webpointmorpheus Network Info
IEEE & The OSI Model

     Overview      IEEE Specification Outline      OSI Layers
     Layer Operation      OSI Layer Details

©2005 - material compiled by Bob Carnaghi, www.webpointmorpheus.com

Overview     Top of Page

This page offers an introduction to the methods and definitions that outline computer networking. The networking standards were initially defined by the IEEE, which is outlined in next paragraph. The IEEE standards are in a constant state of refinement, and take their form based upon the OSI Seven Layer Model, which is outlined below. The entire spectrum of computer networking hangs upon the frame of definitions provided by the IEEE and the OSI Layer Model.

If you have a difficult time with the acronymn drenched terms used in this document, check out the Network Definitions Page.

The IEEE     Top of Page

The IEEE - Institute of Electrical and Electronics Engineers - is a standards organization that oversees the quality and inter-operability of today's computing applications and hardware. The IEEE vision is stated: "To advance global prosperity by fostering technological innovation, enabling members' careers and promoting community world-wide." The IEEE Mission: "The IEEE promotes the engineering process of creating, developing, integrating, sharing, and applying knowledge about electro and information technologies and sciences for the benefit of humanity and the profession." See the IEEE Home Page for more information on the structure and operation of the IEEE.

IEEE 802 Subcommittees
Subcommittee	Scope
IEEE 802	LAN/MAN Overview & Architecture
IEEE 802.1	LAN/MAN Bridging and Management (Higher Layer LAN Protocols)
IEEE 802.1s	Multiple Spanning Tree
IEEE 802.1w	Rapid Reconfiguration of Spanning Tree
IEEE 802.1x	Port-based Network Access Control
IEEE 802.2	Logical Link Control (LLC)
IEEE 802.3	CSMA/CD access method (Ethernet)
IEEE 802.3ab	1000BaseT - Gigabit Ethenet
IEEE 802.3ae	10 Gigabit Ethernet
IEEE 802.3z	1000BaseCX, 1000BaseSX, 1000BaseLX
IEEE 802.4	Token Passing Bus access methods & Physical layer specifications
IEEE 802.5	Token Ring access methods & Physical layer specifications
IEEE 802.6	Distributed Queue Dual Bus (DQDB) access method and Physical layer specifications (MAN)
IEEE 802.7	Broadband LAN
IEEE 802.8	Fiber Optic
IEEE 802.9	Isochronous LANs (this standard is not currently used)
IEEE 802.10	Interoperable LAN/MAN Security
IEEE 802.11	Wireless LAN Medium Access Control (MAC) and Physical layer specifications
IEEE 802.12	Demand-priority access method, Physical Layer, and repeater specifications
IEEE 802.13	Not currently in use
IEEE 802.14	Cable Modems (this standard is not currently used)
IEEE 802.15	Wireless Personal Area Network (WPAN)
IEEE 802.16	Wireless Metropolitan Area Network (Wireless MAN)
IEEE 802.17	Resilient Packet Ring (RPR) Access

 

The OSI Seven Layer Model     Top of Page

The OSI^a layer model was developed by the International Standards Organization in 1984. The objective of the model is to provide a common standard for the complex relationships in computer networks. It divides the different functions and services provided by network hardware and software into 7 separate compartmentalized layers. This approach facilitates modular engineering, simplifies network technologies, and helps to isolate problems when troubleshooting. Software and hardware vendors can focus on only the specific layer(s) in which their hardware or software is implemented. This approach makes possible products that are compatible, standardized and interoperable.

