Introduction to Local Area Network (LAN)

eLearning >> Networking



Introduction
 

This module introduces the Local Area Network, its protocols, topologies and transmission methods.

Upon completion of this module, you will be able to:

 
  • Define and describe the operations of the three LAN transmission methods.
  • Identify and compare the most common LAN topologies.
  • Identify and compare the most common LAN implementations.


LAN Overview

Local Area Network (LAN) Overview
 

A LAN is a group of computers, printers and other interconnected devices.

Characteristics of a LAN include:

  • Limited geographic operation
  • High speed data transfer rates
  • Full time connectivity to local services
  • Generally lower in cost than a WAN
  • Cabling is primary transmission medium

 

Major LAN Issues
 
Two major issues when designing and implementing a LAN include:
 
    1. Distance
    2. Cost
Distance
 

Local Area Networks are limited by distance.  The main reason is due to the transmission media.  At the most, most cabling can carry a data signal roughly a mile before the signal attenuates completely.  In order to keep the signal at a quality transmission level, it must be boosted by repeaters along the way.

Imagine trying to connect your network from one coast of the United States to another. After installing transmission cable across the country, devices to boost the data signal would need to be set up at specific intervals (depending on the cable used, up to every mile) to ensure proper signal strength and quality. Do you see the distance and cost issue already at hand?

Cost
 
At some point, cost is a major consideration in the design of a LAN and how large it can grow.

Illustrated with Images Text
The larger a LAN becomes, the more equipment it needs to function properly.
Eventually, a LAN must segment due to bandwidth overloading and other issues.

How far can a digital signal on a LAN travel on typical media (cat 5) before the signal attenuates completely?
 
 
100 meters
 
 
1000 feet
 
1 mile
 
 
10 miles
  Correct Answer: 3


What are the two major factors when designing and implementing a LAN? (choose all that apply)
 
 
Users
 
Cost
 
 
Time
 
Distance
  Correct Answer(s): 2, 4



LAN Transmission Methods

LAN Transmission Methods
 
LAN transmission methods fall into 3 main categories:
 
  • Unicast transmission
  • Multicast transmission
  • Broadcast transmission
Unicast Transmission
 
In unicast transmissions, a single data packet is sent from a source to a single destination on the network.

Unicast Process

  • The source addresses the packet with the destination address.
  • The packet is sent into the network.
  • The network delivers the packet to the destination.
Multicast Transmission
 
In multicast transmissions, a single data packet is copied and sent to specific destinations on the network.

Multicast Process

  • The source addresses the packet using a multicast address.
  • The packet is sent into the network.
  • The network copies the packet.
  • A copy is delivered to each destination that is included in the multicast address.
Broadcast Transmission
 
In broadcast transmissions, a single data packet is copied and sent from a source to all destinations on the network.

Broadcast Process

  • The source addresses the packet with the broadcast address.
  • The packet is sent into the network.
  • The network copies the packet.
  • The packet copies are delivered to all destinations on the network.

Which LAN transmission method is characterized by a single data packet that is copied and sent to specific destinations on the network.
 
 
Unicast Transmission
 
Multicast Transmission
 
 
Broadcast Transmission
 
 
Broadband Transmission
  Correct Answer: 2


Which LAN transmission type is characterized by a single data packet that is copied and sent from a source to a all destinations on the network.
 
 
Unicast Transmission
 
 
Multicast Transmission
 
Broadcast Transmission
 
 
Broadband Transmission
  Correct Answer: 3


 
Physical and Logical Topology

LAN Topologies
 

Before defining LAN topologies, the distinction needs to be made between Physical Topology and Logical Topology.

Physical Topology
 

Physical topology refers to the real, visual topology, or architecture of the network. It describes how network devices are physically connected by the media.

Logical Topology
 

Logical topology refers to the method the media access protocol uses the determine device priority. It creates a simple view of the basic network structure, so a determination may be made about the order in which devices transmit.


Physical and Logical topologies are typically the same.
 
 
True
 
False
  Correct Answer: 2


Which topology refers to the method the media access protocol uses the determine device priority.
 
 
Physical
 
Logical
 
 
Star
 
 
Ethernet
  Correct Answer: 2



Common LAN Topologies

LAN Logical Topologies (Architecture)
 

The most commonly implemented LAN topologies are Bus, Ring, and Star. Each layout has particular advantages and disadvantages.

These topologies have evolved as the transmission media used in LANs improved. The advancements in transmission media also led to an improvement in LAN media access protocols.

Bus Architecture
 

In a bus topology:

  • a single cable connects each workstation in a linear, daisy-chained fashion.

  • signals are broadcasted to all stations, but stations only act on the frames addressed to them.
Bus Topology Pros/Cons
 
  • The main advantage of the bus topology is the ease of initial set-up.

