1. Network Analysis and Design
Introduction to Network Design

2. Network Design A network design is a blueprint for building a network
The designer has to create the structure of the network [and] decide how to allocate resources and spend money

3. Elements of Good Network Design
Deliver the services requested by users
Deliver acceptable throughput and response times
Cost efficiency
Reliable
Expandable
Manageable
Well-documented

4. Network Design Issues User requirements Locations of devices
Characteristics of applications
Types of traffic
Topologies
Routing protocols
Budget
Performance
Etc.

5. Classifications of Network Design
Build a new network
Expand or upgrade the existing network
Create the overlay network
Virtual Private Network (VPN)

6. Types of Networks Access network: Backbone network:
The ends or tails of networks that connect the small sites into the network
LAN, campus network
Backbone network:
The network that connects major sites
Corporate WAN

7. Objectives How to design a network using the correct techniques?
Some common guidelines applicable for all types of network design

8. Top-Down Network Design Methodology
A complete process that matches business needs to available technology to deliver a system that will maximize an organization’s success
Don’t just start connecting the dots
In the LAN, it is more than just buying a few devices
In the WAN, it is more than just calling the phone company

9. Top-Down Network Design Methodology (Contd.)
Analyze business and technical goals first
Explore divisional and group structures to find out who the network serves and where they reside

10. Top-Down Network Design Methodology (Contd.)
Determine what applications will run on the network and how those applications behave on a network
Focus on applications, sessions, and data transport before the selection of routers, switches, and media that operate at the lower layers

11. Network Design Phases Requirement analysis Logical network design
Physical network design

12. Phase I - Requirement Analysis Phase
Analyze goals and constraints
Characterize the existing network
Characterize network traffic

13. Phase II - Logical Network Design Phase
Map the requirements into the conceptual design
Design a network topology
Node locations
Capacity assignment

14. Phase III - Physical Network Design Phase
Select technologies and devices for your design
Implementation

15. Business Goals Increase revenue Reduce operating costs
Improve communications
Shorten product development cycle
Expand into worldwide markets
Build partnerships with other companies
Offer better customer support or new customer services

16. Recent Business Priorities
Mobility
Security
Resiliency (fault tolerance)
Business continuity after a disaster
Networks must offer the low delay required for real-time applications such as VoIP

17. Business Constraints
Budget
Staffing
Schedule
Politics and policies

18. Information Goals of the project Scope of the project
What problem are they trying to solve?
How will new technology help them be more successful in their business?
Scope of the project
Small in scope: Allow sales people to access network via a VPN
Large in scope: An entire redesign of an enterprise network
Does the scope fit the budget, capabilities of staff and consultants, schedule?

19. Information (Contd.) Applications, protocols, and services
Current logical and physical architecture
Current performance

20. Technical Goals Scalability Availability Performance Security
Manageability
Usability
Adaptability
Affordability

21. Scalability Scalability refers to the ability to grow
Network must adapt to increases in network usage and scope in the future
Flat network designs don’t scale well
Broadcast traffic affects the scalability of a network

22. Availability
Availability is the amount of time a network is available to users
Availability can be expressed as a percent up time per year, month, week, day, or hour, compared to the total time in that period
24/7 operation
Network is up for 165 hours in the 168-hour week
Availability is 98.21%

23. Availability (Contd.)
Different applications may require different levels
Some enterprises may want % or “Five Nines” availability

24. Availability (Contd.) An uptime of 99.70 % An uptime of 99.95 %
Downtime = x 60 x 24 x 7
30.24 mins per week
An uptime of %
Downtime = x 60 x 24 x 7
5.04 mins per week
An uptime of %
Downtime = x 60 x 24 x 365
5.256 mins per year

25. Availability (Contd.)
System availability (R) is calculated from the component availability (Ri)
Series:
R =  Ri
Parallel:
R = 1 – (1 – Ri)

26. Availability (Contd.) R1 = 99.95%, R2 = 99.5% Series: Parallel:
R = x = 99.45%
Decreases system availability
Parallel:
R = 1 – [(1 – ) x (1 – 0.995)] = %
Increases system availability

