The Physical Layer

The Physical layer in the OSI model is responsible for binary transmission. This involves electronic circuits, wires, connector, voltages and data rates or speed of transmission. One of the most commonly used device that belongs to this layer is the NIC (Network Interface Card). There are different NICs for different types of networking protocol. There are also different NICs for different types of networking media or wires. For laptops that do not have a built-in network interface card, a special device called PCMCIA (Personal Computer Memory Card International Association) is used.

There are three things that you need to consider in selecting a NIC. Two of which is already mentioned, the type of Protocol your want to use in your network, the type of Media, and lastly the type of System Bus your computer have. It is called Bus because it is a collection of wires on the motherboard which carries data and timing signals from one part of a computer to another.

Types of Protocols you need to consider:

• Ethernet
• Token Ring
• FDDI

 

Types of Media:

• Twisted-pair
• Coaxial
• Wireless
• Fiber-optic

Type of System Bus:

• PCI - Peripheral Component Interconnect
• ISA - Industry Standard Architecture

CCNA Exam tip:

This would possibly come out in the CCNA Exam. Situations that require NIC installation:

• Installation of a NIC on a PC that does not already have one
• Replacement of a malfunction or damaged NIC
• Upgrade from a 10Mbps NIC to a 10/100/1000Mbps NIC
• Change to a different type of NIC, such as wireless
• Installation of secondary, or backup, NIC for network security reasons

Another very common Physical Layer device is the Modem (Modulator - Demodulator). This device provides the computer with connectivity to a telephone line. It converts the data in digital format to analog format and vice versa.

CCNA Exam tip:

This would possibly come out in the CCNA Exam. The ping command is the command used to test network connectivity. It is a basic program that can verity whether the specified IP address exist in the network and is reachable. In checking IPv6 address, you may use the ping6 command. Going deeper inside the ping command, this command will send an ICMP/ICMPv6 ECHO_REQUEST packets to elicit an ICMP/ICMPv6 ECHO_REPLY from a network host/computer.

The basic ping command synopsys:
ping <ip address> - where <ip address> is the address of the host/computer that you want to test connectivity with.

ping 127.0.0.1 - this is an internal loopback test. If your TCP/IP network configuration configuration is working well, then you will get a reply from 127.0.0.1. If you want to try IPv6, its loopback address is ::1 which is a shortcut for 0000:0000:0000:0000:0000:0000:0000:0001.

The OSI Model

In the beginning, when man discovered computer networks and its importance... there was chaos. Different brilliant people created their own brilliant ways of sending data electronically from one computer to another computer. As an end result, computers made from one vendor could not directly communicate with computers made from a different vendor for they do not follow the same communication protocols. A protocol is a set of rules or an agreement that determines the format and transmission of data.

With this the ISO (International Organization for Standardization) passed a standard to different vendors a set of standards that ensure greater compatibility and interoperability of various computer network devices.

This new standard is called the Open System Interconnection Model (or OSI Model or commonly known as the 7 OSI Layers).

The 7 OSI Layers:

1. Physical


2. Data Link


3. Network


4. Transport


5. Session


6. Presentation


7. Application

The benefits of the OSI Model are:

• It reduces complexity of the entire computer network design by slicing it into 7 different areas. You can then set your focus to one area instead of the whole computer network system.


• It standardizes interfaces. You can now clearly determine which type of interface you are going to adopt for your network so that all computers can commuication with each other without problems.


• It facilitates modular engineering for you can now focus your design to one particular layer at a time.


• While one particular vendor focuses only its development or enhancements to a particular layer, others can also enhance other layers. Thus accelerates the evolution of computer networks.


• Lastly, since it reduces the complexity of the design, it also simplifies the teaching and learning of computer networking.

CCNA Exam Tips

In taking CCNA exams, there are about 60+ questions and you are to answer them within 90 minutes. Doing a quick math, it means that you have 90 seconds to answer each question. There are simple questions where you can answer them in just 10 to 15 seconds, and there are those that would probably take you more than 90 seconds to answer. Most of these questions involve conversion from decimal to binary or from binary to decimal.

This is what I want to focus in this article. Please forgive me if this post is so elementary, I just want to share my techniques in doing decimal to binary conversions and vice-versa in order for you to possibly save time in answering CCNA questions.

Decimal to Binary Conversion

This is what I do, in a scratch pad, I wrote the following place values:

____ ____ ____ ___ ___ ___ __ __ __ _ _ _ _ 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 1

Okay, so now I will show you how to use these values. Suppose you want to convert 202 decimal to binary, how are we going to do it? Doing it the conventional way is to divide 202 by 2 then get the answer and the remainder, then divide the answer by 2 again and so on...

I am not a math wizard, so for me, I can solve faster in addition or subtraction than in division. So let me show you how to convert 202 decimal to binary using subtraction and using the values presented in Figure 1.

First, look for the HIGHEST value in Figure 1 that we can subtract to 202 that will not result to a negative answer. So what will it be? It will be 128 right? And the answer to 202-128 is 74. So write 1 above 128 in Figure 1.

____ ____ ____ ___ ___ _1_ __ __ __ _ _ _ _ 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 2

So instead of doing a repetitive division, we will be doing a repetitive subtraction. Next, look for the HIGHEST value in Figure 1 that we can subtract to 74 that will not result to a negative answer. This time, it will be 64 right? And 74-64 is 10. So write 1 above 64 in Figure 2.

____ ____ ____ ___ ___ _1_ _1 __ __ _ _ _ _ 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 3

Next, look for the HIGHEST value in Figure 1 that we can subtract to 10 that will not result to a negative answer. It's 8 right? And 10-8 is 2. So write 1 above 8 in Figure 3.

____ ____ ____ ___ ___ _1_ _1 __ __ 1 _ _ _ 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 4

Next, look for the HIGHEST value in Figure 1 that we can subtract to 2 that will not result to a negative answer. It will be 2! And 2-2 is ... So write 1 above 2 in Figure 3.

____ ____ ____ ___ ___ _1_ _1 __ __ 1 _ 1 _ 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 4

Since our previous answer was 0, let's now stop and fill in the remaining blanks with 0, and that's the binary equivalent of 202 decimal.

__0_ __0_ __0_ _0_ _0_ _1_ _1 _0 _0 1 0 1 0 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 5

The answer is 0000011001010 or simply 11001010. That's it! very elementary right? :)

Binary to Decimal Conversion

Now, let's do the reverse. Suppose we want to convert 11101011011 binary to decimal, how will we do it using Figure 1? This is so elementary again, just place the binary bits to our Figure 1 starting from the rightmost bit.

____ ____ __1_ _1_ _1_ _0_ _1 _0 _1 1 0 1 1 
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
               Figure 6

Then add all the values that carries a 1 on top. That will be:

   1024
  + 512
  -----
   1536
  + 256
  -----
   1792
  +  64
  -----
   1856
  +  16
  -----
   1872 
  +   8
  -----
   1880
  +   2
  -----
   1882
  +   1
  -----
  =1883
               Figure 7

That's it! The answer is 1883.

I hope that in a little way, you have gained something from this article. If you are not planning to take the CCNA exam, perhaps you can use this to teach your 3 year old child to do conversion from decimal to binary and binary to decimal.