Ethernet

Ethernet is a family of computer networking technologies for local area networks (LANs). Ethernet was developed at Xerox PARC between 1973 and 1974 by Robert Metcalfe. Metcalfe developed the physical method of cabling that connected devices on the Ethernet as well as the standards that governed communication on the cable. Ethernet has since become the most popular and most widely deployed network technology in the world.  Ethernet facilitates communication over a single cable shared by all devices on the network. Once a device is attached to this cable, it had the ability to communicate with any other attached device. This allows the network to expand to accommodate new devices without requiring any modification to those devices already on the network.

Ethernet Topology:

The topology of an Ethernet network has changed out of necessity over the years as the networking technologies have evolved. Where in the 90's Bus topology was very popular today this has made way for what is called Star Topology. In Bus topology the whole LAN was considered one big collision domain which meant only one device could transmit data at any one point in time. Today's Star Topology, or switched ethernet, splits any LAN up into segments which runs its own separate ethernet protocol, which allows devices to transmit simultaneously either in duplex or half duplex.

Half Duplex:

Bi-directional transmission but not at the same time using one collision domain.

Duplex:

Bi-directional transmission at the same time using two collision domains.
Never a collision when devices are connected using full duplex Ethernet switch.


Advantages of Star topology:

• Easy to install and wire
• No disruptions to the network when connecting or removing devices
• Easy to detect faults and remove parts

Disadvantages of Star topology:

• Requires more cable length than bus topology
• If switch fails, devices attached are disabled and cannot participate in network communication
• Expensive due to cost of switches and longer cables

Ethernet Media types:

Ethernet Medium Access Control:

Ethernet is a broadcast medium where in one device is a sender while every other device receives the data. Each device listens to everything going up and down the wire, identifies the start of the frame, and copies the data on wire into a buffer. Here it examines destination MAC address to see if it is for that device and if yes it continues reading the rest of the frame otherwise it drops the frame. (CSMA/CD)

Cisco Packet Tracer

So basically in this post i'm going to introduce you to and talk a little bit about Cisco packet Tracer.

Cisco Packet Tracer is a powerful network simulation program that allows students to experiment with network behavior and ask “what if” questions.
It allows students to create networks with an almost unlimited number of devices and to experience troubleshooting without having to buy real Cisco routers or switches. The tool is created by Cisco Systems. The purpose of Packet Tracer is to offer students a tool to learn the principles of networking as well as develop Cisco technology specific skills.

Ok so here is a beginners tutorials on how to set up a basic network topology using packet tracer from the CiscoStudents YouTube channel:

PACKET TUTORIAL #1

Further on in the blog we will be coming back to learn a little more about Cisco Packet Tracer and building a Larger network topology using interconnected switches in a hierarchy.

Switches - Layer 2 Devices

A network switch or a MAC bridge is a computer networking device that connects devices together on a computer network. Unlike less advanced network hubs, a network switch has the ability to forward data only to the one or multiple devices that need to receive it, rather than broadcasting the same data out of each of its ports.

There are three main types of switches:

• Store and forward switch
• Cut through switch
• Adaptive switch

Store and forward:

A store and forward switch accepts the input data and then buffers or stores it briefly. It then checks the checksum to verify data and then it outputs the framed data to a specific interface or port. This method of switch is very error resistant but can be quit slow by comparison.

Cut through switch:

A cut through switch is a little bit more intelligent as it recognises that the destination address will always be at the start of the frame of data. This allows a cut through switch to to begin forwarding incoming frame onto the output port as soon as the destination address is recognised. This greatly improves the speed at which the switch processes data however it also may propagate some bad frames or errors.

Adaptive switch:

An adaptive layer 2 switch basically combines the functionalities of a store and forward switch and a cut through switch. An adaptive switch will normally operate in cut through mode however if a particular port's error rate becomes to high, the switch will automatically reconfigure the port to run in store and forward mode. This adaptive switching is usually done on a port by port basis.

Switches are commonly used to connect devices within a LAN  which we discussed in a an earlier post(small geographical area, limited number of nodes). It is because of this limitation on the scope of a network connected to a switch which allows the switch itself to learn and store information about the network that it connects. 

