CS-772 COMPUTER COMMUNICATION AND NETWORKS 3 3 -0

CS-772 COMPUTER COMMUNICATION AND NETWORKS 3(3 -0)

Protocols u The OSI model u Functions of the layers u

Protocols To communicate, entities must agree on protocol. u Protocol is a set of rules that govern communication. u The layered functionality of the different protocols in the OSI Mode is called a Protocol Stack u

Standards standards that allow computers to connect to one another and enable computers and peripheral devices to exchange information with as little error as possible u The first Ethernet standard was published in 1980. u

Network Protocols • A communication protocol is a set of rules that specify the format and meaning of messages exchanged between computers across a network. • A set of related protocols that are designed for compatibility are called protocol suite.

Human and Computer Protocols Human Protocol Hi Hi Got the time? Computer Protocol Web client open connection OK send me data 2: 00 time Web server

Protocols and Standards • Industry standard • Open standard • Free of Charge • Changes are Hard sometimes • By one company or Organization • Close Standard

Layering u u What is it? Building complex systems is hard! • Approach: “Divide and conquer”. • Split job into smaller jobs, or layers. Analogy to other fields. • Building a house: digging, foundation, framing, etc. • Car assembly line… Basic idea: each step dependent on the previous step but does not need to be aware of how the previous step was done.

Analogy: Air Travel The problem: air travel. u Decomposed into series of steps: u Arrival at airport Departure from airport Check-in Baggage claim Boarding Deplane Takeoff Landing Traveling

More on the air travel analogy… Departure Check-in Baggage claim Boarding Deplane Takeoff Landing intermediate air traffic sites Airplane routing Traveling Airplane routing Arriving airport Departing airport Arrival

Layered Protocol Design • Layering model is a solution to the problem of complexity in network protocols • The model divides the network protocols into layers, each of which solves part of the network communication problem – Each layer has its own protocol! • Each layer implements a service to the layer above – Relying on services provided by the layers below.

Layers u u Layers are the different components that need to be designed/implemented when designing/implementing networks. Each layer responsible for a set of functions. Top layer relies on services provided by bottom layer. Layer makes it service available to higher layer through an interface.

The OSI Model OSI stands for Open Systems Interconnection u It is a model that covers all aspects of network communication u Purpose is to open communication between different systems without requiring changes to the underlying hardware and software u

The OSI Model (cont. ) u u It is a layered framework for the design of network systems Please Do Not Touch Steve’s Pet Alligator All People Seem To Need Data Processing Please Do Not Throw Sausage Pizza Away

The OSI Model (cont. ) u u Each layer has its own set of welldefined functions Layers communicate and interact with the layers immediately above and below

The OSI Model (cont. ) u u Each layer in the sending machine adds its information in the form of header and trailer. Each layer in the receiving machine gets information related to it and passes the rest to upper layers.

Data Name at Different OSI model layers u u u u Application Presentation Session Transport Network Data Link Physical u u u u Message Packets Datagram and segment Datagram Frames Bits

Functions of the layers u 1: Physical Layer • Coordinates the functions required to transmit a bit stream over a physical medium. • Deals with the mechanical and electrical specs of the medium • Defines the procedures that physical devices had to perform for trans. to occur

Functions of the layers (cont. ) u Physical layer concerns with the following; • • Physical characteristics of interfaces and media Representation of bits Data rate (the transmission rate) Synchronization of bits Line configuration Physical topology Transmission mode

Functions of the layers (cont. ) Data Link Layer u This layer breaks the input data into frames, transmits the frames sequentially, and processes the acknowledged frames sent back by the receiver. It adds a header and trailer to the frames it creates. u It makes it to appear error free to upper layer u Data Link Layer is Sub divided u u MAC (Media Access Control) LLC (Logical Link Control)

Functions of the layers (cont. ) u Data link layer responsibilities are; • • Framing Physical addressing Flow control Error control It Adds a header and trailer to the frames its creates

