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OSI Model and Standards ITNW 1325, Chapter II

OSI Model and Standards ITNW 1325, Chapter II. Understanding the OSI Model. Understanding the OSI Model. Overview : Open Systems Interconnection (OSI) – a layered reference model comprised of seven functional layers

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OSI Model and Standards ITNW 1325, Chapter II

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  1. OSI Model and Standards ITNW 1325, Chapter II

  2. Understanding the OSI Model

  3. Understanding the OSI Model Overview: • Open Systems Interconnection (OSI) – a layered reference model comprised of seven functional layers • Developed by the International Organization for Standardization (ISO) in 1984 – based on their analysis of TCP/IP, IBM SNA, and DECNET protocols • Governed by the ISO Standard 7498 – some vendors build their products according to it (Novell) • Ensures compatibility and solves communication issues among different implementations of network hardware and software

  4. Understanding the OSI Model Overview (continued): • Uses the divide-and-conquer approach to networking from low-level hardware to the high-level software • Constructs a series of independent but interconnected layers – breaks the big problem of communications into smaller problems that are isolated from each other • Individual layers encapsulate specific independent functions – changes to one layer don’t affect other ones • Implements the “peer communication” principle – only identical remote layers communicate to each other

  5. Understanding the OSI Model Overview (continued): • “Universal” resembles “imperfect” – some network functions operate at several layers, while some do not require services from every layer • Practical usage is questioned by many because of its complexity and lack of flexibility The OSI networking model remains a great tool for learning networks – protocols, devices security, and other models

  6. Understanding the OSI Model Reasons for Layering: • Divides communications into a finite number of logical blocks – simplifies comprehension and use • Provides design modularity – allows upgrades to a specific layer to remain separate from the other ones • Allows programmers to specialize in a particular layer of the networking model, with open set of specifications • Encourages interoperability by promoting balance between different networking models • Allows vendors to produce standardized interfaces

  7. Understanding the OSI Model Seven Layers:

  8. Understanding the OSI Model

  9. Understanding the OSI Model

  10. Understanding the OSI Model Peer Communication, Overview: • Each layer is unaware of the activities of all other ones on the same host – doesn’t acknowledge their services • Each layer only communicates logically to an identicallayer on the other side of the communication process – information is passed via headers and trailers added • Headers and trailers added at the sending layer will be read and removed at the peer layer on the other side • Protocol suites combine protocols defined at different layers together to enable network communications

  11. Understanding the OSI Model Peer Communication, Illustration:

  12. Understanding the OSI Model Peer Communication, Advantages: • Allows convenient distribution of networking functions • Permits independent error checking on different layers • Simplifies creation of protocols Peer Communication, Disadvantages: • Results in overhead that grows as data traverses the model from the Application to the Data Link layer • Leads to reduced efficiency of network utilization

  13. OSI Layer Functions

  14. OSI Layer Functions Application (L7): • Defines network services that software applications (browsers, e-mail clients, etc) can request from the network and requests the services on their behalf • Accepts data from applications and interprets their formatting and procedures to the network • Interprets data coming from the network and passes it to proper applications • Facilitates multiple important protocols – HTTP, FTP, DNS, Telnet, SMTP, SNMP, etc.

  15. OSI Layer Functions Presentation (L6): • Receives data from the Application layer and prepares it for transmission over the network • Reformats the incoming data from lower layers for specific machine/application combination • Performs encryption and compression of data for outbound communications – as well as decryption and decompression of data for inbound communications • The only layer that restructures data – other ones add headers and/or trailers without reconfiguring the data

  16. OSI Layer Functions Presentation (continued): • Distinguishes between file extensions and coding schemes – BMP, JPG, WAV, MP3, ASCII, HTML, etc. • Example – Presentation layer protocols encode online music tracks into MP3 format • Example – Presentation layer protocols interpret JPG images so that HTTP is able to understand them • Example – Presentation layer protocols encode text using ASCII and other schemes • Example – Presentation layer protocols encode/decode sensitive data within secure Internet connections

  17. OSI Layer Functions Session (L5): • Allows senders and receivers to establish and manage data transmission session – independently of the actual data flow over the network • Detects if the transmission has been cut off, notifies the client software, and restart its at the appropriate point • Determines the order of communication, maximum duration of transmission, and provides clocking or timing for the session • Assists large data transfers – informs the receiver about the beginning/end of the stream that’s broken in pieces

  18. OSI Layer Functions Session (continued): • Allows information of different streams – that may be originating from different sources – to be properly combined or synchronized • Facilitates NetBIOS, SQL, RPC, and other protocols