   The Seven Layers:^b

1. Application Layer
• The Application layer provides network services and tools for programmers to define network use.
• The Application layer provides network services directly to the user's application. This layer is said to be "closest to the user".
• Some of the software defined at the Application layer are the web browser, email software and Windows Explorer, and other software with which a typical user interacts.
• Application Layer protocols include TELNET, HTTP, FTP, TFTP, SMTP, NTP, etc.
• The Application layer is also referred to as Layer 7.
2. Presentation Layer
• The Presentation Layer incudes a set of standards that assure consistent inter-communication between applications on different systems. This set of protocols assures that a document created on a MAC can be smoothly presented on a PC, etc. This layer defines encryption, compression, conversion and other coding functions. This layer 'represents' the data in a particular format to the Application layer.
• Presentation Layer document formats may include PostScript, PDF formats, .txt, .doc, and others which are commonly shared between applications.
• Specifications defined at this layer include: GIF, JPEG, MPEG, MIME, and ASCII, and others.
• The Presentation layer is also referred to as Layer 6.
3. Session Layer
• The Session Layer establishes, maintains, and terminates a distinct end-to-end connection between communicants: who is sending, who is receiving, who should get what, for how long, etc. This layer controls the dialog between source and node. It also provides error reporting for the Application, Presentation and Session layer.
• The Session Layer uses protocols for sockets, RPC, NETBIOS, etc.
• The Session layer is also referred to as Layer 5.
4. Transport Layer
• The Transport Layer breaks the information stream created by the user (from the upper layers) into managable chunks called segments. The Transport layer is responsible for end-to-end (also called source-to-destination) delivery of entire messages. The segments are formatted for transmission across the network as a continuous message.
• The Transport Layer provides reliable connectivity to the data transfer by sequencing the segments to guarantee that they will be delivered and reassembled in the same order that they were sent. The Transport Layer also provides services such as error checking and software type flow control.
• This is the pivotal layer between the upper layers (layers 5, 6, & 7) that deal with the actual information, and the lower layers (layers 1, 2, & 3) that deal with the transmission of the packets.
• This layer offers two modes of transmission: connection-oriented and connectionless. Connection-oriented transmissions maintain continuity to the message, guaranteeing with an 'ack' (acknowledgement) that the data has arrived safely in its entirety. Connectionless transmissions only send data, they are not concerned with whether it arrives or not.
1. TCP is an example of an connection-oriented transport protocol. Connection-oriented means that a connection (a virtual link) must be established before data can be exchanged successfully and coherently. Delivery is guaranteed by sending an acknowledgement ('ack') back to the source when messages are received. Web pages require a connection-oriented data transfer. Otherwise, the page may not have all the text or images, and the computer/browser would never know.
2. Connectionless is the opposite of connection-oriented; the sender does not establish a connection before it sends data, it just sends without guaranteeing delivery. UDP is a connectionless transport protocol. A good example of a connectionless transmission is a radio station: The DJ just sends the tunes out to the listeners. He rarely knows who, if anyone, receives them successfully.
• Protocols that operate on the Transport Layer are TCP, UDP, NETBEUI, SPX, etc.
• The Transport layer is also referred to as Layer 4.
5. Network Layer
• The Network Layer is defined by a set of rules for adding information to the segments of data that control how it is handled by routers, hubs, switches, etc. during transmission between networks. This layer is not concerned with the quality or continuity of the data being transmitted. The actual transmission is the only concern of this layer.
• This layer converts the segments from the Transport layer into packets (or datagrams) and is responsible for path determination, routing, and the delivery of these individual packets across multiple networks without guaranteed delivery. The network layer treats these packets independently, without recognizing any relationship between those packets. It relies on upper layers for reliable delivery and sequencing. This layer is is responsible for logical addressing (also known as network addressing or Layer 3 addressing.)
• Protocols defined at this layer: IP, IPX, ICMP, RIP, OSPF, BGP.
• Devices that operate on this layer: Routers, Layer 3 Switches.
• The Network layer is also referred to as Layer 3.
6. Data Link Layer
• The Data Link Layer defines a series of protocols for the use of and access to the Physical Layer. This layer defines an instantaneous communication that operates on a per-transmission basis, and must be re-created for each new tranmission.
• The Data Link Layer allows for the methods of individual machine identification on the network, collision detection and avoidance on the network, error detection and correction, flow control, etc. These functions are typically NIC specific.
• The Data Link Layer provides transparent network services to the Network layer so the Network layer can be ignorant about the physical network topology and and provide access to the physical networking media. This layer is responsible for reassambling bits taken off the wire by the Physical layer into frames, makeing sure they are in the correct order, and requesting retransmission of missing frames in case errors occur. Error checking is provided by adding a CRC (Cyclic Redundancy Check) to the frame, as well as flow control.
• IEEE 802 divided the Data Link layer into two sublayers: the LLC and the MAC sub layer.
1. LLC sublayer - Logical Link Control. The Logical Link Control is the upper sublayer of the Data Link layer. LLC masks the underlying network technology by hiding the differences between the layers. A single interface is then provided to the network layer. The LLC sublayer uses Source Service Access Points (SSAPs) and Destination Service Access Points (DSAPs) to help the lower layers communicate to the Network layer protocols acting as intermediaries between the different network protocols (IPX, TCP/IP, etc.) and the different network types (Ethernet, Token Ring, etc.) This layer is also responsible for frames sequencing and acknowledgements. The LLC sublayer is defined in the IEEE standard 802.2.
2. MAC sublayer - Media Access Control. This sub-layer takes care of physical addressing and allows upper layers access to the physical media. It also handles frame addressing and error checking. This layer controls and communicates directly with the physical network media through the network interface card. It converts the frames into bits to pass them on to the Physical layer which puts them on the wire. IEEE LAN standards such as 802.3, 802.4, 802.5 and 802.10 define standards for the MAC sublayer as well as the Physical layer.
• Switches and Bridges work at the Data Link Layer.
• The Data Link layer is also referred to as Layer 2.
7. Physical Layer
• The Physical Layer communicates directly with the physical media, it is responsible for activating, maintaining and deactivating the physical link. It handles a raw bit stream and places it on the wire to be picked up by the Physical layer at the receiving node. It defines electrical and optical signaling, voltage levels, data transmission rates and distances as well as mechanical specifications such as cable lengths and connectors, the amount of pins and their function, and other physical specifications.
• The Physical Layer defines electrical and fiber optical signaling, voltage levels, data transmission rates and distances as well as mechanical specifications such as cable lengths and connectors, the amount and configuraion of pins, and their function.
• The Physical Layer provides a set of protocols for the electrical transformation of data for transmission. Data is transformed into pulses, (Ethernet,) light, (Fiber-optics,) or Radio Waves, (Wireless).
• Devices and protocols that operate on the Physical Layer are HUBs/concentrators, repeaters, NICs, and LAN and WAN interfaces such as RS-232, OC-3 and BRI, etc.
• The Physical layer is also referred to as Layer 1.

