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					Chapter 9
Network
 Design



   Networking
      in the
  Internet Age
   by Alan Dennis


                                           1
Copyright © 2002 John Wiley & Sons, Inc.
Copyright 2002John Wiley & Sons, Inc. All rights reserved.
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contained herein.

                                                                 2
   Chapter 9. Learning Objectives
• Be familiar with the overall process of design
  and implementing a network
• Be familiar with techniques for developing a
  logical network design
• Be familiar with techniques for developing a
  physical network design
• Be familiar with network design principles

                                               3
                 Chapter 9. Outline
• Introduction
   – Traditional Network Design, Building Block Network
     Design
• Needs Analysis
   – Geographic Scope, Application Systems, Network Users,
     Categorizing Network Needs, Deliverables
• Technology Design
   – Designing Clients and Servers, Designing Circuits and
     Devices, Network Design Tools, Deliverables
• Cost Assessment
   – Request for Proposal, Selling the Proposal to Management,
     Deliverables
• Designing for Network Performance
   – Managed Networks, Network Circuits, Network Devices,
     Minimizing Network Traffic
                                                            4
Introduction




               5
    Traditional Network Design
• The traditional network design approach
  follows a structured systems analysis
  and design process similar to that used to
  build application systems.
  – The network analyst meets with users to determine
    the needs and applications.
  – The analyst estimates data traffic on each part of
    the network.
  – The analyst designs circuits needed to support this
    traffic and obtains cost estimates.
  – Finally, a year or two later, the network is
    implemented.
                                                          6
     Traditional Network Design
• Three forces are making the traditional
  design approach less appropriate for many
  of today’s networks:
  – 1. The underlying technologies used in
    computers, networking devices and network
    circuits are rapidly changing.
  – 2. Network traffic is growing rapidly.
  – 3. The balance of costs has changed
    dramatically over the last 10 years.
                                                7
     Building Block Network Design
              (Figure 9-1)
• While some organizations still use the traditional
  approach, many others use a simpler approach to
  network design, the building block approach.
• This approach involves three phases: needs
  analysis, technology design, and cost assessment.
• When the cost assessment is initially completed,
  the design process returns to the needs analysis
  phase and cycles through all three phases again,
  refining the outcome of each phase.
• The process of cycling through all three design
  phases is repeated until a final design is decided
  on (Figure 9-2).
                                                       8
Figure 9-1 Network Design   9
Fig. 9-2 The cyclical nature of network design 10
Needs Analysis




                 11
                Needs Analysis
• The first step is to analyze the needs of network users
  along with the requirements of network applications.
• Most efforts today involve upgrades and not new network
  designs, so most needs may already be understood.
• LAN and BN design issues include improving
  performance, upgrading or replacing unreliable or aging
  equipment, or standardizing network components to
  simplify network management.
• At the MAN/WAN level, circuits are leased and upgrades
  involve determining if capacity increases are needed.
• The object of needs analysis is to produce a logical
  network design, which describes what network elements
  will be needed to meet the organization’s needs.

                                                            12
   Geographic Scope (Figure 9-3)
• Needs analysis begins by breaking the
  network into three layers based on their
  geographic and logical scope:
  – The access layer which lies closest to the user
  – The distribution layer which connects the
    access layer to the rest of the network
  – The core layer which connects the different
    parts of the distribution layer together.


                                                      13
Figure 9-3 Geographic Scope   14
         Application Systems
• The designers must review the applications
  currently used on the network and identify
  their location so they can be connected to
  the planned network (baselining).
• Next, applications expected to be added to
  the network are included.
• It is also helpful to identify the hardware
  and software requirements and protocol
  type for each application.
                                                15
               Network Users

• In the past, application systems accounted for the
  majority of network traffic. Today, much network
  traffic comes from Internet use (i.e., e-mail and
  WWW).
• The number and type of users that will generate
  network traffic may thus need to be reassessed.
• Future network upgrades will require
  understanding how the use of new applications,
  such as video, will effect network traffic.
                                                   16
     Categorizing Network Needs

• The next step is to assess the traffic generated in
  each segment, based on an estimate of the relative
  magnitude of network needs (i.e. typical vs. high
  volume). This can be problematic, but the goal is a
  relative understanding of network needs.
• Once identified, network requirements should be
  organized into mandatory requirements, desirable
  requirements, and wish list requirements.

                                                    17
               Deliverables

• The key deliverable for the needs
  assessment stage is a set of network maps,
  showing the applications and the circuits,
  clients, and severs in the proposed network,
  categorized as “typical” or “high volume”.




                                             18
Figure 9-4 Sample needs assessment   19
Technology Design




                    20
          Technology Design

• After needs assessment has been completed,
  the next design phase is to develop a
  technology design (or set of possible
  designs) for the network.