MEDIA ACCESS CONTROL u u It is responsible for physical addressing and access to the network media, only one device at a time may transmit on any type of media. . Addressing Contention Deterministic Addressing The Data Link layer is responsible for the physical addresses of devices on the network. Every device on a network has a hard-coded address attached to it. u Example The address for Ethernet card would be 00 -AA-00 -59 -65 -71. u

u u Contention Any device can transmit whenever it needs to. It allows equal access to the network media, but at the expense of possible collisions. Collisions occur when two devices try to transmit at the same time and disrupt each other’s signaling. Devices listen for other signals on the media before transmitting. Collisions are not totally eliminated, but they are kept down to manageable form. It is called CSMA (Carrier Sense Multiple Access. It has two types CSMA/CD A stands for Carrier Sense Multiple Access/Collision Detection. CSMA/CA A stands for Carrier Sense Multiple Access/Collision Avoidance

Deterministic Unlike a contention-based network, where devices are free to transmit whenever they want, a deterministic network u The two types of deterministic networks are token passing and polling u

LOGICAL LINK CONTROL u u The Logical Link Control (LLC) sublayer of the Data Link layer establishes and maintains data link connections between network devices. It is responsible for any flow control and error correction found in this layer The following are connection services the LLC sublayer supplies Unacknowledged connectionless service Connection-oriented service

Functions of the layers (cont. ) 3: Network Layer • The third layer of the OSI model is the Network layer. This layer is responsible for routing information from one network device to another. The Network layer decides what path data will take if the destination device is located on another network.

Functions of the layers (cont. ) u Network layer responsibilities are; • Routing u One of the functions of the network layer is to route the packets to their final destination in an internetwork environment.

Functions of the layers (cont. ) u u SWITCHING: Datagram switching describes how data is forwarded across an inter network. There are three main methods Circuit Switching: In circuit switching, a dedicated connection is made between the two communicating devices. Two advantages of this method are no congestion (because the link is dedicated) and almost no channel-access delay. The disadvantages are inefficient use of the media and a possible long wait to establish a connection.

Network Layer u u Message Switching: With message switching the data is sent from device to device in whole across the network. This is also known as store and forward. Devices must store all the information as it is sent in whole. The media is used more efficiently with this method, and congestion can be controlled. Packet Switching: A combination of circuit switching and message switching. With packet switching, data is broken into small pieces and routed from device to device. Devices that forward the data only need to keep the information in memory instead of in physical storage, because data was split

Functions of the layers (cont. ) u 4: Transport Layer • Responsible for the source to destination delivery of the entire message. • Ensures that whole message arrives in order.

Functions of the layers (cont. ) u Transport layer responsibilities are; • Service-point addressing u Not only from computer-to-computer connection, but also from process to process, so it gets the entire message to correct process(running program) on that computers • Segmentation and reassembly u Divided into transmittable segments with a unique sequence number • Connection control u • • Can be connectionless or connection oriented Flow control Error control

Transport Layer u u Provide acknowledgment of successful transmission Flow control • Partition data into packets and assign each one a sequence number • Provide service to assemble the received packets back into their original order • Error detection and correction u Lowest layer to which application programs are typically written

Functions of the layers (cont. ) u 5: Session Layer • Is the network dialog controller which establishes, maintains and synchronizes the interaction between communicating systems.

Functions of the layers (cont. ) u Session layer responsibilities are; • Manage who can transmit data at a certain time and for how long • Synchronization u Adds checkpoints into a stream of data.

Session Layer u Enable data exchange between application to application • • • Establishment Synchronization Re-establishment

Functions of the layers (cont. ) u 6: Presentation Layer • Concerned with the syntax of the information exchanged between two systems.

Functions of the layers (cont. ) u Presentation layer responsibilities are; • Translation u Changes information into a stream of bits which is a common format for different systems. • Encryption u The sender transforms the original information to another form and receiver transforms the message to its original form for privacy reasons. • Compression u Data compression reduces the number of bits to be retransmitted, it is important at the transmission of multimedia data.

Functions of the layers (cont. ) u 7: Application Layer • Enables the user whether human or software to access the network and provides user interfaces.

