Layered Architecture or Network ArchitectureThe layered architecture consists of the set of layers, as explained in the class, each layeris nothing but a collection of functions combined in one group. So, bottom line is, all thelayers make the communication process achievable, by dividing functionality among thelayers. Also each layer provides services to the layer above it. In the last lecture, therewere 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 anothermachine, a kind of indirect communication (by taking help of intermediate layers), exceptthe physical layer. Because the physical layer of one host connects with the physical layerof another host through the medium directly. Those layers that communicate with thecorresponding layer on another machine are called as peer-to-peer layers and thecommunication 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 examplewe are assuming that there are only two routes, whereas in practical there can be multiplerouters 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 thelayer below it i.e. to layer 6 or take message from the layer below it (i.e. layer 6) anddeliver to the application. Application may be interacting with the various applicationsrunning on the computer. Layer 7 is only concerned about interacting with the applicationexecuting 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 respectiveheader 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 thelayer 6, whereas each layer except layer 1 has some function to perform, which they dowhen 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 thetake the message from the applications in execution or delivers the message to theapplication 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; Sothat 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 inboth 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. Theprocess starts at layer 7, and then moves from layer n to layer n-1 in descending order atsending side and ascending order at receiving side. At each layer (except layer 7 andlayer 1), a header is added and at layer 3, a header and trailer are added. When theformatted data unit passes through the layer 1, it is changed into electromagnetic signaland transported along a physical link. Upon reaching the destination, the signal passes into layer 1; layer1 transforms the received signal back into digital data. The data units then move back upfrom layer n to layer n+1 through the layered architecture. As each block of data reachesthe next higher layer, the headers and trailers attached to it at the corresponding layer areremoved and actions appropriate to that layer are taken. By the time message reacheslayer 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 anacronym 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 observerthat standards are being followed by everybody. This ensures compatibility andscalability in future.Q. What is OSI?Answer: As told above, OSI is an acronym for Open Systems Interconnection. Tounderstand it, you need to break these into three words and understand each wordindividually.Open: Open means one who is ready to accept any thing and give any thing but is anindividual. For example: I am open to smoking and drinking, i.e. I don’t mind taking harddrink and smoke.System: System, you all know is a computer. But here systems are Open, which meansthere are autonomous or individual in nature and can receive messages and sendmessages.Interconnection: Interconnection means connecting in between.So, if we combine all three of them, what we can make out is, interconnectingautonomous systems together.Q. How many layers are in the ISO/OSI Model?Answer: There are seven layers in the ISO/OSI Model. 1. Application Layer 2. Presentation Layer 3. Session Layer 4. Transport Layer
5. Network Layer 6. Data Link Layer 7. Physical LayerApplication LayerQ. What is the purpose of Application Layer?Answer: As explained before, this is the topmost layer in ISO/OSI model, and it interactswith 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 someapplication, for example chat application or telnet application or ftp application) andgives that message to the layer below it i.e. presentation layer (for other functions to beperformed on this message) through an interface present between application layer andpresentation layer called as service access point. Note that no header is attached to themessage at this layer.In figure (ii), Application layer takes messages from the layer below it i.e. presentationlayer, here also messages pass between presentation layer to the application layer throughan interface called as service access point, and deliver it to the appropriate application inexecution 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 twohosts or between two systems, Since two systems are open as explained below, they areautonomous or individual in nature, so each one of them can have their own language orcharacter set. Thus one of the jobs that Presentation Layer does is Translation.Actually there are three functions that Presentation layer does: 1. Translation 2. Encryption 3. Data CompressionQ What is Translation, and why is it required?Answer: The process in two hosts or two systems are usually exchanging information inthe form character strings, numbers etc. The information is changed to bit, thisconversion of character strings and numbers into bits can be done in two ways, one wayis ASCII and second way is EBCDIC. So let us assume that Host A uses ASCIIrepresentation 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, hewill not be able to understand that. The solution to this is, presentation layer at the senderconverts