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OSI Reference Model

Application -File, printing, message, database, and application services.
Presentation -Data encryption / decryption, compression, and translating services.
Session -Dialog control.
Transport -End to end connection.
Network -Routing.
Data Link -Framing.
Physical -Physical topology.

Advantages of Using a Layered Model
  1. Allows a layer to be changed without impacting the rest of the model.
  2. Interoperability between network applications is improved by using a standard interface.
  3. Design and development efforts can be made in a modular fashion.
  4. Network operations and troubleshooting can be simplified.

Five Conversion Steps of Data Encapsulation     Data >> Segments >> Packets >> Frames >> Bits
  1. Upper layers convert and format the information into data and send it to the Transport Layer.
  2. The Transport layer turns the data into segments and adds headers then sends them to the Network layer.
  3. The Network layer receives the segments and converts them into packets and adds header information (logical addressing) and sends them to the Data Link Layer.
  4. The Data Link layer receives the packets and converts them into frames and adds header information (physical source and destination addresses) and sends the frames to the Physical Layer.
  5. The Physical layer receives the frames and converts them into bits to be put on the network medium.

Application Layer

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application.  This layer interacts with software applications that implement a communicating component.  Such application programs fall outside the scope of the OSI model.  Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.

When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit.  When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist.  In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.

Two key types of application-layer implementations are TCP/IP applications and OSI applications.  TCP/IP applications are protocols, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP), that exist in the Internet Protocol suite.  OSI applications are protocols, such as File Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and Common Management Information Protocol (CMIP), that exist in the OSI suite.

Internetworking Applications


-Connects countless servers presenting diverse formats: multimedia, graphics, text, sound, and video.  Applications such as Netscape Navigator, Internet Explorer, and Mosaic simplify accessing and viewing web sites. 


-Versatile can use SMTP or X.400 to deliver messages between different email applications.

Electronic Data Interchange

-Composite of specialized standards that facilitates the flow of tasks such as accounting, shipping / receiving, and order and inventory tracking between business.

Bulletin Boards

-Includes Internet chat rooms, and sharing public domain software.

Internet Navigation Utilities

-Includes Gopher, WAIS, and search engines, e.g. Yahoo, Excite, and Alta Vista.  Helps users locate resources and information on the Internet. 

Financial Transaction Services

-They gather and sell information pertaining to investments and credit data to their subscribers.

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Presentation Layer

The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system will be readable by the application layer of another system. Some examples of presentation-layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes.

Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination.  Presentation-layer implementations are not typically associated with a particular protocol stack.  The following serve to direct graphic and visual image presentations:


-Picture format used by Mac and PowerPC programs for transferring Quick draw graphics.


-Tagged Image File Format, a standard graphics format for high-resolution bitmapped images.


-Joint Photographic Experts Group standards.


-Musical Instrument Digital Interface, used for digitized music.


-Moving Picture Experts Group, standard for compression and coding of motion video.  Digital storage and bit rates up to 1.5 Mbps.


-Mac and PowerPC audio and video applications.

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Session Layer

The session layer establishes, manages, and terminates communication sessions between presentation layer entities.  Communication sessions consist of service requests and service responses that occur between applications located in different network devices.  These requests and responses are coordinated by protocols implemented at the session layer.  Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session-layer protocol.  Also provides dialog control between devices or nodes.  Coordinates and organizes communications between system by offering three different modes: simplex, half-duplex, and full-duplex.  The layer basically keeps different applications' data separate from other applications' data.

Session Layer Protocols and Interfaces


-Network File System, developed by Sun Microsystems and used with TCP/IP and Unix workstations to allow transparent access to remote resources. 


-Developed by IBM to provide users with a simpler way to define their information requirements on both local and remote systems.


-A broad client / server redirection tool used for disparate service environment.  Its procedures are created on clients and performed on servers. 

X Window

-Widely used by intelligent terminals for communications with remote Unix computers, allowing them to operate as though they were locally attached monitors.

AppleTalk Session Protocol

-A client / server mechanism which establishes and maintains sessions between AppleTalk client and server machines.  

Digital Network Architecture Session Control Protocol

-A DECnet session layer protocol.

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Transport Layer

The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery.  Services located in Transport layer both segment and reassemble data from upper layer applications and unite it onto the same data stream.  They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an Internetwork.  It also hides details of any network dependent information from the higher layer by providing transparent data transfer. 

Flow Control

-Data integrity is ensured by maintaining flow control and allowing users the option to request reliable data transport between systems.  Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process.  Reliable data transport employs a connection-oriented communication session between systems.  The protocols ensure that the following are achieved: 

-segments delivered are acknowledged to sender upon delivery.

-non acknowledged segments are re-sent.

-segments are put back in sequence upon arrival at their destination.

-a manageable data flow is maintained to avoid congestion, overloading, and data loss.


The Transport layer is responsible for providing mechanisms for multiplexing upper layer applications.  Multiplexing enables data from several applications to be transmitted onto a single physical link.

Virtual Circuits

Virtual circuits are established, maintained, and terminated by the transport layer.

Error Checking and Recovery

Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur.


-Positive acknowledgement with retransmission ensures that reliable data delivery by requiring a receiving machine to send an acknowledgment message to the sender when it receives data.  The sending machine documents each segment sent and waits for an acknowledgment before sending the next segment.  Using windowing, the machine will transfer an agreed upon number of segments.  If the receiving machine receives all the segments intact, it will request the next segment of the next window.  If it misses a segment, it will request the missing segment and will transmit a request for the next segment of the next window, when the first window's segments are all received.