27. Availability (Contd.) 99.999% may require high redundancy (and cost)
ISP 1
ISP 2
ISP 3
Enterprise

28. Availability (Contd.)
Availability can also be expressed as a mean time between failure (MTBF), and mean time to repair (MTTR)
Availability = MTBF / (MTBF + MTTR)
A typical MTBF goal for a network that is highly relied upon is 4000 hours. A typical MTTR goal is 1 hour.
4000 / 4001 = 99.98% availability

29. Network Performance Common performance factors include Bandwidth
Throughput
Bandwidth utilization
Offered load
Accuracy
Efficiency
Delay (latency) and delay variation
Response time

30. Bandwidth Vs. Throughput
They are not the same thing
Bandwidth is the data carrying capacity of a circuit
Usually specified in bits per second
Fixed
Throughput is the quantity of error free data transmitted per unit of time
Measured in bps, Bps, or packets per second (pps)
Varied

31. Other Factors that Affect Throughput
The size of packets
Inter-frame gaps between packets
Packets-per-second ratings of devices that forward packets
Client speed (CPU, memory, and HD access speeds)
Server speed (CPU, memory, and HD access speeds)
Network design
Protocols
Distance
Errors
Time of day
etc.

32. Throughput of Devices
The maximum PPS rate at which the device can forward packets without dropping any packets
Theoretical maximum is calculated by dividing bandwidth by frame size, including any headers, preambles, and interframe gaps

33. Throughput of Devices (Contd.)
Frame Size
(Bytes)
Theoretical Max PPS
(100-Mbps Ethernet) 64
148,800
128
84,450
256
45,280
512
23,490
768
15,860
1024
11,970
1280
9,610
1518
8,120

34. Bandwidth, Throughput, Load
100 % of Capacity
Throughput
Actual
Ideal
100 % of Capacity
Offered Load

35. Throughput Vs. Goodput
Most end users are concerned about the throughput for applications
Goodput is a measurement of good and relevant application layer data transmitted per unit of time
In that case, you have to consider that bandwidth is being “wasted” by the headers in every packet

36. Utilization The percent of total available capacity in use
For WANs, optimum average network utilization is about 70%
For hub-based Ethernet LANs, utilization should not exceed 37%, beyond this limit, collision becomes excessive

37. Utilization (Contd.)
For full-duplex Ethernet LANs, a point-to-point Ethernet link supports simultaneous transmitting and receiving
Theoretically,
Fast Ethernet means 200 Mbps available
Gigabit Ethernet means 2 Gbps available
100% of this bandwidth can be utilized
Full-duplex Ethernet is becoming the standard method for connecting servers, switches, and even end users' machines

38. Efficiency Large headers are one cause for inefficiency
How much overhead is required to deliver an amount of data?
How large can packets be?
Larger better for efficiency (and goodput)
But too large means too much data is lost if a packet is damaged
How many packets can be sent in one bunch without an acknowledgment?

39. Efficiency (Contd.) Small Frames (Less Efficient)
Large Frames (More Efficient)

40. Delay from the User’s Point of View
Response Time
The time between a request for some service and a response to the request
The network performance goal that users care about most
A function of the application and the equipment the application is running on, not just the network
Most users expect to see something on the screen in 100 to 200 ms
The 100-ms threshold is often used as a timer value for protocols that offer reliable transport of data

41. Delay from the Engineer’s Point of View
Propagation delay
Signal travels in a cable at about 2/3 the speed of light in a vacuum
Relevant for all data transmission technologies, but especially for satellite links and long terrestrial cables
Geostationary satellites: propagation delay is about 270 ms for an intercontinental satellite hop
Terrestrial cables: propagation delay is about 1 ms for every 200 km

42. Delay from the Engineer’s Point of View (Contd.)
Transmission delay
Also known as serialization delay
Time to put digital data onto a transmission line
Depends on the data volume and the data rate of the line
It takes about 5 ms to output a 1,024 byte packet on a Mbps T1 line

43. Delay from the Engineer’s Point of View (Contd.)
Packet-switching delay
The latency accrued when switches and routers forward data
The latency depends on
the speed of the internal circuitry and CPU
the switching architecture of the internetworking device
the type of RAM that the device uses
Routers tend to introduce more latency than switches
QoS, NAT, filtering, and policies introduce delay

