Tutorial Class 6: UPPER LAYER OF THE OSI MODEL

In our last class, we had a brief introduction to the OSI model—the seven-layer model used to help application developers design applications that can run on any type of system or network. Each layer has its special jobs and select responsibilities within the model to ensure that solid, effective communications do occur.

In this tutorial class, you’ll get to know the functions defined at each layer of the upper layer of the OSI model.

The Application Layer

The Application layer of the OSI model is the layer where users actually communicate to the computer. This layer only comes into play when access to the network is going to be needed soon. It’s just like browsing the internet with Internet Explorer or Mozilla Firefox. The browser will respond to requests such as using HTTP by attempting to access the Application layer.

The Application layer acts as an interface between the actual application program and the next layer (presentation layer) by providing ways for the application to send information down through the protocol stack. This means that the browser doesn’t truly reside within the Application layer—it interfaces with Application layer protocols when it needs to deal with resources on the network.
The Application layer is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist.

The Application layer acts as an interface between the actual application programs. Simply put, applications like Microsoft Word do not reside at the Application layer but instead interfaces with the Application layer protocols, such as FTP and TFTP.

The Presentation Layer
The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting.
The Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one. The data received from the application layer is translated into a language (code) that the presentation layer understands. Computers receive this generically formatted data and then convert the data back into its native format for actual reading (for example, EBCDIC to ASCII). Data compression, decompression, encryption, and decryption are associated with this layer.

The Session Layer
The Session layer basically keeps different applications’ data separate from other applications’ data. It sets up, manages, and then tears down sessions between Presentation layer entities. This layer also provides dialog control between devices, or nodes.

So, now you know the three upper layers of the OSI model; the application, presentation and session layer and the respective roles they play in information and communication transfer.
In summary, the following services are available at the layers:
•    Application layer: File, print, message, database and application services.
•    Presentation layer: Data encryption, compression and translation services.
•    Session layer: Dialog control.

In our next tutorial class, you’ll get to know the functions defined at each layer of the lower layer of the OSI model.

Tutorial Class 5: The OSI Reference Model

In our last class, we got to understand the basics of an internetwork through examples. In this tutorial class, I’ll be introducing you to internetworking models. This simply talks about the Open Systems Interconnection (OSI) reference model which was created by the International Organization for Standardization (ISO) to break the barrier of computers typically communicating only with computers from the same manufacturer.
The OSI model is the primary architectural model for networks. It describes how
data and network information are communicated from an application on one computer through the network media to an application on another computer by breaking it into layers.

What is a reference model?
A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers. When a communication system is designed in this manner, it’s known as layered architecture.
To understand it better, let’s take the following scenario:

You and some friends want to start a company. One of the first things you’ll do is sit down and think through what tasks must be done, who will do them, the order in which they will be done, and their relationship with each other. These tasks will be grouped into departments. Each of your departments has its own unique tasks, keeping its staff members busy and requiring them to focus on only their own duties.
These departments are simply the layers of the communication system. For things to run smoothly, the staff of each department will have to trust and rely heavily upon the others to do their jobs and competently handle their unique responsibilities. In your planning sessions, you would probably take notes, recording the entire process to facilitate later discussions about standards of operation that will serve as your business blueprint, or reference model.

Similarly, software developers can use a reference model to understand computer communication processes and see what types of functions need to be accomplished on any one layer. If they are developing a protocol for a certain layer, all they need to concern themselves with is that specific layer’s functions, not those of any other layer. Another layer and protocol will handle the other functions. The technical term for this idea is  binding . The communication processes that are related to each other are bound, or grouped together, at a particular layer.

Advantages of Reference Models
The OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. The primary purpose of all such models, especially the OSI model, is to allow different vendors’ networks to interoperate.

Advantages of using the OSI layered model include, but are not limited to, the following:
•    It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
•    It allows multiple-vendor development through standardization of network components.
•    It encourages industry standardization by defining what functions occur at each layer of the model.
•    It allows various types of network hardware and software to communicate.
•    It prevents changes in one layer from affecting other layers, so it does not hamper development.