ASCII message to some common format which when received by Host B isconverted into EBCDIC. What that common format is? That is another question, we wontget into that.Q What is Encryption? Why is it required?Answer: Encryption is encoding, i.e. converting message from intelligible form tounintelligible form. This is done to ensure security, so that while some confidentialmessage 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 beforetransmission, he can make out the meaning of it. Presentation layer at the sender endencrypts the message and the encrypted message is transmitted over the medium. Aftertraveling through the medium, encrypted message reaches the physical layer of thedestination, 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 definedecryption as, converting message from unintelligible form to intelligible form, so thatone 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. Tounderstand why data compression is done or what the advantages of doing datacompression 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 isnot shared by anyone else). The link can carry 10 bits/sec that means Host A can place 10bits on the link in 1 sec. If Host A has a packet of size 20 bits, how much time will it taketo transmit it? Obviously the answer is 2 seconds. If presentation layer of A can compressthat message to half, i.e. after compression the size of the packet is 10 bits, now howmuch time will it take? This time the answer is 1 sec, which is less than time taken tosend the same uncompressed packet. So transmission time is reduced by compression.So, Presentation layer at the source compresses the message and compressed message isgiven to the next layer down and finally reaches the physical layer of the source, whichtransmits the message on the medium, medium travels through the medium and isreceived by the receiver’s physical layer. Physical layer of the receivers gives themessage to the layer in upward direction. Finally message reaches the presentation layerwhere message is decompressed to retrieve the original message. (Remember that when amessage travels down the layer, a header is attached to the message at every layer exceptthe 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 theapplication layer and physical layer)So after message is translated, encrypted and compressed at the presentation layer, it ispassed to the lower layer i.e. Session layer, through interface present between thepresentation 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 orfull-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 isadvisable to add checkpoints after every 100 pages. Checkpoint helps in acknowledgingthat after every 100 page is received, an acknowledge goes to the sender. In this case, if acrash happens during the transmission of page 523, the only pages that need to be resentafter 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 entiremessage. What I mean by that, transport layer is not concerned how data travel fromsource to destination, the only concern of transport layer is that it should reach there without any error and without any loss. In must simpler words, transport layer ensures thatdata reaches at the destination intact overseeing error control and flow control. Theconcern 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 fromsource to destination i.e. network layer, data link layer and physical layer. Theresponsibilities of transport layer are: 1. Service Point Addressing 2. Segmentation and Reassembly 3. Connection Control 4. Flow Control 5. Error Control
Network LayerQ. What is the purpose of Network Layer?Answer: The network layer is responsible for source to destination delivery of a packetpossibly across multiple networks; again network layer relies on the layer below it foractual transmission. The layer below network layer is data link layer. But yes data linklayer define the route that the packet will follow or packet can also take dynamic routedefined by the intermediate routers on the run. But for sending a packet from one routerto another, there is a medium in between, to travel through the medium and data linklayer and physical layer comes into picture, so network layer without the help of data linklayer and physical layer is incomplete. Similarly transport layer relies on network layer,and transport layer is incomplete without the network layer and layers below networklayer.The responsibilities of network layer are 1. Routing 2. Logical AddressingData 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 thedata link layer which actually do the job of sending frame from one end of link to otherend of the link (in a point to point link) obviously the actual transmission of frame isdone by the physical layer i.e. putting the bits of the frame on the link. Data link layer hasto take care of flow control, error control between two links that falls in the whole pathfrom source to destination. The responsibilities of data link layer are 1. Framing 2. Physical Addressing 3. Flow Control 4. Error Control 5. Access ControlPhysical LayerQ. What is the purpose of Physical Layer?Answer: Physical layer is the lower layer, and converts the message into bits; put the bitson to the medium. The number of bits that the physical layer can put on the medium ortransmit through the medium is called as the transmission rate. Below is the figure thedepicts the transformation done to the message at physical layer.
The physical layer is concerned with the following 1. Physical characteristics of the interfaces and medium 2. Representation of bits 3. Data Rate or Transmission Rate 4. Link Configuration 5. Physical topologySummary of ISO/OSI Model