1. COMPUTER NETWORK AND DESIGN
CSCI 3385K

2. What is a network?
A network is a connected collection of devices and end systems, such as computers and servers, which can communicate with each other. Networks carry data in many types of environments, including homes, small businesses, and large enterprises. In a large enterprise, there may be a number of locations that need to communicate with each other, locations in terms of where the workers are located is as follows:
Main Office:
a site where everyone is connected via a network and where the bulk of corporate information is located. Main offices can have hundreds or even thousands of people who depend on network access to do their jobs. A main office may use several connected networks, which can span many floor in an office building or cover a campus that contains several buildings.
Remote Location:
Branch Offices: Small group of people work and communicate with each other via a network. Although some information may be stored in the local branch office, it is more likely that they need to have local network resources such printer, etc. but must access information directly from the main office.
Home Offices: Users are often require to have on-demand connection to the main offices
Mobile Users: Users connect to the main office network while at the main office, branch, or traveling

3. What is a network?
You may use a network in your home office to communicate via the Internet to locate information, place orders for merchandise and send messages to friends. Or you may have a small office that is set up with a network that connects other computers and printers in the office. Or you may work in a large enterprise in which there are many computers, printers, storage devices and servers that are used to communicate and store information from many departments over large geographic areas.

4. Common Physical Components
Four major categories of physical components in a computer network:
Personal computers (PCs): The computer serve as end points in the network, sending and receiving data
Interconnections: the interconnections consist of components that provide means for data to travel from one point to another point in the network. This category includes components such as the following:
Network Interface Cards: translate the data produced by the computer into a format that can be transmitted over the local network
Network media: cables or wireless media that provides the means by which the signals are transmitted from one networked device to another
Connectors: provides the connection points for the media
Switches: Switches are devices that provide network attachment to the end systems and intelligent switching of the data within the local area network
Routers: Routers interconnect networks and choose the best path between networks

5. Common Physical Components

6. Interpreting a Network Diagram
The network diagram is used to capture information related to the network. The amount of information and the details differs from organization. The network topology is represented by a series of lines and icons

7. Interpreting a Network Diagram

8. Resources-sharing function and benefits
Network allows end users to share both information and hardware resources. The major resources that are shared in a computer network include the following:
Data and applications: when users are connected through a network, they can share files and even software application programs, making data more easily available and promoting more efficient collaboration on work projects
Resources: resources that can be shared include both input devices, such cameras and output devices such printers
Network storage: today there are several ways in which the network makes storage available to users. Direct attached storage (DAS) directly connects physical storage to a PC or a shared server. Network attached storage (NAS) makes storage available through special network appliance. Finally, storage area networks (SANs)
Backup devices: a network can also include backup devices, such tape drives, that provide central means to save files from multiple computers. Network storage is also used to provide archive capabilities, business continuance and recovery disaster.

9. Resources-sharing functions and benefits

10. Network storage

11. Network user applications
The most common network user applications include the following:
very valuable application for most users, users can communicate information (messages and files) electronically in a timely manner, to not only users in the same network but also outside the network.
Web browser: allows access to the Internet through a common interface. The Internet provides a wealth of information and it is vital to the productivity of both home and business users. Browsers allows to communicate with suppliers, customer, handling orders, and locating information saving time and increasing overall productivity.
Instant messaging: started as personal user-to-user space; however provided considerable benefit in the corporate world. There are many instant messaging applications that provide data encryption and logging, which are essential for corporate use.
Collaboration: working together as individuals or groups is greatly facilitated when collaborators are on the network. Users creating separate parts of an annual report or business plan, for example data files can be sent to a central resource for later compilation.
Database: this type of application allows users on a network to store information in central location (file servers) so that other users on the network can easily retrieve selected information in the format that are most useful to them.