The OSI Reference Model

One of the greatest functions of the OSI specifications is to assist in data transfer between hosts from different vendors—meaning, for example, that they enable us to transfer data between a Unix host and a PC or a Mac.
The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end. Figure 1 shows the three upper layers and their functions, and Figure 2 shows the four lower layers and their functions.

The user interfaces with the computer at the Application layer and also the upper layers are responsible for applications communicating between hosts.

The four bottom layers are responsible for networking or network addresses. They define how data is transferred through a physical wire or through switches and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host’s application.

The following network devices operate at all seven layers of the OSI model:
•    Network management stations (NMSs)
•    Web and application servers
•    Gateways (not default gateways)
•    Network hosts

The OSI reference model has seven layers:
•    Application layer (layer 7)
•    Presentation layer (layer 6)
•    Session layer (layer 5)
•    Transport layer (layer 4)
•    Network layer (layer 3)
•    Data Link layer (layer 2)
•    Physical layer (layer 1)

So, I guess with this short class, you’ve gotten a brief introduction to the OSI model—the seven-layer model used to help application developers design applications that can run on any type of system or network. Each layer has its special jobs and select responsibilities within the model to ensure that solid, effective communications do occur.

In our next tutorial class, you’ll get to know the functions defined at each layer of the upper layer of the OSI model.

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Tutorial Class 4

In this class, I’ll be giving you the answers to the self tests in the previous class and shed more light on internetworking with some examples.

The answers to the previous self tests are as follows:

For the first question, in order for host A to ping (that is, communicate with ) host B, you need a crossover cable to connect the two switches together. I hope you remember from our Ethernet cabling class that to connect two hosts, you use a crossover cable.

For the second question, there are a variety of cables in use. For the connection between the switches,
we’d obviously use a crossover cable. We have a console connection between the router console and the computer that uses a rolled cable. Plus, the connection from the router to the switch is a straight-through cable, which is the same for the hosts to the switches.

I hope you understand those answers. If you don’t,  just go over the previous classes on Ethernet cabling and networking devices.

We’ll be understanding more about internetworking with the figure below.




 

I guess you remember this figure from our last class. Looking at the figure, how many collision domains and broadcast domains are in this internetwork?

Hopefully, you answered nine collision domains and three broadcast domains! Remember, only routers break up broadcast domains by default. And since there are three connections, that gives you three broadcast domains. But where are the nine collision domains?
The all-hub network is one collision domain; the bridge network equals three collision domains. Add in the switch network of five collision domains—one for each switch port—and you’ve got a total of nine.


The figure below which shows switched networks creating an internetwork is what is typically found today.




Okay, here LAN switches are placed at the center of the network so the routers are connecting only logical networks together. If I implemented this kind of setup, I’ve created virtual LANs (VLANs), something I’m going to tell you about in later classes. But it is really important to understand that even though you have a switched network, you still need a router to provide your inter-VLAN communication, or internetworking. Don’t forget that!
Obviously, the best network is one that’s correctly configured to meet the business requirements of the company it serves. LAN switches with routers, correctly placed in the network, are the best network design.

Now each port on the switch is a separate collision domain and each VLAN is a separate broadcast domain. But you still need a router for routing between VLANs. How many collision domains do you see here? I’m counting 10—remember that connections between the switches are considered a collision domain!



So now that you’ve gotten an introduction to internetworking and the various devices that live in an internetwork, in our next class, I will be introducing you to internetworking models. This simply talks about the  Open Systems Interconnection (OSI) reference model which was created by the International Organization for Standardization (ISO) to break the barrier of computers typically communicating only with computers from the same manufacturer.

YOUR BUSINESS AND ITS NETWORK

Hey, I know wireless networking is what’s popping now, but not everyone has the luxury of using a wireless network. So, I’ll be telling you about how to use networking cables in your business.
For the basic business network where every computer system can communicate with the others, you’ll need two basic things; a network (Ethernet cable) and a networking device (see tutorial for more info).

Suppose you have ten employees, each using a computer system, you’ll need yards of Ethernet cable and a router. Now when I say ‘yards’, I don’t mean just get really long cables without measuring your business/office space. It’ll be good to note that the longer the cable, the more money you spend. Also, the networking device you get depends on your budget. A router is more expensive than a switch, which in turn costs more than a hub. I suggested a router because of its capabilities, but if your hands are a bit tied, a few switches would equally do a great job.

