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   

Saturday, May 8, 2010

Introduction to Network Concepts II( OSI Model )

Today I'm going to talk about concept that very important in CCNA . This is a actually model that describe all the functionality of communication between two computers. This model created by International Organization for Standardization (ISO) in 1984 and its called OSI Model (Open System Interconnection Model).Don't confuse with following words.
  • ISO - International Organization for Standardization (Remember this as International Standard Organization)
  • OSI Model - Open System Interconnection Model
  • IOS - Inter networking Operating System.
Before 1984 in networks operate as one unite and all the equipments that use to computer have to made by one vendor. Like IBM or some other companies produce all the parts of the computer (both hardware and software ) and one IBM computer could communicate with only another IBM computer. so this make big restriction for improvement of global communication. As a solution for this problem ISO introduce this ISO/OSI reference model.

This OSI model is a layered model and its have seven layers purpose of this layers are provide clearly define functions to each layer. Every layer have own functionality and standard for both inputs and outputs. This standards helps to manufacturing companies to create interoperable network devices and software.
In Cisco CCNA mainly talking about layer 2 and layer 3 functionality on OSI model but OSI model will be use throughout your networking career.Following are the layers of the OSI model and you should remember this sequence of OSI layers.

To remember this sequence of OSI layers you can use following sentences.

All People Seems To Need Data Processing.
or other way
Programmer Do Not Throw Sausage Pizza Away.

This top-down explanation of ISO model explain, from starting with user interact with application layer to entering bit stream into physical medium from physical layer in between this two layers other five layer perform their own functions see bottom video.

Later on I'm going describe more about this ISO model and functions of each layer now lets see some main functions and protocols of each layers.

1.Application Layer (L7)
This is the layer where the end users themselves interact with the network in this layer interact with operating system and applications whenever the user want to transfer files ,read massage or perform any other activity related to network.
  • L7 Protocols: Email protocols SMTP and POP3, Telnet, HTTP [Hyper Text Transfer Protocl], FTP[File Transfer Protocol], SNMP[Simple Network Management Protocol]
  • L7 Network Devices: Gateway redirecter working on this layer.
2.Presentation Layer(L6)
This layer ensures properly formatting data also converting user data to bits that coming from application layer and other end controlling how data present to application layer. other activities of this layer are

- Compatibility with the operating system
- proper encapsulation of data for network transmission.
- Data encryption, Decryption, Compression, and translation.

If you open a file in a word processing application, and you got pages of unrecognizable characters, that's a presentation layer issue.
  • L6 Network Devices: Gateway redirecter working on this layer
3.Session Layer (L5)
This layer is the manager of the two way communication between two remote hosts.this is the layer that handles the creation, maintennance, and teardown of communications between those two hosts. The overall communication itself is referred to a session.This offering three different modes Simplex, Half Duplex, Full Duplex.
  • L5 Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC
  • L5 Network Devices: Gateway
above three layers on OSI model known as Application set of OSI model and other 4 layers known as Transport set.

4.Transport Layer (L4)
This layer mainly provide reliable(TCP) or unreliable(UDP) services for data transfer and segmentation of upper layer data. other functions of this layer.

- Establish end to end connectivity between application
- Defines flow control
- Transport layer also provides the acknowledgement of the successful data transmission and retransmits the data if no error free data was transferred.
- error handling and connectionless oriented data deliver in the network.
  • L4 Protocols:TCP,UDP, SPX, NETBIOS, ATP and NWLINK.
  • L4 Network Devices:The Brouter, Gateway and Cable tester work on the transport layer.
5.Network Layer (L3)
This is the layer some time known as "Cisco layer". Defines logical addressees associated with a specific protocol using that it decide how to data transmit between network devices. Network layer routes the packets according to the unique network addresses. Router works as the post office and network layer stamps the letters (data) for the specific destinations.
  • L3 Protocols:IP, ICMP, ARP, RIP, OSI, IPX and OSPF
  • L3 Network Devices:Router, Brouter, Frame Relay device and ATM switch devices
6.Data Link Layer (L2)
Phisycal source and destination addresses are perform on this layer.Only error detection error correction is not perform.
-Identifies the higher layer protocol(Type or SAP)
-Frame sequencing
-Flow control
-Logical Link Control (LLC)performs Link establishment
-Media Access Control (MAC) Performs Access method
  • L2 Protocols:802.3 CSMA/CD (Ethernet) ,802.4 Token Bus (ARCnet),802.5 Token Ring,802.12 Demand Priority
  • L2 Network Devices:Bridge Switch, ISDN, Router, Intelligent Hub, NIC
7.Physical Layer (L1)
Physical layer defines and cables, network cards and physical aspects only understand 'ones' and 'zeros'.It also provides the interface between network and network communication devices.It defines the physical characteristics of the network such as connections, voltage levels and timing.
  • L1 Protocols:ISDN, IEEE 802 and IEEE 802.2
  • L1 Network Devices:Hubs, Repeaters
In this post I only try to give some introduction to functions of OSI layers next post I will talk more about OSI model layers , its protocols and its devices.

Sunday, April 25, 2010

Introduction to Network Concepts I

CCNA networkingIn 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 ,ccnaLet's get some brief idea about some important devices in network
  1. Router ,ccna ,ccnaRouter 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 ,ccna ,ccna
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 ,ccna ,ccna
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 ,ccna ,ccna
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 ,ccna ,ccnaA 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.

Friday, April 23, 2010

Overview of Cisco and CCNA

CCNA is a professional level certification in Cisco certifications.professional level means these certifications designation earned by a person to assure qualification to perform a job or task. so these Cisco certification are created for give expert knowledge about Cisco products, mainly Cisco routers and Switches. so briefly following things we are going to learn under CCNA tutorials from here.
Cisco Certified Network Associate (CCNA) validates the ability to install, configure, operate, and troubleshoot medium-size route and switched networks, including implementation and verification of connections to remote sites in a WAN. CCNA curriculum includes basic mitigation of security threats, introduction to wireless networking concepts and terminology, and performance-based skills. This new curriculum also includes (but is not limited to) the use of these protocols: IP, Enhanced Interior Gateway Routing Protocol (EIGRP), Serial Line Interface Protocol Frame Relay, Routing Information Protocol Version 2 (RIPv2),VLANs, Ethernet, access control lists (ACLs)

Ok lets talk little bit about job opportunities for CCNA when we talk about this truth is along with CCNA we have nothing. All you need is ability to do some thing just it. using CCNA you can start your network career and you can make beautiful it. SO I think that CCNA is important as a door opener,also CCNA is a prerequisite to CCNP as well-as a lot of recruitment agencies won't even consider you regardless of how much experience you have if you don't have CCNA/CCNP that recognition have it.Following are some links you can find about job opportunities and about average salaries offered in IT jobs that have cited CCNA.

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