When a switch receives a frame of data it learns the location of the sender and stores or records this location in a switch table sometimes called a MAC Table. Then if the frame destination is unknown it floods the network. During this flooding the switch learns addresses by examining the source mac address of each frame received, then it updates the MAC table as necessary. 

Then if the frame destination of a particular frame of data is known it simply sends it directly to the destination MAC address.

Switches can be interconnected in a hierarchy to accommodate bigger networks of interfaces.

In a hierarchy the switches work exactly the same with the same self learning capabilities.

Data Link Layer

As i have mention before the data link layer or layer 2 is the protocol layer which is responsible for the transfer of data within a LAN/WAN or between physically connected nodes on an network such as switches and hubs.
The data link layer provides the means to transfer data between network entities and may also provide the detection and correction of errors that may occur in layer 1.

In this post we will be outlining some of the protocols used, their functions and also we will be talking Switches which are a layer 2 device.

Data Link Protocols:

The Data Link Protocols in general performs three functions:

• Flow control - controls when to transmit data (MAC or media access control)
• Detects and corrects errors (Error Control)
• Identifies the start and end of a message(Message Delineation)

Data Link Layer devices encapsulate layer 3 data with a header and a trailer to create what is called a frame. Layer 2 devices deliver these frames using unique hardware addresses. A frame's header contains source and destination addresses that identify which device originated the frame and which device is expected to receive and process it.

The data link layer is only concerned with local delivery of frames between devices on the same LAN. Data-link frames, as these protocol data units(PDU) are called, do not cross the boundaries of a local network.
When devices attempt to use a medium at the same time, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and can provide ways to reduce or prevent them.

Logical link control (LLC) sublayer:

The first sublayer, LLC, multiplexes protocols running on the data link layer, and can provides flow control, acknowledgment, and error notification. The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the source and destination machines.

Media Access Control (MAC) sublayer:

MAC can refer to the sublayer that determines who is allowed to access the media at any one time or it can also refer to the structured framing of packets of data which are delivered based on the  MAC addresses inside.

There are many many protocols used in layer 2 to complete theses various flow control, framing and error handling functions. Here I am just going to list a few of these protocols

• Address Resolution Protocol(ARP)
• Token Ring 
• Ethernet
• Point-to-point (PPP)
• Fibre Distributed Data Interface(FDDI)
• IEEE 802.11 wireless LAN
• CSMA/CD 
• CSMA/CA

The Physical layer

Ok so in this post i am going to be talking about the physical layer and the various configurations and types of medium used at this layer. I'll also be outlining the limitations, advantages and disadvantages of each of the main types of physical medium in use today.

So as I have said in my previous post the physical layer is the cabling and or wireless signals that carries the data from source to destination. Within this layer we have two broad categories of media which are Guided Media and Unguided Media. Guided media is where the message or data flows through a physical medium such as wiring or cables. On the other hand then when the message or data is broadcast through a medium, such as over the air or through a vacuum. we call this Unguided Media. The key issues in the selection of one medium over another are data rate (bps) and distance.

Guided Media:

• TWP (Twisted Wire Pair)
• Co-Axial
• Optical Fiber

TWP - Twisted Wire Pair:

Twisted wire pair consists of two copper wires(or other conductors) twisted together and is the most common medium in use today. 

TWP is widely used in the telephone network and local area networks in various specifications.

There are two main types of TWP available which are Unshielded Twisted Pair and Shielded Twisted Pair although the shielded variety has become commonplace due to its metal braid or sheathing which increases its performance. TWP is available in many different specifications or categories(Cat 3, Cat 5, Cat 5E etc.) to cater for different uses and network requirements.

The advantages of TWP:

• Cheap
• Fast over short range
• Easy to work with 

Disadvantages of TWP:

• Low data rates
• Susceptible to interference
• Short range between amplifiers

Co-Axail:

Co-Axial cabling is more expensive than TWP but less susceptible to interference and can carry much more data. Mainly used for the connection of video equipment due to the high data transfer rates the most common use today is the connection of many cable tv providers set-top boxes to the antennae or satellite dishes.

