Business Data Communications and Networking, 6th ed. FitzGerald and Dennis

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Chapter 10 Network Design

Objectives of Chapter 10 Become familiar with… the overall process of design and implementing a network commonly used MAN and WAN designs Understand commonly used backbone designs commonly used LAN designs

INTRODUCTION

Introduction The traditional network design approach follows a structured systems analysis and design process similar to that used to build application systems. The network analyst met with users to determine the needs and applications. The analyst estimated data traffic on each part of the network. The analyst designed circuits needed to support this traffic and obtains cost estimates. Finally, a year or two later, the network is implemented.

Introduction Three forces are making the traditional design approach less appropriate for many of today’s networks: 1. The underlying technology of computers, networking devices and the circuits themselves if rapidly changing. 2. The growth in network traffic is immense. 3. The balance of costs has changed dramatically over the last 10 years.

Introduction While some organizations still use the traditional approach, many others use a simpler approach to network design, the building block approach. Begin by classifying users as “typical” or “high volume.” The network is then planned by adding circuits using a few standard network designs, also classified as “typical” or “high volume.”

THE NETWORK DESIGN PROCESS

The Network Design Process The basic process involves three steps that are performed repeatedly: Needs analysis Technology design Cost assessment By cycling through these three processes, the network design settles on the final network design.

The Network Design Process

The Network Design Process

Needs Analysis The goal of needs analysis is to understand why the network and what users and applications it will support. Much of the work may have already been done monitoring existing systems, which can provide a baseline against future design requirements.

Geographic Scope The first step is to define the geographic scope of the network. A data communication network can have four basic levels of geographic scope: Wide area networks (within several states, provinces or countries) Metropolitan area networks (within a city) Campus area networks (within a series of buildings located in the same general area) Local area network (within one building)

Application Systems The designers must review the list of applications that currently use the network and identify the location of each one so that all of them will be interconnected by the planned network (baselining). In many cases, the applications will be relatively well defined. It is also helpful to identify the hardware and software requirements of each application that will use the network, and , if possible, the message type each application uses.

Network Users In the past, applications system accounted for the majority of network traffic. Today, much network traffic is produced by the discretionary use of the Internet (I.e. e-mail and WWW). Therefore, you must also assess the number and type of users that will generate and receive network traffic.

Categorizing Network Needs The next step is to assess the relative amount of traffic generated in each segment, based on some rough assessment of the relative magnitude of network needs (I.e. typical vs. high volume).This assessment can be problematic, but the goal is some relative understanding of the network needs. Once the network requirements have been identified, they also should be organized into mandatory requirements, desirable requirements, and wish list requirements.

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

Technology Design Once the needs have been defined, the next step is to develop a technology design (or set of possible designs) for the network.

Designing Clients and Servers “Typical” users are allocated the “base level” client computers, as are servers supporting “typical” applications. “High volume” users and servers are assigned some “advanced” computers.

Designing Circuits and Devices There are two interrelated decisions in designing network circuits and devices: the fundamental technology and protocols the capacity of each circuit. Designing the circuit capacity means capacity planning, 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 (average vs peak).

Designing Circuits and Devices The designer usually starts with the total characters transmitted per day on each circuit, or if possible, the maximum number of characters transmitted per two second interval if peaks must be met. Although no organization wants to overbuild its network and pay for more capacity than it needs, in most cases, going back and upgrading a network significantly increases costs.

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.

Network Design Tools 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.

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 part is the design of the network circuits.

Cost Assessment The purpose of cost assessment is to assess the costs of various network alternatives produced from the previous step. Some of the costs to consider are: Circuit costs Internetworking devices Hardware costs Software costs Network management costs Test and maintenance costs

Request for Proposal (RFP) Although some network components can be purchased “off-the-shelf”, most organizations develop an RFP before making large network purchases. Once the vendors have submitted their proposals, the organization evaluates them against specific criteria, and selects the winner(s). One of the key decisions in the RFP process is the scope (one vendor or multi-?).

Information in a Typical RFP Request for proposal Information in a Typical RFP Background Information Organizational Profile Overview of current network Overview of new network Goals of new network Network Requirements Chose 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

Request for proposal Service Requirements Bidding Process 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 lists

Selling the Proposal to Management One of the main problems in network design is obtaining the support of senior management. The key to gaining senior management acceptance lies in speaking their language. A focus on network usage, budgets, and reliability are easily understandable issues.

Deliverables There are three key deliverables for this step: An RFP that goes to potential vendors. After the vendor has been selected, the revised set of network set of maps with the technology design component complete. The business case that provides support for the network design, expressed in business objectives.

COMMON WIDE AREA NETWORK DESIGNS

Common Wide Area Network Designs Most organizations do not build their own WANs by laying cable, building microwave towers, or sending up satellites. Instead most organizations lease circuits from interexchange carriers, and use those to transmit their data. Once the major connection points one the WAN have been identified, the next step is to design the circuits that will connect those locations.