12. Impact of users applications on the network
Batch application: FTP and TFTP, inventory updates would be initiated by the user then there is no further direct human interaction. Bandwidth is important but no critical as long as the time it took the application to complete does not become excessive
Interactive applications: Inventories queries, updates required human interaction, the user will request some type of information from the server and wait for a reply. Bandwidth become more important as uses become impatient with slow responses but still is not critical. In most cases QoS (Quality of Service) could overcome bandwidth limitation by giving priority over batch applications

13. Impact of users applications on the network
Real-time application: VoIP and video application involve human interaction and because the amount of information that is transmitted, bandwidth has become critical. In addition because this application are time-critical, latency (delay through the network) has become critical, even variations in the amount of delay can affect the network and not only bandwidth is mandatory but QoS is mandatory as well. VoIP and video must be given the highest priority. With the proper network design issues as poor QoS and latency can be resolved.

14. Characteristics of a Network
Network can be described and compared according to network performance and structure as follows:
Speed: Measure of how fast data is transmitted over the network. A more precise term would be data rate.
Cost: Indicates the general cost of component, installation and maintenance of the network.
Security: Indicates how secure the network is, including the data is transmitted over the network. The subject of security is important and constantly evolving. You should consider security whenever you take actions that affect the network
Scalability: Indicates how well the network can accommodate more users and data transmission requirements. If a network is design and optimized for just the current requirement, it can be very difficult and expensive to meet new needs when the network grows.
Reliability: Indicates the dependability of the components (routers, switches, PCs, etc.) that make up the network. This is measure as probability of failure, or mean time between failures (MTBF)

15. Characteristics of a Network – cont.
Availability: Measure of the probability that the network will be available for use when it is required. For networks to be used 24 hours a day, 7 days a week, 365 days a year, the availability is calculated by dividing the time is actually available by the total time in a year and the multiplying by 100 to get a percentage.
For example: if a network is unavailable for 15 minutes a year because of network outages its percentage availability can be calculated as follows:
([number of minutes in year – downtime] / [number of minutes in year]) * 100 = Percentage availability
([ – 15] / [525600]) * 100 =

16. Characteristics of a Network – cont.
Topology: In networks, there are two types of topologies
Physical: which is the arrangement of cables, network devices and end systems (PCs and servers)
Logical: which is the path the data signals take through the physical topology
These characteristics and attributes will provide a means to compare different networking solutions

17. Logical Topology
Refers to the logical path that the signal use to travel from one point on the network to another, that is, the way in which data accesses the network media and transmits packets across it.
Physical and logical topologies could be same but at the same time could have different topologies
Star topology is by far the most common implementation of LAN, Ethernet uses logical bus topology in either a physical bus or physical star.

18. Logical Topology

19. Physical Topologies

20. Physical Topology – cont.
Refers to the physical layout of the devices and cabling.
You must match the appropriate physical topology to the type of cabling (twisted pair, coaxial, fiber, and so on) that will be installed.
You must understand the type of cabling used is important in understanding each type of physical topology
Three primary physical categories
Bus
Ring
Star

21. Physical Topology – Bus Topology
Consist of cables connecting PCs or file servers together like a chain
Terminator attached to each end of bus cable segment
There is a beginning and an end to each segment
The main cable segment must end with a terminator that absorbs the signal when it reaches the end of the line of the wire
If there is no terminator the electrical signal representing the data bounces back at the end of the wire, causing errors in the network.
When transmitting a packet across a bus:
It is detected by all nodes on the segment
Given time limit to reach destination or it is considered late
IEEE (Institute of Electrical and Electronics Engineers)
Develops standards for network cabling, transmission

22. Physical Topology – Bus Topology
Early bus topologies were cable together in line using coaxial cable
Modern bus topologies establish the bus in a hardware device and connect the host devices to the bus using twisted-pair wiring
Advantages:
Requires less cable than other topologies
Easy to extend bus with a workstation
Disadvantages:
High management costs
Single defective node can take down entire network
Can become quickly congested with network traffic