If the computer systems are in different offices, you might want to place your networking device at the center of the whole network so as to reduce the length of the networking cable used and the distance between the devices and the computer systems.

YOUR BUSINESS AND ITS NETWORK

Tutorial Class 3: Networking Devices

In this post, you’ll get to understand the basics of internetworking by focusing on how to connect networks together using Cisco routers and switches. Since you  know exactly what an internetwork is.

You create an internetwork when you connect two or more LANs or WANs via a router and configure a logical network addressing scheme with a protocol such as IP (you’ll get to know more of that later).

I’ll be covering these the following topics in this post:

• ·        Network segmentation

• ·        How bridges, switches, and routers are used to physically segment a network

• ·        How routers are employed to create an internetwork

Networks cannot just stay as a simple computer to computer connection, it grows larger everyday. This means that at some point you’ll have to break up one large

network into a bunch of smaller ones because good communication would have reduced to a slow crawl as the network grew and grew. And with all that growth, your LAN’s traffic congestion has reached  epic proportions. The answer to this is breaking up a really big network into a number of smaller ones—something called  network segmentation . You do this by using devices like routers, switches,

and bridges . Before I go on, there are two terms which I’ll be using in the remaining part of this class that you need to understand; a collision domain and a broadcast domain.

A collision domain is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. At the same time, a different device tries to transmit, leading to a collision, after which both devices must retransmit, one at a time. Not very efficient!

A broadcast domain is a set of all devices on a network segment that hear all the broadcasts sent on that segment.

Now let’s look at the different types of networking devices;

Hubs: hubs don’t segment a network; they just connect network segments together. So basically, it’s an inexpensive way to connect a couple of PCs together, which is great for home use and troubleshooting, but that’s about it!

 Taking a look at the figure above, you get a picture of a basic LAN network that’s connected together using a hub. This network is actually one collision domain and one broadcast domain.

Switches: switches aren’t used to create internetworks (they do not break up broadcast domains by default); they’re employed to add functionality to a network LAN. The main purpose of a switch is to make a LAN work better—to optimize its performance—providing more bandwidth for the LAN’s users. Also, they only “switch” frames from one port to another within the switched network. You’ll get to know more about frames later.

By default, switches break up collision domains . Switches create separate collision domains but a single broadcast domain. 

 

The hub used in the figure above just extended the one collision domain from the

switch port, while all the switch ports have different collision domains (meaning one host doesn’t have to wait for another to transmit before it can transmit) but are on the same broadcast domain (they are all on the same network, thus, they can communicate).

Here’s a list of some of the things that commonly cause LAN traffic congestion:

• ·        Too many hosts in a broadcast domain

• ·        Broadcast storms

• ·        Multicasting

• ·        Low bandwidth

• ·        Adding hubs for connectivity to the network

Bridges: The term bridging was introduced before routers and hubs were implemented, so it’s pretty common to hear people referring to bridges as switches. That’s because bridges and switches basically do the same thing—break up collision domains on a LAN, so switches can still be called multiport bridges, still there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only had 2 or 4 ports. This is nothing compared to the hundreds available on some switches!

Routers: routers are used to connect networks together and route packets of data from one network to another. Routers, by default, break up a broadcast domain. The figure below shows a router that creates an internetwork and breaks up broadcast domains.

The network in the figure above is a pretty cool. Each host is connected to its own collision domain, and the router has created two broadcast domains. And don’t forget that the router provides connections to WAN services as well! The router uses something called a serial interface for WAN connections, specifically, a V.35 physical interface on a Cisco router.

Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast—unless you’ve got a router. When the router’s interface receives this broadcast, it can respond by basically saying, “Thanks, but no thanks,” and discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, it’s important to remember that they break up collision domains as well.

There are two advantages of using routers in your network:

• ·        They don’t forward broadcasts by default.

• ·        They can filter the network based on layer 3 (Network layer) information (e.g., IP address).

Four router functions in your network can be listed as follows:

• ·        Packet switching

• ·        Packet filtering

• ·        Internetwork communication

• ·        Path selection

Remember that routers are really switches; they’re actually what we call layer 3 switches (we’ll talk about layers later in this tutorial).