                                           21
    Designing Clients and Servers
• In the building block approach, the technology
  design is specified by using “standard” computer
  units:
   – “Typical” users are allocated “base level” client
     computers, as are servers supporting “typical”
     applications.
   – “High volume” users and servers are assigned
     “advanced” computers.
   – The definition for a standard unit, however,
     keeps changing as hardware costs continue to
     fall.

                                                     22
   Designing Circuits and Devices
• Two interrelated decisions in designing network
  circuits and devices are: 1) deciding on the
  fundamental technology and protocols and 2)
  choosing the capacity each circuit will operate at.
• Capacity planning means estimating the size and
  type of the “standard” and “advanced” network
  circuits for each type of network.
• This requires some assessment of the current and
  future circuit loading in terms of average vs. peak
  circuit traffic.

                                                    23
       Estimating Circuit Traffic
• The designer often starts with the total
  characters transmitted per day per circuit, or
  if possible, the maximum number of
  characters transmitted per two second
  interval if peak demand must be met.
• While no organization wants to overbuild
  its network and pay for unneeded capacity,
  going back and upgrading a network often
  significantly increases costs.
                                               24
         Network Design Tools
• Network modeling and design tools can perform a
  number of functions to help in the technology design
  process.
• Some modeling tools require the user to create the
  network map from scratch. Other tools can “discover”
  the existing network.
• Once the map is complete, the next step is to add
  information about the expected network traffic and see
  if the network can support the level of traffic that is
  expected. This may be accomplished through
  simulation models.
• Once simulation is complete, the user can examine the
  results to see the estimated response times and
  throughput.
                                                            25
             Deliverables

• The key deliverables at this point are a
  revised set of network maps that
  include general specifications for the
  hardware and software required.
• In most cases the crucial issue is the
  design of the network circuits.



                                             26
Figure 9-5 Physical Network Design   27
Cost Assessment




                  28
                Cost Assessment
• Cost assessment’s goal is to assess the costs of
  various network alternatives produced as part of
  technology design. Costs to consider include:
   Circuit costs for both leased circuits and cabling.
   Internetworking devices such as switches and routers.
   Hardware costs including servers, memory, NICs & UPSs.
   Software costs for operating systems, application software
     and middleware.
   Network management costs including special hardware,
     software, and training.
   Test and maintenance costs for monitoring equipment and
     supporting onsite repairs.
   Operations costs to run the network.
                                                           29
      Request for Proposal (RFP)
• While some components can be purchased “off-
  the-shelf”, most organizations develop an RFP
  before making large network purchases.
• The RFP creates a competitive environment for
  providing network equipment and services (see
  Figure 9-6).
• Once vendors have submitted network proposals,
  the organization evaluates them against specific
  criteria and selects the winner(s).
• Multi-vendor selections have the advantage of
  maintaining alternative equipment and services
  sources, but are also more difficult to manage.
                                                     30
              Figure 9-6. Request for Proposal
• Background Information
   – Organizational profile; Overview of current network; Overview of new
     network; Goals of the new network
• Network Requirements
   – Choice sets of possible network designs (hardware, software, circuits);
     Mandatory, desirable, and wish list items, Security and control requirements;
     Response time requirements; Guidelines for proposing new network designs
• Service Requirements
   – Implementation time plan; Training courses and materials; Support services
     (e.g., spare parts on site); Reliability and performance guarantees
• Bidding Process
   – Time schedule for the bidding process; Ground rules; Bid evaluation criteria;
     Availability of additional information
• Information Required from Vendor
   – Vendor corporate profile; Experience with similar networks; Hardware and
     software benchmarks; Reference list
                                                                                  31
Selling the Proposal to Management
• An important hurdle to clear in network design is
  obtaining the support of senior management.
• Gaining acceptance from senior management lies
  in speaking their language and presenting the
  design in terms of easily understandable issues.
• Rather than focusing on technical issues such as
  upgrading to gigabit Ethernet, it is better to make a
  business case by focusing on organizational needs
  and goals such as comparing the growth in
  network use with the growth in the network
  budget.
                                                     32
                Deliverables

• There are three key deliverables for this step:
  1. An RFP issued to potential vendors.
  2. After the vendor has been selected, the revised
    set of network maps including the final
    technology design, complete with selected
    components.
  3. The business case written to support the
    network design, expressed in terms of business
    objectives.