  19. OSI Layer Functions Transport (L4): • Accepts data from the Session layer services and provides messaging service for them • Facilitates connection-oriented (guarantee of delivery) and connectionless (delivery not guaranteed) protocols • Connection-oriented protocols ensure data delivery – used for sensitive data transmissions over the Internet • Connectionless protocols don’t ensure data delivery – but impose much lower overhead onto the network • Submits data with its header added to the Network layer for further handling

  20. OSI Layer Functions Transport, Connection-Oriented Protocols: • Explicitly establish a session (“connection”) before allowing data to be sent • Ensure data delivery by requiring and acknowledgement (ACK) of the receipt of data packets – retransmit in case an ACK is not timely returned • Negotiate for the highest number of data segments to be sent before an acknowledgement is required • Provide data integrity via checksums – unique character strings attached to data that allow the receiving node to determine if a data unit was modified during delivery

  21. OSI Layer Functions Transport, Connection-Oriented Protocols (continued):

  22. OSI Layer Functions Transport, Connection-Oriented Protocols (continued): • Ensure reliable data delivery by breaking large data units into multiple smaller segments (segmentation) – with segment size related to the MTU size • The MTU size is the maximum data size that nodes on the way can place into their memory buffers • Identify segments that belong to the same message, determine the order of segments (sequencing), and reconstruct the segmented units (reassembly) • Gauge appropriate rate of transmission based on how fast the recipient can accept data (flow control)

  23. OSI Layer Functions Transport, Connectionless Protocols: • Do not establish a connection before sending data • Do not require acknowledgements for data sent – don’t ensure the that the data was properly received • Define a special term for data carried – datagrams • Do not perform error check • Much less sophisticated and have less transmission and processing overhead than connection-oriented ones • Used in cases when data needs to be sent quickly • Example – streaming video and audio transmissions over the network

  24. OSI Layer Functions Transport, Protocols:

  25. OSI Layer Functions Network (L3): • Accepts data from the Transport layer – wraps segments into packets that carry addressing information • May brake large packets into smaller ones – according to capacity of the network (fragmentation) • Defines protocol-dependent logical addressing schemes that uniquely identify nodes within interconnected networks and enable network segmentation • Establishes the best delivery path (routing) considering addressing, delivery priorities, network congestion, quality of service, and cost of the paths (routes)

  26. OSI Layer Functions Network (continued): • Implements congestion control by sensing delays associated with routes and managing how much traffic is sent across them – helpful within busy networks • Internet Protocol (IP) is the most common L3 protocol

  27. OSI Layer Functions Data Link (L2): • Encapsulates packets received from the Network layer into frames – complete packages to be transmitted • Defines the format of the header and/or trailer added to packets received – depend on the network type in use • Common network types are Ethernet and Token Ring – use different frames and can not be used together • Frame format and maximum size map onto the carrying capacity of the network medium • Performs verification of data integrity using checksum mechanism – to detect transmission errors

  28. OSI Layer Functions Data Link (continued): • Implies error correction upon the receiver’s request for retransmission in case a frame is dropped or altered • Manages point-to-point transmission across the medium within the same logical or physical cable segment • Splits into two sublayers with separate duties – Logical Link Control (LLC) and Media Access Control (MAC)

  29. OSI Layer Functions Data Link, Sublayers:

  30. OSI Layer Functions Data Link, Sublayers, LLC: • Interfaces the Network layer – implies intelligence • Packages data frames differently for different networks • Manages flow control and issues requests for retransmission for data with errors Data Link, Sublayers, MAC: • Defines a unique physical identifier – MAC address – for network cards (every frame carries a destination and source MAC addresses) • Defines and manages the access to the physical medium

  31. OSI Layer Functions Data Link, MAC Addresses: • 48-bit non-replaceable, “burned-in” addresses (BIA) - represented using twelve hexadecimal characters • Consist of two parts – a block ID and a device ID • A block ID (“Organizational Unit Identifier, OUI”) – a six-character (24-bit) sequence that uniquely identifies each vendor (managed by IEEE), with large vendors assigned several different block IDs • A device ID (“serial number”) – a six-character (24-bit) sequence that uniquely identifies the device (managed by the manufacturer)

  32. OSI Layer Functions Data Link, MAC Addresses (continued):

  33. OSI Layer Functions Data Link, Frame Integrity: • Before a frame is sent, the sender performs a cyclic redundancy check (CRC) on all of its fields – generates a unique 4-byte frame check sequence (FCS) code • The FCS code is attached to the frame being sent – to be detached and regenerated by receiver • The generated code is compared to the one received – no error is assumed in case the two codes match and a retransmission request is issued in case of mismatch