44. Delay from the Engineer’s Point of View (Contd.)
Queueing delay
The average number of packets in a queue on a packet-switching device increases exponentially as utilization increases

45. Queuing Delay and Bandwidth Utilization
Number of packets in a queue increases exponentially as utilization increases

46. Delay Variation (Jitter)
The amount of time average delay varies
Users of interactive applications expect minimal delay in receiving feedback from the network
Users of multimedia applications require a minimal variation in the amount of delay
Delay must be constant for voice and video applications
Variations in delay cause disruptions in voice quality and jumpiness in video streams

47. Delay Variation (Jitter) (Contd.)
Short fixed-length cells, for example ATM 53-byte cells, are inherently better for meeting delay and delay-variance goals
Packet size tradeoffs
Efficiency for high-volume applications versus low and non-varying delay for multimedia

48. Delay Variation (Jitter) (Contd.)
Audio/video applications minimize jitter by providing a buffer that the network puts data into
Display software or hardware pulls data from the buffer

49. Accuracy
Data received at the destination must be the same as the data sent by the source
Error fames must be retransmitted, which has a negative effect on throughput
In IP networks, TCP provides retransmission of data
For WAN links, accuracy goals can be specified as a bit error rate (BER) threshold
Fiber-optic links: about 1 in 1011
Copper links: about 1 in 106

50. Accuracy (Contd.)
On shared Ethernet, errors often result from collisions
Collisions happen in the 8-byte preamble of the frames (not counted)
Collisions happen past the preamble and somewhere in the first 64 bytes of the data frame (legal collision)
Collisions happen beyond the first 64 bytes of a frame (late collision)

51. Accuracy (Contd.)
Late collisions are illegal and should never happen (too large network)
A goal for Ethernet collisions: less than 0.1% affected by a legal collision
Collisions should never occur on full-duplex Ethernet links
In wireless LAN CSMA/CA, collisions can still occur

52. Security
Security design is one of the most important aspects of enterprise network design
Security problems should not disrupt the company's ability to conduct business
The cost to implement security should not exceed the cost to recover from security incidents

53. Security (Contd.) Network Assets Hardware Software Applications Data
Intellectual property
Trade secrets
Company’s reputation

54. Affordability Affordability is sometimes called cost-effectiveness
A network should carry the maximum amount of traffic for a given financial cost
Financial costs include nonrecurring equipment costs and recurring network operation costs
Campus networks: low cost is often more important than availability and performance.
Enterprise networks: availability is usually more important than low cost

55. Affordability (Contd.)
Monthly charges for WAN circuits are the most expensive aspect of running a large network
How to save
Use a routing protocol that minimizes WAN traffic
Improve efficiency on WAN circuits by using such features as compression
Eliminate underutilized trunks
Use technologies that support oversubscription

56. Adaptability
Avoid incorporating any design elements that would make it hard to implement new technologies in the future
Change can come in the form of new protocols, new business practices, new traffic patterns

57. Usability
The ease of use with which network users can access the network and services
Usability might also include a need for mobility
Some design decisions will have a negative affect on usability:
Strict security, for example

58. Characterizing a Network (Why?)
Verify that a customer's technical design goals are realistic
Understand the current topology
Locate existing network segments and equipment
Locate where new equipment will go
Develop a baseline of current performance

59. Characterizing a Network (What?)
Infrastructure
Addressing and naming
Wiring and media
Architectural and environmental constraints
Health

60. Infrastructure Develop a set of network maps
Learn the location of major internetworking devices and network segments

61. Infrastructure (Contd.)
Information to collect
Geographical locations
LAN, WAN connections
Buildings and floors, and possibly rooms
Location of major servers or server farms
Location of routers and switches
Location of mainframes
Location of major network-management stations
Location and reach of virtual LANs (VLANs)
Etc.

