Summary of 2nd Chapter (Module)

Summary
This page summarizes the topics discussed in this module.


Computer networks developed in response to business and government computing needs. Applying standards to network functions provided a set of guidelines for creating network hardware and software and provided compatibility among equipment from different companies. Information could move within a company and from one business to another.

Network devices, such as repeaters, hubs, bridges, switches and routers connect host devices together to allow them to communicate. Protocols provide a set of rules for communication.

The physical topology of a network is the actual layout of the wire or media. The logical topology defines how host devices access the media. The physical topologies that are commonly used are bus, ring, star, extended star, hierarchical, and mesh. The two most common types of logical topologies are broadcast and token passing.

A local-area network (LAN) is designed to operate within a limited geographical area. LANs allow multi-access to high-bandwidth media, control the network privately under local administration, provide full-time connectivity to local services and connect physically adjacent devices.

A wide-area network (WAN) is designed to operate over a large geographical area. WANs allow access over serial interfaces operating at lower speeds, provide full-time and part-time connectivity and connect devices separated over wide areas.

A metropolitan-area network (MAN) is a network that spans a metropolitan area such as a city or suburban area. A MAN usually consists of two or more LANs in a common geographic area.

A storage-area network (SAN) is a dedicated, high-performance network used to move data between servers and storage resources. A SAN provides enhanced system performance, is scalable, and has disaster tolerance built in.

A virtual private network (VPN) is a private network that is constructed within a public network infrastructure. Three main types of VPNs are access, Intranet, and Extranet VPNs. Access VPNs provide mobile workers or small office/home office (SOHO) users with remote access to an Intranet or Extranet. Intranets are only available to users who have access privileges to the internal network of an organization. Extranets are designed to deliver applications and services that are Intranet based to external users or enterprises.

The amount of information that can flow through a network connection in a given period of time is referred to as bandwidth. Network bandwidth is typically measured in thousands of bits per second (kbps), millions of bits per second (Mbps), billions of bits per second (Gbps) and trillions of bits per second (Tbps). The theoretical bandwidth of a network is an important consideration in network design. If the theoretical bandwidth of a network connection is known, the formula T=S/BW (transfer time = size of file / bandwidth) can be used to calculate potential data transfer time. However the actual bandwidth, referred to as throughput, is affected by multiple factors such as network devices and topology being used, type of data, number of users, hardware and power conditions.

Data can be encoded on analog or digital signals. Analog bandwidth is a measure of how much of the electromagnetic spectrum is occupied by each signal. For instance an analog video signal that requires a wide frequency range for transmission cannot be squeezed into a smaller band. If the necessary analog bandwidth is not available the signal cannot be sent. In digital signaling all information is sent as bits, regardless of the kind of information it is. Unlimited amounts of information can be sent over the smallest digital bandwidth channel.

The concept of layers is used to describe communication from one computer to another. Dividing the network into layers provides the following advantages:

• Reduces complexity
• Standardizes interfaces
• Facilitates modular engineering
• Ensures interoperability
• Accelerates evolution
• Simplifies teaching and learning

Two such layered models are the Open System Interconnection (OSI) and the TCP/IP networking models. In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function: application, presentation, session, transport, network, data link, and physical. The TCP/IP model has the following four layers: application, transport, Internet, and network access.

Although some of the layers in the TCP/IP model have the same name as layers in the OSI model, the layers of the two models do not correspond exactly. The TCP/IP application layer is equivalent to the OSI application, presentation, and session layers. The TCP/IP model combines the OSI data link and physical layers into the network access layer.

No matter which model is applied, networks layers perform the following five conversion steps in order to encapsulate and transmit data:

1. Images and text are converted to data.
2. The data is packaged into segments.
3. The data segment is encapsulated in a packet with the source and destination addresses.
4. The packet is encapsulated in a frame with the MAC address of the next directly connected device.
5. The frame is converted to a pattern of ones and zeros (bits) for transmission on the media.