Tuesday, August 3, 2010

Introduction to Network Concepts III(Functions of OSI Layers)

Today I'm going to talk about functions of OSI layers those I covered on previous post.In CCNA we want to learn first 4 layers (Physical, Data-Link, Network and Transport ) deeply and other three not that mach of important because final three layers are associate with Software developers and not with Network people. So let start this with Physical Layer. 
  • Physical Layer Implementation and functions.
When we considering the implementation of physical layer its have mainly three type of implementations.
  1. Ethernet 
  2. Token Ring (Used with IBM computers )
  3. Fiber Distributed Data Interface (FDDI- Dual optical fiber ring at 100Mbps )
Now a days Ethernet is a most popular and most common Local Area Network architecture invented by Dr. Robert M.Metcalfe at Xerox Palo Alto Research Center in 1972.physical layer encoding and transmission methods have become more complex over time so Ethernet specifications is broken into different category as different network media to be supported by the same technology. Also another imporant thing is around the same time the OSI model was developed, the IEEE developed the 802-standards such as 802.5 Token Ring and 802.11 for wireless networks. Both organizations exchanged information during the development which resulted in two compatible standards. The IEEE 802 standards define physical network components such as cabling and network interfaces.

Following are the characteristics of different Ethernet specifications.

 Summary of Ethernet 802.3 Characteristics 
Standard Speed Maximum Distance Media Type Connector Used
10BASE-2 10Mbps 185m RG-58 coaxial BNC
10BASE-5 10Mbps 500m RG-58 coaxial BNC
10BASE-T 10Mbps 100m Category 3, 4, or 5 UTP or STP RJ-45
10BASE-FL 10Mbps Up to 2km Fiber-optic SC or ST

The name 10BASE-2 breaks down as follows:
  • 10—10Mbps data transmission speed
  • Base—Represents baseband, the signaling mode where the media can only send one signal per wire at a time
  • 2—Actually refers to 185m or the maximum segment length (where 185 is rounded up to 200 and 2 is a multiple of 100m)
Traditional Ethernet supports data transfers at the rate of 10 Megabits per second (Mbps). Over time, as the performance needs of LANs have increased, the industry created additional Ethernet specifications for Fast Ethernet and Gigabit Ethernet. Fast Ethernet extends traditional Ethernet performance up to 100 Mbps and Gigabit Ethernet up to 1000 Mbps speeds.  

Summary of  of Fast Ethernet 802.3u Characteristics
Standard Speed Maximum Distance Media Type Connector Used
100BaseT4 100Mbps 100m Category 3, 4, or 5 UTP or STP RJ-45
100BaseTX 100Mbps 100m Category 5 UTP or STP RJ-45
100BaseFX 100Mbps 412m with half-duplex MM fiber Fiber-optic SC or ST
For more informations see here.

First Ethernet was commercially implemented by using thick coaxial cable so called it Thick Ethernet.

Thick Ethernet ( 10Base-5)

As In images all the computers are connected to the main backbone thick coaxial cable through AUI cables. Ethernet signals convert into AUI signals in MAU( Media Access Unite) or Transceiver.

Disadvantages Of Thick Ethernet
  • Inflexible-It can be very difficult to add or move a node once it is connected to the coaxial cable.
  • Fault Intolerant-Since Thick Ethernet use common physical cable to interconnect all the nodes, the failure of any part of the coaxial cable or any node has the ability to cause the entire network to go down.
  • Susceptible To Ground Loops- a ground loop occurs when a network cable is used to interconnect devices which are powered from different sources, and therefore a difference in voltage exists between two points on the network. The result is an electrical current flowing through the shields of the cable, which causes considerable noise to be introduced into the center conductor.
  • Very Difficult Troubleshooting- Troubleshooting such a failure can be extremely frustrating, as the only way to do it is to check each node and the cabling between them one at a time.
Thick Ethernet ( 10Base-2)
Thin Ethernet got its name because the coaxial cable it runs on is thinner than the cable used for Thick Ethernet so Thin Ethernet system has a much more flexible cable that makes it possible to connect the coaxial cable directly to the Ethernet interface in the computer. The Network Interface Card (NIC) performs the functions of a transceiver so that no external transceiver is needed for workstations.

Disadvantages Of Thin Ethernet
  • Difficult To Change-Any changes to the network will result in at least some "down time," as the bus must be broken and a new section spliced in at the point of the break.
  • Fault Intolerant-If any device or cable section attached to the network fails, it will most likely make the entire network go down.
  • Difficult Troubleshooting
  • Specialized Cable-The RG-58A/U coaxial cable used in 10 Base-2 networks can not be used for any other purpose. In the event that the network is changed to another type, then the cable will have to be replaced.
Because of all these disadvantage and considering implementation feasibility ,Twisted pair Ethernet was take place main advantage of this twisted pair is in office wiring we can use same wire for both  telephone and network wiring.

Twisted-Pair Ethernet (10Base-T)  

The twisted-pair Ethernet system operates over two pairs of wires, one pair used for receive data signals and the other pair used for transmit data signals. The two wires in each pair must be twisted together for the entire length of the segment, which is a standard technique used to improve the signal carrying characteristics of a wire pair.

Fiber Optic Ethernet (10BaseF)

The fiber optic media system use pulses of light instead of electrical currents to send signals, which provides electrical isolation for equipment at each end of a fiber link. The electrical isolation provides immunity from the effect of lightning strikes and the different ground currents found in separate buildings. This is essential when segments must travel outside a building to link separate buildings

To learn more about Ethernet check this Ethernet Guide .Next post I will cover connecting two devices, Cabling and Connectors   


අයේෂ් said...

nice post....thank you

හිරන්ත ප්‍රදීප් වීරරත්න said...

great one machan..write more posts.we are waiting..

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