Advantages of Co-Axial:

• Supports multiple channels
• Greater data rates
• Lower error rates
• Greater range between amplifiers

Disadvantages of Co-Axial:

• More expensive than TWP
• Harder to work with

Fibre Optics:

An optical fibre cable contains one or more optical fibres that are used to transmit converted data in the form of light pulses. Fibre Optics allow for much greater point to point transmission speeds than any of the other media available today with data rates of up to 1 Pbs(Petabits per second) with is the equivalent of 125,000,000 Mbps. 

The core itself transfers data at the speed of light in a vacuum but due to the effect of what is called total internal refraction (shown in diagram below) over extended distances this continuous refraction can slow the signal down. This calls for the use of repeaters or extenders like in TWP and Co-Axial however with a far greater distance between them.

Advantages of Fibre Optics:

• Very fast data rates
• Greatly increased bandwidth and capacity
• Lower signal loss
• Immune to electromagnetic and radio frequency interference
• Overall system economy

Disadvantages of Fibre Optics:

• Fibre optic components are very expensive
• Very specialized and expensive to repair

Unguided Media:

• Radio-waves
• Microwave 

Both of these methods of transmission provide a means of transmitting data over a network without the use of physical means to define the path it takes. Some examples of unguided media include the cellular 3G/4G networks, Wi-Fi and Wi-Max.

All of us are very familiar with Wi-Fi at this stage as this particular unguided medium has become very widespread as it allows the connection of many different users on varying devices to connect to a network simultaneously and wirelessly.

Advantages of Unguided Media:

• Ease of access
• Multiple users and devices
• Wireless connection

Disadvantages of Unguided Media:

• Congestion (more users one connection point)
• Signal absorption, reflection and obstruction.

OSI Model animation

Hey guys i was researching a little bit about the OSI model and i came across this video on YouTube from an uploader called J Kenneth Lim and i thought the way in which he explains it is simple and easy to understand so have a look and hopefully it will clear things up a little.

OSI Reference Model

The Open System Interconnection (OSI) model is a standard reference model for communication between two end users in a network which was developed by the International Organisation for Standardisation (ISO) in 1984.

This is a theoretical system which divides the problems of communication into subsections called layers with each layer performing a subset of the required tasks or functions. In this model each layer relies on the next layer down to perform more primitive or basic functions and tasks performed in each layer are exclusive to that particular layer.

Here is a diagram of the OSI model layers:

Ok so now i will explain each layer in a little more detail so we will start at the top.

The Application Layer:

The application layer is the OSI layer closest to you and me, the user. As we interact with an application that implements a communicating component, such as a web browser, the application layer of the ISO model is also interacting with the application. The functions of the application layer are typically functions like identification of available communication partners, assessing the quality of the network available and synchronising communication. Notice these are all application specific functions.

The Presentation Layer:

This layer provides encryption of application layer data before sending it down the stack. It either formats and encrypts data to be sent across the network or transforms received data into the form the application above accepts.

Session Layer:

The session layer basically monitors, manages and terminates the connections between the local and remote application.

Transport Layer:

The transport layer facilitates the transferring of data sequences from source to destination.
It does this by what is called segmentation/desegmentation which means that it creates packets out of the data received from he application layer. Packetization is basically the process of dividing up the received information into smaller bundles (packets) of information. The Transport layer also keeps track of data transmission, re-sending those packets that fail and acknowledging successful transmission.

Network Layer:

The network layer provides the means of transferring the packetized data from the transport layer to the destination location on the same connecting network. This is basically the routing layer which uses IP addresses and routing protocols to transfer the data from source to destination.

Data Link Layer:

The Data Link Layer is responsible for the transfer of data within a LAN/WAN or between physically connected nodes on an network such as switches and hubs. This layer is normally divided into two sublayers the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.
The MAC layer controls how a device on a network accesses and transmits data.
The LLC layer identifies the protocols, and controls error checking and packet synchronisation.

The Physical Layer:

The physical layer is basically the cabling or wireless signals that connects the source to destination. This is where we see the data packets as "bits on the wire".

Here is an image of the OSI model in action between source and destination:

So as you can see the exact same steps that are performed at source during the transmission of the data are performed in reverse at destination.

During transmission each layer adds header to the outgoing packet and then at destination the same layer removes the header on incoming packet until original user data is presented in destination application as we can see in the image below: