OSI Reference Model PDF

Chapter Five-OSI Reference Model Overview & functions of each layer 1 OSI Prep. by Werkneh E. Outline  Layered Framework of OSI  OSI Upper layers  Overview & functions of each layer  Transport layer...

Chapter Five-OSI Reference Model Overview & functions of each layer 1 OSI Prep. by Werkneh E. Outline  Layered Framework of OSI  OSI Upper layers  Overview & functions of each layer  Transport layer  Packetizing  OSI lower layers  Addressing  Physical layer  Multiplexing and Demultiplexing  Data link layer  Connection control  Framing  Session layer  Addressing  Presentation layer  Error detection and correction  Application  Data link control and protocol  Clientserver model  Flow control  Addressing  Error control  Application layers services  Multiple access  Network Layer  Internetworking  Packetizing and framing  Addressing  Network layer protocols 2 OSI Prep. by Werkneh E.  Layered frame work of OSI  Open Systems Interconnection  A theoretical model-not implemented (reference)  Developed by International Standards Organization (ISO)  Open – to connect open systems  Contains seven layers 3 OSI Prep. by Werkneh E. OSI Lower Layers Physical Data Link Network 4 OSI Prep. by Werkneh E.  OSI Physical Layer  Responsible for transmission of bits  Bit to signal  Layer actually interact to the physical medium  Signal encoding: 0 and 1 representation  Medium used  Transmission type: serial/parallel  Transmission mode: simplex, duplex, 1/2  Topology and multiplexing: star, bus, TDM, FDM ?  Interface: device connection  Bandwidth: which bandwidth used  Signal type: analog or digital signal ? 5 OSI Prep. by Werkneh E.  Note The unit of communication at the physical layer is a bit. 6 OSI Lower Layers Physical Data Link Network 7 OSI Prep. by Werkneh E.  OSI Data Link Layer 1. Addressing – Headers and trailers are added, containing the physical addresses of the adjacent nodes, and removed upon successful delivery. 2. Flow control – This avoids overwriting on the receiver’s buffer by regulating the amount of data that can be sent. 3. Media Access Control (MAC) – In LANs, it decides who can send data, when and how much. 4. Synchronization – Headers have bits, which tell the receiver when a frame is arriving. It also contains bits to synchronize its timing to know the bit interval to recognize the bit correctly. Trailers mark the end of a frame, apart from containing the error control bits. 5. Error control – It checks the CRC to ensure the correctness of the frame. If incorrect, it asks for retransmission. Again multiple schemes (positive acknowledgement, negative acknowledgement, go- back-n, sliding window, etc.) exist here. 6. Node to node delivery – Finally, it is responsible for error-free delivery of the entire frame/ packet to the next adjacent node (node-to-node delivery). 8 OSI Prep. by Werkneh E. Communication at the data link layer Legend Source Destination D Data H Header A R1 R3 R4 B Data link Data link Physical Physical Link 1 Link 3 Link 5 Link 6 D2 H2 Frame D2 ame Fr H2 D2 H2 D2 H2 Frame Frame 9 OSI Prep. by Werkneh E.  Note The unit of communication at the data link layer is a frame. 10 Framing  Data-link layer takes packets from Network Layer and encapsulates them into Frames.  Then, it sends each frame bit-by-bit on the physical layer.  At receiver’ end, data link layer picks up signals from physical layer and assembles them into frames. 11 OSI Prep. by Werkneh E.  Addressing  Data-link layer provides layer-2 hardware addressing mechanism.  Hardware address is assumed to be unique on the link. It is encoded into hardware at the time of manufacturing.  Hardware address is a 6-byte(48 bits) physical address (or MAC- Media Access Control address) in hexadecimal;  written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:  Hardware address or a NIC’s address is permanent - a LAN address is burned into its ROM during manufacturing 12 OSI Prep. by Werkneh E.  Error Detection and Correction  mainly a data link layer function  networks must be able to transfer data from one device to another with complete accuracy - our wish  data can be corrupted during transmission - many factors exist - like transmission impairments  hence, reliable systems must have a mechanism for detecting and correcting errors  two types of errors: single-bit and burst  single-bit error: only one bit in a data unit (byte, character, packet,...) has changed; while Burst is occurred when more bit are affected 13 OSI Prep. by 13 Werkneh E.  Error Detection and Correction cont’d  Two types of error-detecting codes: parity and CRC  parity check: most common and simple; two varieties single parity check 2-D parity check: Can also detect (but not correct) any combination of two errors in a packet ❖detect single bit errors 14 OSI Prep. by Werkneh E. Cyclic Redundancy Check 15 OSI Prep. by Werkneh E. Error correction error correction by retransmission: when an error is detected, the receiver will tell the sender to retransmit the entire data unit; see next: Data Link Control forward error correction: the receiver can use an error-correcting code to correct certain errors; the hamming code is used to detect and correct errors; you can read more if you are planning to implement error correction 16 OSI Prep. by Werkneh E.  Flow Control  refers to a set of procedures used to restrict the amount of data that the sender can send before receiving an acknowledgement  A technique for speed-matching of transmitter and receiver 17 OSI Prep. by 17 Werkneh E. Flow Control Protocols: Stop-and-Wait Flow Control  sender sends one frame, stops until it receives confirmation from the receiver and then sends the next frame.  only one frame can be transmitted at a time  Inefficiency 18 OSI Prep. by Werkneh E. Flow Control Protocols: Sliding Window Flow Control  sender can transmit several frames continuously before needing an acknowledgement (ACK)  Each frame is numbered with k-bit sequence number, allowing for 2k sequence numbers  Each frame’s number is 1 greater than the previous frame and each ACK’s number is the number of the next frame expected by the receiver 19 OSI Prep. by Werkneh E. Error Control  refers to both error detection and error correction  in the data link layer, error control refers primarily to methods of error detection and retransmission  anytime an error is detected, specified frames are retransmitted; this process is called automatic repeat request (ARQ) 20 OSI Prep. by Werkneh E. Error Control (Cont’d) Requirements for error control mechanism:  Error detection - The sender and receiver, either both or any, must ascertain that there is some error in the transit.  Positive ACK - When the receiver receives a correct frame, it should acknowledge it.  Negative ACK - When the receiver receives a damaged frame or a duplicate frame, it sends a NACK back to the sender and the sender must retransmit the correct frame.  Retransmission: The sender maintains a clock and sets a timeout period. If an acknowledgement of a data-frame previously transmitted does not arrive before the timeout the sender retransmits the frame, thinking that the frame or it’s acknowledgement is lost in transit. 21 OSI Prep. by Werkneh E. Error Control: Stop-and-Wait ARQ Normal operation Lost or damaged frame Lost ACK frame 22 OSI Prep. by Werkneh E. Error Control: Stop-and-Wait ARQ (Cont’d) Stop-and-Wait ARQ efficiency After each frame sent the host must wait for an ACK ❖inefficient use of bandwidth To improve efficiency ACK should be sent after multiple frames Alternatives: Sliding Window protocol ✓ Go-back-N ARQ ✓ Selective Repeat ARQ 23 OSI Prep. by Werkneh E. Error Control: Sliding Window protocols a.Go-Back-N-ARQ b. Selective Repeat ARQ 24 OSI Prep. by Werkneh E.  Multi-Access  When host on the shared link tries to transfer the data, it has a high probability of collision.  Data-link layer provides random access mechanism such as CSMA/CD and CSMA/CA to equip capability of accessing a shared media among multiple Systems.  There are other multiple access protocols such as  Controlled-Access Protocols - get permission  Reservation  Polling  Token Passing Reading(Try your best to understand each)  Channelization Protocols - simultaneous use  FDMA - Frequency-Division MA  TDMA - Time-Division MA  CDMA - Code-Division MA 25 OSI Prep. by 25 Werkneh E. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)  Carrier Sense Multiple Access/Collision Detection is a technique for multiple access protocols.  If no transmission is taking place at the time, the particular station can transmit.  If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. The devices will wait for random amount of time.(Back off)  After a random time interval, the stations that collided attempt to transmit again.  If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.  Specially designed for ethernet not for Wi-Fi due to ❑Collision detection is difficult for Wi-Fi network due to limited radio range and hidden node problem 26 OSI Prep. by Werkneh E.  CSMA/CA NB: Both RTS & CTS frame includes the duration of time that source needs to occupy the channel-transfer duration NAV(network animation vector): a period the other station should wait before starting checking for channel idleness 27 OSI Prep. by Werkneh E. OSI Lower Layers Physical Data Link Network 28 OSI Prep. by Werkneh E. OSI Network Layer  A key design issue is determining how packets are routed from source to destination→ routing  path determination  Routes can be based on static tables that are ''wired into'' the network and rarely changed, and can be highly dynamic, being determined a new for each packet  When two network connected for communication:  There may be addressing difference  Protocol difference  Packet size of each can be different  It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected. 29 OSI Prep. by Werkneh E. Network Layer has 4 basic functions:  Internetworking  Addressing  Packetizing  Fragmentation  Routing  Main functions of routers (network layer devices)  Routing (Path determination): selecting best path b/n end devices  Routing protocols(RIP, OSPF, BGP)  Forwarding: move packets from router’s input to appropriate router output 30 OSI Prep. by Werkneh E. Communication at the network layer Legend Source Destination D Data H Header A R1 R3 R4 B Network Network Data link Data link Physical Physical D3 H3 Datagram 31 OSI Prep.D3 by H3 Datagram Werkneh E.  Note The unit of communication at the network layer is a datagram 32  See Journey of a Packet in Network >> play 33 OSI Prep. by Werkneh E.  Internetworking ▪ internetworking refers to the logical gluing of heterogeneous physical networks together to look like a single network to the upper transport and application layers ▪ the above internetwork is made up of 5 networks: 4 LANs and 1 WAN ▪ Si: switch or router; fi: interface ▪ data sent from A to D passes through 3 links ▪ nomenclature: ▪ the source and the destinations are usually referred to as hosts ▪ a host or a router is referred to as a hop 34 OSI Prep. by Werkneh E. Packetizing and Fragmenting Packetizing: encapsulates packets received from upper-layer protocols and makes new packets out of them; done by the IP protocol in the Internet model Fragmenting: a datagram can travel through different networks; each router decapsulates the IP datagram from the received frame, processes it, and then encapsulates it in another frame the format and size of the received frame depends on the protocol used by the physical network from which the frame has just arrived departing frame depends on the protocol used by the physical network to which the frame is going OSI Prep. by 35 Werkneh E. Addressing To uniquely and universally identify each device on the internet to allow global communication between all devices Analogous to the telephone system; the department of computer science: 251 011 1222922, wherever you are on the globe Each address belongs to a single host, but a single host can have multiple addresses (if it has multiple connections to the internet) The identifier used in the network layer of the internet model is called the internet address or IP address It is a 32-bit binary address (in ipv4) OSI Prep. by 36 Werkneh E. Network layer protocols  There are 5 network layer protocols in the TCP/IP  The main protocol is IP, the glue that holds the whole internet together and responsible for host-to-host delivery  It needs the services of other protocols  ARP (address resolution protocol) - maps an IP address to a MAC address (of the next hop)  RARP (reverse ARP) - maps a MAC address to an IP address; usually used in some situations such as when a diskless host is booted; it gets the binary image of its operating system from a remote file server but does not know its IP address; obsolete, replaced by DHCP- dynamic host configuration protocol  ICMP (internet control message protocol) - to handle unusual situations such as the occurrence of an error  IGMP (internet group management protocol) - for multicasting since IP is designed for unicast delivery; 37 OSI Prep. by Werkneh E. OSI Upper Layers Transport Session Presentation Application 38 OSI Prep. by Werkneh E. OSI Transport Layer  Isolates messages from lower and upper layers  Segmentation: Breaks down message size  Error control and flow control: Monitors quality of communications channel  Selects most efficient communication service(connection oriented and connection less transmission service) necessary for a given transmission 39 OSI Prep. by Werkneh E. Transport layer Functions  Process to process communication  Addressing: port numbers  Encapsulation and Decapsulation  Multiplexing and demultiplexing  Flow control  Error control  Congestion control 40 OSI Prep. by Werkneh E. data link layer: node-to-node delivery network layer: host-to-host delivery transport layer: process-to-process delivery several processes may be running on the source as well as the destination; an addressing mechanism is required 41 OSI Prep. by Werkneh E. Packetizing the transport layer creates packets out of the message received from the application layer it divides a long message into smaller ones called segments; they are then encapsulated into the data field of the transport-layer packet and headers are added Addressing at the data link layer, we need a MAC address at the network layer, we need an IP address at the transport layer, we need a transport-layer address, called a port number, to choose among several processes [0-65,535] 42 OSI Prep. by Werkneh E.  Socket address ❖ process-to-process delivery needs two addresses: IP address and port number at each end ❖ the combination of an IP address and a port number is called a socket address 43 OSI Prep. by Werkneh E. Multiplexing and Demultiplexing  the addressing mechanism allows multiplexing and demultiplexing by the transport layer since there may be several processes that need to send packets, but there is only one transport-layer protocol (UDP or TCP) 44 OSI Prep. by Werkneh E.  Connection Control Services: Connection less and connection oriented UDP (User Datagram Protocol) TCP (Transmission Control Protocol) connectionless and unreliable reliable, but complex no flow or error control, no retransmission if data is Error control, Loss control, Sequence control, corrupted or lost Duplication control, convenient for Convenient for: File Transfer Protocol (FTP), Remote multimedia and multicasting applications login – TELNET, Email – SMTP, World Wide Web – for client-server situations HTTP etc. e.g., DNS can use UDP, give me the IP address of the host name www.hu.edu.et Some well known ports used by UDP(more : Some well known ports used by TCP(more : www.iana.org ) www.iana.org ) DNS 53 DNS 53 Echo 7 Telnet 23 TFTP 69 FTP:Data: 20 45 OSI FTP: Control: 21 Prep. by Werkneh E. TCP Connection management  Connection setup (three-way handshake) and connection termination in TCP look as follows:  TCP requires connection establishment before data transfer begins.  For a connection to be established or initialized, the two hosts must synchronize their Initial Sequence Numbers (ISNs). 46 OSI Prep. by Werkneh E. TCP Transmission  Reliable delivery in TCP is realized using acknowledgement  Stop-and-wait protocol scenarios 47 OSI Prep. by Werkneh E. TCP Retransmission Scenarios  Different TCP retransmission scenarios are shown below 48 OSI Prep. by Werkneh E. OSI Upper Layers Transport Session Presentation Application 49 OSI Prep. by Werkneh E. OSI Session Layer  Establishes logical connections between systems  Manages log-ons, password exchange, log-offs  Terminates connection at end of session 50 OSI Prep. by Werkneh E. OSI Upper Layers Transport Session Presentation Application 51 OSI Prep. by Werkneh E. OSI Presentation Layer  Provides format conversion or translation (original message → machine understandable language)  Data compression  Maintain data Integrity through encryption and decryption  SSL(Secure socket layer ) is a protocol used for data encryption 52 OSI Prep. by Werkneh E. OSI Upper Layers Transport Session Presentation Application 53 OSI Prep. by Werkneh E. OSI Application Layer  Provides access to network for end-user  receives services from the transport layer and provides services to users (humans or software)  provides user interfaces and support services such as email, remote file access and transfer, access to the WWW 54 OSI Prep. by Werkneh E.  