Communication Networks NETW 501 Lecture 2 Network Architecture: OSI Reference Model Course Instructor: Dr.-Ing. Maggie Mashaly maggie.ezzat@guc.edu.eg C3.220

Layers and Protocols Fundamental idea: A piece of software/hardware provides a service to its users but keeps the details of its internal state and algorithms hidden from them Sounds familiar?? YES! Used all the time in computer science & programming E.g: Abstract data types, Data Encapsulation & Object-oriented programming The same approach is implemented in networks To reduce their design complexity, networks are organized as a stack of layers, each one built upon the one below it

Layers and Protocols Function of each layer is to: Offer certain services to the higher layers Make use of services provided by underlying layers For machines to communicate: Layer n on one machine carries a conversation with layer n on another machine Rules and conventions of this conversation are known as Layer n Protocol Protocol: is an agreement between communicating parties on how communication is to proceed

Multilayer Communication 1. Guy 1 speaks only Urdu and English 2. Guy 2 speaks only Chinese and French Help of translators is needed! 3. Guy 1 passes message to translator 7. Translator translates message, forwards it to Guy 2 Translators agree on common language: Dutch 4. Translator passes message to secretary for transmission 6. Secretary passes message to translator 5. Message sent through fax

The OSI Reference Model How communication occurs? In reality, no data are directly transferred from layer n on one machine to layer n on another machine Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached Lowest layer is the Physical Layer, through which communication actually occurs

Physical Layer Function: 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…

Data Link Layer Data Link Layer Function: Two Sub-Layers Divides data into Frames Responsible for detecting /correcting transmission errors Two Sub-Layers Medium Access Control (MAC) Logical Link Control (LLC) LLC MAC Network Physical Data Link Layer

1. Medium Access Control (MAC) What problems does MAC sub-layer handle? 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 Token Passing Random Access Reservation Carrier Sense Multiple Access (with Collision Detection) CSMA/CD

2. Logical Link Control (LLC) What problems does LLC sub-layer handle? Detection of bit errors; Retransmission of frames Correction of bit errors Flow Control Simplest Error Detection: Parity Checks Even Parity : Sum of 1s is even Odd Parity : Sum of 1s is odd Single Error Detected Ask for Retransmission Double Error Goes undetected 1011000 1011000 LLC LLC LLC LLC 10110001 10110101 10110001 10100101 Even Parity Assumed Even Parity Assumed

Network Layer Functions of Network Layer: Routing Addressing Transfers packets across multiple links and/or multiple networks Nodes jointly execute routing algorithm to determine paths across the network Addressing Each node in the network must have its own unique address (IP address) Must scale to large networks Congestion Control To deal with traffic surges

Routing Example D B F A E C 5 7 4 9 3 5 7 6 6 Route Cost ABF 13 AB DF 16 ABEF 12 ACF ACEF 17

Issues in Routing Requirements for efficient routing algorithms: Computation of routes - What are possible routes from source to destination Cope with dynamic changes in costs - Links might fail or their cost might increase/decrease Avoid congestion - Since a good route may attract a lot of traffic and overload the computers on it Avoid route oscillations - To guarantee that all data is sent on one route

Transport Layer Functions of Transport Layer: Provide a reliable and efficient network connection Allowing the upper three layers to 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

Session Layer Functions of 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)

Presentation Layer Different computers have different ways of representing the same information The presentation layer establishes a common ground of communication Functions of Presentation Layer: 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

Application Layer Function of 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

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

Peer-to-Peer Communications Layer n+1 entity Layer n+1 entity n-SDU n-SDU n-SAP n-SAP n-PDU n-SDU H Layer n entity Layer n entity H n-SDU n-PDU SDU: Service Data Unit PDU: Protocol Data Unit SAP: Service Access Point

Peer-to-Peer Communications: Protocol 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 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

Connection-Oriented & Connectionless Services Established in three- phases: Connection setup between two SAPs to initialize state information SDU transfer 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! e.g.: TCP operates over IP, IP operates over ATM

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 Receiver side: multiple PDUs are re- assembled into one SDU Segmentation n-SDU n-PDU n-PDU n-PDU Re-assembly n-SDU n-PDU n-PDU n-PDU

Multiplexing Sharing of layer n service by multiple layer n+1 users Layer n+1 entity Layer n+1 entity Layer n+1 entity Layer n+1 entity n-SAP n-SAP n-SAP n-SAP n-PDU n-SDU H Layer n entity Layer n entity H n-SDU n-PDU 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

References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy “Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013 “Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International