Basic Communications Model Standards are needed at all Layers User Layer Application Layer Computer (Transport) Layer Transmission Layer

Subnet Layer Transmission of Messages 1. Within a Single Subnet, or Station A Station B 2 2 1 1 4 3 Transmission of Messages 1. Within a Single Subnet, or 2. in Point-to-Point Links Between Subnets

Across multiple subnets in an internet Internet Layer Station A Station B 2 1 4 3 Routing of Messages Across multiple subnets in an internet

Internetting Station A Station B 2 1 4 3 internet: collection of subnets such that any station on any subnet can communicate with any other station on any other subnet simply by giving the internet address of the other station.

Layering in Major Architectures Basic Model TCP/IP IPX/SPX OSI Application Application Application Application (7) Presentation (6) Computer (Transport) Transport: TCP, etc. Complex: SPX, etc. Session (5) Transport (4) Internet Internet Internet Network (3): Internet & Subnet Subnet (OSI) Subnet (OSI) Subnet (OSI) Data Link (2) Physical (1)

OSI Architecture Reference Model for Open Systems Interconnection (OSI) Standards Agencies ISO (computers) Don’t confuse OSI and ISO! ITU-T (telecommunications) De jure standards (official/open)

OSI Architecture Standards are sophisticated Sophistication brings complexity Slow standards development Products are late to market and expensive The result: low acceptance of OSI standards in general But OSI dominates at the single network (subnet) transmission layer

Physical Layer (OSI Layer 1) Physical transmission standards Physical Connector Transmission media Electrical Voltage levels Meanings of various voltage levels

The Data Link Layer (OSI Layer 2)

Data Link Layer (OSI Layer 2) Organize data for transmission over a data link Data packages at various layers are called protocol data units (PDUs) The Data Link Layer PDU is called a frame Organization allows addressing on shared data links Organization allows error checking and other meta delivery processes Media Access Control Controls when stations may transmit in shared media LANs and other shared data links

TCP/IP Architecture Internet Engineering Task Force (IETF) Of the Internet Society De jure (official/open)

TCP/IP Architecture Standards are simple Widely Used Often have “Simple” in the name Standards and products are fast to market Products are inexpensive and fast to market Widely Used On the Internet In many corporate networks Dominates at internet and transport layers