-During a transfer, congestion can occur because high speed computers can generate data faster that the network can transfer it or because many computers are using the network and sending datagrams through a single gateway.  When a machine receives a flood of datagrams, it stores them in a buffer.  If the buffer fills, all additional datagrams are discarded.  Transport can issue a "not ready" signal to stop a device from transmitting additional segments.  Once the buffer is emptied, it sends a "ready" transport indicator.  When the waiting machine receives this "go" signal, it continues where it left off.  To avoid failures in data transfers, the receiving host acknowledges every segment it receives.

Connection-Oriented Communications

-In reliable transport operations


-A window is the number of segments that can be sent without receiving an acknowledgement.  Windowing can increase the throughput for data exchanges by limiting the number of acknowledgments needed for total segments transferred.  Example: if the window size is three then an acknowledgment is required after the third segment is transferred.

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Network Layer

The Network layer provides routing and related functions that enable multiple data links to be combined into an internetwork.  This is accomplished by the logical addressing (as opposed to the physical addressing) of devices.  The network layer supports both connection-oriented and connectionless service from higher-layer protocols. Network-layer protocols typically are routing protocols, but other types of protocols are implemented at the network layer as well.  Routers work at this level and provide the routing services for an internetwork.

Routing a Packet

  1. The router receives the packet and looks up the destination IP address.
  2. If the packet isn't destined for the router, the router looks for the destination address in the routing table.
  3. Once the destination interface is found, the packet will be sent to the interface.
  4. At the destination interface, the packet is framed and sent out on the local network.

-There are two types of packets at the Network layer.

Data Packets

-Used to transport user data through internetwork.

-Uses routed protocols such as: IP and IPX.

Router Update Packets

-Used to update neighbor routers about networks connected to routers on the internetwork.

-Routing protocols: RIP, EIGRP, OSPF.

-Builds and maintains routing tables on each router.

Routing Table

Network Address

-Protocol specific network addresses.  A table is maintained for individual routing protocols since each protocol keeps track of a network with a different addressing scheme.


-The interface the packet is sent out on when destined for a particular network.


-The distance to the remote network.

-Routers breakup broadcast domains by not forwarding broadcast or multicast packets through a router.  They also breakup collision domains as each interface is a separate network.

-Routers use logical addresses in a network layer header to determine the next hop router to forward the packet to.

-Routers can use access lists to control security on packets entering or leaving an interface.

-Routers can provide layer 2 bridging and can simultaneously route through the same interface

-Routers provide connections between Virtual LANs. (VLANs)

-Routers can provide Quality of Service for specific types of network traffic.

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Network vs. Data Link Layer Addresses

Network layer addressing is referred to as logical addressing, whereas Data Link layer addressing uses physical addresses. The physical address of a device can't be changed without removing or replacing the hardware (physical address is burned into a NIC's ROM); while a logical address is configured in software and can be changed as needed.

Data Link Layer

The Data Link layer provides reliable transit of data across a physical network link.  Different Data Link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control.  The Data Link layer translates messages from the Network layer into bits for the Physical layer to transmit.  It formats messages into data frames and adds a customized header containing the source and destination hardware addresses.  Data Link layer is responsible for uniquely identifying each device on a local network.

-When a packet is sent between routers, it is framed with control information at the Data Link layer.  The information is removed at the destination router and only the original packet remains.  If the packet is to go to another router, the framing process is repeated until it gets to the receiving host.  The packet is never altered, only encapsulated with control information to be passed on to the different media type.

The IEEE has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).

MAC (Media Access Control)

The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.

--The MAC describes how a station schedules, transmits and receives data on a shared media environment.

--Ensures reliable transfer of information across the link, synchronizes data transmission, recognizes errors (doesn't correct them), and controls the flow of data.

--Defines how packets are placed on the media.

--Physical addressing is defined here as well as local topologies.

--MAC example is Ethernet/802.3 and Token Ring/802.5

--Line discipline, error notification, ordered delivery of frames, and optional flow control can be used at this layer.

--In General, MACs are only important in shared medium environments where multiple nodes can connect to the same transmission medium.

LLC (Logical Link Control)

The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network.  LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols.  IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link.

--Responsible for identifying Network layer protocols and encapsulating them.

--A LLC header tells the Data Link layer what to do with a packet once it is received.

Switches and Bridges

-Layer 2 devices (Switches / Bridges) propagate broadcast storms and the only way to prevent them is with a router.

-Each port on a switch is in its own collision domain.

-Switches allow all segments to transmit simultaneously.

-Switches can't translate different media types.

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Physical Layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.  Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors.  Physical-layer implementations can be categorized as either LAN or WAN specifications.

-The Physical layer has two responsibilities, send and receive bits (bits have a value of 1 or 0).

-The interface between DCEs and DTEs is defined at the Physical layer.

-The DCE is on the service provider side.

-The DTE is the attached device, the services available to a DTE are accessed through a CSU/DSU.

-HSSI Peer-based communications assumes intelligence in DCE and DTE devices.

Hubs and Repeaters

-Hubs are multiple port repeaters.  A repeater receives a signal, regenerates the digital signal, and forwards it on all active ports.  An active hub does the same thing.  All devices plugged into a hub are on the same collision and the same broadcast domains.  Hubs don't look at any traffic that enters, it just forwards all traffic to all ports.  Every device connected to the hub must listen if a device transmits.

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