Understanding the OSI Model

• Computer network operations are defined by two models.
• TCP/IP protocol suite
• OSI model

Protocol Layering

• In basic communication, only one protocol might be sufficient.
• Complex communication may require dividing tasks between different layers, necessitating a protocol at each layer.

Layered Tasks

• The concept of layers can be seen in everyday activities.
• Sending postal mail between two friends demonstrates this concept, where each step represents a layer.
• The process is complex without post office services.

Principles of Protocol Layering

• Bidirectional communication requires each layer to perform two opposite tasks.
• Protocol layering requires identical objects under each layer at both sites.
• Logical connection helps understanding task layering in data communication and networking

The OSI Model

• Network devices connect using wired or wireless transmission media.
• The International Standards Organization (ISO) created the Open Systems Interconnection (OSI) model.
• The ISO was established in 1947.
• Aims to create international standards for global agreement.
• The OSI model is an ISO standard.
• It covers all aspects of network communications and was introduced in the late 1970s.
• ISO is the organization, and OSI represents the model.
• There are seven layers to the OSI model:
• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

Layers of the OSI Model

• The physical layer's coordinate function carries a bit stream over a physical medium.
• Deals with mechanical and electrical specifications of interfaces.
• Data rate refers to the duration of a bit.
• Synchronization of bits is between sender and receiver clocks.
• Line configuration includes P2P, MP2P, and shared link.
• Physical topology and Transmission mode are considered.
• The data link layer transforms the physical layer into a reliable link.
• It makes the physical layer appear error-free to the upper layer. ( network layer)
• The data link layer divides bit streams into manageable data units called frames.
• It uses physical addressing.
• The flow control mechanism avoids overwhelming the receiver.
• It detects and retransmits damaged or lost frames.
• It also uses a mechanism to recognize duplicate frames.
• Access control is necessary when multiple devices connect to the same link.
• Data link layer protocols determine which device controls the link.
• The network layer delivers individual packets from source to destination.
• It uses logical addressing when a packet crosses a network boundary.
• The transport layer handles process-to-process delivery of the entire message.
• It recognizes packet relationships.
• Service point addressing uses port addresses.
• Segmentation and reassembly ensures correct message reassembly at the destination.
• Connection, flow, and error control is included
• The session layer provides dialog control and synchronization.
• Dialog control allows communication between two processes in half or full duplex mode.
• Synchronization allows adding checkpoints to the data stream.
• The presentation layer deals with the syntax and semantics of exchanged information.
• Translation provides interoperability between different encoding methods.
• Encryption/Decryption ensures privacy for sensitive information.
• Compression reduces the number of bits contained in the information.
• The application layer provides services to the user, such as:
• Network virtual terminal allows users to log on to remote hosts.
• Mail services
• Directory services: provides global information about various objects and services.

TCP/IP Protocol Suite

• A hierarchical protocol made of interactive modules.
• Hierarchical means each upper level protocol is supported by services from lower level protocols.
• The original TCP/IP protocol suite was defined as four software layers built upon hardware.
• Today, TCP/IP is thought of as a five-layer model.

TCP/IP and OSI Model

• Several application protocols are under the Application Layer
• The Transport Layer manages several transport protocols.
• Internet Protocol is used for the Network Layer. Underlying LAN and WAN technology are used for the Data Link layer

Critique of the TCP/IP Reference Model

• Service, interface, and protocol distinctions may not have been made.
• It is not a general model
• The host-to-network "layer" is not really a layer.
• There is no mention of physical and data link layers. Minor protocols deeply entrenched are hard to replace.

Addressing

• Four levels of addresses are in use in the internet using TCP/IP protocols.
• Physical
• Logical
• Port
• Specific Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits.
• Every byte (2 hexadecimal digits) is separated by a colon. An internet with two routers connects the three LANs.
• Each device has a logical and physical address for each connection.
• If a computer connects to one link, it has one pair of addresses.
• Each router connects to three networks and each router, needs three pairs of addresses, one for any connection that happens.
• IP Addresses are logical addresses
• The sending machine has three processes at a, b, and c ports running at said time.
• The receiving machine can have two processes running at j and k ports at same time.
• A port address is a 16-bit address represented by one decimal number as shown.

Network Hardware

• Local Area Networks Metropolitan Area Networks Wide Area Networks Wireless Networks Home Networks Internetworks

Network Software

• Protocol Hierarchies
• Design Issues for the Layers
• Connection-Oriented and Connectionless Services
• Service Primitives
• Relationship of Services to Protocols

Design Issues for the Layers

Addressing Error Control Flow Control Multiplexing Routing

Service Primitives (connection oriented)

The service primitives are:

• LISTEN : Block waiting for an incoming connection
• CONNECT: Establish a connection with a waiting peer
• RECEIVE: Block waiting for an incoming message
• SEND: Send a message to the peer
• DISCONNECT: Terminate a connection