v  Physical layer defines cables, network cards and physical aspects. It defines raw bit stream on the physical media. It also provides the interface between network and network communication devices.

v  It is also responsible for how many volts for 0 and how many for 1. Physical layer also checks the number of bits transmitted per second and two ways or one way transmission. Physical layer also deals with the optical, mechanical and electrical features.

v  The physical layer is concerned with the transmission of raw data bits over communication lines.

v  The Physical layer defines all the electrical and physical specifications for devices.

v  In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.

v  To understand the function of the physical layer in contrast to the functions of the data link layer, think of the physical layer as concerned primarily with the interaction of a single device with a medium, where the data link layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium.

v  The physical layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium).

v  Physical layer standards such as RS-232 do use physical wires to control access to the medium.

v  Protocols that work on the physical layer are ISDN, IEEE 802 and IEEE 802.2
Hubs, Repeaters, Oscilloscope and Amplifier works on the physical layer.

v  The physical layer accounts for much of the tangible components of a network, including cables, satellites, earth stations, repeaters, multiplexers, concentrators, and modems. Physical layer protocols and standards are of mechanical, electrical, functional, and procedural nature.

v  The major functions and services performed by the physical layer are:

                Establishment and termination of a connection to a communications medium.

                Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.

                Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

v  Modifies the simple digital signal pattern (1s and 0s) used by the pc to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization.

v  The data link layer hides the above details from the higher layers. To the network layer, it appears as a reliable communication channel which can send and receive data packets as frames.

v  The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer.

 

v  Protocols:

       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

§  Communicates with the adapter card

§  Controls the type of media being used:

       802.3 CSMA/CD (Ethernet)

       802.4 Token Bus (ARC net)

       802.5 Token Ring

       802.12 Demand Priority

v  Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system.

v  Both WAN and LAN services arrange bits, from the physical layer, into logical sequences called frames. Not all physical layer bits necessarily go into frames, as some of these bits are purely intended for physical layer functions. For example, every fifth bit of the FDDI bit stream is not used by the data link layer.

v  The data link layer is concerned with the reliable transfer of data over the communication channel provided by the physical layer.

v  To do this, the data link layer breaks the data into data frames, transmits the frames sequentially over the channel, and checks for transmission errors by requiring the receiving end to send back acknowledgment frames.

v  Data link protocols are concerned with the following issues:

             How to divide the data into frames.

             How to delimit frames by adding special bit patterns to the beginning and end of each frame. This allows the receiving end to detect where each frame begins and where it ends.

             Error detection:

§  Some form of error check is included in the frame header. This is constructed by the transmitting end based on the contents of the frame, and checked for integrity by the receiving end. A change in the frame bits can be detected in this way.

             Error correction:

§  When a frame arrives corrupted or is for any reason lost in the network, it is retransmitted. Lost acknowledgment frames may result in duplicate frames, which need to be detected and corrected as well.

             Flow control:

§  In general, not all communication devices in a network operate at the same speed. Flow control provides a means of avoiding a slow receiver from being swamped by data from a fast transmitter.

v  The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer.

v  The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.

v  Network devices including Router, Brouter, Frame Relay device and ATM switch devices work on the network layer. 

v  These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.

v  The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it’s easier to visualize this layer as managing the sequence of human carriers taking a letter from the sender to the local post office, trucks that carry sacks of mail to other post offices or airports, airplanes that carry airmail between major cities, trucks that distribute mail sacks in a city, and carriers that take a letter to its destinations. Think of fragmentation as splitting a large document into smaller envelopes for shipping, or, in the case of the network layer, splitting an application or transport record into packets.

v  The network layer is concerned with the routing of data across the network from one end to another.

v  To do this, the network layer converts the data into packets and ensures that the packets  are delivered to their final destination, where they can be converted back into the original data.

v  Network layer protocols are concerned with the following issues:

                                              The interface between a host and the network.

                                              The interface between two hosts across the network.

          Routing of packets across the network, including the allocation of a route and handling of congestion.

                                              Correct ordering of packets to reflect the original order of data.

          Collection of statistical information (e.g., number of transmitted packets) for performance measurement and accounting purposes.

                                              Internetworking: communication between two or more networks.

v  The network layer hides the above details from the higher layers. To the transport layer, it appears as a uniform data transfer service, regardless of the location of the communicating devices and how they are connected.

Network layer provides:

v  Routing:

     Routes frames among networks.

v  Subnet Traffic Control:

Routers can instruct a sending station to “throttle back “ its frame transmission when the router’s buffer fills up.

v  Frame Fragmentation:

If it determines that downstream router’s maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-   assemble at the destination station.

v  Logical-Physical Address Mapping:

Translates logical addresses, or names, into physical addresses.

v  Subnet Usage Accounting:

It has accounting functions to keep track of frames forwarded by subnet        Intermediate systems, to produce billing information.

v  The aim of the transport layer is to isolate the upper three layers from the network, so that any changes to the network equipment technology will be confined to the lower three layers (i.e., at the node level).  

v  The Brouter, Gateway and Cable tester work on the transport layer. 

v  These protocols work on the transport layer TCP, SPX, NETBIOS, ATP and NWLINK.

v  The Transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/de segmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail.

v  Although it was not developed under the OSI Reference Model and does not strictly conform to the OSI definition of the Transport Service, the best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets.

v  Transport layer protocols are concerned with the following issues:

             Establishment and termination of host-to-host connections.

             Efficient and cost-effective delivery of data across the network from one host to another.

             Multiplexing of data, if necessary, to improve use of network bandwidth, and demultiplexing at the other end.

             Splitting of data across multiple network connections, if necessary, to improve throughput, and recombining at the other end.

             Flow control between hosts.