 

When you look at the figure above, you’ll notice that the router is found at center stage and that it connects each physical network together? We have to use this layout because of the older technologies involved–—bridges and hubs.

On the top internetwork in the figure, you’ll notice that a bridge was used to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub.

Notice something else: The three hubs at the bottom that are connected also connect to the router, creating one collision domain and one broadcast domain. This makes the bridged network look much better indeed!

SELF TESTS:

1. We’ve taken a look at the various RJ45 cables. Keeping this in mind, what cable is used between the switches in the figure below.

 2.   In the figure below, what cable is used between;

·        the switches and the hosts,

·        the switches and the router and

·        the router and the console?

Check the next tutorial class for the answers. More examples and figures will be given in the next class for more clarification on the topic.

YOUR BUSINESS AND ITS NETWORK

In this post, I'll be introducing you to networking your business according to its structure.

As a  start-up owner, by now you must have considered the location and the kind of business you want to go into. The kind of business will determine how you'll set up the IT structures within. You also should have considered your budget, the kind of people you are working with and the kind of person you are. If you have employees that forget to turn of the lights after a work day then you know you'll have problems with your power management. If they are the kind that are addicted to the internet, you'll definitely have problems with productivity and work ethics, especially if your business is the type that has little to do with the internet. If this is the case, then I guess you might want to enroll such employees into a training course on, "The Internet-phobic Personnel and How To Become One." 

Whatever the business, you'll need to consider the business structure with respect to its departments and how you want your "intradepartmental" and "interdepartmental" communication to be via the networked computers.

Of course, employees within a department should be able to share files with each other without having  to move from their seats. It now depends on whether you want different departments to be on the same network. For example, you might want the sales department to be connected to the marketing department and not to the research department or the transport department to be connected to the sales department and not the human resources department.

Another thing to put into consideration is the number of employees you have and wish to have in the next few months. This is necessary because it will determine the number of computer systems, the kind and number of networking devices you need to get. Learn more about departmental networking in the upcoming network devices and subnetting tutorials.

Coming in my next free services: Basic network design for a small office. You don't just mount computer systems in an office and connect them, you have to take cost into consideration. See the different ways computers can be arranged in an office to reduce cost and increase performance.

Tutorial 2: Ethernet Cabling

Ethernet cabling is an important discussion, especially if you are planning on taking the Cisco exams. These cables are simply used to connect two computer systems, using an RJ 45 connector. An Ethernet cable consists of eight standalone wires having different colours and arranged in different ways depending on the devices to be connected. These cables are firmly inserted into the RJ 45 connector with the aid of a crimping tool, and is plugged into the network card on the computer system.

Three types of Ethernet cables are available: 

•  Straight-through cable 
•  Crossover cable
•   Rolled cable

·      

We will look at each in the following sections.

 Straight-Through Cable

The  straight-through cable  is used to connect

• ·         Host to switch or hub
•  Router to switch or hub

·         Host here refers to a standalone computer system.

Eight wires are used in this cable to connect devices, although not all eight are used to send information.

The diagram below shows the arrangement of a straight through cable.

Crossover Cable

The  crossover cable  can be used to connect

• ·         Switch to switch
•  Hub to hub
•  Host to host
•  Hub to switch
•  Router direct to host

·        

·        

·       

Eight wires are used in this cable to connect devices, although not all eight are used to send information.

The diagram below shows the arrangement of a crossover cable.

 

Colours 1-8 at the left is simply the arrangement of the wires at one end of the cable and colours 3,6,1,4,5,2,7,8 at the right is the arrangement of the wires at the other end of the cable.

Rolled Cable

Although  rolled cable  isn’t used to connect any Ethernet connections together, you can use a

rolled Ethernet cable to connect a host to a router console serial communication (com) port.

If you have a Cisco router or switch, you would use this cable to connect your PC running

HyperTerminal to the Cisco hardware (that is, when  you want to configure your Cisco device. Eight wires are used in this cable to connect serial devices, although not all eight are used to send information, just as in Ethernet networking.

These are probably the easiest cables to make because you just cut the end off on one side

of a straight-through cable, turn it over, and put it back on (with a new connector, of course).