23. Physical Topology – Bus Topology

24. Physical Topology – Star Topology
Multiple nodes attached to central device (hub, switch, router)
Cable segments radiate from center like a star
Example: Workstations/Servers connected to a switch
Most common physical topology in Ethernet LAN
Advantages:
Start-up costs are much lower than other topologies
Easier to manage, defective nodes quickly isolated
Easier to expand by connecting nodes or networks
Offers better equipment and high-speed options
Cabling and connector are less expensive
Disadvantages:
Failure of central device may cause network failure
Requires more cable than bus

25. Physical Topology – Star Topology

26. Physical Topology – Extended-Star Topology
When a star network is expanded to include an additional network device that is connected to the main network devices
Disadvantage:
The problem with a pure extended-star topology is that if the central node point fails, large portions of the network can be isolated

27. Physical Topology – Extended-Star Topology

28. Physical Topology – Ring Topology
Continues data path with no beginning or ending point
Workstations attached to cable at points around ring
Transmitting data across ring topology
Goes around ring to reach destination
Continues until ends at source node
Advantages:
Easier to manage than bus
Suited to transmitting signal over long distances
Disadvantages:
More expensive to implement than bus
Has become outdated so there are fewer equipment options

29. Physical Topology – Ring Topology

30. Physical Topology – Dual-Ring Topology
In dual-ring topology two rings allow data to be sent in both directions.
This setup creates redundancy (fault tolerance), meaning that if one ring fails, data can be transmitted on the other ring

31. Physical Topology – Dual-Ring Topology

32. Physical Topology – Full Mesh Topology
Every node is connected to every other node in the network
Provides fault tolerance
Fault tolerance: built-in protection against failure
If link breaks, nodes can still communicate
Alternate communication path increases as number of nodes increases
Mesh is used less on cabled LANs
Expensive and difficult to implement
Often used in MANs and WANs due to reliability
How to calculate the number of connections in a mesh
n(n-1) 2

33. Physical Topology – Full Mesh Topology

34. Physical Topology – Partial Mesh Topology
At least one device maintains multiple connections to all other devices, without being fully mesh
This methods trades off the cost of meshing all devices by allowing the network designer to choose which nodes are the most critical and properly interconnect them

35. Physical Topology – Partial Mesh Topology

36. Connection to the Internet
Three common methods of connecting the small office to the internet
DSL (Digital Subscriber Line)
Uses the existing telephone lines and a DSL modem
Modem is a device that convert the incoming digital encoding into Ethernet format
Cable
Uses cable television (CATV) infrastructure and a Cable modem
Serial
Uses the classic digital local loop and a CSU/DSU (Channel service unit/Data service unit)
CSU/DSU is a device that converts a digital data frame from the LAN technology to a frame appropriate to the WAN technology or vise versa
All three methods the Ethernet output is sent to a router that is part of the customer premises equipment (CPE)

37. Connection to the Internet

38. Network Design Introduction
Step 1: Understand protocols, access methods, topologies
Example: Telecommunications-based WAN vs satellite-based WAN
Step 2: Understand physical equipment used
Example: Different media is used for backbone and internal network nodes
Step 3: Understanding basic network design principles
Structured wiring and networking techniques
Designing for multimedia and client/server applications
Taking advantage of LAN and WAN characteristic

39. Network Design Introduction
Step 4: Assess characteristics of the planned network
Types of computers used as well as location
Software applications used and resources required
Pattern in organization relative to network use
High and low network use periods
How simplify troubleshooting and maintenance
Determine security need for the network
Anticipate how growth will affect network resources

40. Designing a simple LAN
Scenario: Small law office with 4 attorneys & 1 secretary
Four components of solid design
Star topology
Switch connecting computes in middle of star layout using network cable
Ability to share certain information on network
Ability to share printers on network

41. Designing a simple LAN Rationale for the design:
Star economical to implement and maintain
Using cable instead of wireless due to sources of interference and building construction
Use of switch satisfies need for fast communication
Resource sharing using peer-to-peer network instead of using a server
Clients decide which files to share
Printers can be shared
Network will be simple and easy to maintain
Internet access easily added

42. Designing a simple LAN