network change. So you'd arrange to have the application owners create a transaction or operate the application. 

17. Components of a Test Plan
Test objectives and acceptance criteria
The types of tests that will be run
Network equipment and other resources required
Testing scripts
The timeline and milestones for the testing a project

18. Components of a Test Plan
Test objectives and acceptance criteria
Objectives and acceptance should be based on a customer’s business and technical goals
Acceptance of test results are acceptable by both the customer and the tester.
Measure response time
Measure applications throughput
Measure the amount of time it takes to hear a dial tone using Voice over IP
Establish a baseline measurement of CRC errors

19. Test Objectives and Acceptance Criteria
Specific and concrete
Based on business and technical goals
Clear criteria for declaring that a test passed or failed
Avoid biases and preconceived notions about outcomes
If appropriate, reference an established baseline

20. Test Plan Considerations and Implementation
Network Connectivity Section
Is Layer 2 set up appropriately? (VLANs on the right trunks, PVCs, etc.)
Do your router tables have the proper routes? (Check the next hops and ages, too.)
Can you ping everywhere in the network? (Performance: Are the times acceptable?)
Do trace routes show paths you would expect?
If you load balance across the core of your network, verify each link is being used.
Is DHCP handing out addresses?
DNS resolving names properly?
Does your remote access still work?

21. Test Plan Considerations
Application Connectivity Section
Does VOIP work? Is it showing up in the right queues?
Are your firewalls and proxies blocking and allowing traffic appropriately?
Can you browse the Web?
Are your network management and logging systems working?
Do your business applications work? (And do transactions complete in an acceptable time?)
Are backup jobs still working?

22. Achieve Success with a New Network Design
Your chances of success are much greater if you perform several simple tests along the way, rather than waiting until you think you're done and discovering that something doesn't work. Performing simple incremental tests along the way will help testers and customers maintain a sense of truthfulness and confidence about in the system being tested.

23. Types of Tests Application response-time tests with terminal
Throughput tests (I/O)
Availability tests (failure test)
Regression tests (does the network perform similarly after changes were implemented)
Regression testing makes sure the new system doesn’t break any applications or components that were known to work and perform to a certain level before the new system was installed. Regression testing does not test new features or upgrades. Instead, it focuses on existing applications. Regression testing should be comprehensive, and is usually automated to facilitate comprehensiveness.

24. Types of Tests What is Protocol Testing?
To understand the behavior of a protocol, it must be tested to observe the protocol’s functionality
Verify every phase of testing life cycle for
Functionality testing
Interoperability testing (IOT) is the process of testing to determine the interoperability of a software product
Performance
Obtain tools to generate and test the protocol messages
Regression testing makes sure the new system doesn’t break any applications or components that were known to work and perform to a certain level before the new system was installed. Regression testing does not test new features or upgrades. Instead, it focuses on existing applications. Regression testing should be comprehensive, and is usually automated to facilitate comprehensiveness.

25. Types of Tests Why is Protocol Testing required?
Different vendor products need to communicate with each other.
If any product is using this standards in their devices they are interoperable with other vendor devices as both must meet compliance to Standards of IETF/RFC to study the network through their packet data.
Protocol testing ensures proper functionality of various elements of a message. It also ensures whether it was designed as per specification.
Regression testing makes sure the new system doesn’t break any applications or components that were known to work and perform to a certain level before the new system was installed. Regression testing does not test new features or upgrades. Instead, it focuses on existing applications. Regression testing should be comprehensive, and is usually automated to facilitate comprehensiveness.

26. Resources Needed for Testing
A Test Plan should include a network topology drawing for tester to be able to reference.
A list of switches, routers, bridges, firewalls, servers, telephone equipment, and wireless access points.
A list of documented version numbers for hardware and software.
Scheduled time in a lab either at your site or the customer’s site
Power, air conditioning, rack space, and other physical resources
Help from co-workers or customer staff
Help from users to test applications
Network addresses and names

27. Resources Needed for Testing
How it is carried out?
Objective: To test the protocol
i.e. to check every node with their packet data.
Tools: Protocol Analyzer or WireShark and simulator.