                                                   33
Designing for Network
    Performance




                        34
  Network Management Software
• Network management software is designed
  to provide automated support for some or
  all of the network management functions
  (Figure 9-8 shows an example).
• There are three fundamentally different
  types of network management software:
  – Device management software
  – System management software
  – Application management software
                                             35
Figure 9-8 Network management software (Source: HP OpenView)
                                                        36
   Network Management Software

• One major problem is ensuring that hardware
  devices from different vendors can understand and
  respond to the messages sent by the network
  management software of other vendors.
• The two most commonly used network
  management protocols are:
   – Simple Network Management Protocol (SNMP, part of
     the TCP/IP protocol suite)
   – Common Management Interface Protocol (CMIP,
     developed by ISO)

                                                     37
   Simple Network Management Protocol
             (See Figure 9-9)
• SNMP: TCP/IP suite protocol for network
  management that allows agents to communicate
  with each other and other network devices
• Agents: programs residing on network devices
  that gather and share network status information
• Management Information Bases (MIBs):
  databases of network status statistics such as
  traffic levels, error rates & data rates
• Network Management Console: when requested,
  data from the MIBs is sent to a Network
  Management Console.
                                                 38
Fig. 9-9 Network management with SNMP   39
      Policy-based Management
• In policy-based management, the network
  manager uses special software to set priority
  policies for network traffic.
• These take effect when the network
  becomes busy.
• For example, videoconferencing might be
  given a high priority since delays will have
  the highest impact on the performance of
  that application.

                                             40
             Traffic Analysis
• The easy way to manage network traffic growth is
  simply to upgrade heavily used circuits.
• A more sophisticated approach is to do traffic
  analysis. Consider the network in Figure 9-10:
   – The Toronto-Dallas network segment is
     heavily used (thick line), but the traffic is
     mostly moving between LA and NY.
   – The solution is to create a new LA to NY
     segment (dashed line).
                                                 41
Figure 9-10 Simple WAN   42
      Service Level Agreements
• More organizations establish service level
  agreements (SLAs) with common carriers.
• SLAs specify the type of performance and
  fault conditions for their leased circuits.
• For example, a 99.9% availability means
  the circuit will be down for 8.76 hours/year.
• The SLA also often includes the maximum
  allowable response time.

                                              43
            Network Devices
• Since network devices vary in their
  characteristics, a network’s performance
  will be influenced by the devices selected to
  operate on it.
• Three important factors to network
  performance that are related to network
  device characteristics are:
  – Device latency
  – Device memory
  – Load balancing
                                              44
               Device Latency
• Latency is the delay (waiting time) that occurs
  when a device processes a message.
• Slow speed devices have high latency, while high
  speed devices have low latency.
• The fastest devices run at wire speed.
• Latency becomes a critical issue under high traffic
  conditions since high latency devices can create
  traffic congestion.
• This is similar to the way that long lines of traffic
  form at tollbooths on highways during rush hour.

                                                      45
              Device Memory
• Memory and latency are related, since any device
  that operates at less than wire speed may need to
  store newly arrived packets.
• Otherwise packets will need to be retransmitted,
  making the traffic situation worse.
• Memory is also important for servers since
  memory access speeds are many times faster than
  hard disk access times.
• The larger the memory a server has, the more
  likely it is able to process a request quickly, so
  Web and file servers should have the greatest
  amount of memory practical.
                                                       46
            Load Balancing
• Load balancing means sharing the
  processing load between servers.
• A separate load balancing server is usually
  needed to allocate the work between
  processors.
• The load-balancing server then allocates
  tasks to the other processors, using an
  algorithm such as a round robin formula.
• An example of this is shown in Figure 9-11.

                                            47
Figure 9-11 Network with load balancer   48
    Minimizing Network Traffic
• An alternative way of improving network
  performance is to minimize network traffic.
• This can be done by shifting some of the
  data so it resides closer to the users (for
  example, a mirrored web site).
• Two current approaches to minimize traffic
  flow are content caching and content
  delivery.

                                            49
   Content Caching (Figure 9-12)
• Content caching means storing frequently used
  web pages locally, using a cache engine.
• Web requests do not go out directly, they are first
  shunted by the router to the cache engine to see if
  they are available locally.
• Traffic volume is lowered since many frequently
  requested web sites, such as yahoo.com, can be
  retrieved from the cache and don’t need to go out
  on the Internet.

                                                    50
Figure 9-12 Network with cache engine   51
Content Delivery (see Figure 9-13)
• Another way to minimize network traffic is for
  web site operators to move content closer to users,
  called content delivery, is done by operating web
  servers near NAPs, MAEs and other exchanges to
  minimize network traffic.
• If a web page of one of the content deliverers
  client’s is accessed, it checks if any web page
  components are located on a server near the
  requesting computer and sends those.
• This benefits both the Web provider by lessening
  demand on its Web servers, the ISP by lowering
  demand on its Internet circuits, as well as
  decreasing Internet traffic overall.
                                                    52
Figure 9-13 Network with content delivery   53
End of Chapter 9




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