Layered Architecture or Network ArchitectureThe layered architecture consists of the set of layers, as explained in the class, each layer is nothing but a collection of functions combined in one group. So, bottom line is, all the layers make the communication process achievable, by dividing functionality among the layers. Also each layer provides services to the layer above it. In the last lecture, there were five points why layers are required.

Q. Why layers are required, cannot we make only one layer?Answer: No, as explained in previous lecture.

Q. What is peer-to-peer communication?Between machines layer ‘x’ on one machine, communicates with the layer ‘x’ on another machine, a kind of indirect communication (by taking help of intermediate layers), except the physical layer. Because the physical layer of one host connects with the physical layer of another host through the medium directly. Those layers that communicate with the corresponding layer on another machine are called as peer-to-peer layers and the communication between peer to peer layers is called as peer-to-peer communication.

Circles with X represent routers. A and B are hosts are communicating, in this example we are assuming that there are only two routes, whereas in practical there can be multiple routers in between.

Q. What is Encapsulation?Answer:

Layer 7 which is the topmost layer in layered architecture interacts with the application, and the purpose of this layer is to get the message from the sender and forward to the layer below it i.e. to layer 6 or take message from the layer below it (i.e. layer 6) and deliver to the application. Application may be interacting with the various applications running on the computer. Layer 7 is only concerned about interacting with the application executing on the machine. Layer 7 and Layer 1 don’t attach any header to the message. All the layers expect layer 7 transform the received message and attach the respective header or trailer.

Q Why all the layers except layer 7 transform the message?Answer: Because layer 1 only takes message from the application and forward it to the layer 6, whereas each layer except layer 1 has some function to perform, which they do when they receive message from the layer above it, and then attach the header also. Also, there is no layer above layer 1, so layer 1 has nothing to do for any one else, except the take the message from the applications in execution or delivers the message to the application in execution.

Q What is layer to layer interface or Service Access Point?Answer: Messages are passed from one layer to another layer through an interface; So that means an interface exist between every pair of layers, i.e. Layer 7-Layer 6 interface, between layer 7 and layer 6Layer 6-Layer 5 interface, between layer 6 and layer 5, and so on.

The upper layers are implemented in the software and lower layers are implemented in both software and hardware, except for the physical layer which is completely hardware.

Q How layered technology helps in achieving communication between two hosts?Answer: As shown in above figure, with gives an over all view of layered architecture, L7 data means the data unit at layer 7, L6 data means data unit at layer 6 and so on. The process starts at layer 7, and then moves from layer n to layer n-1 in descending order at sending side and ascending order at receiving side. At each layer (except layer 7 and layer 1), a header is added and at layer 3, a header and trailer are added. When the formatted data unit passes through the layer 1, it is changed into electromagnetic signal and transported along a physical link.

Upon reaching the destination, the signal passes into layer 1; layer 1 transforms the received signal back into digital data. The data units then move back up from layer n to layer n+1 through the layered architecture. As each block of data reaches the next higher layer, the headers and trailers attached to it at the corresponding layer are removed and actions appropriate to that layer are taken. By the time message reaches layer 7, the message is in a form that it can be delivered to the application in execution.

ISO/OSI ModelQ What do you mean by ISO/OSI Model?Answer: ISO is an acronym for International Standards Organization and OSI is an acronym for Open Systems Interconnection.

Q. What is this ISO for? (You will not be asked this question in exam)Answer: ISO is a word wide organization that takes formulates the standard and observer that standards are being followed by everybody. This ensures compatibility and scalability in future.

Q. What is OSI?Answer: As told above, OSI is an acronym for Open Systems Interconnection. To understand it, you need to break these into three words and understand each word individually.Open: Open means one who is ready to accept any thing and give any thing but is an individual. For example: I am open to smoking and drinking, i.e. I don’t mind taking hard drink and smoke.System: System, you all know is a computer. But here systems are Open, which means there are autonomous or individual in nature and can receive messages and send messages.Interconnection: Interconnection means connecting in between.

So, if we combine all three of them, what we can make out is, interconnecting autonomous systems together.

Q. How many layers are in the ISO/OSI Model?Answer: There are seven layers in the ISO/OSI Model.

1. Application Layer2. Presentation Layer3. Session Layer4. Transport Layer

5. Network Layer6. Data Link Layer 7. Physical Layer

Application LayerQ. What is the purpose of Application Layer?Answer: As explained before, this is the topmost layer in ISO/OSI model, and it interacts with the applications in execution as shown in figure below.

(i) (ii)

In figure (i), Application layer takes message from the user (here user is like some application, for example chat application or telnet application or ftp application) and gives that message to the layer below it i.e. presentation layer (for other functions to be performed on this message) through an interface present between application layer and presentation layer called as service access point. Note that no header is attached to the message at this layer.

In figure (ii), Application layer takes messages from the layer below it i.e. presentation layer, here also messages pass between presentation layer to the application layer through an interface called as service access point, and deliver it to the appropriate application in execution for which that message is meant for.