28. Example Test Script Workstations Server 1 Firewall Network A Network B
Protocol Analyzer
Protocol Analyzer

29. Example Test Script (continued)
Based on the previous slide
Test objective.
Assess the firewall’s capability to block Application ABC traffic, during both light and moderately heavy load conditions.
Acceptance criterion.
The firewall should block the TCP SYN request from every workstation on Network A that attempts to set up an Application ABC session with Server 1 on Network B. The firewall should send each workstation a TCP RST (reset) packet.

30. Example Test Script (continued)
Start capturing network traffic on the protocol analyzer on Network A.
Start capturing network traffic on the protocol analyzer on Network B.
Run Application ABC on a workstation located on Network A and access Server 1 on Network B.
Stop capturing network traffic on the protocol analyzers.

31. Example Test Script (continued)
5. Display data on Network A’s protocol analyzer and verify that the analyzer captured a TCP SYN packet from the workstation. Verify that the network layer destination address is Server 1 on Network B, and the destination port is port 1234 (the port number for Application ABC). Verify that the firewall responded to the workstation with a TCP RST packet.

32. Example Test Script (continued)
Display data on Network B’s protocol analyzer and verify that the analyzer did not capture any Application-ABC traffic from the workstation.
Log the results of the test in the project log file.
Save the protocol-analyzer trace files to the project trace- file directory.
Gradually increase the workload on the firewall, by increasing the number of workstations on Network A one at a time, until 50 workstations are running Application ABC and attempting to reach Server 1. Repeat steps 1 through 8 after each workstation is added to the test.

33. Example Test Script (continued)
Host A sends a TCP SYNchronize packet to Host B
Host B receives A's SYN
Host B sends a SYNchronize-ACKnowledgement
Host A receives B's SYN-ACK
Host A sends ACKnowledge
Host B receives ACK.
TCP socket connection is ESTABLISHED.

34. Tools for Testing a Network Design
Network-management and monitoring tools. These monitoring tools are used to alert network management to problems and report significant network problems.
Traffic generation tools
Modeling and simulation tools
QoS and service-level management tools
Protocol analyzer

35. Tools for Testing a Network Design
The following list of products are probably more related to network monitoring than network design, but don't forget that two important steps in the top-down network design methodology are characterizing the existing network and testing the new network.
Big Brother Professional Edition
Ixia IxN2X Multiservice Test Solution
LANSurveyor
Multi Router Traffic Grapher
Nagios
NetIQ

36. Tools for Testing a Network Design
Online Erlang Traffic Calculators
OPNET
Orion NetFlow Traffic Analyzer (NTA)
NetMRI
Tivoli
Visio Enterprise Network Tools
WANDL's Network-Planning and Analysis Tools
WhatsUp Gold

37. Protocol Analyzer Tool
A protocol analyzer is used to analyze traffic behavior, errors, utilization, efficiency, and rates of broadcast and multicast packets.
A protocol analyzer can be a computer program (WireShark) or a piece of computer hardware that can intercept and log traffic passing over a digital network or part of a network. As data streams flow across the network, the sniffer captures each packet and, if needed, decodes the packet's raw data, showing the values of various fields in the packet, and analyzes its content according to the appropriate RFC or other specifications.

38. Simulation Tool
A simulation tool enables you to develop a model of a network, estimate the performance of the network and compare alternatives for implementing the network.
iTrinegy Network Emulator (INE) products enable you to realistically recreate a wide variety of network conditions like latency, jitter, packet loss/error/reordering and bandwidth restrictions so that you can simulate environments such as Wide Area Networks (WANs), Wireless LANs, GPRS, 3G, IP over Radio/Radio over IP(RoIP), Satellite or MPLS networks.

39. Command Tools Test Tools: Command Format ipconfig
ping <IP address> ping
ping <DNS name> ping yahoo.com
tracert <DNS name> tracert yahoo.com
nslookup <DNS name> nslookup yahoo.com
netstat netstat -a

40. Reasons to Optimize Meet key business and technical goals
Use bandwidth efficiently
Control delay and jitter
Reduce serialization delay
Support preferential service for essential applications
Meet Quality of Service (QoS) requirements (IP Multicast)

41. IP Multicast
Server
Server

42. IP Multicast Router/MCS
The Miscellaneous Control Subsystem (MCS) works with its companion Routing Engine provides control and monitoring functions for router components. It also generates a clock signal for the SONET/SDH interfaces on the router.

43. IP Multicast Applications
Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.

44. IP Multicast Helps Optimize Bandwidth Usage
With IP multicast, you can send a high-volume multimedia stream just once instead of once for each user
Requires support for
Multicast addressing
Multicast registration (IGMP)
Multicast routing protocols

45. IP Multicast Addresses
IPv4 Multicast Addresses to
IPv6 Multicast Addresses
FF02:0:0:0:0:0:0:1 All Nodes Address
FF02:0:0:0:0:0:0:2 All Routers Address

46. IP Multicast Helps Optimize Bandwidth Usage
To map an IP multicast address to a MAC- layer multicast address, the low order 23 bits of the IP multicast address are mapped directly to the low order 23 bits in the MAC-layer multicast address. Because the first 4 bits of an IP multicast address are fixed according to the class D convention, there are 5 bits in the IP multicast address that do not map to the MAC-layer multicast address. 

47. IP Multicast Addressing
Uses Class D multicast destination address to
Converted to a MAC-layer multicast destination address
The low-order 23 bits of the Class D address become the low-order 23 bits of the MAC-layer address
The top 9 bits of the Class D address are not used
The top 25 bits of the MAC-layer address are 0x01:00:5E followed by a binary 0

48. Internet Group Management Protocol (IGMP)
Allows a host to join a multicast group
Host transmits a membership-report message to inform routers on the segment that traffic for a group should be multicast to the host’s segment
IGMPv2 has support for a router more quickly learning that the last host on a segment has left a group

49. Multicast Routing Protocols
Becoming obsolete
Multicast OSPF (MOSPF)
Distance Vector Multicast Routing Protocol (DVMRP)
Still used
Protocol Independent Multicast (PIM)
Dense-Mode PIM
Sparse-Mode PIM

50. Multicast Routing Protocols

51. Multicast Routing Protocols
What is PIM?
Protocol-Independent Multicast (PIM) is a family of multicast routing protocol for Internet Protocol (IP) networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN
or the Internet. It is termed protocol - independent because PIM does not include its own topology discovery mechanism, but instead uses routing information supplied by other routing protocols.

52. PIM (Protocol Independent Multicast)
What is PIM Dense Mode?
Dense mode is used when there are many members (employees listen to a company president).
Dense PIM does not require the computation of routing tables.

53. PIM (Protocol Independent Multicast)
What is PIM Dense Mode?
Dense mode PIM is the older and simpler PIM mode. It works well in small networks where there are a large number of listeners, but is inefficient in larger network.

54. PIM Dense Mode

56. PIM (Protocol Independent Multicast)
What is PIM Sparse Mode?
Sparse mode utilizes a rendezvous point (RP). A rendezvous point provides a registration service for a multicast group.
Sparse mode PIM relies on IGMP which let a host join a group by sending a membership- report message, and detach from a group by sending a leave message.

57. PIM (Protocol Independent Multicast)
What is PIM Sparse Mode?
PIM Sparse Mode (PIM-SM) explicitly builds unidirectional shared trees rooted at a rendezvous point (RP) per group, and optionally creates shortest-path trees per source. PIM-SM generally scales fairly well for wide-area usage.

59. Serialization What is serialization?
Serialization is the process of translating data structures or object state into a format that can be stored (for example, in a file or memory buffer, or transmitted across a network connection link) and reconstructed later in the same or another computer environment. When the resulting series of bits is reread according to the serialization format, it can be used to create a semantically identical clone of the original object.

60. Serialization Delay
What is serialization delay? Serialization delay is the time it takes for a unit of data, such as a packet, to be serialized for transmission on a narrow channel such as a cable. Serialization delay is dependent on size, which means that longer packets experience longer delays over a given network path. Serialization delay is also dependent on channel capacity ("bandwidth"), which means that for equal- size packets, the faster the link, the lower the serialization delay.

61. Reducing Serialization Delay
Link-layer fragmentation and interleaving
Breaks up and reassembles frames
Multilink PPP
Frame Relay FRF.12
Compressed Real Time Protocol
RTP is used for voice and video
Compressed RTP compresses the RTP, UDP, and IP header from 40 bytes to 2 to 4 bytes

62. Reducing Serialization Delay

63. A Few Technologies for Meeting QoS Requirements
IETF controlled load service
IETF guaranteed service
IP precedence
IP differentiated services

64. IP Type of Service Field
The type of service field in the IP header is divided into two subfields
The 3-bit precedence subfield supports eight levels of priority
The 4-bit type of service subfield supports four types of service
Although IP precedence is still used, the type of service subfield was hardly ever used

65. IP Type of Service Field
Type of Service Subfield
Bit 3 4 5 6 7
D = Delay
T = Throughput
R = Reliability
C = Cost
Precedence D T R C 8 15 24 31
Bit
Version
Header Length
Type of Service
Total Length
Identification
Flags
Fragment Offset
Time to Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options
Padding

66. IP Differentiated Services (DS) Field
RFC 2474 redefines the type of service field as the Differentiated Services (DS) field
Bits 0 through 5 are the Differentiated Services Codepoint (DSCP) subfield
Has essentially the same goal as the precedence subfield
Influences queuing and packet dropping decisions for IP packets at a router output interface
Bits 6 and 7 are the Explicit Congestion Notification (ECN) subfield

67. IP Differentiated Services (DS) Field6
Differentiated Services Codepoint
Explicit Congestion Notification 8 15 24 31
Header Length
Version
Differentiated Services
Total Length

68. Resource Reservation Protocol (RSVP)
RSVP complements the IP type-of-service, precedence, DSCP, and traffic-class capabilities inherent in an IP header.
RSVP supports mechanisms for hosts to specify QoS requirements for individual traffic flow.
RSVP can be deployed on LANs and enterprise WANs to support multimedia applications or other types of applications with strict QoS requirements.

69. Resource Reservation Protocol (RSVP)
IP header type-of-service capabilities and RSVP are examples of QoS signaling protocols.

70. Classifying LAN Traffic
IEEE 802.1p
Classifies traffic at the data-link layer
Supports eight classes of service
A switch can have a separate queue for each class and service the highest-priority queues first

71. Cisco Switching Techniques
Process switching is the slowest switching method
Fast switching allows highest throughput by switching a packet using an entry in the fast-cache that was created when a previous packet to the same destination was processed.
NetFlow switching is optimized for environments where services must be applied to packets to implement security, QoS features, and traffic accounting. Example Internet and enterprise network environment boundary.
These are technologies for “switching” (forwarding) packets through a router.

72. Cisco Switching Techniques
Cisco Express Forwarding (CEF) is a technique for switching packets quickly across large backbone networks and the Internet.
CEF depended on a forwarding information base (FIB), rather than caching techniques.
FIB allows CEF to use less CPU resources compared to other Layer 3 switching methods. FIB contains forwarding information for all routes in the routing tables.
These are technologies for “switching” (forwarding) packets through a router.

73. Cisco Switching Techniques
Why did CEF evolve?
With the introduction of web-based applications and other interactive applications that are characterized by sessions of short duration to multiple addresses.
It became very apparent that the cache- based system could not deliver the needed performance for these applications.
These are technologies for “switching” (forwarding) packets through a router.

74. Cisco Queuing Services
First in, first out (FIFO) queuing store packets when the network is congested and forward the packets in the order they arrived in when there is no congestion. Disadvantage: No packet priority scheme.
Priority queuing ensures that important traffic is processed first. Priority is based on the type of protocol, incoming interface, packet size, and source or destination address. The priorities are high, medium, normal, and low.

75. Cisco Queuing Services
Custom queuing is designed to allow the network to be shared among applications with different minimum bandwidth or latency requirements. Custom queuing provides different amounts of queue space to different protocols and handles the queues in round-robin manner. A particular protocol can be prioritized by assigning it more queue space.
Custom queuing can be used to guarantee bandwidth at a potential congestion point.

76. Cisco Queuing Services
Custom queuing helps ensure that each traffic type receives a fixed portion of available bandwidth and that when the link is under stress, no application receives more than a predetermined proportion of capacity.
Weighted fair queuing (WFQ) operates from algorithms designed to reduce delay variability and provide predictable throughput and response time for traffic flows. Applications with small payloads are not starved of bandwidth by applications that send large packets.

77. Cisco Queuing Services
Class-based Weighted Fair Queuing (CBWFQ) combines the best scenarios of priority, custom, and weight-fair queuing.
Class-based WFQ allows you to define traffic classes based on matching criteria such as protocol, access control lists, and input interfaces.
Low latency queuing (LLQ) combines priority queuing with CBWFQ. LLQ brings strict priority queuing to CBWFQ. Strict priority queuing allows delay-sensitive data such as voice to be sent before packets in other queues are sent.

78. Priority Queuing START NO Packet in high queue? NO
Packet in medium queue?
YES NO
Packet in normal queue?
YES NO
Packet in low queue?
YES
YES
Dispatch Packet
Continue

79. Reached transmission window size?
Custom Queuing
START (with Queue 1)
Packet in Queue? NO
YES
Reached transmission window size?
YES NO
Next Queue
Dispatch Packet

80. Low-Latency Queuing One queue always gets the green light
Use this for voice
Combine this with class-based weighted fair queuing
Define traffic classes based on protocols, access control lists, and input interfaces
Assign characteristics to classes such as bandwidth required and the maximum number of packets that can be queued for the class

81. Random Early Detection (RED)
Congestion avoidance rather than congestion management
Monitors traffic loads and randomly discards packets if congestion increases
Source nodes detect dropped packets and slow down
Works best with TCP
Weighted Random Early Detection
Cisco’s implementation uses IP precedence or the DS field instead of just randomly dropping packets

82. Traffic Shaping
Manage and control network traffic to avoid bottlenecks
Avoid overwhelming a downstream router or link
Reduce outbound traffic for a flow to a configured bit rate
Queue bursts of traffic for that flow
In summary, traffic shaping is the manipulation and prioritization of network traffic to reduce the impact of heavy users or machines from effecting other users.

83. Committed Access Rate (CAR)
Cisco feature for classifying and policing traffic on an incoming interface
Supports policies regarding how traffic that exceeds a certain bandwidth allocation should be handled
Can drop a packet or change the IP precedence or DSCP bits

84. Security Penetration
A penetration test is a proactive and authorized attempt to evaluate the security of an IT infrastructure by safely attempting to exploit system vulnerabilities, including OS, service and application flaws, improper configurations, and even risky end-user behavior. Such assessments are also useful in validating the efficacy of defensive mechanisms, as well as end-users’ adherence to security policies.

85. Security Penetration
Penetration tests are typically performed using manual or automated technologies to systematically compromise servers, endpoints, web applications, wireless networks, network devices, mobile devices and other potential points of exposure. Once vulnerabilities have been successfully exploited on a particular system, testers may attempt to use the compromised system to launch subsequent exploits at other internal resources, specifically by trying to incrementally achieve higher levels of security clearance and deeper access to electronic assets and information via privilege escalation.

86. Summary An untested network design probably won’t work.
It’s often not practical to test the entire design.
However, by using industry testing services and tools, as well as your own testing scripts, you can (and should) test the complex, risky, and key components of a network design.

87. Summary
Optimization provides the high bandwidth, low delay, and controlled jitter required by many critical business applications
To minimize bandwidth utilization by multimedia applications, use IP multicast
To reduce serialization delay, use link fragmentation and compressed RTP
To support QoS and optimize performance, use IP precedence, DSCP, 802.1p. advanced switching and queuing methods, RED, CAR, etc.

88. Review Questions Why is it important to test your network design?
Why is regression testing important?
What are some characteristics of well-written acceptance criteria?
What are some characteristics of a good network simulation tool?

89. Review Questions Why is it important to optimize your network?
What has become of the IP type of service field?
What are some methods for marking packets to identify the need for priority handling?
Compare and contrast Cisco queuing services.

90. This Week’s Outcomes Industry Tests Build and Test a Prototype
Write and Implement a Test Plan
Tools for Testing a Network Design
Multicasting
QoS
Queuing and Traffic Shaping

91. Due this week 12-1 – Concept questions 9
1-5-3 – Network design project
New office network

92. Next week Review chapters 12 and 13 in Top-Down Network Design
13-1 – Concept questions 10
4-2-3 – Networking practical experiences
Basic network troubleshooting

93. Q & A
Questions, comments, concerns?