As LANs became more and more popular, attempts were made to trunk one network to another. These attempts were unsuccessful as each company marketing their own network would create and use their own standards of communication over the network. To solve this problem the International Standards organization (ISO) proposed a seven Layer model that would provide a common basis for the development if standards that would allow different systems to be connected. Each layer provides services needed by the next layer in a way that frees the upper layer from concern about how these services are provided. This simplifies the design of each layer.

This provides

• Less complex elements
• Standard Interfaces
• Design specialization
• Promotes functional symmetry
• Multi-vendor integration
• Modular interoperability
• Independent evolution
• More easily learned.

This model is called the Open Systems Interconnection Model. The ISO-OSI seven Layers are defined below:

1. Physical Layer (layer 1)

• This layer provides mechanical, electrical, functional, and procedural means to activate and deactivate physical transmission connections between datalinks.

• Physical Trunk describes the function of the electrical and mechanical characteristics of the network.

• Functional
• Electrical
• MBps - MHz,
• Manchester Encoding,
• Noise,
• Grounding
• Procedural
• Documentation,
• Installation,
• Certification,
• Administration
• Mechanical
• Copper Cable,
• Fiber Optic,
• Wireless,
• Cell
• Radio
• Microwave
• Infra Red

1. Datalink Layer (layer 2)

• This layer provides functional and procedural means for connectionless-mode transmission among networks. SUPER-UX supports CSMA/CD for Ethernet.
• Data Trunk How the data is packaged for transmission.
• Provides for the Physical
• Operations
• Physical addressing
• Network Topology
• Line Discipline
• Error Notification
• Flow Control
• Access to Media
• Physical Layer
• MAC Address
• Logical Layer
• LLC

• Ethernet 802.3 CSMA/CD
• Token Ring 802.5
• FDDI/ANSI

• Network Topology
• WAN
• HDLC
• LAPB
• PPP
• ISDN
• Frame Relay
• ATM

1. Network Layer (layer 3)

• This layer provides a means of connectionless-mode transmission among transport entities. It makes transport entities independent of routing and relay considerations associated with connectionless-mode transmission. SUPER-UX supports Internet Protocol (IP) for Ethernet and HYPERchannel-DX.

• Network how communication is established, maintained and terminated between stations.
• Network Addressing - two part, host & network
• All protocols
• IP
• IPX
• DECnet
• AppleTalk
• Banyan Vines
• X25
• Path Determination
• Routing Table
• Initialization
• Maintenance
• Routing Metrics
• Update Process
• Convergence

1. Transport Layer (layer 4)

• This layer provides transparent data transfer between sessions and relieves them of concern about achieving reliable and cost effective data transfer.

• SUPER-UX supports Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). SUPER-UX also supports UltraNet (TP-4).
• Transport How data is to be transmitted and received
• Multiplexing, Capability of multiple applications to share a transport connection e.g. TCP Port Number.
• Defines end – end connectivity between host applications
• Segments – reassembles upper layer data stream.
• Provides logical connectivity between end systems
• Sends Segments from end – end
• Error control
• CRC
• Flow Control
• Congestion avoidance
• Windowing
• Positive ACK
• Ensures Data reliability

1. Session Layer (layer 5)

• This layer provides the services needed by protocols in the presentation layer to organize and synchronize their dialogue and manage data exchange.

• Session Organizes, synchronizes and manages the dialogue between users.
• Establishes Manages and terminates communications between applications

1. Presentation Layer (layer 6)

• This layer manages the representation of the information that application layer protocols either communicate or reference during communication.

• Presentation how information is to be translated from the station's format in to that understandable by the user.
• Provides code format and conversion services
• Data
• ASCII
• EBCIDIC
• HTML
• MIDI
• MPEG
• Graphics
• PIC
• TIFF
• JPEG
• GIF

1. Application Layer (layer 7)

• This layer serves as the window between corresponding application processes that are exchanging information.
• Application/User how information is to be exchanged between the user and the station.
• Service to user applications, not the applications themselves
• Services
• Email
• FTP
• RAS
• SNMP
• SMTP
• EDI
• WWW
• Transaction Services

Transport - Transport Layer = Segments

Network – Network Layer = Packets

Data Link – Data Link Layer = Frames

Physical – Physical Layer = Bits