  • The primary disadvantage is that a break in the cable affects the entire LAN.

  • Note

    Bus topology is the traditional topology for the Ethernet standard.
    Ring Architecture
     
    In a ring topology:

  • Unidirectional links connect the transmit side of one device to the receive side of another device.

  • Devices transmit frames to the next device (downstream member) in the ring.

  • Illustrated with Images Text
    Both the Toronto and Fiji offices are examples of the ring topology.
    The Toronto office, with its single ring, is an example of the Token Ring standard.
    The Fiji office has a little variation in its layout. It is known as a FDDI Ring standard.
    Ring Topology Pros/Cons
     
     
    • The only advantage of the ring topology is that it uses the network's lengths, which are shorter, using less cabling.
    • The main disadvantage is that a cable break between devices affects the entire LAN (only in the Token Ring Standard). Cost is another factor. The ring topology usually construes higher implementation costs.
    Star Topology
     
    In a star topology, each station is connected to a central hub or concentrator that functions as a multi-port repeater. Each station broadcasts to all of the devices connected to the hub. Physical LAN topologies are usually characterized as either bus or ring. Note that a LAN's Physical and Logical topology often differs.
    Star Topology Pros/Cons
     
  • The primary advantage of the star topology is that a cable break usually affects only one workstation. Workstations can be added easily, without affecting the entire network.

  • The main disadvantage is the amount of cable used.
  • Topology Comparison
     

    Topology Use Pros Cons
    Bus Central logical topology for Ethernet

    Ease of initial set-up

    Low implementation costs

    Cable breaks affect the entire LAN
    Ring Central logical topology for Token Ring and FDDI Uses less cabling Cable breaks between devices affect the entire LAN (only in the Token Ring Standard).

    Higher implementation costs.

    Star Central physical topology for various logical implementations Cable break usually affects only one workstation

    Workstations can be added easily, without affecting the entire network

    Uses excessive cabling due to physical distance factors


    Tip

    In an Ethernet network, devices communicate as though they are on a physical bus topology; although they may actually be wired together in a physical star topology.

    Which topology is the standard for an Ethernet network?
     
    Bus
     
     
    Ring
     
     
    Star
     
     
    Ethernet Ring
      Correct Answer: 1


    Which topology is characterized by having each station connected to a central hub or concentrator that functions as a multiport repeater?
     
     
    Bus
     
     
    Ring
     
    Star
     
     
    Ethernet Ring
      Correct Answer: 3

     
    Common LAN Implementations

    Introduction
     
    There are three LAN technologies that account for virtually all deployed networks:
     
    • Ethernet/IEEE 802.3
    • Token Ring/IEEE 802.5
    • FDDI / CDDI
    Ethernet / IEEE 802.3
     
    • Ethernet is a LAN specification developed by Xerox, Intel, and Digital, which is based upon CSMA/CD and runs over various cable types. Ethernet and IEEE 802.3 are very similar protocols.

    • Both can run at 10, 100 and 1000 Mbps.

    • Both Ethernet and IEEE 802.3 run on a logical bus topology .
    Token Ring / IEEE 802.5
     

  • Token ring is a token-passing LAN development that runs on a logical ring topology .

  • Token Ring / IEEE 802.5 run at 4 and 16 Mbps

  • An active monitor device is responsible for managing a Token Ring network. This reduces the possibility of error and eliminates the possibility of collision.

  • Tip

    Don't let the number of Mbps, in comparison between Ethernet and Token Ring mislead you! A 4 or 16 Mbps Token Ring network can be just a fast (and even more stable) as a 10 or 100 Mbps Ethernet network, respectively.

    FDDI / CDDI
     

  • FDDI (Fiber Distributed Data Interface) is a fiber optics based protocol which runs on a 100 Mbps logical ring topology.

  • CDDI (Copper Distributed Data Interface) is a copper based protocol that also runs on a 100 Mpbs logical ring topology.

  • Both FDDI and CDDI use a dual-ring architecture to provide a redundant environment for data transfer.


  • A 4MB Token Ring LAN is typically a slower connection than a 10MB Ethernet LAN.
     
     
    True
     
    False
      Correct Answer: 2


    Which Common LAN implementation is based on IEEE 802.5?
     
     
    Ethernet
     
    Token Ring
     
     
    FDDI
     
     
    CDDI
      Correct Answer: 2



    Summary

    In Summary...
     
    After working through the LAN Features module, you should have a better understanding of why and how internet works came about, as well as implementation challenges.

    In review, see if you are able to answer the following:

     
    • Define and describe the operations of the three LAN transmission methods.
    • Identify and compare the most common LAN topologies.
    • Identify and compare the most common LAN implementations.


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