62. Infrastructure (Contd.)
Medford
Fast Ethernet
50 users
Roseburg
Fast Ethernet
30 users
Frame Relay
CIR = 56 Kbps
DLCI = 5
Frame Relay
CIR = 56 Kbps
DLCI = 4
Gigabit Ethernet
Grants Pass HQ
16 Mbps
Token Ring
Grants Pass HQ
Fast Ethernet
75 users
FEP (Front End Processor)
IBM Mainframe T1
Web/FTP server
Eugene
Ethernet
20 users T1
Internet

63. Addressing and Naming
IP addressing for major devices, client networks, server networks
What to consider?
Private/public address
Classless/classful addressing
Variable-length subnet mask (VLSM)
Route aggregation or supernetting
Discontiguous subnets

64. Discontiguous Subnets
Area 0
Network
Router A
Router B
Area 1
Subnets
Area 2
Subnets

65. Wiring and Media
Document the types of cabling in use as well as cable distances
Distance information is useful when selecting data link layer technologies based on distance restrictions

66. Wiring and Media (Contd.)
Single-mode (SM) fiber
Multi-mode (MM) fiber
Shielded twisted pair (STP) copper
Unshielded-twisted-pair (UTP) copper
Coaxial cable
Microwave
Laser
Radio
Infra-red

67. Architectural Constraints
Make sure the following are sufficient
Air conditioning
Heating
Ventilation
Power
Protection from electromagnetic interference

68. Architectural Constraints (Contd.)
Make sure there’s space for:
Cabling conduits
Patch panels
Equipment racks
Work areas for installing and troubleshooting equipment

69. Wireless Installations
Reflection
Signal bounces back and interferes with itself
Metal surfaces such as steel girders, scaffolding, shelving units, steel pillars, and metal doors
Implementing a WLAN across a parking lot can be tricky because of metal cars that come and go

70. Wireless Installations (Contd.)
Absorption
Energy of the signal can be absorbed by the material in objects through which it passes
Reduces signal level
Water has significant absorption properties, and objects such as trees or thick wooden structures can have a high water content
Implementing a WLAN in a coffee shop can be tricky if there are large canisters of liquid coffee

71. Wireless Installations (Contd.)
Refraction
RF signal is bent when it passes from a medium with one density into a medium with another density
The signal changes direction and may interfere with the nonrefracted signal
It can take a different path and encounter other, unexpected obstructions, and arrive at recipients damaged or later than expected

72. Wireless Installations (Contd.)
Diffraction
Similar to refraction
Like refraction, the signal is bent around the edge of the diffractive region and can then interfere with that part of the signal that is not bent

73. Wireless Installations (Contd.)
Boost the power level to compensate for variable environmental factors
The additional power added to a transmission is called the fade margin

74. Health Performance Availability Bandwidth utilization Accuracy
Efficiency
Response time
Status of major routers, switches, and firewalls

75. Develop a Performance Baseline
How much better the new internetwork performs once your design is implemented
Baseline of normal performance should not include nontypical problems caused by exceptionally large traffic loads
The decision whether to measure normal performance, performance during peak load, or both, depends on the goals of the network design

76. Characterize Availability
Date and Duration of Last Major Downtime
Cause of Last Major Downtime
MTBF
MTTR
Enterprise
Segment 1
Segment 2
Segment n

77. Utilization
Measurement of how much bandwidth is in use during a specific time interval
Different tools use different averaging windows for computing network utilization
Trade-off between amount of statistical data that must be analyzed and granularity

78. Utilization in Minute Intervals

79. Utilization in Hour Intervals

80. Utilization (Contd.)
The size of the averaging window depends on your goals
When troubleshooting network problems, keep the interval very small, either minutes or seconds
For performance analysis and baselining purposes, use an interval of 1 to 5 minutes
For long-term load analysis, to determine peak hours, days, or months, set the interval to 10 minutes

81. Bandwidth Utilization by Protocol
Relative Network Utilization
Absolute Network Utilization
Broadcast Rate
Multicast Rate
Protocol 1
Protocol 2
Protocol 3
Protocol n

82. Accuracy Bit error rate (BER) Frame error rate (FER) Packet loss
Collision
Runt (partial) frame
Healthy network should not have more than one bad frame per megabyte of data

83. Characterize Packet Sizes
Increasing the maximum transmission unit (MTU) on router interfaces can also improve efficiency
Increasing MTU can increase serialization delay

84. Characterize Packet Sizes (Contd.)

85. Characterize Packet Sizes (Contd.)
Small frames consist of control information and acknowledgments
Data frames fall into the large frame-size categories
Frame sizes typically fall into what is called a bimodal distribution

