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   

Introduction to Network Concepts I

In this post I'm going to talk about important concepts of networking that really help you to do CCNA with great knowledge.also in this section I'm describing extra things that not in CCNA syllabus but these basic concepts are help you to get clear idea about networking.

A computer network is a group of computers. Network may be classified according to a wide variety of characteristics.

First Computer Network:-The Advanced Research Projects Agency (ARPA)designed "Advanced Research Projects Agency Network"(ARPANET) for United states Department of Defence as first Network during the Cold war in December 5,1969.

Network Classification

1.Using Connection method.

In this, we can classify Network according to the hardware and software technology that is used to interconnect the individual devicesin the Network. such as Optical fiber,Ethernet,Wireless LAN,HomePNA,Power line communication.

Ethernet use physical wiring to connect devices. frequently deployed devices include Hubs,Switches, Bridges, or Routers.
Wireless LAN technology is designed to connect devices use Radiowaves or infrared signales as transmission.

2.Using Scale

Network also classified according to their Scale as

• Local Area Network (LAN)
• Wide Area Network (WAN)
• Metropolitan Area Network(MAN)
• Personal Area Network(PAN)
• Virtual Private Network Area Network (VPN)
• Campus Area Network CAN)
• Storage Area Network (SAN)

These classification depending on their Scale I'm going to describe these things deeply in later Posts but here you want to get clear idea about differences between LAN and WAN.In short and sweet we can say "You own the LAN and You hire the WAN" this mean if we get LAN it spread over same geographical area and all the stuff in network belongs to that organization. it may be one building ,several building, campus or one small island.WAN exists over many office buildings separated by a vast distance the office buildings in a WAN may be in different countries or even continents. important thing is WAN not belongs to organization it may use some third party services like ISP services. The other difference between LAN and WAN, is the speed of the network. The maximum speed of a LAN that most likely to be based on switched IEEE 802.3 Ethernet, or on Wi-Fi technology can be 1000 megabits per second, while the speed of a WAN can go up to 150 megabits per second. This means the speed of a WAN, is one-tenth of the speed of a LAN. A WAN is usually slower because it has lower bandwidth.

3.Functional relationship (Network architecture)

also network can classify according to functional relationship which exist among the elements of the network. Active Network, Client-sever Network, Peer to Peer(e.g.torrent)

4.Network topology

Network topology signifies the way in which devices in the network see their logical relations to one another.
According to Network topology we can classify Network as Bus Network, Star Network,Ring Network, Mesh Network, Star-bus Network, tree or hierarchical Network

Cisco standard symbols in networking

Let's get some brief idea about some important devices in network

1. Router
   

Router used to forward data among computer networks beyond directly connect devices so routers route packets of abstracts from one arrangement to another.Routers are most commonly related to Network layer of the OSI model (layer 3 ), but arguably also operate on Data Link (layers 2)

2. Switch

Manage network connection between any pair of star wired devices on a network so Switches add more intelligences to data transfer management than hub.also Switches work at Data link layer(layer 2) of OSI model.

3. Hub

Pass electronic signals to the network and connect group of host together. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.In OSI model hub work at physical layer (layer 1).

4. Bridge

A bridge device filters data traffic at a network boundary. Bridges reduce the amount of traffic on a LAN by dividing it into two segments. also bridges working at the Data link layer (Layer 2) of OSI model.

5. Network interface card

A network interface card is used to connect a computer to an Ethernet network. The card provides an interface to the media.The card usually also contains the protocol control firmware and Ethernet Controller needed to support the Medium Access Control (MAC) data link protocol used by Ethernet. NIC work at Physical(Layer 1) and Data link(Layer 2) Layers of OSI model.
Next post I will talk more about these important concepts in networking thanks for reading.