             Addressing of messages to their corresponding connections. The address information appears as a part of the message header.

             Type of service to be provided to the session layer (e.g., error-free versus error prone connections, whether messages should be delivered in the order received or not).

v  The transport layer hides the above details from the higher layers. To the session layer, it appears as a customized data transfer service between two hosts, isolating the underlying network technology from it.

The transport layer provides

v  Message segmentation:

Accept a message from the (session) layer above it, splits the message into smaller units and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.

v  Message acknowledgment:

Provides reliable end-to-end message delivery with acknowledgments.

v  Message Traffic Control:

Tells the transmitting station to “back-off” when no message buffers are available.

v  Session Multiplexing:

Multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions.

v  Responsible for packet creation.

v  Provides and additional connection level beneath the session layer.

v  Ensures that packets are delivered error free, in sequence with no losses duplications.

v  Unpacks, reassembles and sends receipt of messages at the receiving end.

v  Provides flow control, error handling, and solves transmission problem.

v  Typically the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network layer.

v  Consequently, the transport layer must break up the messages into smaller units, or frames.

v  The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries.

v  In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handling the received message up to the layer above.

v  Gateways operate at this layer.

v  Transmits segments.

v  The session layer provides a structured means for data exchange between user processes on communicating hosts.

v  The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures.

v  The OSI model made this layer responsible for “graceful close” of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite. Session layers are commonly used in application environments that make use of remote procedure calls (RPCs).

 

v  Gateway used in Session layer

 

 

v  The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC

v  Session layer protocols are concerned with the following issues:

             Negotiating the establishment of a connection (a session) between user processes on communicating hosts, and its subsequent termination. This includes the setting of various communication parameters for the session (e.g., synchronization and control).

             Correct ordering of messages when this function is not performed by the transport layer.

             Recovery from interrupted transport connections, if necessary.

             Grouping of messages into a larger message, if necessary, so that the larger message becomes available at the destination only when its constituent messages have all been delivered successfully.

v  The session layer hides the above details from the higher layers. To the presentation layer, it appears as an organized communication service between user processes.

The Session layer provides:

v  Session Establishment:

Maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session

v  Session support:

Performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

v  Allows two application running on different computers to establish use nad end a connection called a session

v  Performs name recognition and security.

v  Provides synchronization by placing checkpoints in the data stream.

v  Implements dialog control between communicating processes.

v  The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes.

v  Provides error reporting for the application and presentation layer.

v  Gateways operate at this layer.

v  Transmits data.

v  The Presentation layer establishes a context between application layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.

v  The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML. ASN.1 has a set of cryptographic encoding rules that allows end-to-end encryption between application entities.

v  The presentation layer provides a mutually-agreeable binary representation of the application data communicated between two user processes. Since there are many ways of encoding application data (e.g., integers, text) into binary data, agreement on a common representation is necessary.

 

v  Gateway Redirector used in presentation layer:

v  Presentation layer protocols are concerned with issues such as the following:

          Abstract representation of application data.

          Binary representation of application data.

          Conversion between the binary representation of application data and a common format for transmission between peer applications.

          Data compression to better utilize network bandwidth.

          Data encryption as a security measure.

v  The presentation layer hides the above details from the higher layers. To the application layer, it appears as a universal communication service between user processes, regardless of their system-specific idiosyncrasies, allowing them to converse in a common syntax.

The presentation layer provides:

v  Character Code Translation

For example ASCII to EBCDIC   

v  Data Conversion:

Bit order, integer-floating point, and so on.

v  Data Compression:

Reduces the number of bits that need to be transmitted on the network.

v  Data Encryption:

Encrypt data for security purposes. For example, password encryption.

v  Determines the format used to exchange data among the networked computers.

v  Translates data from a format from the application layer into an intermediate format.

v  Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.

v  Redirector operates at this level.

v  Defines coding and conversion functions.

v  Ensures that information sent from the application layer of one system is readable by the application layer of another system.

v  Includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

             MPEG, ASCII, EBCDIC, GIF, JPEG

v  Gateways operate at this layer.

v  Transmits data.

v  It is the top most layer of OSI model.

v  It provides services that directly support user application such as database access, e-mail and file transfer

v  The application layer is concerned with the semantics of data, i.e., what the data means to applications.

v  The application layer interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.

v  Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer.

v  Gateway network device is operated on the application layer.

v  FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer.

v  The OSI model does not include human interfaces. The common application services sub layer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Control, and Transaction Processing (according to the ACID requirements).

v  The application layer provides standards for supporting a variety of application-independent services.

v  Examples include:

          Virtual terminal standards to allow applications to communicate with different types of terminals in a device-independent manner.

                                              Message handling system standards used for electronic mail.

          File transfer, access, and management standards for exchanging files or parts thereof between different systems.

          Transaction processing standards to allow different companies with different systems to access each other’s on-line databases (e.g., in banking and airline reservation).

          On-line directory standards for storing details of individuals, organizations, and network components.

                                              Standards for exchanging formatted documents.

v  Application layer standards have paved the way for open software systems, in which data can be communicated between incompatible base systems (i.e., different hardware and software architectures) without loss of meaning or usefulness.

 

The application layer provides:

v  Resource sharing and device redirection

v  Remote file access

v  Remote printer access

v  Inter-process communication.

v  Network management.

v  Directory services.

v  Electronic messaging(Such as E-mail)

v  Network virtual terminals.

v  Serves as window for applications to access network services.

v  Handles general network access, flow control and error recovery

v  Provides network services directly to applications. Type of software programs varies a lot: from groupware and web browser to video game. Software programs itself are not part of the OSI model.

v  Determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication.

v  This layer is closest to the user.

v  Gateways operate at this layer.

v  Transmits data.