Ring-Based WAN Design A ring-based WAN design connects all computers in a closed loop, with each computer linked to the next, usually with a series of point-to-point dedicated circuits. One disadvantage is of the ring topology is that messages can take a long time to travel from the sender to the receiver. In general, the failure of one circuit or computer in the network means that the network can continue to function.

Ring-Based WAN Design

Star-Based WAN Design A star-based WAN design connects all computers to one central computer that routes messages to the appropriate computer, usually via a series of point-to-point dedicated circuits. It is easy to manage because the central computer receives and routes all messages in the networks. In general, the failure of any one circuit or computer affects only the one computer on that circuit.

Mesh-Based WAN Design Mesh-based WAN designs: full or partial mesh. The effects of the loss of computers or circuits in a mesh network depend entirely on the circuits available in the network. In general, mesh networks combine the performance benefits of both ring networks, and star networks. The drawback is that mesh networks use decentralized routing so that each computer in the network performs its own routing.

Mesh-Based WAN Design Cloud-based mesh designs are becoming very popular. With this design all computers are simply connected into a packet switched network provided by a common carrier. Cloud-based designs are simpler for the organization because they move the burden of network design and management from the organization to the common carrier.

Fish & Richardson’s WAN

COMMON BACKBONE NETWORK DESIGNS

Common Backbone Network Designs With WANs, the most important issues are centered around the geographic layout of the network and the basic topology. With backbone networks, the most important characteristic is the way in which packets are moved across the backbone. There are three basic approaches used in backbone networks to move packets from one segment to another: Routing, Bridging and Switching

Routed Backbone Design Routed backbones move packets based on their network layer address. The primary advantage is that routed backbones clearly segment each part of the network connected to the backbone. There are two disadvantages to routed backbones. 1. The routers in the network impose time delays 2. Routed networks require a lot of management.

Bridged Backbone Design Bridged backbones move packets based on their data link layer address. Advantages: Bridges tend to be less expensive than routers. Bridged backbones tend to be simpler to install. Disadvantages: Individual segment management is difficult. Network speed is slower than routed backbones since broadcast messages must be permitted to travel everywhere.

Switched Backbone Design Most switched backbones use the data link layer address to move packets . A collapsed backbone is the most common form. Advantages: Improved performance and far fewer networking devices. Potential disadvantages: More broadcast traffic and difficult to isolate and separately manage individual LANs. They also use more cable and if the switch fails, so does the network.

Department of Education’s financial aid network

COMMON LOCAL AREA NETWORK DESIGNS

Traditional LAN Design Client computers are connected to a hub that provides the physical connection. This design has dominated for years because it is simple, easy to install and manage. The introduction of Switches greatly improved response time in LANs with large traffic flows.

Traditional LAN Design

Virtual LAN Design Switches also have enables the creation of Virtual LANs (VLANs). VLANs are usually faster and provide greater opportunities to manage the flow of traffic on the LAN. VLANs are groups of computers in an intelligent switched network.

Basic Switches The front of the switch contains a series of ports exactly like the ports on the front of a hub. These ports are connected inside the switch by a switching fabric which provides connections between any two ports. It is possible to have ports running at different speed, but this could overwhelm the slower port.

Basic Switches

Intelligent Switches Intelligent switches support larger networks than the basic switch’s 8- or 16- port LANs. As well as being able to support far more computers or network connections, the key advantage is in the modularity of intelligent switches. These switches often can support several hundred ports spread over a dozen or more different modules.

Intelligent Switches

Intelligent Switches Since there is not enough capacity in the backplane to support all ports if they become the switch forms groups of connections and assigns capacity using time division multiplexing. This means that the switch no longer guarantees simultaneous transmission on all ports, but will accept simultaneous input and will switch incoming data to outgoing ports as fast as possible.

Port-Based VLANs (Layer-1 VLANs) Port-based VLANs use the physical layer port address to form the groups for the VLAN. It is logical to connect computers that are physically close together on the LAN into ports that are physically close together on the switch, and to assign ports that are physically close together into the same VLAN. This is the approach used in traditional LAN design: physical location determines the LAN, but is not always the most effective approach.

Port-Based VLANs

Building VLANs

VLANs used to balance capacity against network traffic

MAC-Based VLANs Layer-2 VLANs MAC-based VLANs use the dame data link layer addresses to form the VLAN groups. The advantage is that they are simpler to manage when computers are moved.

IP-Based VLANs Layer-3 VLANs IP-based VLANs use the network layer address (i.e. TCP/IP address) to form the VLAN groups. Layer-3 VLANs reduce the time spent reconfiguring the network when a computer is moved as well. Some layer-3 VLANs can also use the network layer protocol to create VLAN groups. This flexibility enables manager even greater precision in the allocation of network capacity.

Application-Based VLANs Layer-4 VLANs Application-based VLANs use the application layer protocol in combination with the data link layer and network layer addresses to form the VLAN groups. The advantage is a very precise allocation of network capacity.

End of Chapter 10