  34. OSI Layer Functions Data Link, Frame Handling: • All NICs connected to the same physical segment of the network receive and process frames sent • Only NIC with matching destination MAC address passes the payload to the Network layer – other nodes would drop the frame • Broadcast frames are sent to and processed by all nodes on the physical segment – costs performance • Reducing the number of nodes on a physical network – segmentation– improves performance by reducing the number of frames sent and processed

  35. OSI Layer Functions Physical (L1): • Accepts frames from the Data Link layer and turns frame bits into the medium pulses on the sending end • Transforms pulses to bits and passes them to the Data Link layer on the receiving end • Defines mechanical, electrical, and procedural characteristics of the network hardware and medium • Determines data transmission rates and timing intervals • Non-intelligent layer – does not read data handled, adds no header or trailer, and performs no error correction

  36. OSI Layer Functions

  37. OSI Model at Work

  38. OSI Model at Work Encapsulation, Overview: • Each lower layer accepts data from the layer above and performs encapsulation – adds a protocol data unit (PDU) composed of layer-specific header and/or trailer • A PDU enables logical communication between a layer at the source computer and the identical layer at the destination computer • Headers are layer-specific labels, trailers carry error-detection/correction information and end-of-PDU flags • The encapsulated data is passed to the layer below

  39. OSI Model at Work Encapsulation, Layer PDU: • Application, Presentation, and Session layer PDUs come in a variety of types and are referred to as Application, Presentation, and Session PDUs • Transport, Network, and Data Link layer PDUs are referred to as segments, packets, and frames • Physical layer PDUs consist of series of pulses that match bit patterns for Data Link layer frames

  40. OSI Model at Work Encapsulation, Process: • Begins at the at the upper three layers – the data is converted into a standard networking format • Transport layer forms segments by adding a header with port information – ensure proper delivery • The Network layer forms packets by adding a header with logical addressing information – ensures routing • The Data Link layer forms frames by adding a header with physical addressing information and a trailer • The Physical layer encodes frames and transmits them as pulses along the physical network

  41. OSI Model at Work Encapsulation, Illustration:

  42. OSI Model at Work Decapsulation: • The receiver’s Physical layer accepts the data from the physical network – transforms pulses into bits, passes to the layer above where bits are read as a frame • Headers and trailers are removed as data travels up the OSI model’s layers at the destination computer • Ultimately, the original data is passed to the receiving application by the receiver’s Application layer – with no headers or trailers present

  43. OSI Model at Work Encapsulation/Decapsulation:

  44. OSI Model at Work Relevance: 1984Today Physical Medium (wireless, copper, fiber-optics) Data Link Ethernet (frame format, access to the medium) Network IP (packet format, address format) Transport TCP (segment format, reliable procedures)

  45. Networking Standards

  46. Networking Standards Advantages: • Creation of competition – everybody may create technological devices based on a standard, as opposed to proprietary, apart from standards, patented devices • Lower cost for consumers – via lower product startup costs, time due to lower manufacturing costs, and healthy competition • Protection of investment into technology – lower costs and clarity of equipment upgrades due to backward compatibility of newer products • Interoperability – all devices from various vendors

  47. Networking Standards Disadvantages: • International standards – open domestic markets to competition from countries with lower production costs • Political conflicts – can be caused by standards or result in rejection of standards proposed by a nation by others The advantages outweigh the disadvantages

  48. Networking Standards Types, De Facto: • Common practices followed by industry for a variety of reasons – ease of use, established habits, costs, etc. • Primary influencing factor – success in the marketplace • Examples – MS Windows, Intel x86 architecture Types, De Jure: • Official, entrusted standards established by a body or an organization – with different subcommittees overseeing different technologies • Subject to lengthy development and acceptance process • Published and accessible to everyone online

  49. Networking Standards Types, De Jure (continued): • First step – working groups of industry experts propose the initial draft that gets published • Second step – requests for comments (RFCs) are sought from all interested developers, users, and specialists • Third step – the comments are reviewed and may be incorporated into a draft of the standard • Finally, the entire organization reviews the draft before it gets published as an official standard • A De Facto standard may become De Jure one upon approval by a committee or other authorized entity

  50. Networking Standards Types, Consortia: • Introduced by industry-sponsored organizations that want to promote a specific technology within a short period of time • Example – World Wide Web Consortium (W3C) that involves Microsoft, Sun, and IBM (developed Internet standards such as HTML, CSS, DOM) • Imply membership that may be open or not Standards can be enforced by the market De Jure standards are enforced by a regulatory authority

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