Functions of the layers (cont. ) u Application layer responsibilities are; • Network Virtual Terminal u Is a software version of the physical terminal and allows a user to log on to a remote host. • File transfer, access and management(FTAM) u Allows a user to access files in a remote computer and to manage or control them. • Mail services u Provides the basis for email forwarding and storage.

Application Layer u u Dictate the semantics of how requests for services are made, such as requesting a file or checking for email. The container for all applications and protocols • Telnet, HTTP, POP, SMTP, Finger, FTP, etc. u u Virtually all distributed systems are applications In Java, almost all network software written will be for the application.

OSI Layers u A typical message as it appears on the network. 2 -2

Functions of the layers (cont. )

Layer Application User Interface Presentation Translation Session "syncs and sessions" Function used for applications specifically written to run over the network allows access to network services that support applications; directly represents the services that directly support user applications handles network access, flow control and error recovery Example apps are file transfer-mail, Translates from application to network format and vice-versa all different formats from all sources are made into a common uniformat the rest of the OSI model can understand responsible for protocol conversion, character conversion, data encryption / decryption, expanding graphics commands, data compression sets standards for different systems to provide seamless communication from multiple protocol stacks not always implemented in a network protocol establishes, maintains and ends sessions across the network responsible for name recognition (identification) so only the designated parties can participate in the session provides synchronization services by planning check points in the data stream => if session fails, only data after the most recent checkpoint need be transmitted manages who can transmit data at a certain time and for how long Examples are interactive login and file transfer connections, the session would connect and re-connect if there was an interruption; recognize names in sessions and register names in history Protocols Network Component s DNS; FTP; SMTP; TELNET; etc Gateway Redirector Net. BIOS Mail Slots Gateway

Transport packets; flow control & error-handling Network addressing; routing additional connection below the session layer manages the flow control of data between parties across the network divides streams of data into chunks or packets; the transport layer of the receiving computer reassembles the message from packets "train" is a good analogy => the data is divided into identical units provides error-checking to guarantee error-free data delivery, with on losses or duplications provides acknowledgment of successful transmissions; requests retransmission if some packets don’t arrive error-free provides flow control and error-handling translates logical network address and names to their physical address (e. g. computername ==> MAC address) responsible for oaddressing odetermining routes for sending omanaging network problems such as packet switching, data congestion and routing if router can’t send data frame as large as the source computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, the network layer reassembles the data think of this layer stamping the addresses on each train car TCP, Net. BIOS / Net. BEUI ATP Gateway Advanced Cable Tester Brouter IP; ARP; RARP, ICMP; RIP; OSFP; IGMP; IPX NWLink Net. BEUI OSI DDP DECnet Brouter Router Frame Relay Device ATM Switch Advanced Cable Tester

Data Link data frames to bits Physical hardware; raw bit stream turns packets into raw bits 100101 and at the receiving end turns bits into packets. handles data frames between the Network and Physical layers the receiving end packages raw data from the Physical layer into data frames for delivery to the Network layer responsible for error-free transfer of frames to other computer via the Physical Layer this layer defines the methods used to transmit and receive data on the network. It consists of the wiring, the devices use to connect the NIC to the wiring, the signaling involved to transmit / receive data and the ability to detect signaling errors on the network media transmits raw bit stream over physical cable defines cables, cards, and physical aspects defines NIC attachments to hardware, how cable is attached to NIC defines techniques to transfer bit stream to cable Logical Link Control error correction and flow control manages link control and defines SAPs 802. 1 OSI Model 802. 2 Logical Link Control Media Access Control Bridge Switch ISDN Router Intelligent Hub NIC Advanced Cable Tester communicates with the adapter card controls the type of media being used: 802. 3 CSMA/CD (Ethernet) 802. 4 Token Bus (ARCnet) 802. 5 Token Ring 802. 12 Demand Priority IEEE 802. 2 ISO 2110 ISDN Repeater Multiplexer Hubs Passive Active TDR Oscilloscope Amplifier