Layer Operation     Top of Page

The Application, Presentation and Session layer are known as the Upper Layers. Those layers are implemented in software. The Transport and Network layer are concerned with protocols for delivery and routing of packets to a destination. They are implemented in software as well. The Data Link Layer works in hard- and software. The Physical layer is operable in hardware only. The Data Link and Physical Layers define LAN and WAN specifications.

Details     Top of Page

Listed below is a condensed description of each layer. This is a model of the process of data passing from Host A to Host B

1. The Application, Presentation and Session layer take user input and convert it into data.
2. The Transport layer adds a segment header converting the data into many tiny pieces called segments.
3. The Network layer adds a network header and converts the segments into packets, or datagrams.
4. The Data Link layer adds a frame header converting the packets/datagrams into frames.
5. The MAC sublayer converts the frames into a bits, which the Physical layer can then place on the wire or radio frequency for transmission.

This process is known as data encapsulation. When the bit stream arrives at the destination, the Physical layer takes it off the wire and converts it into frames. Each layer will then work in reverse to remove their corresponding header while the data packets flow up the OSI model. Finally it is converted back to data and presented to the user. This reverse process is known as decapsulation.

Notes     Top of Page

1. Open System Interconnection Model.
2. The numbering scheme appears to be backwards. However, the layers as shown are how they are often referred to in technical circles. The top layer - the application layer, where data is actually applied to the end user, is referred to as 'Layer 7'.