Functions & Components of LAN

Exploring the Functions of Networking

(Compiled by Vladislav Spector)

What is a LAN

A local area network is a computer network covering a small geographic area, like a home, office, or group of buildings e.g. a school.

The defining characteristics of LANs, in contrast to Wide Area Networks (WANs), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines.

History of LAN

The first LAN put into service occurred in 1964 at the Livermore Laboratory to support atomic weapons research. LANs spread to the public sector in the late 1970s and were used to create high-speed links between several large central computers at one site. Of many competing systems created at this time, Ethernet and ARCNET were the most popular.

Initially, LANs were limited to a range of 185 meters or 600 feet and could not include more than 30 computers. Today, a LAN could connect a max of 1024 computers at a max distance of 900 meters or 2700 feet.

Functions of a LAN

· Data and applications

· Resources (file sharing, print sharing)

· Network storage

· Backup devices

· Communication path to other networks

Network User Applications

· E-mail (Microsoft Outlook, Yahoo, GMail and so on)

· Web browser (IE, Firefox, and so on)

· Instant messaging (Skype, Microsoft Messenger, and so on)

· Collaboration (Whiteboard, Netmeeting, WebEx, and so on)

· Databases

LAN Components

§ Computers, PCs, Servers

§ Interconnections: NICs, Media

§ Network devices: Hubs, Switches, Routers

§ Protocols: LAN, Network

Network card

A network card, network adapter, LAN Adapter or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network.

It is both an OSI layer 1 (physical layer) and layer 2 (data link layer) device, as it provides physical access to a networking medium and provides a low-level addressing system through the use of MAC addresses.

Network hub

A network hub or concentrator is a device for connecting multiple twisted pair or fiber optic Ethernet devices together, making them act as a single network segment.

Network switch

A network switch is a computer networking device that connects network segments.

Low-end network switches appear nearly identical to network hubs, but a switch contains more "intelligence" than a network hub. Network switches are capable of inspecting data packets as they are received, determining the source and destination device of that packet, and forwarding it appropriately.

Router

A router is a computer whose software and hardware are usually tailored to the tasks of routing and forwarding, generally containing a specialized operating system (e.g. Cisco's IOS).

Network Topology

Network topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network, especially the physical (real) and logical (virtual) interconnections between nodes

Physical Topology Categories

· Bus Topology - All devices receive the signal.

· Star Topology - Transmission through a central point, Single point of failure.

· Ring Topology - Signals travel around ring, Single point of failure.

· Full-Mesh Topology - Highly fault-tolerant, Expensive to implement.

· Partial-Mesh Topology - Trade-off between fault tolerance and cost.

Summary of Network

§ A network is a connected collection of devices that can communicate with each other. Networks carry data in many kinds of environments, including homes, small businesses, and large enterprises.

§ There are four major categories of physical components in a computer network: the computer, interconnections, switches, and routers.

§ The major resources that are shared in a computer network include data and applications, peripherals, storage devices, and backup devices.

§ The most common network user applications include e-mail, web browsers, instant messaging, collaboration, and databases.

§ User applications affect the network by consuming network resources.

§ The ways in which networks can be described include characteristics that address network performance and structure: speed, cost, security, availability, scalability, reliability, and topology.

§ A physical topology describes the layout for wiring the physical devices, while a logical topology describes how information flows through a network.

§ In a physical bus topology, a single cable effectively connects all the devices.

§ In a physical star topology, each device in the network is connected to the central device with its own cable.

§ When a star network is expanded to include additional networking devices that are connected to the main networking device, it is called an extended-star topology.

§ In a ring topology, all the hosts are connected in the form of a ring or circle. In a dual-ring topology, there are two rings to provide redundancy in the network.

§ A full-mesh topology connects all devices to each other; in a partial-mesh topology, at least one device has multiple connections to all other devices.

§ There are three common methods of connecting the small office to the Internet: DSL using the existing telephone lines, cable using the CATV infrastructure, and serial links using the classic digital local loops.

Understanding the Host-to-Host Communications Model

Understanding Host-to-Host Communications

§ Nonstandards-based Older model (before OSI)

– Proprietary (IBM SNA, Digital DECnet)

– Application and combinations software controlled by one vendor

§ Standards-based model (OSI)

– Multivendor software

– Layered approach

Why a Layered Network Model?

§ Simplifies teaching and learning

§ Reduces complexity

§ Facilitates modular engineering (free changing of Protocols)

§ Standardizes interfaces

Communication between Hosts

(Horizontal Communication)

Communication between Layers
(Vertical Communication)

q Data Encapsulation

Moving from the top, down - messages get larger and larger ! !  

A message is passed down, and the lower layer adds a header to it. 

This is called encapsulation, because it is like placing an object into a capsule. 

The header is sometimes called a wrapper. 

Each successive lower layer encapsulates what it receives from the layer above it. 

q De-Encapsulation

Moving from the bottom, up - messages get smaller and smaller ! ! 

A message is first stripped of it's header, and then the inner contents (the "data" portion) is passed up. 

This is "decapsulation" but no one uses that term. 

Each successive upper layer receives the data message from the layer below, and then strips off it's own header and passes the data up.

Although there are seven layers in the OSI model,

they can be grouped into three areas:

q High-level Protocols (layers 5, 6 and 7  -  Session, Presentation, and Application) - how the data is presented, displayed, and summarized for the user  -  and in the reverse direction, how the user prepared data is assembled into meaningful data structures (high-level protocols).

q Medium-level Protocols (Layers 3 and 4 - Network and Transport) - how the data is assembled into packets and frames and how error checking and flow control is implemented - and in the reverse direction, how the received packets and frames are assembled into structures such as files and databases (medium-level protocols)

q Low-level Protocols (Layers 1 and 2 - Physical and DataLink) - how the data is converted into electrical pulses of ones's and zero's (bits) and sent across cables or the physical medium, and in the reverse direction, how the electrical pulses are taken off the cable and converted to ones and zero's.

TCP/IP Stack

§ Defines four layers

§ Uses different names for Layers 1 through 3

§ Combines Layers 5 through 7 into single application layer

Summary

§ The OSI reference model defines the network functions that occur at each layer.

§ The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

§ The data link layer defines how data is formatted for transmission and how access to the physical media is controlled.

• The network layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.

Troubleshooting networking using the OSI model

When troubleshooting networking it is always sensible to approach the problem from the perspective of the OSI model. The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. The beauty of this model is the fact that you can individually troubleshoot every layer using simple methods. I suggest working from layer 1 upwards until you find the problem.

Physical (Layer 1)

This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level.

It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.

Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Data Link (Layer 2)

Ethernet, ATM, Frame Relay, etc.  At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization.

The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.

The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it.

The LLC layer controls frame synchronization, flow control and error checking.

Network (Layer 3)

Typically IP (the bottom half of TCP/IP).  This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node.

Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Transport (Layer 4)

Usually TCP (the top half of TCP/IP).

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control.

It ensures complete data transfer.

Session (Layer 5)

This layer establishes, manages and terminates connections between applications.

The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end.

It deals with session and connection coordination.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa.

The presentation layer works to transform data into the form that the application layer can accept.

This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

This layer looks at things like JPEG, MPEG, MIDI, QUICKTIME and other files of the same nature. Most of your troubleshooting will be with the applications that create them (at layer 7) but be aware that you can hex files to look at the structure and change them.
Application (Layer 7)

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
If all of the other layers are working and have been tested, then this is usually just a matter of applying patches to software or reinstalling. Everyone probably has experience troubleshooting problems in windows. Telnet is an excellent tool for connecting to virtually any port to check to see if the above layers are functioning properly