network analysis and design introduction to network design
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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: The ends or tails of networks that
connect the small sites into the networkLAN, campus network
Backbone network:The network that connects major sitesCorporate 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 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 99.999% or “Five Nines” availability
24
Availability (Contd.)
An uptime of 99.70 % Downtime = 0.003 x 60 x 24 x 7 30.24 mins per week
An uptime of 99.95 % Downtime = 0.0005 x 60 x 24 x 7 5.04 mins per week
An uptime of 99.999 % Downtime = 0.00001 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)
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Availability (Contd.)
R1 = 99.95%, R2 = 99.5%
Series: R = 0.9995 x 0.995 = 99.45% Decreases system availability
Parallel: R = 1 – [(1 – 0.9995) x (1 – 0.995)] =
99.99975% Increases system availability
27
Availability (Contd.)
99.999% may require high redundancy (and cost)
Enterprise
ISP 1 ISP 2 ISP 3
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
SizeHeaderSizeFrame
BandwidthPPS
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
Offered Load
Throughput
Actual
Idea
l
100 % of Capacity
100 % of Capacity
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 1.544 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
0
3
6
9
12
15
0.5 0.6 0.7 0.8 0.9 1
Average Utilization
Ave
rage
Que
ue D
epth
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 802.11 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 f
eatures 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
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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.)
Gigabit Ethernet
Eugene Ethernet20 users
Web/FTP server
Grants PassHQ
16 MbpsToken Ring
FEP (Front End Processor)
IBMMainframe
T1
MedfordFast Ethernet
50 users
RoseburgFast Ethernet
30 usersFrame Relay
CIR = 56 KbpsDLCI = 5
Frame RelayCIR = 56 Kbps
DLCI = 4
Grants PassHQ
Fast Ethernet75 users
InternetT1
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 1Subnets 10.108.16.0 -
10.108.31.0
Area 0Network
192.168.49.0
Area 2Subnets 10.108.32.0 -
10.108.47.0
Router A Router B
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 troubleshooti
ng equipment
69
Wireless Installations
Reflection Signal bounces back and interferes with its
elf Metal surfaces such as steel girders, scaff
olding, 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 p
erforms 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
Enterprise
Segment 1
Segment 2
Segment n
MTBF MTTRDate and Duration of Last Major Downtime
Cause of Last Major Downtime
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
Network Utilization
0 1 2 3 4 5 6 7
17:10:00
17:07:00
17:04:00
17:01:00
16:58:00
16:55:00
16:52:00
16:49:00
16:46:00
16:43:00
16:40:00
Tim
e
Utilization (%)
79
Utilization in Hour Intervals
Network Utilization
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
17:00:00
16:00:00
15:00:00
14:00:00
13:00:00
Tim
e
Utilization (%)
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
Protocol 1
Protocol 2
Protocol 3
Protocol n
Relative Network Utilization
Absolute Network Utilization
Broadcast Rate
Multicast Rate
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 ter
minal 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 exce
pt 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 b
usiness 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 e-mail: 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: 18 + 8-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
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