 

Once you have the correct cable connected from your PC to the Cisco router or switch, you can start HyperTerminal to create a console connection and configure the device. Set the con-

figuration as follows:

1.  Open HyperTerminal and enter a name for the connection. It is irrelevant what you name

It.Hyper Terminal is located in assessibility menu in the start menu bar.Then click OK.

 

2.  Choose the communications port—either COM1 or COM2, whichever is open on your PC.

 

3.  Now set the port settings. The default values (2400bps and no flow control hardware) will not work; you must set the port settings as shown in Figure 1.25.

Notice that the bit rate is now set to 9600 and the flow control is set to None. At this point,

you can click OK and press the Enter key and you should be connected to your Cisco device

console port.

Port settings for a rolled cable connection

The figure below shows an RJ 45 connector

 RJ 45 and cable connected

 Crimping tool

 

In our next tutorial, we will be looking at networking devices, how to connect them and we'll have a few self tests.

Tutorial 1: A brief intro.

Before we really go into networking proper, you’ve got to understand the big picture and then
the answer to the key question, Why is it so important to learn networking, Cisco internetworking to be precise?

Networks and networking have grown exponentially over the past few years. They’ve had to evolve at light speed just to keep up with huge increases in user needs such as sharing data and printers as well as more advanced demands such as videoconferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), the challenge is to connect the sometimes many relevant networks together so all users can share the networks’ wealth.

A basic LAN (Local Area Network) network can be connected together using a hub. This network is actually one collision domain and one broadcast domain. I’m sure you have no idea what this means. Not to worry, more light will be shed on this as the turorial goes on.

In the next tutorial, I’ll be introducing you to basic internetworking. I mean Ethernet cabling and how to use it.  For clarity on issues, just leave a comment.

Free Networking Tutorials Coming Soon!

Ever wondered why wanna be networkers pay for tutorials? It's because whenever they look at the free materials and try to read them, it all looks like gibberish. I guess this is due to the fact that they have little or no idea of what they want to do, trust me, I'm a CCNA, I know what I'm talking about.
In the following posts, free tutorials, downloadable materials, a comprehensive guide and more will be available to you.

YOU CAN DO IT TOO

Dear CEO,
As you requested, I'm writing to present to you my quotation for my services as your network administrator. After my previous consults for your feasibility report , the cost of purchasing the office networking devices and the network configuration of your computer systems, I'll be charging xxx,xxx....

Take it or leave it, a good network administrator who knows the value of his service would not collect anything less than  a six-figure salary, depending on the size of the company and the magnitude of his service. Therefore, as the owner of a business start-up, considering all other expenses you have to make in order to get the business up and running, do you have that kind of money for starters?

You might say, "oh, if that's the case, then I don't need to have my computers networked." If that's your mentality in this day and age, throw it away. Which do you think is better; your employee running from one end of the building to the other countless number of times just to deliver some documents to you or she just staying in her office and sending the documents via the computer. The latter would save a whole lot of time and as the saying goes,"time is money."

Getting an insider to do the job for you would save you a lot of money. That insider doesn't have to be a professional networker to do the surface networking for your company. He/She just needs to know the basics. In the following posts, I'll be giving you insights into the kind of network your business can have depending on its nature, its arrangement, configuration, and more.
You really don't need some professional zapping cash from your account when you can equally do it yourself.

For Starters

DON'T FORGET YOUR POWER

Consider this situation in a company:
Technician: The server is down!!! The server is down!!!
Boss: What's all the fuss about? Go put on the generator.
Technician: (After a few minutes) Sir, something is wrong with the generator.
Boss : (Starting to panic) What of the inverters?
Technician: There's been a break in connection, there's no output.
Boss: (Now in a state of dilemma) The UPS?
Technician: They'll be off in less than 2 minutes.
Boss: Ahhhhhh!!

        This must have been a 15th century company. They don't know about backup and must be ignorant of the saying,"Precaution is better than cure." If your company falls in this category, then you'll need a whole lot of restructuring to do. Believe me, it's nothing personal, it's just for your own good.
        By now you must have guessed that I'm not referring to big companies that have multiple branches or multinationals or corporations. This mainly goes to business startups who have a little 'know-how' of IT or have limited capital to get the right office power appliances.