86. Characterize Response Time
A more common way to measure response time is to send ping packets and measure the round-trip time (RTT)
Variance measurements are important for applications that cannot tolerate much jitter
You can also document any loss of packets

87. Characterize Response Time (Contd.)
Node A
Node B
Node C
Node D
Node A
Node B
Node C
Node D X X X X
node = router, server, client, or mainframe

88. Checking Status of Major Devices
CPU utilization
How many packets it has processed
How many packets it has dropped
Status of buffers and queues
You can use SNMP or commands in the devices

89. Characterizing Network Traffic (Why?)
Analyze network traffic patterns to help you select appropriate logical and physical network design solutions to meet a customer's goals

90. Network Traffic Factors
Location of traffic sources and sinks
Traffic load
Traffic behavior

91. Traffic Flow
Information transmitted between communicating entities during a single session
Flow attributes:
addresses for each end of the flow
direction
symmetry
path
number of packets or bytes

92. Traffic Flow Types Terminal/host Client/server Peer-to-peer
Server/server
Voice over IP

93. Terminal / Host Examples: Telnet, ssh
Usually asymmetric: terminal sends a few characters and the host sends many characters
In some full-screen terminal applications, the terminal sends characters typed by the user and the host returns data to repaint the screen
The screen is usually 80 characters wide by 24 lines long, which equals 1920 characters
The full transfer is a few thousand bytes

94. Client / Server Examples: FTP, HTTP
Usually bidirectional and asymmetric
Requests are typically small frames except when writing data to the server
Responses range from 64 bytes to 1500 bytes or more, depending on the MTU of the data link layer

95. Peer-to-Peer Examples: Workgroup, videoconferencing, P2Ps
No hierarchy and no dedicated server
Usually bidirectional and symmetrical
Another example is a meeting between business people at remote sites using videoconferencing equipment
Information dissemination in a class is a client/server model

96. Server / Server
To implement directory services, to cache heavily used data, to mirror data for load balancing and redundancy, to back up data, and to broadcast service availability
Generally bidirectional
With most server/server applications, the flow is symmetrical, but in some cases there is a hierarchy of servers, with some servers sending and storing more data than others

97. VoIP
The flow associated with transmitting the audio voice is separate from the flows associated with call control
The voice flow for transmitting the digital voice is essentially peer-to-peer
The call control flow for call setup and teardown is a client/server flow

98. Traffic Load Network capacity is sufficient to avoid bottleneck
Key parameters:
Number of stations
Average time that a station is idle between sending frames
Time required to transmit a message once medium access is gained
Application usage patterns

99. Traffic Load (Contd.) Traffic load caused by applications
Terminal screen: 4 Kbytes
Simple 10 Kbytes
Simple web page: 50 Kbytes
High-quality image: 50,000 Kbytes
Database backup: 1,000,000 Kbytes or more

100. Traffic Load (Contd.) Protocol overhead IPX: 30 bytes TCP: 20 bytes
IP: 20 bytes
Ethernet: byte preamble + 12-byte interframe gap (IFG)
HDLC: 10 bytes

101. Traffic Behavior Broadcast Goes to all network stations on a LAN
All ones data-link layer destination address
FF: FF: FF: FF: FF: FF
Doesn’t necessarily use huge amounts of bandwidth
But does disturb every CPU in the broadcast domain

102. Traffic Behavior (Contd.)
Multicast
Goes to a subset of stations
01:00:0C:CC:CC:CC (Cisco Discovery Protocol)
Should just disturb NICs that registered to receive it
Requires multicast routing protocol on internetworks

103. Traffic Behavior (Contd.)
Broadcast/multicast traffic is necessary and unavoidable
share topology information
advertise services
locate services
addresses and names
No more than 20% of the network traffic, otherwise segment the network using routers or VLANs

104. Traffic Behavior (Contd.)
Layer 2 devices, such as switches and bridges, forward broadcast and multicast frames out all ports
Router does not forward broadcasts or multicasts
All devices on one side of a router are considered part of a broadcast domain
VLANs can also limit the size of a broadcast domain based on membership