1. 1. Layered Architecture of Communication Networks: OSI Reference Model

2. © Tallal Elshabrawy 2 The OSI Reference Model

3. © Tallal Elshabrawy 3 Physical Layer Transfer of bits across communication link Transmission Medium Twisted-Pair cable, Coaxial Cable, Optical Fiber, Radio, Satellite, Infrared Aspects of a communications link Mechanical: cable, plugs, pins,... Electrical/optical: modulation, signal strength, voltage levels, bit times, … Functional/procedural: how to activate, maintain, and deactivate physical links…

4. © Tallal Elshabrawy 4 Data Link Layer Transfers frames across direct connections Two Sub-Layers Logical link control (LLC) Medium access control (MAC) LLC MAC Data Link Layer Network Physical

5. © Tallal Elshabrawy 5 Medium Access Control (MAC) Contention Two or more nodes want to transmit over the same medium at the same time Medium access control Defines the rules for accessing the medium Common MAC Schemes Carrier Sense Multiple Access (with Collision Detection) CSMA/CD Token Passing Random Access Reservation

6. © Tallal Elshabrawy 6 Logical Link Control (LLC) Detection of bit errors; Retransmission of frames Correction of bit errors Flow Control Simplest Error Detection: Parity Checks Even Parity : Sum of 1 s is even Odd Parity : Sum of 1 s is odd 1011000 LLC Even Parity Assumed 10110001 LLC 10110101 Single Error Detected Ask for Retransmission 1011000 LLC Even Parity Assumed 10110001 LLC 1010010110100101 Double Error Goes undetected

7. © Tallal Elshabrawy 7 Network Layer Routing: Transfers packets across multiple links and/or multiple networks Addressing must scale to large networks Nodes jointly execute routing algorithm to determine paths across the network Forwarding transfers packet across a node Congestion control to deal with traffic surges

8. © Tallal Elshabrawy 8 Routing Example 4 6 5 7 9 3 6 7 5 A F E C D B RouteCost ABFABF13 AB DFAB DF16 ABEFABEF12 ACFACF13 ACEFACEF17

9. © Tallal Elshabrawy 9 Issues in Routing Efficient routing algorithms are needed to perform Computation of routes Cope with dynamic changes in costs Avoid congestion since a good route may attract a lot of traffic and overload the computers on it Avoid route oscillations

10. © Tallal Elshabrawy 10 Transport Layer Provide a reliable and efficient network connection Allowing the upper three layers operate independent of underlying networks Deals with Packet loss Packet Delay Packet reordering Performs flow control and buffering Port numbers enable multiplexing Message segmentation and reassembly Connection setup, maintenance, and release

11. © Tallal Elshabrawy 11 Session Layer Establishes and maintains a session between two end- users Dialog management Full duplex Half Duplex In case of connection disruption during large file transfers. It enables the communication to resume from synchronization points What is the difference between the session layer and transport layer? The session layer establishes the connection in conjunction with the application Example: A video session might have multiple transport connections for Video Audio Data (e.g., captions, messages)

12. © Tallal Elshabrawy 12 Presentation Layer Different computers have different ways of representing the same information The presentation layer establishes a common ground of communication Data compression Decreasing as much as possible the amount of bits used to represent information Security If an un-authorized person intercepts the message, it is unintelligible

13. © Tallal Elshabrawy 13 Application Layer Application Layer: Provides services that are frequently required by applications: Example application layer residents E-mail File Transfer Protocol (FTP) Hypertext Transfer Protocol (HTTP) Telnet

14. © Tallal Elshabrawy 14 Headers & Trailers Each protocol uses a header that carries addresses, sequence numbers, flag bits, length indicators, etc… CRC check bits may be appended for error detection Application Presentation Session Transport Network Data Link Physical CRC Bit stream Application Presentation Session Transport Network Data Link Physical Data AH PH SH TH NH DH

15. © Tallal Elshabrawy 15 Peer-to-Peer Communications Layer n+1 entity Layer n entity n-SAP n-SDU H H n-PDU

16. © Tallal Elshabrawy 16 Layer n in one machine interacts with layer n in another machine to provide a service to layer n+1 The entities comprising the corresponding layers on different machines are called peer processes. The machines use a set of rules and conventions called the layer-n protocol. Layer-n peer processes communicate by exchanging Protocol Data Units (PDUs) Peer-to-Peer Communications: Protocol

17. © Tallal Elshabrawy 17 Communication between peer processes is virtual and actually indirect Layer n+1 transfers information by invoking the services provided by layer n Services are available at Service Access Points (SAP’s) Each layer passes data & control information to the layer below it until the physical layer is reached and transfer occurs The data passed to the layer below is called a Service Data Unit (SDU) SDU’s are encapsulated in PDU’s Peer-to-Peer Communications: Services

18. © Tallal Elshabrawy 18 Connection-Oriented & Connectionless Services Connection-Oriented Three-phases: 1.Connection setup between two SAPs to initialize state information 2.SDU transfer 3.Connection release E.g. TCP, ATM Connectionless Immediate SDU transfer No connection setup E.g. UDP, IP Layered services need not be of same type TCP operates over IP IP operates over ATM

19. © Tallal Elshabrawy 19 Segmentation & Reassembly A layer may impose a limit on the size of a data block that it can transfer for implementation or other reasons Thus a layer-n SDU may be too large to be handled as a single unit by layer-(n-1) Sender side: SDU is segmented into multiple PDUs Segmentation n-SDU n-PDU Re-assembly n-SDU n-PDU

20. © Tallal Elshabrawy 20 Multiplexing Sharing of layer n service by multiple layer n+1 users Multiplexing tag or ID required in each PDU to determine which user an SDU belongs to Layer n+1 entity Layer n entity n-SAP n-SDU H H n-PDU Layer n+1 entity n-SAP