Q. What is the purpose of Presentation Layer?Answer: Presentation layer is concerned about the language between used between two hosts or between two systems, Since two systems are open as explained below, they are autonomous or individual in nature, so each one of them can have their own language or character set. Thus one of the jobs that Presentation Layer does is Translation.Actually there are three functions that Presentation layer does:

1. Translation2. Encryption3. Data Compression

Q What is Translation, and why is it required?Answer: The process in two hosts or two systems are usually exchanging information in the form character strings, numbers etc. The information is changed to bit, this conversion of character strings and numbers into bits can be done in two ways, one way is ASCII and second way is EBCDIC. So let us assume that Host A uses ASCII representation of converting his message (remember message is combination of strings

and numbers) and Host B uses EBCDIC. If any message in ASCII is sent to Host B, he will not be able to understand that. The solution to this is, presentation layer at the sender converts ASCII message to some common format which when received by Host B is converted into EBCDIC. What that common format is? That is another question, we wont get into that.

Q What is Encryption? Why is it required? Answer: Encryption is encoding, i.e. converting message from intelligible form to unintelligible form. This is done to ensure security, so that while some confidential message is in transit and some other third person manages to gain access to that message, he should not be able to know what it means, if the message is not encrypted before transmission, he can make out the meaning of it. Presentation layer at the sender end encrypts the message and the encrypted message is transmitted over the medium. After traveling through the medium, encrypted message reaches the physical layer of the destination, where encrypted message is again decrypted to get the original message. So, decryption is done at receive end, and is opposite of encryption. We can define decryption as, converting message from unintelligible form to intelligible form, so that one can make out what it mean.

Q. What is Data Compression? Why is it required?Answer: Data Compression reduces the number of bits contained in the information. To understand why data compression is done or what the advantages of doing data compression are, let us take one example.Suppose host A wants to transmit to Host B. They both are on point-to-point link (i.e. Host A is on one side of the link and Host B is on the other side of the link, and link is not shared by anyone else). The link can carry 10 bits/sec that means Host A can place 10 bits on the link in 1 sec. If Host A has a packet of size 20 bits, how much time will it take to transmit it? Obviously the answer is 2 seconds. If presentation layer of A can compress that message to half, i.e. after compression the size of the packet is 10 bits, now how much time will it take? This time the answer is 1 sec, which is less than time taken to send the same uncompressed packet. So transmission time is reduced by compression.So, Presentation layer at the source compresses the message and compressed message is given to the next layer down and finally reaches the physical layer of the source, which transmits the message on the medium, medium travels through the medium and is received by the receiver’s physical layer. Physical layer of the receivers gives the message to the layer in upward direction. Finally message reaches the presentation layer where message is decompressed to retrieve the original message. (Remember that when a message travels down the layer, a header is attached to the message at every layer except the Application layer and physical layer, and in data link layer a trailer is also attached. Also when a message travels up the layer, headers are removed at every layer except the application layer and physical layer)

So after message is translated, encrypted and compressed at the presentation layer, it is passed to the lower layer i.e. Session layer, through interface present between the presentation and session layer. Figure below depicts the behavior of presentation layer.

Session layerQ. What is the purpose of Session Layer?Answer: Session layer enable for processes in two different systems to enter into a dialog. It allows communication between two processes to take place either in half-duplex or full-duplex. So Session layer does dialog control.Session layer allows a process to add checkpoints (also called as synchronization points) into a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to add checkpoints after every 100 pages. Checkpoint helps in acknowledging that after every 100 page is received, an acknowledge goes to the sender. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 600, and onwards.

Transport LayerQ. What is the purpose of Transport LayerAnswer: The transport layer is responsible for source to destination delivery of the entire message. What I mean by that, transport layer is not concerned how data travel from source to destination, the only concern of transport layer is that it should reach there with out any error and without any loss. In must simpler words, transport layer ensures that data reaches at the destination intact overseeing error control and flow control. The concern about how data travel is of the lower layer to transport layer, i.e. network layer. As a matter of fact, lower three layers are concerned about how to route the packet from source to destination i.e. network layer, data link layer and physical layer. The responsibilities of transport layer are:

1. Service Point Addressing2. Segmentation and Reassembly3. Connection Control4. Flow Control5. Error Control

Network LayerQ. What is the purpose of Network Layer?Answer: The network layer is responsible for source to destination delivery of a packet possibly across multiple networks; again network layer relies on the layer below it for actual transmission. The layer below network layer is data link layer. But yes data link layer define the route that the packet will follow or packet can also take dynamic route defined by the intermediate routers on the run. But for sending a packet from one router to another, there is a medium in between, to travel through the medium and data link layer and physical layer comes into picture, so network layer without the help of data link layer and physical layer is incomplete. Similarly transport layer relies on network layer, and transport layer is incomplete without the network layer and layers below network layer.The responsibilities of network layer are

1. Routing2. Logical Addressing

Data Link LayerQ What is the purpose of Data Link Layer?Answer: The data link layer is responsible for end to end delivery of frames i.e. it is the data link layer which actually do the job of sending frame from one end of link to other end of the link (in a point to point link) obviously the actual transmission of frame is done by the physical layer i.e. putting the bits of the frame on the link. Data link layer has to take care of flow control, error control between two links that falls in the whole path from source to destination. The responsibilities of data link layer are

1. Framing2. Physical Addressing3. Flow Control4. Error Control5. Access Control

Physical LayerQ. What is the purpose of Physical Layer?Answer: Physical layer is the lower layer, and converts the message into bits; put the bits on to the medium. The number of bits that the physical layer can put on the medium or transmit through the medium is called as the transmission rate. Below is the figure the depicts the transformation done to the message at physical layer.

The physical layer is concerned with the following1. Physical characteristics of the interfaces and medium2. Representation of bits3. Data Rate or Transmission Rate4. Link Configuration5. Physical topology

Summary of ISO/OSI Model