Receives services from the transport layer and provides services to users (humans or software)  Provides user interfaces and support services such as email, remote file access and transfer, access to the WWW  Three general issues related to the application layer:  the client-server paradigm,  addressing, and services 55 55 OSI Prep. by Werkneh E. The Client-Server Model  to do a task, there must be a client and a server  a computer runs a program to either request a service from another computer (client) or to provide a service to another computer (server)  communication takes the form of the client process sending a message to the server process and then waiting for a reply  a client program runs when needed, but the server program runs all the time 56 56 OSI Prep. by Werkneh E.  Addressing A client and a server communicate with each other using addresses Addressing mechanism in the application layer is different from the ones in other layers email address: [email protected] Web page (host name): http://www.hu.edu.et These are aliases convenient for human beings; they must be mapped to IP addresses An application program needs the services of another program for this; this application program is called DNS - domain name system: it uses port 53 It is not directly used by the user; but by application programs to perform the mapping 57 57 OSI Prep. by Werkneh E. Domain Name Resolution 58 58 OSI Prep. by Werkneh E. Application Layer Services  Electronic mail  SMTP: simple mail transfer protocol  MIME: multipurpose internet mail extensions  POP3 : post office protocol 3  IMAP : internet mail access protocol  Webmail  File transfer (FTP - file transfer protocol)  HTTP - hypertext transfer protocol for accessing data on the WWW  Www  Multimedia 59 59 OSI Prep. by Werkneh E. Application Services protocols Remarks File transfer Protocol FTP used to send and received file from a remote host Simple mail Transfer protocol SMTP Used to only send Email over a network Hyper text transfer protocol HTTP Used for Internet to send document that encoded in HTML Post Office Protocol 3 POP3 the whole mailbox will be cleared (transferred) from server to local computer; view mail only once Multipurpose Internet Mail Extensions MIME allows non-ASCII data to be sent using the existing mail programs and protocols it transforms non-ASCII data at the sender site to ASCII and back to non-ASCII at the receiving site Internet Mail Access Protocol IMAP IMAP is similar to POP3 but with the following additional features a user can: check the email header prior to downloading, search the contents of the email for a specific string, partially download, email; allows you to download emails from your email server onto multiple devices. It keeps your messages on the email server. Multimedia the combination of text, graphics, images, video and audio used together; at least one must be continuous (time-dependent like audio, video, animation) Webmail some websites provide email service to anyone examples are Yahoo and Hotmail World Wide Web WWW a repository of information spread all over the world and linked together; strings of text within a page that link to other documents are called hyperlinks 60 OSI Prep. by Werkneh E. Summary of OSI Layers 61 OSI Prep. by Werkneh E. summary 62 OSI Prep. by Werkneh E. Lack of OSI Model’s Success  The OSI model appeared after the TCP/IP protocol suite.  Most experts were at first excited and thought that the TCP/IP protocol would be fully replaced by the OSI model.  This did not happen for several reasons, but we describe only three, which are:  First, OSI was completed when TCP/IP was fully in place and a lot of time and money had been spent on the suite; changing it would cost a lot.  Second, some layers in the OSI model were never fully defined.  Third, when OSI was implemented by an organization in a different application, it did not show a high enough level of performance to entice the Internet authority to switch from the TCP/IP protocol suite to the OSI model. 63 OSI Prep. by Werkneh E. Quiz (10 %) 1. Which device can encapsulate and decapsulate at the same time in one communication? 2. List some of the functionality of each layers of OSI Model ? 3. What is difference between physical address and logical address? 4. What is the difference between fragmentation and segmentation? 64 OSI Prep. by Werkneh E. Quiz 2 (10 %) sec1 1. List some of the functionality of each layers of OSI Model ? 2. What is difference between physical address and logical address? 3. What is the difference between fragmentation and segmentation? 65 OSI Prep. by Werkneh E.